Retrospective dosimetry of Greifswald VVER 440 RPV: Fluence calculations and Nb based fluence measurements

The operation of the four Greifswald units (VVER-440/230) finished in 1991 after 12 – 15 years of operation. The investigation of RPV material from the decommissioned Greifswald NPP offers the possibility to evaluate the state of the standard RPV design and to assess the quality of prediction rules and assessment tools. The different operation conditions (irradiated, annealed, and re-irradiated) are a special advantage of the research program.
In autumn 2005 the first trepans (diameter 120 mm) were gained from the unit 1 of this NPP. A new drilling machine was developed and adopted to the actual plant conditions. The drilling machine allows the following remote controlled actions:

• Labeling the position and orientation of the trepan
• Drilling the trepan and ejection of the trepan into the RPV
• Closing the hole in the RPV

Details of the trepanning procedure will be given.
Fluence calculations using the code TRAMO were based on pin-wise time dependent neutron sources and an updated nuclear data base (ENDF/B-VI release 8). The neutron and gamma fluence spectra were determined at the trepan positions. The integral neutron fluence at the inner RPV wall near the critical weld was found to 4.64* 10^18 n*cm-2. The maximum fluence at the RPV is 50 % higher. In addition, it could be shown that the fluence at the designated reference positions can be neglected (3 orders of magnitude smaller). The gamma fluence was calculated to 1.9 *10^20 photons* cm-2 at the critical weld with a similar axial distribution.
The experimental determination of the neutron fluence was based on Niobium activity measurements. The RPV material contains Niobium as trace element. The analysis of the Nb-content is carried out by ICP-MS (inductive coupled plasma mass spectrometry) after dissolution of the material sample. The radiochemical isolation of Nb was done by an anion exchange column. By this separation all other elements were removed.
The measurement of the 93mNb activity is realized by Liquid Scintillation Spectrometry (LSC). Parallel the chemical yield is determined by the above mentioned ICP-MS method.