Cooling pool
Decay pools (also wet storage , fuel pool or fuel storage ) are water- filled pools in nuclear power plants , in which the initially highly radioactive fuel elements consumed (burned off) in the nuclear reactor are decayed , i.e. to reduce the remaining radiation activity and temperature ( decay heat ) until they can be transported for several years can be stored and refrigerated.
Decay basins are usually located in the immediate vicinity of the reactor, since the fuel elements have to be continuously cooled during transport from the reactor core to the decay basin. For reasons of radiation protection too , the fuel elements must be constantly surrounded by sufficient water. The minimum coverage with water is approx. Two meters.
Decay process
The fuel elements come from the reactor to the spent fuel at a temperature of over 100 ° C due to the heat of decay. Since the water used as the coolant acts as a moderator , additional neutron absorbers must be present in the cooling pool in order to avoid criticality .
During storage , the largely short-lived radionuclides formed during nuclear fission in the reactor decay . The energy released is given off in the form of heat to the surrounding water and dissipated via cooling circuits . The phenomenon of the so-called Cherenkov light can often be observed here: a bluish glow that is caused by the passage of fast electrons through water. Under normal circumstances, the water temperature in the cooling pool is less than 50 ° C (during normal operation under 45 ° C). This water is used, at least in newer nuclear power plants , to increase the efficiency of the secondary circuit as a feed water preheater .
The fuel elements remain in the spent fuel until their radioactivity and thus the resulting decay heat has decreased to such an extent that they can be transported. Both the dose rate and the heat output are limiting here , as there are legally prescribed limit values for the external dose rate and the surface temperature of the transport containers (such as the CASTOR ). After the fuel rods have died down, they are placed in interim storage facilities . To this day, repositories do not exist.
Relocation of the fuel assemblies
For the rearrangement of (spent) fuel elements from the reactor, the concrete cover will first of the biological shield and in reactors in which the spent fuel is not located in the containment, the lid of the containment vessel (engl. Containment , orange in the above scheme, graphics) is opened and put aside. The reactor pressure vessel (RPV) is then filled up to the flange and kept pressureless. Then the 40 to 100-ton RPV cover (yellow dome above No. 41 in the diagram) is lifted upwards by crane (No. 26 in the illustration). The reactor core is therefore accessible from above. After opening the RPV, the transport basin, i.e. the area above the reactor pressure vessel (yellow), is flooded with water until the water level is at the same level as that of the decay basin. If this is the case, a connection between the RPV and the cooling pool is established by removing the storage pool sluice. The highly radiating fuel elements are adequately shielded by the large water cover.
The fuel elements can be lifted with the fuel element changing machine (a special crane on a driving bridge above the basin) through the storage pool lock in the wall of the transport basin (which is dry during normal operation) from the reactor cask and into the adjacent decay basin. There they are then stored in a storage rack (Fig., No. 27).
Storage quantities
For operational reasons and for emergencies, the capacity includes at least one reactor filling of fuel elements; storage racks are used to create the capacity for additional storage quantities ( conventional storage ).
By means of so-called compact storage, the storage capacity is again expanded several times over; the installation of absorber material in the storage racks enables a closer allocation of fuel elements.
In view of the lack of repositories and qualified transport containers , decay pools are used as interim storage facilities for spent fuel elements beyond the operational storage requirements of the respective power plants. The German decay basins are 83% full on average, and that of the Isar I nuclear power plant even 91%.
storage time
G. Schmidt from the Öko-Institut in Darmstadt described a maximum period of four years for so-called wet storage as suitable because of the active cooling and cleaning systems with the energy required for storage; this was confirmed by the head of internal communications at the Grohnde nuclear power plant .
According to Michael Sailer , the former head of the German Reactor Safety Commission , the fuel elements in the decay basins of German nuclear power plants are stored for about 5 years, in those of the Japanese nuclear power plant Fukushima-Daiichi for about 15 years.
In the absence of suitable repositories, the five years stipulated there are also significantly exceeded in the USA.
Risks
In the aftermath of the nuclear disaster in Fukushima , the proposal arose to physically separate the nuclear power plant from the reactor facilities in order to minimize the risks of nuclear power plants.
In the currently (2011) most advanced worldwide planning of a possible repository for nuclear waste in Forsmark , Sweden , one requirement is to have to transport spent nuclear fuel rods as little as possible.
External influences
In the case of nuclear power plants with internal cooling basins, these are always located directly next to the flood chamber of the reactor in order to facilitate the handling of the fuel assemblies and thus within the reactor building. Protection against external influences therefore depends on the construction of the reactor building, which in Germany, for example, has taken into account protection against plane crashes since the mid-1980s. In the case of pressurized water reactors , the basin is located inside the containment .
cooling
In the event of a leak or failure of the cooling system, the pool can (partially) run dry due to leakage or evaporation . In this case, the fuel elements stored there can heat up excessively. If there is still water in the pool, the zircaloy of the cladding tubes can react with the water (vapor) in an exothermic redox reaction to form zirconium oxide and hydrogen at approx. 800 ° C and an explosive oxyhydrogen gas mixture can form within a short time .
