High temperature reactor

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Moderator sphere made of graphite for pebble bed reactors

As a high-temperature reactor ( HTR ) are nuclear reactors referred to the much higher operating temperatures enable than other known reactor types. This is achieved through the use of a gaseous coolant and ceramic instead of metallic materials in the reactor core ( graphite as moderator ).

The term high-temperature reactor is often used synonymously with pebble bed reactor in German . However, this is only one of the various possible designs of the HTR (see below).

Various small high-temperature reactors have been operated for years as test reactors since the 1960s, but this continuous operation is viewed critically in retrospect, among other things because of unusually large disposal problems. Two larger prototypes had to be abandoned in 1989 after a short period of operation. Between 1995 and 2010, pebble bed reactors received further international attention. That ended with the collapse of a South African HTR construction project.

In China, a larger prototype has been built since 2009 (interruption after the Fukushima accident), which (as of October 2019) will go into operation at the end of 2020 and, if successful, will be followed by commercial systems.

Overall, the concept has not caught on due to various difficulties and breakdowns as well as a lack of profitability.

Purpose of the higher temperature

The highest possible coolant outlet temperature (i.e. the temperature at which the coolant leaves the reactor core) is desirable for two reasons:

  • If the reactor is used to generate electricity, a higher coolant outlet temperature - as with any other thermal power plant - makes energy generation more economical, as it enables a higher degree of thermal efficiency when converting heat output into mechanical output .
    • However, due to material properties, the usual water / steam cycle for turbine drives does not allow temperatures higher than approx. 550 ° C, so that an efficiency gain only occurs up to primary coolant temperatures of approx. 650 to 700 ° C. The table also shows that the real efficiency does not only depend on the coolant temperature.
    • Gas turbines would have the advantage of higher working temperatures and could also achieve efficiencies of up to 60% with downstream steam turbines (see gas and steam combined cycle power plant ). Gas turbines for large power plants have been studied intensively for decades, but could not be made ready for use in the nuclear environment.
  • Reactors can not only be used to generate electricity, but also to supply process heat . High-temperature heat (> 1000 ° C), the generation of which is planned with Very High Temperature Reactors (VHT), is particularly valuable. However, the combination of a chemical and a nuclear facility means an increased safety risk.
Maximum coolant temperatures and thus theoretically achievable Carnot efficiency (at 25 ° C ambient temperature) as well as actually achieved efficiencies
Reactor type Temperature in ° C Carnot efficiency Real efficiency
Boiling water reactor 285 47% 34-35%
RBMK 285 47% 31%
CANDU reactor 300 48% 31%
Pressurized water reactor 320 50% 33-35%
Breeder reactor , sodium-cooled 550 64% 39%
Advanced gas-cooled reactor 650 68% 42%
High temperature reactor 750 71% 41%



The previously known HTR constructions use the noble gas helium . In addition to the advantageous higher coolant temperature, the use of gas instead of a liquid as a coolant is intended to reduce mechanical wear and tear and corrosion of the parts around the flow. In the case of pebble-pile HTR, however, abrasion leads to such high erosion and dust formation that this theoretical advantage is of no consequence.

Compared to carbon dioxide (CO 2 ), which is used in other gas-cooled reactors, helium offers the additional advantages that it cannot be chemically changed or decomposed and the main isotope 4 He is not activated by neutron irradiation . However, the small 3 He content of 0.00014% results in almost quantitative tritium . In addition, the protective oxide layers on metals are destroyed in pure helium. Small amounts of corrosive agents such as water vapor in helium can remedy this, but only at the expense of constant corrosion of the graphite components by the water vapor. Attempts to counter this problem by using corrosion-resistant ceramic materials (e.g. silicon carbide) have so far been unsuccessful, even on a laboratory scale. As a monatomic gas, helium diffuses very easily through solid materials, so that airtightness against helium is difficult to achieve. The AVR reactor (see below) lost 1% of its coolant per day, for newer reactors it is calculated at 0.3% per day.

