Spent fuel pool

Cooling ponds (also wet storage or fuel storage ) are water-filled pools at nuclear power plants, consumed in a nuclear reactor ( spent ), initially highly radioactive fuel to decay, that is, to reduce the remaining radiation activity and temperature ( decay heat ) stored for several years in which to transport capacity and can be cooled.

Cooling ponds are usually very close to the reactor because the fuel has to be constantly cooled during transport from the reactor core to the spent fuel pool. Also on radiological protection grounds the fuel elements must be surrounded by enough water constantly. The minimum cover with water is about 2 meters.

Decay process

The fuel assemblies come with a resulting from the decay heat temperature of about 100 ° C from the reactor to the spent fuel pool. Since the water used as a coolant acts as a moderator in the spent fuel pools must also be present neutron absorber to prevent a criticality.

During storage, the disintegrated formed by nuclear fission in the reactor, mostly short-lived radionuclides. The released energy is emitted in the form of heat to the surrounding water and discharged via cooling circuits. It is here often and well the phenomenon of so-called Cherenkov light observed: a bluish luminous phenomenon that is caused faster electrons when passing through water. Under normal circumstances, the water temperature in the cooling pond is less than 50 ° C. and serves, at least in modern nuclear power stations, in order to increase the efficiency of the Speisewasservorerwärmer as a secondary circuit.

The fuel elements remain in the spent fuel pool until their radioactivity and thus the resulting decay heat has decreased so far that they can be transported. Here, both limiting the dose rate and the thermal power, there are statutory limits for the dose rate, and the outer surface temperature of the transport containers (such as the Castor ). After the decay of the fuel rods are placed in interim storage. Repository does not exist today.

Rearrangement of the fuel elements

For the rearrangement of ( spent ) fuel assemblies from the reactor initially be the concrete cover of the biological shield and reactors in which not the spent fuel pool is located in the containment, the lid of the containment (English Containment, orange in the schematic diagram) opened and side down. Subsequently, the reactor pressure vessel (RPV ) is filled up to the flange, and held pressurized. Then the 40-100 ton vessel head (yellow dome above # 41 in the schematic diagram) is lifted by crane (No. 26 in the illustration ) upwards. The reactor core is thus accessible from the top. After the RPV Open the transport pool, so the area above the reactor pressure vessel ( yellow), flooded with water until the water level is at the same level as that of the spent fuel pool. If this is the case, by removing the storage pool locks the connection between the RPV and the cooling basin is produced. The highly radioactive fuel elements are adequately shielded by the large water coverage. The fuel can be lifted from the reactor vessel into the adjacent spent fuel pools with the refueling machine (a special crane on a traveling bridge above the pool ) through the fuel pool gate in the wall of the ( dry during normal operation ) Transport basin. Once there, they are stored in a storage rack ( Fig., No. 27).

Storage quantities

The capacity is for operational reasons and for emergencies at least one reactor fuel filling, by means of storage racks, the capacity for more storage volumes are produced (conventional storage). In the way of so -called compact storage, the storage capacity is further extended several times, in this case a closer assignment is by the incorporation of absorber material in the storage racks made ​​possible with fuel.

Given the lack of repository and qualified shipping container cooling ponds are used on the operationally necessary storage requirements of the respective power plants also as an interim storage for spent nuclear fuel. Thus, the German spent fuel pools are filled to an average of 83%, the nuclear power station Isar I, even to 91 %.

Storage time

G. Schmidt of the Öko-Institut in Darmstadt called because of the need for storing active cooling and cleaning systems with the energy needed for them a maximum of four years for the so-called wet storage as appropriate; this was confirmed by the head of internal communications of a nuclear power plant Grohnde.

According to Michael Sailer, the former head of the German Reactor Safety Commission, are stored in the spent fuel pools German nuclear power plants, fuel for about 5 years, in which the Japanese nuclear power plant Fukushima Daiichi about 15 years.

Be lack of suitable repository, for example, significantly exceeded even in the U.S., provided there 5 years.

Risks

In echoes of the Fukushima nuclear disaster rose to any minimization of the risks of nuclear power plants, the proposal in the future to separate spent fuel pool of reactor plants in space. In the present ( 2011) world's most advanced planning of a possible repository for nuclear waste in Forsmark (S ) is a requirement to have to transport spent nuclear fuel rods as little as possible.

External events

For nuclear power plants with internal cooling ponds are these always right next to the flood area of the reactor, in order to facilitate fuel handling, and thus within the reactor building. The protection against external hazards is therefore dependent on the building structure of the reactor building, which takes into account in Germany since the mid-1980s, for example, the protection against aircraft crash. In pressurized water reactors, the pool is located inside the containment.

Cooling

Event of a leak or failure of cooling the tank can run dry by leakage or evaporation ( partially ). In this case, the fuel stored there can heat excessively. Is there still water in the basin exists at about 800 ° C, the Zircaloy cladding tubes of the can react with the water (vapor ) in an exothermic redox reaction to zirconium oxide and hydrogen to form an explosive oxyhydrogen gas mixture in a short time.

For complete draining of the fuel rods they can catch fire, resulting in destruction of the fuel result. Also in this scenario, the radioactivity is released; in addition to the flue the various present in the spent fuel radionuclides released into the atmosphere (chimney effect, see Chernobyl disaster ). The only countermeasure is the timely refilling cool water to keep the water level in the tank high enough for the necessary cooling. Since the water serves as a shield for the radioactive radiation, of the fuel in the tank in addition to the cooling effect, in the case of a low water level, a padding is further complicated by severe circumstances radioactivity. Also there is a danger that fuel at high temperature by the water replenishment, the above -mentioned water - zircaloy reaction is started.

In a created as a result of the Fukushima nuclear disaster study, the Swiss nuclear regulatory authority Ensi reviewed the security situation for the cooling capabilities of the fuel element, ie the spent fuel pool, in the nuclear power plants Beznau I and II as well as physical City on the Upper Rhine as "insufficient". There retrofit measures have been arranged.

Leaks

Also in the cooling pond is accidents can happen and radioactivity, are, for example, on escaping cooling water released. Studies are currently being observed in the basin of the Indian Point nuclear power plant that lies below the limits amounts of tritium, cesium and strontium enter the groundwater and be deported from there into the Hudson River.

If the cooling water through a major leak escape relatively quickly and should emergency measures for refilling the water tank by about fighting vehicles do not work in time, threatened with heavy emptying the pool with a so-called Zirkoniumbrand, ie the zirconium cladding of the fuel elements react violently with oxygen after its heating. New experiments with individual fuel rods have shown that it takes around 12 hours after an average period of time after removal from the reactor until ignition occurs. The fuel elements were removed from the reactor, however, only recently ( the decay heat is even higher ), this period of time but it can significantly shorten to ignition.

Hydrogen formation

In normal operation can be split by radiolysis in the vicinity of the stored fuel the water into hydrogen and oxygen. Thus, no major accumulations of these two gases can collect ( oxyhydrogen ) under the umbrella of the spent fuel pool, the air must be continuously extracted from there, otherwise, if appropriate, risk of explosion after some time.