A new approach to description of in-service embrittlement of WWER-1000 reactor pressure vessel materials

Abstract

Based on available published data and experimental findings, the in-service embrittlement of materials has been analyzed. The contributions of thermal ageing and neutron irradiation to embrittlement of the base and weld metals of WWER-1000 reactor pressure vessels are discussed. Equations have been derived which describe the shift of the critical brittle temperature depending on the irradiation time and neutron fluence. For a nickel-rich weld metal a relation between the radiation embrittlement coefficient and the amount of alloying elements Ni, Mn, and Si has been defined.Keywords: embrittlement, reactor pressure vessel, shift of critical brittle temperature, irradiation, neutron fluence. Introduction. Investigations [1-25] of embrittlement behavior of WWER-1000 reactor pressure vessel (RPV) materials [the base metal (BM) is 15Kh2NMFA-A steel and the weld metal (WM) -the weld joints of steel 15Kh2NMFA-A] give rather conflicting findings. This applies primarily to the description of the shift of the critical brittle temperature ΔT k k as a function of the neutron fluence [1]:

Fracture toughness prediction for highly irradiated RPV materials: From test results to RPV integrity assessment

“…It should be noted that, as a common case, Eq. (10) allows one to take into account thermal aging of a material if the DBTT shift DT k is taken in the form [30,35] …”

“…The dependence DT F (F) is described by Eq. (11) and the dependence DT t (t) is described by equation [35] DT t ðtÞ ¼…”

“…Numerical values of the coefficients in Eqs. (11) and (14) for WWER-1000 RPV are represented in [35].…”

Analysis of the reasons for the scatter of the data on temperature reference specimens of VVER-1000 base metal

“…2). The parameter ω, equal to 20°C for weld seams and 38°C for VVER-1000 base metal, introduced in order to evaluate the upper limit of the 95% interval for normative dependences of the radiation embrittlement of VVER-1000 vessel material, attests to the large scatter of data as compared with the weld-seam metal [4,5]. Therefore, the parameter ω, which can be regarded as a measure of the scatter of the experimental data relative to the regression line, is almost twice as large for the base metal as for the weld-seam metal, while the scatter of the weld-seam metal properties is much larger than for the casing metal [2,3].…”