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An important function of reactor dosimetry is the verification of fluence values used in the calculation of pressure vessel embrittlement and its impact on (a) the fracture toughness requirements for protection against pressurized thermal shock (Title 10, Code of Federal Regulations [CFR], Part 50.61) and (b) the pressure-temperature operating limits required by 10 CFR 50.60 and Appendix G. The dosimetry surveillance requirements are part of Appendix H to 10 CFR 50.
In recent years there has been significant progress in the computational techniques of neutron transport, making it possible to derive accurate estimates of the fast neutron fluence through the reactor vessel thickness and in the reactor cavity. In addition, a trend has been identified in the material properties and chemical composition of the pressure vessel of recent vintage plants where the materials used are such that the estimated end-of-life fluence is only a very small fraction of the fluence required to meet the 10 CFR 50.61 criteria. For future light water reactors (LWRs), cavity dosimetry combined with analytical computations could give an accurate estimate of the pressure vessel neutron irradiation. This could then be used to estimate the degree of vessel material embrittlement and thus alleviate the need for invessel dosimetry surveillance. The question as to whether the need for archival sample irradiation can be alleviated in the future remains to be answered.
dosimetry, pressure vessel, fluence embrittlement
Nuclear Engineer, U.S. Nuclear Regulatory Commission, Washington, DC