Another disadvantage of helium is that its viscosity increases with increasing temperature. This can lead to less flow through hot areas and therefore less cooling. This effect was discussed as a possible cause for the overheated areas found in the AVR (Jülich) .

Helium cooling in connection with a ceramic core increases the risk of cooling gas bypasses, since the ceramic components used - unlike metals - cannot guarantee a helium-tight enclosure and since a pebble has a high flow resistance. Such bypasses around the core have also been discussed as a cause of the overheated AVR areas.

Like any gas cooling, helium cooling requires high system pressures for adequate heat dissipation. This means that pressure relief accidents caused by leaks in the primary circuit are a significant risk in current pebble bed reactor concepts, none of which contain full pressure containment as an additional barrier. In order to avoid this risk, a liquid salt cooling was proposed as an alternative to helium, which enables pressureless operation. Corresponding studies on the Fluoride Cooled High Temperature Reactor (FHR) are being carried out as part of the Generation IV development program of the Generation IV International Forum .

Fuel, moderator and structural material

The nuclear fuel is used in the form of coated particles (see Pac beads ) whose pyrocarbon and (in later variants) silicon carbide shells are intended to prevent the escape of fission products. This replaces the usual fuel rod casings. In addition, the risk of corrosion should be reduced by using a suitable covering material. The diameter of a coated particle is a little less than 1 mm. The thickness of the shell layers is <0.1 mm, which makes the release of fission products through diffusion a problem in continuous operation at temperatures around 800 ° C. The pellets are coated with additional graphite, i.e. pure carbon, as a structural material and moderator : To manufacture the fuel element , the fuel pellets are placed in a mass of graphite powder and synthetic resin. This is then solidified in the desired shape of the fuel element by pressure and the resin is also converted into coke-like carbon at a high temperature in the absence of air. Graphite is porous (20%) and therefore only makes a small contribution to retaining the fission products.

Two different geometrical shapes of the fuel elements have been tested:

  • in Great Britain, Japan and USA prismatic blocks,
  • In Germany, balls the size of tennis balls that form a loose bed in the reactor vessel ( pebble bed reactor ).

A spherical fuel element with a diameter of 6 cm contains between 10,000 and 30,000 coated particles, depending on the design.

With such fuel elements, higher burn-ups can theoretically be achieved than with standard light water reactors . The elimination of the metallic cladding tubes improves the neutron balance in the reactor, because the neutron absorption in graphite is lower than in the cladding tube materials. However, for reasons of material technology (tightness for fission products), the spherical fuel elements used up to now are not suitable for high burn-ups. The actually achievable and currently targeted (e.g. in the Chinese HTR-PM) burns of approx. 100% FIFA are hardly higher than those of conventional light water reactors, so that there is no better fuel utilization. In addition, the HTR could not be implemented as a thermal thorium breeder , as was originally planned, i.e. That is, it hatched less fissile material than it consumed, while a thermal breeder succeeded with the specially designed light water reactor Shippingport . With a currently achievable breeding ratio of only <0.5, the designation used for pebble bed reactors as near-breeder or upconverter is therefore hardly justified.

In all prototype HTRs, the majority of the fuel globules contained highly enriched, i.e. weapons- grade uranium and natural thorium in a ratio of 1: 5 to 1:10. 233 U is hatched from the thorium through neutron capture and subsequent beta decays. The 233 U is partially split in addition to the 235 U and thus used directly to generate energy; this corresponds to the incubation and combustion of the plutonium when using 238 U as breeding material in the standard fuel.

After the US government banned the export of weapons-grade uranium for HTR in 1977, the development was switched from uranium / thorium to the classic low-enriched uranium fuel (LEU, enrichment approx. 10%). The latter is also intended as a reference fuel for the current HTR. At the moment thorium is again being discussed more intensely as a breeding material; however, pebble bed reactors are hardly involved, since efficient thorium use requires both a breeder reactor and reprocessing : in fact, neither can be guaranteed with pebble bed reactors. Therefore, the liquid salt reactor for thorium utilization is currently mentioned in particular . Overall, the use of thorium in pebble bed reactors turned out to be a dead end.

