Reactor safety

The safety of nuclear power plants is intended to protect people and the environment from the harmful effects of ionizing radiation emanating from nuclear power plants. It is crucially important in the design , approval, construction and operation of the plants. In particular, the release of dangerous radioactive substances must be prevented, accidents must be avoided and the effects of accidents that occur in spite of this must be limited to the facility itself.

This results in three central technical tasks (protection goals):

• Control of reactivity , see reactivity incident , e.g. B. Chernobyl nuclear disaster
• Cooling of the reactor core , see loss of coolant accident , decay heat , core meltdown , e.g. B. Fukushima nuclear disaster
• Safe containment of the radioactive material .

Reactor safety research deals with how these tasks can be fulfilled and how a plant can be protected against threats to these protection goals (e.g. natural disasters, human errors, technical failure, terrorism). The reactor safety is constantly examined and further developed by the manufacturers, supervisory authorities and power plant operators. The authorities usually issue safety requirements, compliance with which the manufacturer and operator must prove.

The specific protective measures depend largely on the technology used (e.g. light water reactor , high temperature reactor , breeder reactor ), the geographical location and national legislation. A distinction is made between organizational, structural and technical as well as between active and passive protective measures and systems. Basic concepts include conservative and redundant design, defense-in-depth, and probabilistic and deterministic security analysis.

Whether sufficient standards are applied to design, construction, operation and control, and whether they can be applied at all, has been the subject of intense public and scientific debates for decades, especially in German-speaking countries. Last but not least, several serious accidents have shown that one hundred percent safety cannot be achieved, there remains a residual risk .

Basics

Almost all commercially operated nuclear power plants are light water reactors . They release very high outputs in the smallest of spaces ( power density up to 100 MW / m³) and work at high operating temperatures and high operating pressures. These characteristics involve high risks. Heavy water reactors , on the other hand, have a comparatively lower power density, but have economic disadvantages.

In the reactor core of a pressurized water reactor , water is typically heated to about 320 degrees Celsius at a pressure of about 150 bar . A large nuclear reactor contains 80–150 tons of radioactive nuclear fuels , which, including their fission products, are only allowed to escape in very small quantities. In addition, almost all nuclear power plants have decay basins in which even larger quantities of radioactive material - mostly spent fuel elements - are stored. Failure to cool the cooling pool can also lead to the escape of radioactive substances.

For many years the pebble bed reactor was considered very safe . His spiritual father was Farrington Daniels ; Rudolf Schulten was responsible for the planning and construction of the test nuclear power plant AVR (electrical net output 13 MW) in Jülich from 1957 to 1964. In 2000, the operators admitted that the beta contamination ( strontium -90) of the AVR reactor is the highest of all reactors and nuclear plants in the world and is also in the most unfavorable form - namely dust-bound. A larger reactor, the THTR Hamm-Uentrop - planned from 1967 and commissioned in 1987 - was shut down a good two years later. Schulten and other supporters repeatedly emphasized the supposedly inherent safety of this type of reactor, which does not have to be "produced" by active measures or techniques. Apparently they ignored or misjudged two serious problems with this type of reactor:

• the spherical fuel elements are combustible (ignition temperature approx. 650 ° C); an accident with air entering the reactor would have resulted in a graphite fire with a high level of radioactivity being released.
• Leakages of the steam generator with water and / or steam ingress into the core lead to chemical reactions with graphite, which produce flammable and explosive gases ( hydrogen and carbon monoxide ). It can also lead to an increase in reactivity .

For more information on the development of the pebble bed concept, see here .

conditions

The design basis accident is referred to as the “largest assumed accident” (GAU). If this GAU is mastered, it was previously believed that all other incidents could also be safely mastered. Today we know that this is by no means always the case. Instead of the one (1) design basis accident, a whole spectrum of design basis accidents has been used, the control of which must be proven individually. In Germany, the requirements are regulated in detail in the so-called safety criteria and accident guidelines . These stipulate that control must always be guaranteed even if a device should be inoperable due to an additional (technical or human) fault that is independent of the triggering fault event ( single fault criterion ) and if a second partial device is currently being repaired ( Repair criterion ). These two criteria represent a more precise definition of the redundancy principle, according to which more facilities for the control of incidents must always be available than are actually required. In addition, the incident control facilities must be separated from the operating facilities and disconnected from one another, i.e. H. they must be arranged independently of one another (without common components) and spatially or structurally separated, and be diversified in order to avoid failures due to the same cause. Together with other requirements, such as the fail-safe principle (an error has an effect in the safe direction as far as possible) and automation (avoidance of personnel actions under time pressure), a high degree of reliability in incident control is sought overall.

The Fukushima nuclear disaster (from March 2011) prompted many countries to look at and assess the risks in a new or more impartial manner than before. The EU produced an extensive study known as the "stress test" (see below ).

Theoretical foundations

Protection goals

The basic protection goal for every nuclear power plant is to protect people and the environment from the harmful effects of ionizing radiation. One can strive to achieve this with the following four sub-goals:

• As long as the first barrier ( crystal lattice of the fuel) is retained, the vast majority of the radioactivity is safely retained. Due to the presence of the other barriers, a destruction of the crystal lattice does not automatically mean the release of large amounts of radioactivity.
• A larger scale destruction of the crystal lattice is technically only possible by melting the reactor core (or a considerable part of it). This results in the second protection goal: cooling the fuel assemblies.
• Since the safety-related cooling systems are only designed to dissipate the decay heat (and not for power operation), the reactor must always be able to be switched off safely . Third protection goal: control of reactivity by interrupting the chain reaction.
• If the reactor is not able to shut down, it must be ensured that the chain reaction does not escalate in an uncontrolled manner. This is guaranteed by "negative reactivity coefficients ". A negative temperature coefficient of reactivity ( Doppler coefficient ) causes z. B. that when the fissile material is heated, its reactivity automatically decreases. A negative Doppler coefficient can be achieved through the reactor construction and the design of the fuel assemblies. The EURATOM agreements stipulate that only nuclear reactors with a negative Doppler coefficient may be permitted to operate in the contracting states.

