Post mortem investigations of Greifswald VVER 440 reactor pressure vessel: Recent progress in dosimetry and material investigations


Post mortem investigations of Greifswald VVER 440 reactor pressure vessel: Recent progress in dosimetry and material investigations

Konheiser, J.; Rindelhardt, U.; Viehrig, H.-W.

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.

Some years ago first fluence calculations were done for the four Greifswald units in the frame of a TACIS project [1]. Neutron fluence integrals E>0.5MeV, E>1MeV and dpa-values at the inner and outer pressure vessel walls were obtained with the help of a Green’s functions method. New improved results will be presented based on new calculations of pin-wise time dependent neutron sources and an updated nuclear data base. The Monte Carlo calculations of neutron and gamma fluence spectra for different time intervals were done at all positions throughout the RPV wall, where the material specimens will be taken. The statistical error was estimated to be smaller than 1%. A comparison between the calculated fluences and experimental values (12th cycle of unit 1) shows a good agreement, the differences were smaller than 5 %.

The fluence at the inner wall of the RPV for E > 0.5 MeV for the critical weld was 4.64* 10^18 n/cm2. The maximum fluence at the RPV is larger by a factor 1.5. In addition, it could be shown that the fluence at the designated reference positions can be neglected (3 orders of magnitude smaller).

Because of changes in the dismantling procedure a new technology was developed to extract trepans from the interesting RPV positions. A special drilling machine is under development, which will be positioned at the ring water tank near the main coolant loops. The RPV will be moved by crane in the wanted positions (height and azimuth). The remotely controlled trepanning will start in 2005 at unit 1.

The testing and investigation program is focussed on the characterization of the material state as received. It comprises chemical analysis, microstructure investigations (by means of metallography, electron microscopy and SANS), and mechanical testing (hardness measurements, tensile, Charpy-V and fracture mechanics testing). The key part of the testing will be focussed on the determination of the reference temperature T0 following the ASTM test standard E1921-03. Trepans from 4 height positions (welds 3 and 4, base material from the maximum load position and from reference position) will be investigated..

[1] TACIS-Project NUCRUS 96601, Final Report, Brussels 2000

Keywords: reactor pressure vessel; neutron dosimetry; neutron embrittlement; VVER-440

  • Lecture (Conference)
    4th International Conference on: Safety assurance of nuclear power plants with WWER, 23.-25.05.2005, Podolsk, Russia
  • Contribution to proceedings
    4th International Conference on Safety Assurance of Nuclear Power Plants with WWER, 23.-25.05.2005, Podolsk, Russia
    CD-ROM, Podolsk

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