Scheme of a pebble bed reactor

The spherical fuel elements can be refilled from above and removed from below during operation. If the fuel is still unused, the fuel elements are added again at the top, otherwise they are discharged from the reactor. The pebble bed reactor has the safety advantage that it is not like other reactors with a larger fuel supply for z. B. must be loaded for a whole year of operation. If this possibility is used, however, the supply and removal of the fuel elements must function continuously so that the reactor does not become subcritical . One disadvantage is that reactors with such an operating mode (similar to CANDU and RBMK ) can basically be used to generate weapons-grade plutonium or 233 U at the same time as generating electricity (see below, risk of proliferation). Another significant disadvantage is that in the case of a constantly moving reactor core with fuel elements of different burnup, there are uncertainties with regard to the fuel distribution.

In the operation of the previous pebble bed reactors, this "ball flow" and the ball removal have proven to be weak points. Stable, vault-like spherical packings often formed above the extraction point, preventing the pouring from flowing and thus making the scheduled extraction impossible. In addition, the spheres flow very unevenly, which leads to additional disturbances in the nuclear fuel distribution.

Current spherical fuel elements, as evaluations of AVR experience and follow-up examinations of irradiated modern fuel elements from 2008-2010 have shown, only allow usable temperatures of below 750 ° C, since otherwise too many radioactive metallic fission products are released from the fuel elements. The cause of this release is diffusion through the less than 0.1 mm thick cladding layers around the nuclear fuel. This means that the previously targeted innovative process heat applications such as coal gasification for fuel production or hydrogen production by chemical water splitting are outside the current possibilities of pebble bed reactors, as they require useful temperatures of approx. 1000 ° C. The same applies to power generation with helium gas turbines, which only offers efficiency advantages at temperatures> 850 ° C. Important unique selling points of pebble bed technology are thus called into question, and the VHTR (Very High Temperature Reactor), which was to be developed as part of the Generation IV nuclear network, has moved further into the distance. It is still unclear whether process heat applications at low temperatures (such as the use of process steam to extract oil shale) with pebble bed reactors can be economical.

Power density and safety properties

As with all graphite reactors, the low power density in the core of the HTR (max. Approx. 6 MW / m³ compared to 100 MW / m³ for pressurized water reactors) influences its safety properties both negatively and positively. The low power density cannot be avoided because of the poorer moderation properties of graphite compared to water, because larger amounts of moderator are required. The advantage of light water reactors, in which water is both a coolant and a moderator, cannot be used, which further reduces the power density. This means, on the one hand, that the HTR core and the entire reactor for a given reactor output are much larger than a comparable reactor of a different type and thus the construction and disposal costs are correspondingly higher. In order to counter the high costs, important safety devices are dispensed with: Current HTR concepts, for example, lack a pressure-maintaining containment , as is standard in conventional reactors. On the other hand, there is a safety advantage in the low power density: The heat capacity of the large graphite mass together with the temperature resistance of graphite means that a small HTR is insensitive to cooling loss accidents and some types of reactivity accidents (“power excursions”). An excessively rapid increase in reactivity would also have serious consequences for the HTR, such as a bursting of the fuel elements, possibly even followed by a container bursting.

