Integrated surveillance specimen program for WWER-1000/V-320 reactor pressure vessels

Surveillance specimen programs play an important role in reactor pressure vessel lifetime assessment as they should monitor changes in pressure vessel materials mainly their irradiation embrittlement. Standard surveillance programs in WWER-1000/V-320 reactor pressure vessels have some deficiencies resulting from their design - nonuniformity of neutron field and even within individual specimen sets, large gradient in neutron flux between specimens and containers, lack of neutron monitors in most of containers and no suitable temperature monitors. Moreover, location of surveillance specimens does not assure similar conditions as the belt line region of reactor pressure vessels. Thus, Modified surveillance program for WWER-1000/V-320 deg. C type reactors was designed and realized in two units of NPP Temelin, Czech Republic. In this program, large flat type containers are located on inner wall of reactor pressure vessel in the belt line region that assures their practically identical irradiation conditions with critical vessel materials. These containers with inner dimensions of 210 x 300 mm have two layers of specimens; using inserts (10 x 10 x 14 mm) instead of fully Charpy size specimens allows irradiation of materials from several pressure vessels at once in one container. Irradiation of these archive materials together with the IAEA reference steel JRQ (of ASTM A 533-B type) and reference steel VVER-1000 will allow to compare irradiation embrittlement of these materials and to obtain more reliable and objective results as no reliable predictive formulae exist up to no due to a higher content of nickel in welds. Irradiation of specimens from cladding region will help in the evaluation of resistance of pressure vessels against PTS regimes. (author)