Neutron fluence at the reactor pressure vessel wall. A comparison of French and German procedures and strategies in PWRs

N. Tricot, U. Jendrich, Eurosafe, Berlin, 4-5 novembre 2002, Rapport IRSN-DES/546

While the neutrons within the core may take part in the chain reaction, those neutrons emitted from the core are basically lost for the energy production. This “neutron leakage” represents a loss of fuel efficiency and causes neutron embrittlement of the reactor pressure vessel (RPV) wall. The latter raises safety concerns, needs to be monitored closely and may necessitate mitigating measures. There are different strategies to deal with these two undesirable effects: The neutron emission may be reduced to some extent all around the core or just at the “hot spots” of RPV embrittlement by tailored core loading patterns. A higher absorption rate of neutrons may also be achieved by a larger water gap between the core and the RPV. In this paper the inter-relations between the distribution of neutron flux, core geometry, core loading strategy, RPV embrittlement and its surveillance are discussed at first. Then the different strategies followed by the German and French operators are described. Finally the conclusions will highlight the communalities and differences between these strategies as different approaches to the same problem of safety as well as economy.