If these four goals are consistently achieved, major radiological accidents will not be possible. If it is injured, it can no longer be reliably excluded.

methodology

The main risk of nuclear power plants is that radioactive substances can escape into the environment through minor or major incidents or accidents . The release of radioactivity in normal operation is so small that its share is negligible compared to natural radiation exposure (essentially cosmic radiation and terrestrial radiation ) and, according to the current state of knowledge, the health damage attributable to it cannot be observed or, in the case of reprocessing plants, explained. In the following, therefore, only the accident safety of nuclear power plants will be discussed.

When considering accidents and malfunctions or when analyzing the causes , the assumption is made that a serious failure of technical equipment does not occur by chance, but rather due to a chain (or several chains) of causes and effects. If these chains of effects are identified, they can be specifically interrupted. If such an interruption is provided several times and with mutually independent measures, a very high level of security can be achieved overall, since errors in individual steps can be caught by the functioning of other steps. It does not matter whether these errors result from a failure of components or systems ("technical error") or from human error (" operating error ", " human error ", or "organizational error") (or both). One speaks of a “ multi-level, forgiving security concept” .

This approach is followed in nuclear power plants worldwide. How successful it is depends on its implementation. In the following, the systematic procedure for modern, western light water reactors is described. With other reactors, especially those from the former Eastern Bloc , the situation is clearly different.

Barriers

In western light water reactors, six barriers serve to hold back the radioactive substances:

• The crystal lattice of the fuel [within 6]

During nuclear fission in a reactor, the fission products arise as foreign atoms in the crystal lattice of the uranium dioxide . As long as this remains intact, most of the fission products are very reliably retained in the crystal lattice. This does not apply to the gaseous fission products (about 5–10% share).

• The gas-tight welded fuel rod casings [6]

The uranium dioxide is pressed into tablets, filled into roughly finger-thick Zircaloy tubes (strength properties similar to steel) and these tubes are then welded gas-tight at the top and bottom. As long as all weld seams are tight and there is no hole in a cladding tube, the cladding tubes keep all fission products inside. However, despite the high neutron permeability, structural changes due to radiation and corrosion occur even during normal operation . They cause cracks in a small part of the cladding tubes, which can lead to the escape of gaseous fission products. These are i. d. R. Isotopes (iodine, xenon, krypton) with medium half-lives.

• The reactor pressure vessel [5] with connecting pipes [8], [10]

The reactor pressure vessel consists of a steel wall approx. 20 to 25 cm thick. Together with the pipelines, it forms a closed cooling system in which any fission products that may emerge from the cladding tubes are also enclosed.

• The thermal shield [4]

This serves primarily to shield against direct radiation from the reactor core. Since it does not have a completely closed construction, it can only partially hold back fission products.

• The security container [2]

This gas-tight and pressure-resistant "containment" made of approx. 4 cm thick steel (sometimes also made of prestressed concrete) is designed in such a way that in the event of a leak in the reactor cooling circuit it can safely absorb the entire escaping water / steam mixture with any fission products it may contain.

• The surrounding reinforced concrete shell [1]

The entire containment is surrounded by an approximately 1.5 to 2 m thick reinforced concrete shell, which is primarily exposed to external influences - such as B. Destruction by a plane crash - to prevent, but can also hold back radioactive materials inside.

Strictly speaking, only the gas-tight barriers of fuel rod cladding, reactor pressure vessel and containment meet the requirement to contain radioactivity. Only these three barriers guarantee the containment of volatile radioactive substances (e.g. iodine or cesium). The other barriers mentioned have a "supporting" effect in that they protect the gas-tight barriers from external and internal influences.

Other reactors, especially those of the former Eastern Bloc , have z. Sometimes fewer and qualitatively poorer barriers. But not all western (or German) reactors are protected, for example, by a reinforced concrete shell [1] that would be strong enough to withstand the impact (e.g. crash) of a larger aircraft.

Multi-level, forgiving security

The key points of western light water reactors are the multi-barrier concept (containment of the radioactive materials in several enclosing barriers) and staggered measures to guarantee the adequate integrity and function of the barriers : If the protective measures on one level fail, protective measures on the next level should absorb this. Only if the measures fail at all levels is the (planned) retention function of a barrier impaired or destroyed. Only when all barriers fail can larger amounts of radioactive substances escape.

Four measures complement this concept:

• The principle of "quality despite multiple levels": For each individual barrier and security level there are definitions of the functions and tasks as well as the required quality .
• The principle "Assume errors in spite of quality": Despite generally high quality, a (technical or human) failure is generally assumed and appropriate catching measures are provided.
• The reactor core is designed in such a way that the chain reaction and thus power generation behave in a self- stabilizing manner ( negative feedback , " inherent stability"; this also serves, in particular, to decouple the individual levels of security).
• Finally, the entire safety concept is checked for effectiveness and balance using probabilistic safety analyzes.

Levels of security

There are four levels of safety in German nuclear power plants: The first level corresponds to normal operation of the power plant. Disturbances should be avoided here as far as possible. Nevertheless, it is assumed that malfunctions will occur. In the second level, the "abnormal operation", the aim is to contain these disruptions and prevent them from expanding into disruptions. Here, too, it is systematically assumed that this goal will not be achieved, and on the third level, the level of incident control, incidents are absorbed as far as possible by very reliable safety systems. But here, too, a failure is systematically assumed and in the fourth level an attempt is made with "internal emergency protection measures" to limit the effects of the incident to the system itself and to avoid the need for drastic measures in the area (especially evacuation).

Residual risk

The security concept described aims to achieve a very high level of security against both technical failure and human error. There is always a certain residual risk, since the design of the safety precautions is based on certain technical assumptions (e.g. no very severe earthquake) and a simultaneous failure of several or all safety precautions is possible despite redundant and spatially separated system parts and is estimated by the probabilistic safety analysis but never can be ruled out entirely. The risk remaining with a selected design is often incorrectly referred to as residual risk .

