A comparison of Western and Eastern nuclear reactor pressure vessel steels

The task was essentially to compare the irradiation response of `East' and `West' steels. Since the plates and forgings of pressure vessels must be welded together, it is obvious that the strength requirements of the welds and heat affected zones (HAZ) can be no less demanding than those of the plates and the forgings themselves, particularly as experience has shown that the most likely location for flaws is in the welds or their HAZs. These and the highly stressed regions of the reactor pressure vessel (RPV) are important because neutron irradiation degrades the mechanical properties of steels.After comparing the various designs, manufacture and materials of the various RPVs, a comparison was made of the irradiation response of these different steels. The role of mitigating the change in mechanical properties on irradiation by thermal annealing was also considered.Particular codes/guides could only be used for the predicting results underpinning their own database because a major difference between these national codes/guides is that the elements conferring irradiation sensitivity are different for the two cases considered, i.e. Russian codes [1] (PNAE G-7-002-86) and the USNRC guide [2] (RG 1.99 Rev. 2). In the former, copper and phosphorus are significant, while copper and nickel are identified as significant in the latter case.Predictions were compared for `real' materials used in NPPPVs whose compositions were known. The irradiation response of these steels is coincidentally similar. The essential difference in behaviour is in the lifetime fluence. Eastern steels are irradiated to a much higher fluence than Western steels. Differences in the predictions of the Eastern–Western codes/guides are a reflection of differences in the concentration of deleterious elements and pessimisms of the various codes/guides, particularly at low concentrations of deleterious elements where they are most conservative. Thirdly, and on a `fitness for purpose' basis, the shift in transition temperature produces a limitation to the lifetime of the earlier Eastern RPVs. However, by thermally annealing the RPV to mitigate the effect of neutron irradiation, where the conditions to recover the mechanical properties of both Eastern and Western steels are nearly the same, the operational life of these older Eastern plants has been extended. Life assurance of these plants has, therefore, become practicable.This aspect of RPV technology, which is currently being considered in the US, could extend the operational life of nuclear power plants and thereby reduce the cost of the electricity generated.