In the design of the Soviet 1000-MW pressurized water reactor (PWR), developed to replace the former Soviet 440-MW PWR, the irradiation condition is much less severe. This condition permits the use of a new pressure vessel steel (PVS) 15KH2NMFAA containing nickel (Ni) rather than the previously used PVS 15KH2MFAA, thereby achieving better weldability, higher strength (σ^y350 > 450 MPa at 350°C), and a lower initial transition temperature (Tk = -30°C). PVS 15KH2NMFAA is manufactured from natural, clean, raw materials to increase its radiation stability. Statistical analysis shows that the transition temperature shift after irradiation (ΔTf) at irradiation temperature (Tir) 290°C and fluence (F) of 6 × 10²³ n/m²³, E > 0.5 MeV is defined by copper (Cu) and phosphorus (P) content: ΔTf=230[(Cu+10P)+20]±25°C

Values under typical practice are: Cu + 10P = 0.10 to 0.15%∙ΔTf. Values calculated using both this equation and the U.S. Nuclear Regulatory Commission (NRC) Regulatory Guide 1.99 (Rev. 2) gave similar results. On the basis of this equation, demands for the permitted impurity level of PVS 15KH2MFAA have been developed as follows: Cu < 0.1 and P < 0.01%.

The well-known Soviet equation ΔTf = Af(F × 10^-22)^-1/3 for PVS 15KH2NMFAA irradiated at a Tir of 290°C is correct only at F < 1 × 10²⁴ n/m²∙ Af = 23°C (n/m²)^-1/3 at the permitted impurity level.

The radiation embrittlement quasi-steady-state model that is presented here provides a sufficient interpretation of the combined effect of both Tir under temperatures ranging from 100 to 350°C and F up to 4 × 10²⁴ n/m² (E > 0.5 MeV) on the reference PVS 15KH2NMFAA (Cu + 10P = 0.13%). 1ΔTf=1508+22.41Fexp(-5900RT)

The influence of nickel (Ni) on the PVS 15KH2NMFAA RE is decreased by increasing Tir and lowering F.