Hazards affecting safety

Loss of coolant

A fault that could impair the removal of residual heat and thus lead to a core meltdown is water loss due to the leakage of water from a leak, e.g. B. by rupture of a pipeline or bursting of the reactor pressure vessel . Such a leak has to be controlled by adequate replenishment of water. In the early days of the use of nuclear energy, it was assumed that the worst event to be taken into account to endanger residual heat removal was the double-ended break in the largest pipeline : By definition, such a design accident would be an event that should still be controlled without any serious effects on the environment to have.

See also : Loss of coolant accident

External influences

This article or section needs to be revised. More details should be given on the discussion: Reactor safety # External influences . Please help to improve it , and then remove this flag.

When designing the safety devices of a nuclear power plant, not only disturbances within the plant but also external influences are taken into account. Modern German nuclear power plants therefore also have protective devices against explosion pressure waves, floods, plane crashes and terrorist attacks and a statics that was designed with a view to possible earthquakes. The requirements for these devices and their design are specified on a site-specific basis; compliance with them is demonstrated in the approval process. These requirements are unreliable; According to earthquake maps, the Fukushima 1 nuclear power plant was quite cheap, while the major earthquakes were expected in other regions. The design requirements for plane crashes have tightened over the decades. In older nuclear power plants, emergency control stations (also called emergency systems) that are protected against aircraft crashes have been retrofitted, from which the system can be safely shut down in the event that the control room is destroyed. After the terrorist attack on the World Trade Center , the question was asked whether the existing design is also sufficient against large-capacity aircraft that are intentionally crashed. Only three of the 19 German nuclear reactors in Germany at the time would withstand a targeted plane crash. In all other nuclear power plants, according to critics, a “severe to catastrophic release of radioactive substances is to be expected”.

Operational disruptions

In nuclear power plants, as in any technical system, malfunctions can occur. The safety of a system cannot be inferred from the occurrence of faults alone; this requires a careful analysis of the faults and their accompanying circumstances.

staff

A job in the nuclear industry does not seem attractive to graduates, and many engineers working there are about to retire. The lack of experienced nuclear engineers and construction workers is a key risk and also a cost driver for new projects.

Valuation methods

Statistical research

General security

Measured statistics on the safety of NPPs are only partially available, namely for smaller accidents that actually occurred and were reported in the past. In 1993 the central reporting and evaluation center for incidents and malfunctions was set up, which has been putting the malfunction reports online in an internet portal since 1999.

In order to make representative statistical statements about a certain type of accident (e.g. GAU), this type of accident would have to have occurred at least once. However, the probability of an accident of a certain size occurring cannot be read from the past. Instead, it is calculated in probabilistic safety analyzes (at least as an upper limit):

Probabilistic safety analyzes

So-called probabilistic safety analyzes (PSA) try to quantify the risk of nuclear power plants. This determines the probability with which assumed faults ("triggering events") will occur and with what reliability they can be "controlled as planned" with the existing safety devices. The results are not very suitable for making absolute statements about safety as a whole, since exceeding the "planned control" does not say anything about the consequences that then occur. Due to the existing design reserves, there will usually be no consequences at all if the values ​​are exceeded slightly, but this area is not examined in the usual PPE. A PSA always provides an upper limit for the remaining risk , but does not quantify the risk itself.

Nonetheless, PPE has proven itself well for comparative safety considerations in terms of identifying possible weak points and evaluating planned changes. The PPE tries to identify particularly critical risks that lead to a simultaneous failure of various safety devices, e.g. B. to what extent a simultaneous failure of a) power grid (blackfall) or connection of the power plant and b) emergency power supply facilities (tanks, control, ...) for residual heat removal is possible due to fire, storm, flood, tsunami wave or earthquake ... In contrast, the corresponding preventive measures must be assessed, such as the existing or missing redundant , multiple and spatially separated design of system parts. The PSA of a nuclear power plant is dynamic over its service life: Safety deficiencies can be remedied by retrofitting, on the other hand wear and material fatigue must be taken into account - especially in the areas of the plant that are affected by radioactivity .

Every nuclear power plant has a history and plant-specific PPE in which similar physical laws and components operate. Therefore, experiences in other systems are conditionally transferable and are exchanged in the non-public IRS database (International Reporting System for Operating Experience , also: IAEA / NEA Incident Reporting System) of the faults.

The PSA of a nuclear power plant must be supplemented by regular safety tests that use theoretical simulations or emergency exercises to determine the effects of risks, e.g. B. Check a failure of the power grid, as well as the operability of emergency facilities and thereby train the fault management to uncover system-specific safety deficiencies.

Compared to power generation from other types of energy, nuclear power plants have the structural risk of residual heat removal (" decay heat "), since the energy output of the fuel - unlike conventional power plants - cannot simply be switched off.

About the risks of nuclear power plants and nuclear facilities to meet through appropriate regulations and controls, a network operates national and international organizations, at the United Nations , the International Atomic Energy Agency IAEA (Engl. IAEA), the United Nations Scientific Committee on the Effect of Atomic Radiation UNSCEAR and the World Health Organization WHO ; the Nuclear Energy Agency NEA of the OECD , the International Commission on Radiological Protection, ICRP . At national level in Germany the Federal Ministry for the Environment, Nature Conservation and Nuclear Safety with the Federal Office for Radiation Protection and the Federal Environment Agency and the environmental ministries of the federal states with the respective nuclear supervisory authority . The Swedish radiation protection authority Strålsäkerhetsmyndigheten (before 2008 SKI ) is a national authority with a strong international focus .

Compare to other energy sources

The statistical number of immediate fatalities from known nuclear accidents in OECD countries for the period from 1969 to 2000 per gigawatt year from NPPs is zero according to statistics from the Swiss Paul Scherrer Institute (PSI) for nuclear and reactor research. In comparison, the above-mentioned PSI study lists 0.13 deaths / GW year for coal-fired power plants in OECD countries, and zero for hydropower plants in the EU15. The study lists the Chernobyl catastrophe alone for the deaths due to long-term consequences from nuclear power plants and estimates this to be around 10,000 to 100,000 deaths that can be directly attributed to the long-term consequences of Chernobyl to date. (See also the list of accidents in nuclear facilities , which deals only with cases of radioactivity leakage). For hydropower plants in non-OECD countries, it lists 13.77 fatalities / GWyear (cynically, the majority stems from another major disaster; the rupture of 62 dams in China around the Banqiao Dam in 1975 with an assumed 26,000 immediate deaths).

