Ex-vessel neutron dosimetry results in the extended beltline region

Westinghouse Electric Company LLC, Radiation Engineering & Analysis, 1000 Westinghouse Drive, Cranberry Township, PA 16066, USA

^* Corresponding author: hayeset@westinghouse.com

Published online: 11 November 2024

Abstract

Embrittlement of the pressure vessel has been a long-standing concern for pressurized water reactors. Historically, the beltline region of the pressure vessel, or the region of the pressure vessel located opposite the active fuel, has been the primary focus of embrittlement concerns. Embrittlement evaluations typically include a calculation of the neutron flux incident on the pressure vessel, which is in part validated with measurements. In the context of life extension, the beltline region, which is technically defined as the portion of the pressure vessel experiencing fast neutron fluence (E > 1.0 MeV) equal to or greater than 10¹⁷ n/cm², is growing axially. Present fluence calculations for many plants indicate that the beltline region at the end of the 60 years of operation may extend to the bottom of the pressure vessel nozzles. Calculations in the extended beltline are difficult to validate as there are few dosimetry measurements available. At one Westinghouse 4-loop plant, ex-vessel neutron dosimetry sensor sets were irradiated in the extended beltline region. The measurements in the extended beltline region compare well with the discrete ordinates (RAPTOR-M3G) transport calculations and indicate the net uncertainty may be on the order of 30% in the extended beltline region.