Spectral conditions in surveillance specimen container of Czech VVER-1000 reactors

Irradiation conditions of material specimens in containers placed on the internal surface of the reactor pressure vessel that are used for the evaluation of material changes during the lifetime of the reactor should correspond to those which the reactor pressure vessel is exposed to. The Czech Modified Surveillance Program for VVER-1000/V-320 C type of reactor vessels in Temelin NPP specifies the following principles that must be applied: (1) the locations of containers should well monitor the conditions at the beltline region of the reactor pressure vessel (RPV) wall , i.e. the specimen temperatures should be as close to the real ones as possible and the lead factor should be less than 5, (2) the whole set of the specimens for one testing curve should be put in identical neutron fluence conditions, (3) the sets of the specimens should be located at positions of similar/near neutron fluence for the behavior of different materials to be comparable, (4) the withdrawal scheme of the containers should assure monitoring the pressure vessel material as well as neutron fluence during the whole RPV lifetime. For the assessment of the influence of neutron fluence on the change of the material characteristics the integral neutron fluence with energy above 0.5 MeV is used. Spectral index SI05 defined as the ratio of the neutron flux with energy above 0.5 MeV to the neutron flux with energy above 3 MeV is used as the measure of the neutron spectrum change. The article presents the analysis of the spectral index SI05 in the specimens and in the reactor pressure vessel at the beltline region. Special interest was focused on the region under the beltline weld up to the depth equal to one tenth of the thickness (0.1 T) of the pressure vessel as this depth of the postulated defect is used in the calculations according to the VERLIFE methodology. (author)