The underlying study (Hirschberg et al. (1998): Severe accidents in the energy sector ) by the Paul Scherrer Institute cited above does not deal with the actual effects of a catastrophe in terms of nuclear power plants (pp. 137-182), such as concrete deaths or the extent of environmental damage, or with the costs for increasing the safety of an individual nuclear power plant, but mainly with the estimated costs incurred for the possible damage limitation in the worst possible fictitious accident scenarios in nuclear power plants with the highest safety standards (those in the study also in Western countries described as rarely fulfilled) and maximum distance from human settlements. The results can therefore also be interpreted in such a way that, in the case of nuclear power plant accidents, less or less expensive measures can be used at all or would be effective in avoiding significant pollution and damage.

Cases of illness related to radioactivity

Lawsuits against power plant operators due to increased cases of illness following accidents that have become known, as well as the proven accumulation of certain types of cancer around certain power plants known to be incidents (also in Germany) occur again and again. During normal operation, small amounts of radioactive material escape from the nuclear power plant into the environment. This material includes radioactive noble gases (e.g. krypton -85) and the unstable hydrogen isotope tritium , the escape of which is measured and subject to conditions. Nevertheless, they are suspected of being carcinogenic by being absorbed into the human organism . This was shown in an epidemiological study commissioned by the Federal Office for Radiation Protection in 2007. The leukemia rate in children was significantly increased in the vicinity (5 km) of nuclear power plants. The exact cause of this increased rate of leukemia in the vicinity of nuclear power plants is not yet known - see also Leukemia in the Elbmarsch ; The final report published by the commission of experts in place in November 2004, which examined the possible connections between the Elbmarschleukemia cluster and the local nuclear power plant, ended with the words: "We have lost confidence in this state government." Investigations by the Deutsches Ärzteblatt (1992) and the British Medical Journal (1995) have also found increased leukemia rates in children in the vicinity of nuclear facilities - but also generally in the vicinity of larger construction sites in rural areas. The latter therefore suggests that at locations that u. a. are also suitable for nuclear power plants, there are factors which by themselves already entail an increased risk of disease; It is assumed, for example, that the increased incidence of special types of cancer can be explained by the fact that they are contagious and that the pathogens are introduced by construction worker families through labor migration.

An important part of the scientific debate about cases of illness due to nuclear power plants also relates to the taking of soil samples in their immediate vicinity to measure the local contamination deviation with radioactive material, especially with so-called Pac beads made of plutonium , americium and curium . Increased contamination is also repeatedly found (see, for example, Leukemia Cluster Elbmarsch ); However, there is disagreement among the opposing scientific groups as to whether this increased contamination in the immediate vicinity of the power plants can actually come from the nuclear power plants, since such globules are not used there, or whether it is more due to nuclear weapon tests or the Chernobyl disaster . A large amount of plutonium has been shown to have escaped from Chernobyl, but the graphite-moderated RBMK reactor type there did not contain any americium or curium, which, due to the reactor design, could not have been produced during the accident or due to natural decay processes afterwards.

The main problem with the statistical ( epidemiological ) evidence of such effects is that the assumed influences (e.g. cancer caused by radiation exposure) due to the low number of cases and the low radiation doses are not differentiated with sufficient certainty from the other influences with the same effect (e.g. Smoking, stress, diet, population migration etc.) and the natural probability of occurrence can be separated. The assignment of a certain cancer disease and a possibly resulting death to a certain cause is also fundamentally not possible because of the many known cancer-inducing parameters.

A study of the cancer risk in the vicinity of nuclear power plants by the Federal Office for Radiation Protection came to the conclusion that, for the period from 1980 to 2003, in the vicinity of 16 locations with a total of 22 nuclear power plants in Germany, cancer was more common among children under five years of age. The increase in risk is mainly evident in leukemia. In the vicinity of nuclear power plants, a risk increase of around 60% for all cancers and a doubling of the disease risk for leukemia, i.e. H. a risk increase of about 100% was observed. However, an increased leukemia rate in children is not statistically considered to be evidence of a potential danger, as these children cannot be proven to have been directly ill through the operation of the power plant, and since illnesses (in contrast to deaths) are not recorded in all statistics on the subject.

Other studies have also found increased leukemia rates in children in the vicinity of nuclear facilities - but also in the vicinity of facilities that were initially planned. The latter therefore suggests that there are factors at locations that are suitable for nuclear power plants that inherently entail an increased risk of disease.

Risk projections

Serious accidents

According to the German Risk Study of the Society for Plant and Reactor Safety (GRS) from 1989, a serious accident can be expected every 33,000 years of operation for a German pressurized water reactor of the second generation (this also includes the possibility of an accident occurring immediately ). Block B of the Biblis nuclear power plant was used as the reference system . The result can be, as with all probabilistic safety analyzes (Engl. Probabilistic safety analysis , PSA), not easily transferred to other nuclear power plants. The Biblis nuclear power plant itself has also carried out numerous retrofits since the German risk study, so that different results would be expected for this power plant with a current PSA.

The GRS study from 1989 was criticized by nuclear-critical experts from the Öko-Institut in a statement commissioned by the then SPD-led state government of Schleswig-Holstein to the effect that the probability of a serious accident was classified as too low here. According to the scientists, some assumptions in the context of the earthquake PSA were made too optimistically.

Other studies, in particular more recent ones by the International Atomic Energy Agency (IAEA), show that the probability of accidents is lower, since retrofitted nuclear power plants and even more recent models have more extensive safety systems. The risk of an accident with reactor damage for the EPR is given as approx. 1 per 1,000,000 operating years ; even this is only a statistical quantity that includes an immediate accident.

The latest study on the topic (from 2012) comes from the Max Planck Institute for Chemistry. This study assesses the risk based on previous experience with accidents and not based on estimates. The study comes to the conclusion that a GAU is much more likely than all theoretical methods predict. One can be expected about every 10–20 years.