On the part of the pebble bed HTR proponents, because of the aforementioned positive safety properties in the case of core cooling accidents and some reactivity accidents, it is often stated that small pebble bed reactors can be constructed inherently safe and even catastrophe -free. This claim is controversial even among the proponents of the use of nuclear technology: A frequent counter-argument is that a pebble-pile HTR does not know a core melt, but other very serious accidents can occur, which in turn do not occur in light water reactors. There is a particular risk of accidents from air and water ingress (see accident at AVR Jülich ). A conceivable fire caused by the large amount of graphite in the presence of air, similar to the one in the Chernobyl disaster , could lead to the widespread distribution of dangerous amounts of radioactivity. Under certain circumstances, water ingress can lead to immediate over- criticality , similar to a positive coolant loss coefficient in reactors with liquid coolant, or to chemical explosions. Prompt overcriticality in the case of water ingress in the pebble bed reactor was intensively investigated after the Chernobyl accident because there are similarities between the RBMK reactor on the one hand and the pebble bed reactor in the case of water ingress on the other. The results show that above a water content of 50 kg / m³ in the empty volume of the reactor core, there is a positive temperature coefficient. Such watertightness is only possible in pebble bed reactors with liquid water in the core. Furthermore, the possible increase in reactivity extends deep into the promptly supercritical range ( k eff up to 1.04), so that a nuclear power excursion can occur. The Doppler broadening would slow down the nuclear power excursion, but in many cases an effect that would take effect before the reactor was destroyed would not be expected: Under unfavorable conditions, the power excursion would only be stopped by a temperature increase in the fuel of around 2500 ° C. It plays a role here that the neutron spectrum becomes softer in the presence of water, which reduces the retarding effect of Doppler broadening. A safety report from 1988 therefore speaks of the Chernobyl syndrome of the pebble bed reactor. Regarding the likelihood of such accident scenarios, on the one hand, moisture detection in the helium is answered by the reactor protection system with an emergency shutdown. On the other hand, due to human error (unauthorized manipulation of the reactor protection system in order to be able to put the reactor into operation despite humidity) in 1978 the Jülich pebble bed reactor AVR went into nuclear operation for about three days while liquid water was flowing into the reactor. Another accident scenario investigated with the potential for immediate supercriticality concerns the start-up of the reactor with fuel elements that are soaked with water due to the accident.

Moormann (see also here ) mentions, among other things, the following as the basic safety problems of pebble bed reactors :

  • The impossible online core instrumentation ( black box character of the reactor core)
  • The inadequate retention of radioactive cesium and silver by the thin silicon carbide layer of the fuel particles
  • The inadequate efficiency of the gas cleaning measures, which leads to high contamination of the cooling circuit surfaces
  • The strong formation of radioactive dust
  • The high reactivity of graphite towards air and water vapor
  • Potential overcriticality in water ingress accidents
  • The poorly understood ball flow behavior in operation, which leads to uncertainties in the distribution of fissile materials
  • The lack of pressure-retaining containment for cost reasons
  • The disadvantageously large volumes of radioactive waste

Moormann considers the characterization of the pebble bed reactor as being catastrophe-free and inherently safe to be scientifically dishonest, among other things because the above-mentioned risks from water and air ingress are disregarded. Other German nuclear scientists also expressed doubts about the safety concept of the pebble bed reactors. Likewise, the concept of alleged inherent safety and freedom from disasters is viewed as ineffective by large parts of the nuclear community. Lothar Hahn already commented in 1986 on the alleged inherent security of the HTR: This cleverly designed advertising strategy has undoubtedly had a certain degree of success because it has led to unprecedented disinformation, even in the atomic energy debate. Like almost no other assertion by the nuclear industry, it is based on scientifically untenable assumptions and on incorrect conclusions.

Use and whereabouts of the spent fuel

The individual steps for treating the actual fuel depend on the degree of enrichment of the uranium used, which can be between 10% and 93% of the 235 isotope. With today's favored LEU fuel (10% enrichment), the reprocessing would largely correspond to that of LWR fuel elements.

A reprocessing of HTR fuel elements would require the incineration of the graphite as a first step, whereby all the radioactive CO 2 produced would have to be captured, solidified as CaCO 3 and finally disposed of. Per fuel element with a mass of approx. 200 g (of which approx. 7 to 11 g nuclear fuel), more than 1.1 kg of CaCO 3 to be disposed of with a high proportion of long-lived 14 C would arise from the graphite content alone . Such a combustion process was developed but was never used because of its high cost.

The reprocessing of HTR fuel elements has so far been generally considered uneconomical and the direct disposal of nuclear waste is therefore currently favored. Since the fuel elements, which predominantly consist of graphite moderator, would then be disposed of as a whole, more than twenty times the volume of highly radioactive waste is generated compared to conventional reactors, which significantly increases the disposal costs compared to conventional reactors.

