Neutron dosimetry study in the region of the support structure of a VVER-1000 type reactor

The support structure of a reactor pressure vessel experiences a high mechanical stress during reactor operations. In addition, supposed accident conditions and ferritic steel properties result in a low limiting value for the ductile-to-brittle transition temperature and, therefore, a low maximal irradiation load is permitted for this region compared with others. In case of a VVER-1000 reactor, the same applies to weld #5, which is closely above the support structure. The Russian Reactor Operation Rules lay down that profound analyses of the components integrity have to be regularly done if the neutron fluence above the neutron energy of 0.5 MeV must be expected to accumulate a value greater than 1·1018 n/cm2 during the projected operation time of forty years. Therefore, it was necessary to reliably clarify whether this area of the pressure vessel falls into the category of permanent inspection or not. To this end, highly precise fluence calculations were carried out. Applied codes and nuclear data sets were verified by special neutron activiation measurements, which were done just in the near neighborhood of the support structure and of weld #5. The calculation results show that in certain subareas of both construction elements the neutron fluence will exceed the limiting value if the reactor is nominally operated during the projected life-time.