Decay heat

With decay heat (english decay heat ) - sometimes simply reheat - is referred to in the nuclear reactor technology, the decay heat that still arises anew after terminating the chain reaction in the fuel elements. Since the neutron flux has come through insertion of the control rods almost to a standstill, for example, hardly any new fission reactions take place after the shutdown. The decay heat comes rather from the fact that the existing, short-lived fission products decay radioactively. Heat through downstream decay processes apply in the normal, continuous reactor operation. With decay heat but is explicitly only those heat meant that newly created in the shutdown and thus even later in the spent fuel pool, in Castor or deposits.

The amount of heat and the vapor pressure range after disconnection generally no longer sufficient for operation of the steam turbine. Decay heat must therefore be removed completely with normal or a special cooling circuit. This heat load is immediately after the switching off of between 5 % and 10 % of the previous thermal power of the reactor, depending on the reactor type, the operating time and the nuclear fuel used. In a large-scale reactor such as the EPR of 1600 megawatts ( MW) of electrical power, that is, about 4,000 MW of thermal power, [note 1] occur one hour after the shutdown around 50 MW thermal output, after four days, another 20 MW.

Due to the decay heat damage can occur after total failure of the cooling system at the reactor up to the meltdown. Therefore, light-water reactors require appropriate emergency cooling systems.

Colloquially, the term is residual heat needed. But this is misleading, since the decay heat does not describe the amount of heat stored the reactor core ( heat capacity ) during turn-off, but the power generation of the ongoing radioactive decays.

Calculation

For each radionuclide, the decay heat decays according to an exponential function. In the fission product mixture in a reactor, the exponential functions overlap to a course, which can be calculated for practical purposes by approximation formulas. The calculation rules are defined in the standards DIN 25463-1 and DIN 25463-2.

An approximate formula for the decrease in the amount of heat was given in 1946 by Way and Wigner. A reactor for the continuous operated with the power, the decay heat is the time after the shutdown of the reactor

Here, T0 and t in seconds to use. The following formula applies (with a sufficiently long operating time T0) for a period from about 10 seconds to about 100 days after the shutdown. From the article of Way & Wigner (1946 or 1948) is seen as the average decay heat of the products of a split event with the help of the Bethe- Weizsäcker formula (liquid -drop model for calculating the energy differences of the decay products in radioactive decay chain) and the Sargent - rule ( mean lifetime of a radiator is approximately inversely proportional to the fifth power of the energy difference of mother and daughter nuclei, which can also be brought into connection with the maximum electron energy) can be calculated. This gives a decay of this Einzelnachzerfallsleistung as if the decay took place at the time. That if the decay has taken place at the time, one obtains

Were cleaved under the assumption that nuclei uniformly distributed over an interval, we obtain the total decay heat by integrating

Since the number of the split cores per second, can be related to the performance of the reactor in conjunction ( on the assumption of time constant power ), ie

Wherein the average energy per fission thermally available ( about 200 MeV per fission), one can also refer to the decay heat, the reactor power driven, as indicated above. The correct prefactor therefore results from the correct average Einzelnachzerfallsleistung and the mean, per fission thermally usable energy.

The derivation of Way and Wigner based on very rough model assumptions that must be met by no means in detail. Therefore, they go even of an error in the range of 15% to 20 %. For example, not all follow - unstable nuclides of the Sargent - rule. There are certain permitted and non- permitted transitions, which can severely affect the life of a radiator. The mass numbers, and the cleavage products were fixed in the derivative at the maxima ( in fact they are characteristically distributed with two maxima; see cleavage product ). Also a mapping for a given mass number to a fixed number of protons is not unique among the fission products, so here (for the light (L) and heavy (H) cleavage products ) was a Gaussian distribution for accepted around a mean value. Here the 15 % come from to 20 % inaccuracy. Finally, Way and Wigner have determined the total decay heat, including decays and kinetic energy of the neutrino. To determine the actual thermal decay heat from it, a few assumptions are needed on the distribution of the decay energy to electrons, neutrinos and photons. It is amazing that in the middle so well matches the theoretical prediction of this model approach with experimental results ( the data of the authors reported ).

Finally, it was completely ignored in the above integration that certain nuclides by neutron capture omitted from the decay chain in part, ie may be inactive, or its further decay accelerating or decelerating (see reactor poison). In this respect, the above formula Gesamtnachzerfallswärme is certainly not very accurate. Exact Nachzerfallsleistungen wear the exponential dependence of radioactive decay into account, ie contain a sum of Expontentialfunktionen, but make yourself so far more difficult than all the relevant decay chains must be considered.

Examples of decay heat after long periods of operation

After 11 months of operation, a typical fuel cycle, near the rated power the following values ​​are obtained from the above formula ( power values ​​and durations are based on the fuel content of a typical large reactor):

Decay after refueling

Even several months after the operation can again reach the melting point of fuel for which no cooling. To dissipate the decay heat of spent ( " spent " ) fuel elements, the fuel must be stored in a power plant belonging to water-filled cooling pond for several years, which dissipate the heat again on cooling circuits.