Risk of proliferation

Particularly in the pebble bed reactor, the short dwell time of the individual fuel element can result in relatively pure plutonium-239 or (if thorium is used as the breeding material) uranium-233, i.e. fissile material suitable for nuclear weapons . As a result, this type of reactor, similar to the CANDU and RBMK reactors, can pose a proliferation risk. DA Powers, a member of the ACRS Proliferation Supervisory Board, concluded in 2001 that “pebble bed reactors are not proliferation resistant” and should be viewed as “tailor-made for the easy production of weapons plutonium”.

The required thorium use in HTR use of highly enriched uranium also results in greater proliferation risks: put in Ahaus located, only partly about spent 600,000 fuel element balls after a brief operation disused 300 THTR probably a significant proliferation risk because they have a high proportion contained in weapons-grade uranium.

Trial and prototype plants in Europe, the USA and Asia

AVR high-temperature reactor at Forschungszentrum Jülich 2009 with material lock for dismantling

In the 1960s the experimental HTR DRAGON went into operation in Winfrith, Great Britain. It had prismatic fuel elements and 20 MW heat output.

Four HTR prototype power plants followed :

and in the 1970s

The aforementioned plants were shut down again between 1974 and 1989. After that there were only small test facilities: In Japan, the HTTR (thermal output 30 MW) with prismatic fuel elements has been in test operation since 1999. In China, the HTR-10 (thermal output 10 MW) with pebble heap core became critical in 2003.

The Association of German Engineers VDI presented a review of the test operation of the AVR from the perspective of the proponents in 1990.

Incidents and problems

A dangerous accident occurred at the AVR in Jülich on May 13, 1978: as a result of a long-neglected leak in the superheater part of the steam generator, 27.5 t of water entered the primary He circuit and thus into the reactor core. This represented one of the most dangerous accidents for a high-temperature reactor: Because of the positive reactivity effect of the water (possibility of a prompt supercriticality of the reactor) and the possible chemical reaction of the water with the graphite, explosive gases can arise. The accident was probably only without serious consequences because the core only had temperatures below 900 ° C and the leak remained small.

In 1999 it was discovered that the AVR floor reflector on which the pebble rests broke during operation and that a few hundred fuel assemblies got stuck in the resulting crack or fell through. Most of the fuel elements could not be removed.

In 2008 a report was published by Rainer Moormann , employee at Forschungszentrum Jülich , in which the excessive radioactive contamination of the reactor is attributed to the in principle inadequate monitoring of the reactor core with this type of reactor and to prolonged operation at inadmissibly high temperatures. This u. a. led to the fact that fission products could escape from the graphite spheres. Moormann asks whether the pebble bed principle is justifiable at all: He sees fundamental problems with pebble bed reactors, not just an AVR problem (see also power density and safety properties ). Moormann received the 2011 Whistleblower Prize for his revelations, made against considerable opposition from proponents of pebble bed technology . Moormann's publications have contributed to the decline in international efforts to develop pebble bed reactors, which began in 2000 and which began in 2010.

The 2014 report by an independent group of experts on the AVR confirmed Moormann's assessments.

Cancer incidence in the vicinity of HTR

There was a significant leukemia cluster around the AVR Jülich around 1990. The incidence of thyroid cancer around the THTR-300 is increased by around 64% in women. In both cases it is disputed whether the cause is radioactive emissions from the HTR.

Development of the pebble bed concept

Until 1990

Rudolf Schulten

The first fundamental work and patents on pebble bed reactors go back to the US scientist Farrington Daniels in the 1940s. He initiated research work at the Oak Ridge National Laboratory on the pebble bed reactor, also known at the time as Daniels pile , which, however, was soon terminated by Alvin Weinberg in favor of the more promising light water reactors and molten salt reactors . In Australia work was also carried out on pebble bed reactors until 1970. The priority of Daniels as the inventor of pebble bed reactors has so far hardly been pointed out in Germany; rather, the pebble bed reactor is mistakenly predominantly seen as an invention by Rudolf Schulten and as the only reactor concept developed in Germany alone.

