Beneficiary Organisation Details

Kozloduy NPP
http://www.kznpp.org

Project Aims

• to conduct an annealing heat treatment on the RPV (in the region of weld, seam No.4) to restore the toughness which was degraded by irradiation;
• to conduct nondestructive and destructive examinations (the latter on small samples taken out of the RPV) to approve the effects of annealing and to develop materials data as an input to the fracture-mechanical safety analysis of the RPV.

Project Results

• An annealing heat treatment was conducted on the reactor pressure vessel (RPV) of NPP Kozloduy Unit 2 to recover irradiation effects primarily on the weld seam No.4 in the belt line region of the RPV;
• In-service inspection before and after annealing was performed to prove that no cracking occurred during annealing;
• Material samples were taken from the inside surface of the RPV, from weld metal of seam No.4 and from base metal in the region of maximum neutron fluence. After sampling, the sampling places were smoothly grinded to avoid any stress concentrations during operation resulting from sample cutting. The surface quality of the grinded regions and the minimum wall thickness were checked and proven to fulfil the requirements;
• From the samples taken out of the reactor vessel wall, analysis of chemical element composition and evaluation of neutron fluence was conducted;
• The materials properties before and after annealing and thus the annealing effect was examined by hardness testing, tensile tests and notched-bar impact tests performed with subsize tensile and Charpy specimens. From the latter the critical temperature TK was evaluated. For weld metal also the initial value TKO was evaluated by interpolation of test results.

The impact of the testing results on the RPV integrity during further operation is analysed within item "Rest Life-Time" of the WANO six months program on NPP Kozloduy.

Further Project Results

Further information on the project results could be sought from the beneficiary organisations.