If the fuel rods are completely drained, they can catch fire, which results in the destruction of the fuel elements. In this scenario too, radioactivity is released; In addition, the various radionuclides present in the spent fuel elements are released into the atmosphere with the resulting smoke (chimney effect, see Chernobyl catastrophe ). The only countermeasure is to top up with cool water in good time to keep the water level in the pool high enough for the necessary cooling. Since the water not only has a cooling effect but also serves as a shield for the ionizing radiation of the fuel assemblies in the basin, if the water level is too low, topping up is also made more difficult by, under certain circumstances, strong ionizing radiation. There is also the risk that the water-zircaloy reaction mentioned above will be started when the fuel temperature is high.
In a study carried out as a result of the Fukushima nuclear disaster , the Swiss nuclear supervisory authority, Ensi, rated the security situation for the cooling options for the fuel element storage , i.e. the decay basins , in the Beznau I and II nuclear power plants and Leibstadt am Hochrhein as "insufficient". Retrofitting measures were ordered.
Leaks
Accidents can also occur in cooling pools and radioactivity can be released, for example via escaping cooling water. In the basin of the American nuclear power plant Indian Point , for example, it is currently being observed that amounts of tritium , cesium and strontium that are below the limit values reach the groundwater and are carried away from there into the Hudson River .
If the cooling water escapes relatively quickly due to a major leak and if emergency measures for replenishing the water, for example by means of tank fire engines, do not work in time, there is a risk of a so-called zirconium fire if the basin is heavily emptied . H. the zirconium cladding tubes of the fuel elements react violently with oxygen after they have been heated. New experiments with individual fuel rods have shown that after an average time after removal from the reactor, it takes around 12 hours for ignition to occur. However, if the fuel elements were only removed from the reactor a short time ago (the decay heat is even higher), this time until ignition can be considerably shortened.
Hydrogen formation
During normal operation, radiolysis in the vicinity of the stored fuel elements can split the water into hydrogen and oxygen . So that no large accumulations of these two gases ( oxyhydrogen ) can collect under the roof of the cooling pool, the air must be continuously extracted from there, otherwise there may be a risk of explosion after a while .
See also
Web links
Individual evidence
- ↑ cooling pond in the lexicon Physics / Scientific American
- ^ Fort Calhoun Nuclear Station. Used fuel storage. Utilities Service Alliance (USA), accessed March 24, 2011 .
- ↑ Safety rules of the Nuclear Technical Committee KTA 3303: Heat removal systems for fuel storage pools of nuclear power plants with light water reactors ( Memento of November 8, 2011 in the Internet Archive ), Section 4.2.1.1
- ↑ Reactor pressure vessel cover ( Memento from October 26, 2007 in the Internet Archive )
- ↑ nux.ch, nux-number 4, September 1978: The matter with the fuel elements in the basin ( Memento of January 21, 2005 in the Internet Archive ) (May 18, 2011)
- ↑ wdr.de, monitor, April 7, 2011, Markus Schmidt, Jan C. Schmitt: Out of Control: The explosive internals of Fukushima ( Memento from April 10, 2011 in the Internet Archive ) (April 11, 2011)
- ↑ dradio.de, Deutschlandfunk, Forschung Aktuell, July 26, 2011, Julia Beißwenger: 200 meters in the Castor: the Akw Grohnde in Lower Saxony (July 31, 2011)
- ↑ Michael Sailer in the podcast interview of March 18, 2011 (from 1:22:20), accessed on March 25, 2011.
- ↑ Typically, waste must sit in pools at least five years before being moved to a cask or permanent storage, but much of the material in the pools of US plants has been stored there far longer than that. online ( Memento of 28 April 2011 Webcite ) message from Associated Press March 22, 2011, Fox News , accessed on 28 April 2011th
- ↑ After the end it is far from over . By Wolfgang Kempkens, Wirtschaftswoche , March 16, 2011.
- ↑ badische-zeitung.de, Lokales, Kreis Waldshut , May 7, 2011, bz: Atomic supervision exercises criticism (May 7, 2011)
- ↑ Lessons Fukushima 11032011. Lessons learned and checkpoints from the nuclear accidents in Fukushima (PDF; 3.1 MB). ENSI report of October 29, 2011, p. 8.
- ↑ Frequently Asked Questions About Indian Point Groundwater Leakage. What are the levels of radioactive contamination seen from the monitoring wells? US Nuclear Regulatory Commission (NRC), accessed March 24, 2011 .
- ↑ ENSI : Experience and Research Report 2011 / "OECD SFP Project"