Development work on the pebble bed concept came from Rudolf Schulten and his colleagues: From 1956, the power plant industry under Schulten's direction planned and built the AVR . From 1964 , after Schulten had become head of the institute there, Forschungszentrum Jülich also dealt with the pebble bed reactor, which remained the central research area here until 1989. The first Jülich plans included pebble bed reactors coupled with magnetohydrodynamic electricity generation (MHD). For this purpose, the large-scale ARGAS experiment was set up in Jülich. Since MHD systems require helium temperatures of more than 1500 ° C, which could not be made available by far, this line of development came to nothing.

The German HTR project suffered its first serious setback as early as 1971, when the Krupp company , which together with BBC and NUKEM formed the industrial basis for HTR, completely closed down due to serious doubts about the pebble bed concept five days before the planned first groundbreaking for the THTR-300 withdrew from HTR technology development. Another setback for pebble bed reactors was the development in the USA: There, HTR experienced an extraordinarily strong upswing in the wake of the first oil crisis, as large oil companies such as Gulf and Shell made a massive financial commitment to HTR and pushed for its market launch. Until 1974 it was possible to obtain orders and options for HTR with prismatic fuel elements with a total electrical output of 10 GW, which under the circumstances at the time was to be rated as a breakthrough for the HTR line. Due to various unsolved technical HTR problems, these orders had to be returned in 1975 with the payment of high contractual penalties, which led to the complete withdrawal of the oil companies from HTR technology. HTR technology was no longer able to overcome this setback, which marked the end of the race for the dominant reactor technology and established the dominance of light water reactors ; the HTR technology had not proven to be sufficiently marketable in comparison to light water reactors. According to Klaus Traube , the HTR failure is also due to the fact that the HTR technology was based on military gas-graphite reactors, which were designed to produce weapons plutonium, but not as a power plant, while LWR technology was designed as a Kraftwerk was planned.

The HTR development work in Germany went almost unabated until the early THTR shutdown in 1989. H. with a total personnel capacity of 2,000 to 3,000 employees, further: In addition to the standard concepts for power generation via a water / steam cycle, Jülich and its industrial partners concentrated on the development of reactors with gas turbines (HHT project) and coal gasification (PNP project) from around 1970 ). In addition, there was a smaller project on nuclear district heating with pebble bed reactors (NFE project), which was discontinued in 1984. The HHT project initially appeared to be promising due to the construction of a thermal power station based on a 50 MW helium turbine with conventional coke oven gas firing ( HKW 2 - Oberhausen-Sterkrade thermal power station of Energieversorgung Oberhausen AG). Ongoing technical problems with the complicated turbine technology, in particular with the helium tightness and the storage of the high pressure group, gradually led to the project being withdrawn from 1983 onwards, as a functioning helium turbine for higher temperatures could not be developed: the Jülich component test facility HHV was also updated shut down only 14 days of high temperature operation. In addition, the problem of contamination of the gas turbine remained unsolved, which means that the necessary turbine maintenance was in fact impossible. The process heat development suffered on the one hand from metallic materials that were not sufficiently temperature-resistant for nuclear requirements: Sufficient long-term resistance was only guaranteed up to 900 ° C, while efficient coal gasification would have required 1000 ° C. Another problem turned out to be strong tritium diffusion from the primary circuit into the process gas at high temperatures.

In the USA, the Fort St. Vrain HTGR (prismatic fuel elements, 330 MW el ), which is similar to the THTR-300 , was shut down in 1988 after an overall unsuccessful, brief operation, which further reduced US efforts to develop HTR. After the THTR-300 was shut down in Hamm in 1989, after only 14 months of full-load operation accompanied by problems, state funding for pebble bed reactors was severely restricted in Germany as well. Reactor building industry and electricity suppliers also showed no further interest in pebble bed reactors after 1990.

