Reactor Pressure Vessel Fluence Evaluation Methodology for Extended Beltline Locations (NUREG/CR-7286, ORNL/TM-2020/1584)

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Publication Information

Manuscript Completed: June 2021
Date Published: May 2022

Prepared by:
J. Risner
A. Alpan
J. Yang (formerly with ORNL)

Oak Ridge National Laboratory
Oak Ridge, TN 37831-6283

J. Wallace, NRC Project Manager

Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington DC 20555-0001

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Abstract

It has become increasingly challenging to accurately predict neutron fluence and displacements per atom (dpa) in reactor pressure vessels (RPVs) as plant life extensions and power uprates expand the area of concern, causing neutron damage to locations in the so-called extended beltline region. At this writing, the only available guidance on RPV fluence calculations is from analyses that only address the traditional beltline region. This study evaluated the impact of multiple physical parameters on fast fluence (E > 1 MeV) estimates to ascertain the degree to which extended beltline fluence evaluations are more sensitive to those parameters compared with traditional beltline evaluations. In addition, key calculational parameters in the widely used discrete ordinates method were evaluated to determine their impact on extended beltline fluence estimates. Hybrid radiation transport calculations, which employ the current state of the art in radiation transport simulations, were used as benchmark solutions in the absence of measured data in extended beltline locations. These hybrid calculations utilize continuous-energy Monte Carlo calculations and eliminate the discretizations in space, energy, and angle that impose accuracy limitations on discrete ordinates calculations. This report details the results of the physical and calculational parameter studies and provides insights into where modifications in analysis methodology may be necessary to obtain calculational uncertainty in the extended beltline region comparable to that specified for traditional beltline fluence analyses.