Fissile materials - so-called nuclear fuels - are used in reactors operated used for electricity generation, but also in research reactors. These materials are usually uranium-235, while in some cases mixed-oxide fuel assemblies including plutonium-239 are also used.
Safe handling of nuclear fuel – from enrichment to disposal
In the field of work of "Radiation and Environmental Protection", GRS deals with fundamental safety issues related to the handling of nuclear fuels outside nuclear reactors, i.e. from uranium enrichment and the production of fuel assemblies up to the interim storage of spent fuel assemblies and their eventual disposal. Drawing on their specialist knowledge in this domain, the experts of GRS support and advise the Federal Environment Ministry (BMU) in safety-related issues. As regards matters of nuclear fuel supply and waste management, they take an active part at national and international level in the development of the relevant regulations and also provide advice to foreign authorities. The associated work covers a broad range of topics.
One essential requirement for safety in connection with the handling of nuclear fuels is that so-called criticality accidents must be avoided. A criticality accident may occur if the critical mass of a nuclear fuel is reached or exceeded upon handling - for example during the processing of enriched uranium, but also in connection with the interim storage or disposal of spent fuel assemblies. In such a case, there will be a nuclear chain reaction during which a great deal of energy is released in the form of radiation and heat. GRS is constantly looking into criticality issues and evaluating corresponding experience from the operation of nuclear installations and also performs accident analyses. The results of this work are among other things included in the "Handbook on Criticality" which GRS has been issuing since 1979 and which is continuously updated. This Handbook contains a compilation of fundamental information on criticality safety upon the handling of nuclear fuels and of criticality data for various nuclear fuels.
For the analysis of the criticality safety of irradiated nuclear fuel it is important to know as exactly as possible the composition of the radionuclides in the fuel. This composition changes during the dwell time of a fuel assembly inside the reactor: the nuclear fission process produces new radionuclides, the so-called fission products. The knowledge about the composition of the radionuclides in a fuel assembly is also a prerequisite for determining the kind and amount of the radioactive radiation emitted by this fuel assembly.
Shielding and activation
Apart from the work relating to criticality safety, GRS also deals with issues relating to the shielding of radiation-emitting nuclear fuels and the so-called activation of materials. Activation means the generation of radioactive materials through irradiation with neutrons, which happens e.g. in a nuclear reactor. Other than in the case of contamination, were radioactive materials accumulate on the surface of component materials, the material itself changes when activated. This leads to the fact that following its activation, the material – e.g. the steel of a reactor pressure vessel (RPV) – will emit radioactive radiation even after decontamination. GRS examines i.a. to what neutron radiation components – such as a RPV – are subjected during their service life and what kind of activation ensues. These findings then make it possible e.g. to determine for how long a RPV has to be kept in so-called decay storage following the final shutdown of a nuclear power plant before it can be dismantled without any major shielding measures.
Regarding the work of GRS relating to the material balance, this is to do with the development of methods that allow the exact tracing and documentation of the nuclear fuels used in Germany – for example in the form of uranium during enrichment and fuel assembly production as well as in the form of spent fuel assemblies. The data collected in this connection form i.a. the basis for the proof of nuclear waste disposal to be furnished by the nuclear power plant operators.
In all the topic areas mentioned above, the development, maintenance and validation of computer codes is an essential part of the work of GRS. The same is true of the further development of data libraries in which the basic data of different nuclear fuels that are necessary for the examination of various different issues are summarised.