From 2000

German research centers and companies are or have been involved in projects in the People's Republic of China , as well as the projects in South Africa and Indonesia , which have since been discontinued , where the technology is known under the international name PBMR ( Pebble Bed Modular Reactor ) and experienced a rebirth from around 2000. The trend is towards smaller, decentralized and supposedly inherently safe reactors. Dangers are to be avoided by particularly low power and power density, and by the modularity and the same structure of the small reactors, these should be able to be produced cheaply in larger quantities. However, low power density increases construction costs and disposal problems due to the inevitably larger volume of waste. Development work on high-temperature reactors (especially on HTR with prismatic fuel elements) was carried out at MIT , General Atomics ( USA ) and AREVA in France .

South Africa

The South African government ended the PBMR pebble bed reactor project (165 MW el ) established with support from Forschungszentrum Jülich in September 2010 after investments of more than approx. 1 billion euros, as neither further investors nor customers could be found, and dissolved the company PBMR Ltd. largely on. Further investments of at least 3.2 billion euros would have been necessary. Unsolved technical and security problems as well as escalating costs had deterred investors and customers. Another reason for the failure of the PBMR may have been that the efforts begun in 2001 to obtain certification of the PBMR by the US supervisory authority NRC were unsuccessful. This can be interpreted to mean that the PBMR did not meet US safety standards; thus exports of the PBMR would have been generally hardly possible. The NRC had u. a. criticized the lack of full pressure containment. The revelations by Rainer Moormann on problems with the German pebble bed reactors are also often cited as an impetus for the failure of the PBMR. The completion of this well-advanced project led to a marked decline in international efforts to develop pebble bed reactors.

United States

In the USA, work has been carried out on an advanced pebble bed reactor (PB-AHTR) since 2005, which is intended to eliminate some of the safety disadvantages of the standard concept: The PB-AHTR should not be cooled with helium but with a molten salt, which enables almost pressureless operation. In addition, the balls should be guided in channels, which allows a more regulated flow behavior. In addition, cooling circuits with liquid coolant can be cleaned relatively easily, unlike in the case of gas cooling. At the end of 2011 it was decided to discontinue the concrete development work for the NGNP project, which had a high-temperature reactor of the 4th generation for hydrogen generation, and to continue the NGNP project only as a research project, which is associated with a considerable reduction in funding. As a consequence, it was decided at the beginning of 2012 to limit the research work to the French ANTARES concept with prismatic fuel elements and to postpone the pebble bed reactor option. In order not to release too many radioactive metallic fission products from the fuel elements , the NGNP reactor originally planned for approx. 2027 should remain limited in the first step to a useful temperature of 750 ° C and thus not yet enable hydrogen generation through water splitting.


In 2003, the Chinese government announced its intention to build thirty pebble-bed nuclear reactors at the Shidaowan site by 2020. In 2010 this goal was initially reduced to just one HTR-PM pebble bed reactor with an electrical output of 200 MW el ; The pebble bed reactors that are no longer needed are to be compensated for by five additional high-capacity LWRs, two of which were approved in early 2014. Preparations for the construction of a prototype power plant (HTR-PM) with a thermal output of 250 MW began in 2008. However, as a result of the nuclear disaster in Fukushima , the first partial construction permit for the HTR-PM was withdrawn and further safety analyzes were requested. Construction was permitted in December 2012. Scientific criticism arose at the HTR-PM. After several years of construction delays, the plant is scheduled to go into operation by the end of 2020 with an electrical output of 210 MW. The Chinese project managers describe the disposal problem of pebble bed reactors as solvable with little effort. Experiments on the small pebble bed reactor HTR-10 near Beijing, which has been in operation since 2003, are the subject of several publications. Since 2005, the HTR-10 has only rarely been in operation, which is attributed to the prioritization of the HTR-PM by proponents of pebble beds, but critics associate this with technical problems with ball rolling. The HTR-PM is often considered to be out of date in terms of security technology and not eligible for approval in western high-tech countries due to various security deficiencies, including the lack of full-pressure containment. The same applies to the disposal strategy planned in China for the HTR-PM. In early 2016, further plans by the Institute of Nuclear and New Energy Technology at Tsinghua University to build a commercial high-temperature pebble bed reactor were announced.