See also : List of accidents in nuclear facilities

Stress test for nuclear power plants

A so-called stress test for nuclear power plants includes an assessment of the safety reserves of nuclear power plants in order to analyze any need for retrofitting . In particular, the effects of extreme events with regard to plant safety and any serious accidents that may result from them are to be investigated. The design limit values ​​are not specified in advance, but rather determined and justified within the respective stress test. The so-called stress test is usually not a direct review by (independent, third-party) inspectors, but is based on information provided by the power plant operators using the specifications for the already approved systems, which are then checked by independent international experts; The age and current condition of the systems are not taken into account.

After the nuclear disaster in Fukushima in March 2011, the responsible EU energy commissioner Günther Oettinger asked the nuclear control authorities of the EU member states to simulate extreme loads for the 143 nuclear power plants in the EU. The ENSREG criteria catalog was subsequently supplemented by him in spring 2012 to include the question of dangers from external technical developments , e.g. B. after a possible hazard from airplane crashes. In autumn 2012 he wanted to discuss the consequences to be drawn from the results together with the responsible committee of the European Parliament and the heads of state and government. On June 15, 2012, the national ministers responsible for energy issues discussed the report at their meeting in Luxembourg.

A 104-page study to review the results of the EU stress test was carried out on behalf of Greenpeace and was published before the ministerial meeting. Using the example of various European nuclear power plants (for Germany the Gundremmingen nuclear power plant as an example ), it complains that, among other things, various environmental catastrophes, aircraft crashes, material aging or the interlinking of several factors (e.g. in Fukushima) were not or not sufficiently taken into account in the safety assessment.

Oettinger wants to present the results in mid-October. Parts of it have already seeped through. According to this, most European nuclear power plants have significant safety gaps, in some of the power plants not even the retrofits that were agreed after the Harrisburg disaster in 1979 and the Chernobyl disaster in 1986 have been carried out. Defects were also discovered in twelve German nuclear power plants. B. adequate earthquake measurement systems, some nuclear power plants are also not designed well enough against earthquakes . Overall, however, German nuclear power plants ranked behind some Eastern European power plants in the first half of the plants examined. Nuclear power plants performed particularly poorly in France; Northern European power plants were also criticized. So stayed z. For example, the operating teams in the Swedish Forsmark nuclear power plant and in the Finnish Olkiluoto nuclear power plant have less than an hour to restore an interrupted power supply to maintain the essential reactor cooling. Overall, the EU assumes that the retrofitting of nuclear power plants will cost between 10 and 25 billion euros.

Environmental associations sharply criticized the stress test and called for the power plants in question to be shut down. The stress test was largely carried out on paper, while only a few power plants were actually examined. In addition, certain risks such as the risk of terrorist attacks or plane crashes were completely disregarded, while only the resistance to extreme natural events and the control of the resulting accidents were examined.

Requirements of the European Working Group on Nuclear Safety (ENSREG)

The European Working Group on Nuclear Safety ( European Nuclear Safety Regulators Group , ENSREG) and its subgroup WENRA published 25 May 2011 inspection requirements which the safety assessments of nuclear power plants from the viewpoint of the already made Fukushima events are intended to supplement for any new buildings. In particular, the following should be considered:

a) Trigger events

• earthquake
• Flooding

b) Consequences

• Power failure , including the so-called station black out (failure of the emergency power supply)
• Failure of the cooling system
• Combination of both cases

c) Actions

• Protection against loss of the cooling function for the nuclear fuel rods
• Protection against loss of the cooling function for the spent fuel storage facility
• Protection against loss of containment integrity

France

In addition to the EU stress test, the French nuclear power plants were subjected to a complementary safety assessment by the country's nuclear monitoring authority (ASN) , which, according to its publication at the beginning of January 2012, demonstrated a considerable need for retrofitting of the plants there. In the spring of 2012, Greenpeace France published a counter-opinion which, among other things, criticized the fact that MOX fuel elements were not given special consideration with their considerably greater damage potential.

Stress test for interim storage

According to the answer of the German federal government to a small request from the Bundestag faction of Bündnis 90 / Die Grünen in early 2012, the German Waste Management Commission (ESK) is also to subject all nuclear interim storage facilities in Germany to a "stress test".

Technical measures

Basic measures

• Water basin [3]

Water is used to shield against radioactivity, as a moderator within the reactor pressure vessel (and to dissipate and use the generated heat). The lower water basins (pump sumps) are normally empty; in the event of a leak, they collect the escaping water and allow it to be fed back into the circuit in order to prevent the reactor from drying out.

• Filtered pressure relief [9]

In the event of a serious accident, inadmissible pressure can arise in the containment due to evaporating water. This pressure can be controlled and filtered through the pressure relief system [9] ( Wallmann valve ).

• Hydrogen degradation

At the high temperatures as a result of an accident with core meltdown , hydrogen is generated through reactions of water with metals in the cladding tubes. For example, the zirconium in the alloy of the fuel rods reacts with water from a temperature of 900 ° C to form zirconium oxide and gaseous hydrogen in the following reaction:

${\displaystyle \mathrm {Zr+2\ H_{2}O\rightarrow ZrO_{2}+2\ H_{2}} }$

This strongly exothermic reaction releases heat energy of 576 kJ / mol H ₂ . As soon as the hydrogen content in the air exceeds a concentration of around 4 percent by volume, this mixture becomes explosive as an oxyhydrogen gas . The Society for Plant and Reactor Safety determined in a study that in the event of a core meltdown in a zirconium inventory of a pressurized water reactor (containment volume approx. 70,000 m³) of 20 tons of zirconium, approx. 5,000 m³ of hydrogen are generated within 6 hours. In the case of pressurized water reactors, because of their small volume, there is also the risk that the additional pressure from the hydrogen overloads the reactor pressure vessel . Until the meltdown accident in Three Mile Island in 1979, this zirconium reaction was not taken into account in the scenarios of possible accidents. Only after the graphite fire in the Chernobyl accident in 1986 pointed out the possible importance of chemical reactions as a result of the core meltdown, facilities were made mandatory in Germany that prevent the formation of an ignitable hydrogen-oxygen mixture. In the containment of pressurized water reactors , catalytic recombiners were then installed at exposed places , on the surface of which the oxyhydrogen gas (even well below the explosion limit) reacts to form water. The safety container of a boiling water reactor is flooded with nitrogen during normal operation, so that in the event of an accident free hydrogen is produced, but there is not enough oxygen to produce oxyhydrogen.

