Pressure vessel investigations of the former Greifswald NPP: Fluence calculations and Nb based fluence measurements

Pressure vessel integrity assessment after long-term service irradiation is commonly based on surveillance program results. Radiation loading, metallurgical and environmental histories, however, can differ between surveillance and RPV materials. Therefore, the investigation of RPV material from decommissioned NPPs offers the unique opportunity to evaluate the real toughness response. A chance is given now through the investigation of material from the decommissioned Greifswald NPP (VVER-440/230) to evaluate the state of a standard RPV design and to assess the quality of prediction rules and assessment tools.

The operation of the four Greifswald units was finished in 1991 after 12 – 15 years of operation. 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 different loading schemes of unit 1 (standard, with 4 or 6 dummy assemblies) were taken into account. The fluences after the annealing procedure (after cycle 13) were separately determined. The statistical error of the integral fluence values (E > 0.5 MeV) was estimated to be smaller than 1%. The comparison between the calculated fluences and experimental values (determined from excore fluence monitors applied during the 12th cycle) showed a good agreement.
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 anion exchange separation. During this separation all other elements of the sample were removed. Finally Nb is stripped from the column. The radiochemical separation is accompanied by determination of the chemical yield of Nb using the above mentioned ICP-MS method.
The measurement of the 93mNb activity is realized by Liquid Scintillation Spectrometry (LSC). A detection efficiency of 91 % was achieved.