In April 2011, shortly after the nuclear disaster in Fukushima , the nuclear scientist Antonio Hurtado from the TU Dresden announced that there were plans in Poland to build a pebble bed reactor on the border with Germany. The Leipziger Volkszeitung reported on discussions between the TU Dresden and Polish representatives. The Saxon Ministry of the Environment stated in October 2011 that it did not have any information on such Polish plans. According to Polish information, these vague considerations only relate to a period from 2045 onwards. The announcement by Hurtado should therefore be seen as part of the advertising campaign Change instead of getting out of the German pebble bed community after the Fukushima disaster.


Currently (2013) research on the globular HTR concept is only carried out on a small scale in Germany, namely at the TU Dresden , the GRS , the University of Stuttgart and at RWTH Aachen / Forschungszentrum Jülich (FZJ): For example, at the FZJ the large-scale test stand NACOK operated to investigate pebble bed reactor problems. In addition to the basic funding of the institutions involved, around € 1 million / year from the Federal Ministry of Economics and EU third-party funds are available. A detailed description of the situation in Germany after 1990 can be found here . The nuclear technology professor Günter Lohnert from the University of Stuttgart, a leading representative of the German Kugelhaufen HTR lobby, came under pressure in 2008 after he had campaigned massively for the controversial cold fusion (see in particular Cold Fusion # Sonofusion ). After a lengthy public discussion, the FZJ's Supervisory Board decided in May 2014 to stop HTR research in Jülich at the end of 2014 and to shut down the test stands.


In the Netherlands, a large-scale pebble bed reactor project called ACACIA was worked on until around 2010. This development work has now largely been discontinued.


The French HTR development focused on an HTR with prismatic fuel elements based on the US model. It is currently only being pursued by the US subsidiary of the French Areva group.

Military application

Some military pebble bed reactor projects have become known:

From 1983 to 1992, as part of the projected US missile defense system SDI, work was carried out under the name Timberwind to develop a nuclear rocket drive with a pebble bed reactor. With the end of SDI, Timberwind was also discontinued.

Pebble bed reactors are particularly suitable for generating tritium for nuclear hydrogen bombs . Siemens and the US company General Atomics worked out an offer for a tritium production reactor for the US Department of Defense on the basis of the HTR-Modul200 pebble bed reactor concept until 1989. According to press reports, one of the reasons why the project was not accepted was that Siemens was simultaneously offering a civilian HTR-Modul200 in the Soviet Union .

In 1991, the South African apartheid government planned to upgrade submarines with a pebble bed reactor drive to nuclear submarines for the purpose of safely storing the six atomic bombs . Despite the low power density, the choice fell on pebble bed reactors because other nuclear technology was not available due to the international embargo. After the dismantling of the six South African atomic bombs in 1993, this military project was transferred to the above-mentioned, meanwhile abandoned, civilian PBMR project , also in order to give the people involved in the manufacture of nuclear weapons a professional perspective.

See also


  • Ulrich Kirchner, The High Temperature Reactor: Conflicts, Interests, Decisions . Campus Research Vol. 667, Frankfurt / Main; New York, 1991, ISBN 3-593-34538-2
  • Luigi Massimo, Physics of High-Temperature Reactors . Oxford etc .: Pergamon, 1976
  • Kurt Kugeler and Rudolf Schulten , high temperature reactor technology . Berlin etc .: Springer, 1989

Web links

Wiktionary: high temperature reactor  - explanations of meanings, word origins, synonyms, translations

Individual evidence

  1. Kugeler et al. Schulten (see literature list) p. 2
  2. a b A MODULAR PEBBLE-BED ADVANCED HIGH TEMPERATURE REACTOR, Report University of Berkeley (2008), archive link ( Memento from January 1, 2014 in the Internet Archive ) (accessed March 28, 2012)
  3. Archive link ( Memento from February 18, 2006 in the Internet Archive )
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