Another way in which hydrogen is produced in nuclear reactors is the splitting of the water by ionizing radiation. This process, called radiolysis , directly produces oxyhydrogen. The speed with which the oxyhydrogen is generated is slow compared to the gas quantities in the zirconium reaction. Even in the event of a core meltdown, there is no risk of the reactor pressure vessel being filled with an ignitable radiolysis gas in a short time. Since the reaction also takes place during normal operation, the oxyhydrogen gas can accumulate over a longer period of time and then be ignited by ionizing radiation. For this reason, the above-mentioned catalytic recombiners are also installed at exposed points in the systems of the primary circuit , on the surface of which the oxyhydrogen gas reacts to form water. Despite these precautions, in November 2001 a pipe connected to the reactor cover at the Brunsbüttel nuclear power plant was destroyed by an oxyhydrogen gas explosion.

Removal of the decay heat

One possible mechanism that can lead to the failure of several barriers is overheating of the reactor core or even the melting of the fuel elements ( core meltdown accident ). This would destroy the first four barriers mentioned and, in the longer term, possibly also the two remaining barriers. Cooling devices are required to prevent such overheating. Since a nuclear power plant still produces decay heat even after it has been switched off due to the decay of the accumulated radioactive fission products , these multiple cooling devices must function reliably in the long term. Immediately after shutdown, decay heat of approx. 5–10% of the previous thermal output must be dissipated. Since the nuclear power plant itself no longer generates any energy, the energy required for this is taken from the power grid. If a nuclear power plant is forced to an emergency shutdown due to a failure of the power grid , a blackout , the lack of an external energy supply means that the need to dissipate the decay heat immediately with the help of the emergency power supply, possibly for days and months.

Further development of security technology

The safety of nuclear power plants depends on how a nuclear power plant is designed, built and operated. Worldwide, the safety of nuclear power plants has increased significantly since their introduction in 1956 through experience and retrofitting. Since 1994, the amended Atomic Energy Act in Germany has also required that incidents that go beyond the design (core meltdown accidents) in the case of new nuclear power plants must be contained to such an extent that their effects are essentially limited to the power plant site and no serious measures are taken in the vicinity are necessary to limit risks (evacuations). The new Franco-German joint development " European Pressurized Water Reactor " (EPR) apparently meets these conditions. One such power plant is currently being built in Finland and one in France: In principle, absolute safety cannot be achieved anywhere, including nuclear power plants.

Since May 2001, 11 countries have been working on further developed reactor concepts in a joint project under the leadership of the USA as part of the Generation IV International Forum for Advanced Nuclear Technology (GIF) . A total of 6 different reactor concepts are being pursued with the aim of increased safety and improved economic efficiency while at the same time improving fuel utilization and increased proliferation security. In addition, possibilities of nuclear hydrogen production are being investigated. Two of these concepts should be ready for construction for demonstration plants in 2015 and the remaining four in 2020. A commercial use could then take place maybe 10 years later.

Retrofitting of German NPPs against beyond design basis events

The existing systems were retrofitted in terms of safety in order to be able to control events beyond the design criteria. The most prominent measures include:

Inerting the containment in boiling water reactors

In many boiling water reactors, the containment vessel is filled with nitrogen during power operation in order to prevent an oxyhydrogen gas explosion in the event of an accident with the release of hydrogen (lack of oxygen). Boiling water reactors have a smaller containment than pressurized water reactors with voluminous steam generators, so this measure is easier to do here.

Filtered pressure relief of the containment

In German-speaking countries, this device is called the Wallmann valve, after the Federal Environment Minister at the time . Thus an increase in pressure in the can in the case of containment the possibly contaminated radioactive and positively pressurized containment air is vented through a filter, in order to avoid exceeding the design pressure (and bursting the containment) (the reactor building in this case). The filter holds back radioactive particles, but not noble gases such as. B. Xenon.

Potter candle

The colloquial Töpfer candle is a catalytic recombiner for hydrogen degradation, named after the former Environment Minister Klaus Töpfer , who had these systems retrofitted.

The component is supposed to break down the hydrogen gas by recombination before the explosion limit is reached , i.e. H. catalytic reaction of hydrogen and oxygen to form water without spark or flame. Alternatively, systems have also been developed to ignite the hydrogen below the explosion limit, which also leads to a "gentle" breakdown of the hydrogen ( deflagration ).

These catalytic recombiners are used in all German pressurized water reactors . In the case of the boiling water reactors , only Gundremmingen B and C (construction line 72) were equipped with it, because the containment there was accessible during power operation and was therefore not inertized with nitrogen. In the other German boiling water reactors (construction line 69), the containment was rendered inert during power operation, which rules out an oxyhydrogen gas explosion.

Public perception

The historian Joachim Radkau criticized the sparse public discussion of both the different nuclear developments and the various safety philosophies and concepts during the entire development period of nuclear power in West Germany. In the discussion about the safety of nuclear power plants in the Federal Republic, he distinguishes between an early and a late phase.

• At first there was a public consensus on the technology, but it had considerable inadequacies and developed in a technically and economically uncoordinated manner: among other things, there was an uncoordinated coexistence of too many reactor lines and a hasty development and commissioning of individual types.
• By the mid-1970s, technical developments had stabilized, but public consensus quickly waned.

In 1984, Radkau explained that the concept of reactor safety was very much restricted to business management "availability" and "reliability". The "reactor uncertainty" (GAU) is bureaucratically trimmed on paper and there is no "safety scale".

Accidents

→ Main article : List of accidents in nuclear facilities

In the history of nuclear energy use, the events of Kyschtym (Mayak, 1957) , Windscale / Sellafield (1957) , Three Mile Island (Harrisburg, 1979) , Chernobyl (1986) and Fukushima-Daiichi (2011) stand out.

