Nuclear power: How the stretching operation of nuclear power plants works

According to current law, the last three nuclear power plants in Germany must be shut down by December 31, 2022 at the latest. But in view of the lack of gas, there are increasing political demands to let the power plants run longer. In this context, the keyword “stretching operation” regularly appears, which would enable the nuclear power plants to produce energy – albeit at a reduced rate – for a longer period of time. But what exactly is that? How long can nuclear power plants run in this mode? And is that really safe?

Like almost all nuclear power plants built and operated in Germany, the last three nuclear reactors that have not yet been shut down are pressurized water reactors. In such reactors, the fuel is introduced in the form of fuel elements. The fuel elements in turn consist of fuel rods in which the actual fissile material is enclosed in pellet form. The individual fuel rods are combined into a fuel assembly by a mechanical framework. A typical pressurized water reactor with an output of 1400 MW contains 193 fuel assemblies, each with 300 fuel rods.

The nuclear fuel essentially consists of uranium 238. Only a small part – about five percent – is uranium 235. This uranium isotope is used as the actual fissile material because it has a high effective cross section for thermal, i.e. comparatively slow, neutrons. The water used as a coolant in pressurized water reactors slows down the fast neutrons released during nuclear fission and thus makes them available for further nuclear fissions.

Freshly loaded reactor cores contain more fissile material than is needed to sustain the nuclear chain reaction. The so-called “excess activity” is compensated for by neutron-absorbing substances in the cooling water – for example boric acid – and by control rods inserted into the reactor core. In the course of operation, the proportion of boric acid is reduced further and further, and the control rods are pulled out. If the proportion of boric acid cannot be further reduced, “the natural end of the cycle has been reached”, writes the Society for Reactor Safety on your website. The proportion of uranium 235 has then dropped to about 0.6 percent, which consumes excess activity.

The reactor would then, as it were, shut itself down. One can slow down this “self-shutdown” process, for example by lowering the coolant temperature in the reactor. This increases the density of the coolant, which means that the neutrons are slowed down better and ultimately more neutrons are available for fission. However, on the one hand the activity decreases in “stretch out” or “stretching operation” and on the other hand the power available at the generator is also lower when the temperature of the coolant is lower. After 80 days of stretching, the reactor still has around 60 percent of its capacity.

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However, this is only a very rough summary of the technical basics. Real operation is much more complex: Not only does uranium 235 contribute to energy generation, but also uranium 238, which is split by fast neutrons that have not yet been slowed down, or only slightly so. In addition, neutron capture and
ß-decay plutonium 239, which can also be split by fast neutrons. At the same time, a wide range of fission products is created, including strongly neutron-absorbing atomic nuclei. The neutron flux in the reactor can therefore vary greatly locally. In order to make optimal use of the nuclear fuel, the operators therefore relocate the fuel elements at regular intervals and replace old, completely spent fuel rods.

The energy released by the nuclear reaction, related to the mass of heavy metal used, is referred to as “burnup” – the higher the degree of uranium-235 enrichment and the longer a fuel element remains in the reactor, the higher the burnup. as technical limiting factor the thickness of the oxide layer, which forms on the cladding tubes during operation, has been proven. The permissible mean burn-up is usually around 50 gigawatt days per tonne of heavy metal.

In order not to waste valuable fuel, the energy suppliers operate their nuclear power plants in such a way that they come as close as possible to this maximum burn-up. “The shutdown date is taken into account in the way the reactor is operated in order to make optimum use of the available nuclear fuel in the given time,” confirms Greenpeace nuclear expert Heinz Smital: “This can be seen clearly from the example of the reactors shut down at the end of 2021. This is how Gundremmingen produced C the highest amounts of electricity ever generated in the shutdown year 2021.”

There the operators went into a stretching operation on October 16, 2021, which lasted 76 days until the final shutdown. “It makes a lot of sense to also use the fuel elements in stretch mode during the permitted period for power production,” writes Smital. “However, the remaining amount of available power is then not very large and the contribution to a power supply is very small.”

In other words, whether and how much energy the three German power plants that are still running can still deliver without getting new fuel elements depends on how the reactors have been used to date. Since the operators have had to assume that the plants will be shut down at the end of the year, it is only natural to assume that the remaining reserves are small. In a report by the TVÜ Süd for the Isar 2 reactor, it assumes “reactivity reserves” that would be sufficient for “continued operation for 80 days”.

Interestingly, these 80 days are exactly the period that GRS estimates for a stretching operation – however, the word “stretching operation” does not appear in the controversial report. However, TÜV Süd did not want to answer technical questions about the stretching operation of nuclear reactors: “Please contact the operators of the nuclear power plants or the GRS (Society for Plant and Reactor Safety) directly with your request.”


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