Efforts to verify neutron fluence calculations at pressure vessels of decommissioned reactors by means of retrospective niobium dosimetry

For the first time, trepans from a decommissioned VVER-440 (Greifswald-1) reactor pressure vessel (RPV) have been examined. Activities of a trepan, taken at the RPV weld with the highest fast-neutron load, were measured and estimated on the basis of fluence calculations by the codes TRAMO and DORT. A maximum fluence of 4.05*1019 n/cm2 (E>0.5 MeV) was calculated. The average deviation between the two codes is 2.6 %. Activities resulting from the reaction 93Nb(n,n’)93mNb were measured, niobium being a trace element in the RPV steel. Unfortunately, 93mNb is also produced by neutron capture in the alloy component 92Mo, the built-up 93Mo decaying by electron capture. The ratios of calculated to measured (C/E) 93mNb gamma activities for several trepan samples are between 0.42 and 0.97. The fact that all C/E ratios are below unity suggests that the measured values may have been additionally heightened by activities from other nuclides.