Three Mile Island has confirmed the effectiveness of the concept with staggered barriers and multiple facilities to protect these barriers: the event was not planned in advance. The first four barriers were destroyed by a chain of essentially two work errors. The remaining two (containment and reinforced concrete shell) held up and prevented serious external effects. The complexity of the safety equipment, however, led to new risks and sometimes unforeseen chains of events: radioactivity reached the environment through a pipeline of the water purification system that was opened by an automatic system and leading out of the safety container. The exhaust gas treatment of the auxiliary system building had failed and leaks had occurred in it. A pressure relief valve that properly released a dangerous pressure but did not close again afterwards caused a dangerous further loss of coolant.

This article or the following section is not adequately provided with supporting documents ( e.g. individual evidence ). Information without sufficient evidence could be removed soon. Please help Wikipedia by researching the information and including good evidence.

The Chernobyl disaster turned out differently, and there were different conditions. In particular, the operating team misinterpreted the behavior of the reactor (undetected " xenon poisoning " of the reactor). The construction of the reactor (type RBMK ) showed serious shortcomings:

• The reactor contained 1700 tons of combustible graphite, the fire of which could only be extinguished after a week. Conventional light water reactors do not contain any combustible materials - unlike the much discussed pebble bed reactors , which also contain large amounts of graphite.
• Of the barriers against the escape of radioactive substances, the last two barriers mentioned, the containment and the reinforced concrete casing, were practically completely absent.
• The control rods , which are supposed to lower the reactivity when they are moved into the reactor core, have a short-term increase in reactivity due to their design when they are moved into the reactor core.
• Vapor bubble formation due to insufficient cooling led to increased reactivity (positive vapor bubble coefficient ).
• In terms of design, it was not impossible to put the reactor in a state in which it would promptly become critical .

In addition, human and organizational errors occurred:

• The accident did not occur during normal operation, but rather during an experiment, but outside of the description of the experiment.
• Some of the safety devices were switched off / bypassed to enable this experiment.
• Important operating regulations were not observed by the operating staff.

During the nuclear disaster in Fukushima, a tsunami as a result of an earthquake affected four reactor blocks at the same time for the first time - to varying degrees. Both the intensity of the earthquake and the height of the tidal wave were well above the design values ​​of these systems. As in the case of Chernobyl and the other cases mentioned above, the immediate and wider area was affected; the local population had to be evacuated. The reactors were automatically shut down by the security system due to the earthquake, but the tsunami put the cooling water systems and the emergency diesel generators out of operation. 37 employees and rescue workers were injured during attempts to temporarily cool the fuel assemblies and prevent them from melting, 19 employees were exposed to a greatly increased radiation dose, and two employees suffered severe radiation damage in the leg area. Two other workers died in the tsunami.

See also

• Nuclear Safety
• Design basis accident
• ECURIE (European Community System for the Exchange of Information in Radiological Emergency Situations)
• Ethics committee for a secure energy supply
• International Atomic Energy Organization (IAEA)
• International Nuclear Events Rating Scale (INES)
• Nuclear Committee
• List of reportable events in German nuclear facilities (up to INES level 3)
• List of accidents in European nuclear facilities (INES levels 2 and 3)
• List of accidents in nuclear facilities (INES levels 4 to 7)
• Convention on Nuclear Safety
• Central reporting and evaluation point for incidents and malfunctions

literature

• Paul Laufs: Reactor safety for power nuclear power plants. Springer-Vieweg 2013, ISBN 978-3-642-30654-9 Comprehensive work, deals with technology and the historical-political environment in the Federal Republic of Germany
• Günter Kessler, Anke Veser, Franz-Hermann Schlueter, Wolfgang Raskob, Claudia Landman, Jürgen Päsler-Sauer: Safety of light water reactors . Springer-Vieweg 2012, ISBN 978-3-642-28380-2
• Hirschberg et al .: Severe Accidents in the Energy Sector . Paul Scherrer Institute, 1998. pp. 241 f.

Web links

• Federal Nuclear Safety Inspectorate ENSI - Website of the independent supervisory authority for nuclear facilities in Switzerland
• THE INTERNATIONAL CHERNOBYL PROJECT - Overview (PDF file; 8 MB)
• Technical report on THE INTERNATIONAL CHERNOBYL PROJECT (PDF file; 41.4 MB)
• Current situation report Gamma local dose rate - Radiological situation in the Federal Republic of Germany: Gamma local dose rate - normal level (ODL Germany of the FSO)
• Nuclear power plants in Germany - Notifiable events since the commissioning of the individual German nuclear power plants (Federal Office for Radiation Protection), as of April 13, 2015 ( Memento from April 25, 2015 in the Internet Archive )
• Risks of old nuclear power plants . 2010, study ( Memento from May 23, 2011 in the Internet Archive ) by Wolfgang Renneberg , PDF, 52 pages. Summary page 6 + 7, conclusions page 44.
• Forschungszentrum Jülich's “Radiation Protection Glossary” contains many definitions and terms on radiation protection , dosimetry , radiology, and monitoring of people and the environment.
• Finding aid for inventory 200: Documents on reactor safety research , (including research project on component safety - FKS) of the materials testing institute University of Stuttgart in the Stuttgart University Archives

