A.A. Chernobaeva

Irradiation, Annealing, and Reirradiation Effects on American and Russian Reactor Pressure Vessel Steels

One of the options to mitigate the effects of irradiation on reactor pressure vessels (RPVs) is to thermally anneal them to restore the toughness properties that have been degraded by neutron irradiation. Even though a postirradiation anneal may be deemed successful, a critical aspect of continued RPV operation is the rate of embrittlement upon reirradiation. There are insufficient data available to allow for verification of available models of reirradiation embrittlement or for the development of a reliable predictive methodology. This is especially true in the case of fracture toughness data. Under the U.S.-Russia Joint Coordinating Committee for Civilian Nuclear Reactor Safety (JCCCNRS), Working Group 3 on Radiation Embrittlement, Structural Integrity, and Life Extension of Reactor Vessels and Supports agreed to conduct a comparative study of annealing and reirradiation effects on RPV steels. The Working Group agreed that each side would irradiate, anneal, reirradiate (if feasible ), and test two materials of the other. Charpy V-notch (CVN) and tensile specimens were included. Oak Ridge National Laboratory (ORNL) conducted such a program (irradiation and annealing, including static fracture toughness) with two weld metals representative of VVER-440 and VVER-1000 RPVs, while the Russian Research Center-Kurchatov Institute (RRC-KI) conducted a program (irradiation, annealing, reirradiation, and reannealing) with Heavy-Section Steel Technology (HSST) Program Plate 02 and Heavy-Section Steel Irradiation (HSSI) Program Weld 73W. The results for each material from each laboratory are compared with those from the other laboratory. The ORNL experiments with the VVER welds included irradiation to about 1 x 10{sup 19} n/cm{sup 2} (>1 MeV), while the RRC-KI experiments with the U.S. materials included irradiations from about 2 to 18 x 10{sup 19} n/cm{sup 2} (>l MeV). In both cases, irradiations were conducted at {approximately}290 C and annealing treatments were conducted at {approximately}454 C. The ORNL and RRC-RI experiments have shown generally good agreement for both the Russian and U.S. steels. While recoveries of the Charpy 41-J transition temperatures were substantial in all cases, significantly less recovery of the lateral expansion and shear fracture in some cases (no recovery in one case) deserves further attention. The RRC-KI results for the U.S. steels showed reirradiation embrittlement rates which are conservative relative to the lateral shift prediction based on Charpy impact energy.

Mechanical Properties Distribution in Welds and Forging of VVER-1000

Study of weld metal and shell metal of VVER-1000 reactor is carried out. Metal of the control welded joint performed to the welds Nos. 3 and 4 and metal of the test ring of supporting shell is used. For each of the analyzed materials the bending tests of Charpy specimens are carried out according to the surveillance specimens testing procedure. Fabrication of specimens and all the tests are performed by TK “OMZ Izhora”, ltd. Leadership of the tests and assessment of brittle-to-ductile transition temperature is performed by RRC “Kurchatov Institute”. It is shown that guaranteed values of brittle-to-ductile transition temperature (TK0 ) for VVER-1000 pressure vessel materials (0°C for weld and −25°C for base metal [1]) are the conservative estimation of TK values obtained for all groups of the tested specimens. However in some cases such estimation is super conservative. Assessment of distribution of brittle-to-ductile transition temperature (TK ) values of the weld in radial direction has shown that TK varies from layer to layer in the range from −55 to −22°C. Variations of TK values are conceivably random and not connected with changes of the chemical composition of metal. It is shown that the situation is possible when brittle-to-ductile transition temperature of “inner grooving” weld metal is higher than that of the “outer grooving” weld metal. This result should be taken into account when planning the surveillance specimens programmes. Metal of “inner grooving” should be included in surveillance specimens. While assessing the TK values distribution in axial direction it has been determined that the maximum brittle-to-ductile transition temperature is specific for the area adjacent to the line of weld-to-base metal alloying. This effect is stably observed already at a distance of 8 mm from the alloying line. Study of distribution of TK values of supporting shell metal has shown that for the analyzed supporting shell the maximum brittle-to-ductile transition temperature value corresponds to the inner side of the shell middle third, the minimum value — to the shell inner surface. On the whole, shell metal is characterized by rather low values of brittle-to-ductile transition temperature.Copyright

The Effect of Post-Irradiation Annealing on VVER-440 RPV Materials Mechanolocal Properties and Nano-Structure Under Re-Irradiation

The present work provides the analyses of embrittlement behavior and atom probe tomography study of nano-structure evolution of VVER-440 RPV materials under irradiation and re-irradiation. Specimens from VVER-440 weld with high level of cupper (0.16 wt.%) and phosphorus (0.027–0.038 wt.%) were irradiated in surveillance channels of Rovno Nuclear Power plant unit 1 (Ro-1). The embrittlement behavior has been assessed by transition temperature shift.Copyright