Individual evidence

1. ↑ Panos Konstantin: Practical book energy industry: Energy conversion, transport and procurement , page 295
2. ↑ Mark Hibbs, Decommissioning costs for German Pebble Bed Reactor escalating, NUCLEONICS WEEK, Vol. 43, no. 27, p. 7 (July 2002)
3. ^ E. Wahlen, J. Wahl, P. Pohl: STATUS OF THE AVR DECOMMISSIONING PROJECT WITH SPECIAL RE-GARD TO THE INSPECTION OF THE CORE CAVITY FOR RESIDUAL FUEL. (PDF) Arbeitsgemeinschaft Versuchsreaktor AVR GmbH, March 2, 2002, accessed on April 7, 2019 (English). 
4. ↑ The planning work was already carried out parallel to the commissioning of the smaller AVR pebble bed reactor in Jülich, so that operating experience of the AVR could hardly be incorporated into the THTR concept.
5. ↑ Rainer Moormann : Air ingress and graphite burning in HTRs: A survey of analytical examinations Performed with the code REACT / THERMIX, Jülich Research Center, Report Jül-3062 (1992)
6. ^ R. Moormann, Phenomenology of Graphite Burning in Air Ingress Accidents of HTRs , Science and Technology of Nuclear Installations, Volume 2011 (2011), Article ID 589747, 13 pages, http://www.hindawi.com/journals/stni/ 2011/589747 / ref /
7. ↑ see also article atomic moratorium
8. ↑ Safety Criteria for Nuclear Power Plants, Module 1 "Basic Safety Criteria", Revision D. In: BMU website. BMU, April 2009, accessed on November 6, 2018 . 
9. ↑ scinexx.de: earthquake risk map
10. ↑ Nuclear power plant safety report - too explosive for the public? ( Memento from September 14, 2010 in the Internet Archive ) Greenpeace article about a secret ILK report from 2002
11. ↑ WELT ONLINE, July 11th, 2009: There is a lack of experts for new construction projects
12. ↑ Technology Assessment Group. Paul Scherrer Institute, accessed April 6, 2019 . 
13. ↑ St. Hirschberger, P. Burgherr, G. Spiekerman, E. Cazzoli, J. Vitazek, L. CHeng: "Comparative Assessment of Severe Accidents in the Chinese Energy Sector" (PDF; 1.6 MB), PSI report no. 03-04, Paul Scherer Institute, March 2003, ISSN  1019-0643
14. ↑ Federal Office for Radiation Protection: Monitoring of emissions from nuclear power plants ( Memento from January 17, 2012 in the Internet Archive ) (pdf)
15. ^ Website of the German Childhood Cancer Register. German Childhood Cancer Registry , accessed on March 19, 2011 . 
16. ↑ taz.de: Experts disagree on the danger of nuclear power plants (December 11, 2007)
17. ↑ Epidemiological study on childhood cancer in the vicinity of nuclear power plants - on behalf of the Federal Office for Radiation Protection 2007 (PDF file; 7.3 MB)
18. ↑ Kinlen LJ et al. Childhood leukemia and non-Hodgkin's lymphoma near large rural construction sites, with a comparison with the Sellafield nuclear site. , in BMJ , 310/1995, pp. 763-7
19. ↑ Michaelis J, Childhood cancer in the vicinity of West German nuclear facilities. , in Deutsches Ärzteblatt , 89/1992, pp. C-1386-90
20. ↑ Epidemiological study on childhood cancer in the vicinity of nuclear power plants - KiKK study ( Memento from February 20, 2011 in the Internet Archive ). PDF document on the results of the study, accessed on August 19, 2010.
21. ↑ Kinlen LJ et al. Childhood leukemia and non-Hodgkin`s lymphoma near large rural construction sites, with a comparison with the Sellafield nuclear site. , in BMJ , 310/1995, pp. 763-7
22. ↑ Bernhard Fischer , Lothar Hahn, Michael Sailer , 1989: Evaluation of the results of phase B of the German risk study on nuclear power plants.
23. ↑ Summary of the previous PPE results for the EPR by the HSE (PDF; 90 kB), a British authority for health and safety at work, which is also in charge of approving reactor concepts
24. ↑ The nuclear disaster is more likely than expected. Prof. Dr. Jos Lelieveld, Max Planck Institute for Chemistry
25. ↑ ^{a b} Suzanne Krause: "In no case a sufficient basis" . In: dradio.de, Research News , March 8, 2012 (May 4, 2012)
26. ↑ Daniela Weingartner: "What if a plane crashes?" . In: badische-zeitung.de, Nachrichten, Wirtschaft, April 28, 2012
27. ↑ spiegel.de June 14, 2012: Environmentalists criticize nuclear power plant stress test
28. ^ Antonia Wenisch, Oda Becker: Critical Review of the EU Stress Test performed on Nuclear Power Plants. Study commissioned by Greenpeace. Vienna, Hanover, May 2012. ( PDF , 2 MB)
29. ↑ Nuclear Stress Tests - Flaws, blind spots and compliance. Greenpeace EU, June 2012. ( PDF ) (short summary of the study by Wenisch and Becker)
30. ↑ spiegel.de October 1, 2012 , welt.de: Europe's nuclear power plants are not safe enough. - European nuclear power plants have appalling safety deficiencies. This is proven by extensive stress tests. French nuclear power plants are doing particularly badly - but German nuclear power plants are also affected.
31. ↑ AKW stress test. Bad grades for Europe's kiln . In: Süddeutsche Zeitung , October 1, 2012. Accessed October 2, 2012.
32. ↑ EU nuclear stress test in the analysis. The fairy tale of safe German reactors . In: Tagesschau.de , October 2, 2012. Accessed October 2, 2012.
33. ↑ AKW retrofitting depending on the term . In: Der Spiegel , October 2, 2012. Retrieved October 2, 2012.
34. ↑ Sönke Gäthke : reactors under stress , Germany radio , science in focus 11 March 2012 (4 May 2012)
35. ↑ EU "Stress tests" specifications (PDF; 1.1 MB), Annex II, page 4, European Nuclear Safety Regulators Group, 2011, Brussels
36. ^ Dapd message : Federal government wants stress test for all interim storage facilities . In: themenportal.de (March 25, 2012)
37. ↑ He himself was not aware of this until a conversation with Ranga Yogeshwar in 2011; see K. Töpfer and R. Yogeshwar: Our future. A conversation about the world after Fukushima. Verl. CH Beck, Munich 2011. ISBN 978-3-406-62922-8 .
38. ↑ Federal Ministry for the Environment, Nature Conservation and Nuclear Safety (Ed.) Report of the Government of the Federal Republic of Germany for the Third Review Conference in April 2005, Bonn 2004, page 96f (PDF; 1.4 MB)
39. ↑ Joachim Radkau: in Bild der Wissenschaft 12/1984, pp. 88–90
40. ↑ Krüger, FW et al .: The course of the Chernobyl 4 reactor accident, in: Bayer, A. et al. (Ed.): Ten years after Chernobyl, a balance sheet. Federal Office for Radiation Protection , Stuttgart 1996, pp. 3–23