This document evaluates how implementing the remote siting recommendations for nuclear reactors (NUREG-0625) made by the Siting Policy Task Force of the u.s. Nuclear Regulatory Commission (NRC) can reduce potential. public risk. The document analyzes how population density affects site-specific risk for both light water reactors (LWRs) and high-temperature gas-cooled reactors (HTGRs). The low-density sites proposed by the Siting Policy Task Force measurably reduce acute fatalities [mostly within 8 km (5 miles)] for low probability ty LWR accident sequences compared to existing u.s. sites of relatively high population density. However, these low-density sites negligibly reduce corresponding latent fatalities. While low-density sites reduce individual risk due to nuclear accidents for persons near the LWR plant, they negligibly impact the overall (including non-nuclear) risk of cancer fatality. The probability is low (odds against) that •even one fatality (acute or latent) 5. HTGR release category characteristics, core heatup scenarios • 3-6 6.• Urban site population distribution assumed in this study.. 3-8 vi. ' \.
Portions of this document may be illegible in electronic image products. Images are produced from the best available original document. FOREWORDThis Licensing Topical Report has been prepared for submittal to the U.S. Nuclear Regulatory Commission as part of a review of issues that may bear on the licensing of commercial high-temperature gas-cooled reactors (HTGRs). More specifically, the reactors addressed herein are characterized as graphite moderated and helium cooled, with ceramic fuel in prismatic fuel elements. This review is to be conducted prior to an application for a construction permit to expedite the licensing process when an application is made and to provide a basis for licensing HTGRs on their own merit. This report proposes General Design Criteria (GDC) for use in licensing of HTGRs based on interpretation of the GDC presented in Appendix A to Part 50, Title 10, of the Code of Federal Regulations, which were derived for light-water-cooled reactors. The GDC for HTGRs are presented as a modified text of Appendix A, which the NRG is requested to endorse as suitable for use in HTGR licensing applications.
Measurements were performed to experimentally determine the reactivity worths of selected samples of the In-Place Shim Adjustment (ISA) materials, and the perturbation of that worth caused by support stem and propellant hydrogen. Three shim materials were investigated: Hf02, TaC, and HfC. These materials were plated onto the outside of stainless steel tubes with nominal coating thicknesses of 3 mils and 1/2 mil. These measurements were performed at four different radial core locations. The reactivity worth of hydrogen was also measured at selected locations in the core. The hydrogen was introduced in the form of polyethylene "wires" of 80-mil nominal diameter with the polyethylene "wires" placed in the shim tubes and also in the central flow channels immediately surrounding the test locations. Analysis of these measurements was performed using the ANISN one-dimensional transport code to generate 16-group forward and adjoint fluxes for input to the PERT-ID to obtain reactivity worths per unit mass for the coated shim tubes vs. radial position. The good agreement between measurements and predictions for the dry, ambient shim tubes of TaC and Hf02 with a nominal 3-mil thickness in the graphite fueled PAX/R-1 indicates that the worth of shim tubes of comparable thickness can be predicted for the R-1 composite and HCTE fuel designs. The experimental results for the nominal 1/2-mil and 3-mil thick coatings also provided self-shielding data which con be extrapolated to determine the reactivity worths of other shim tube thicknesses. In addition, the experimental results confirmed that TaC will provide a greater shim capability than Hf02 for a given coating thiickness. Based on the result of the shim tube experiments, the TaC coating thickness has been specified for two, alternate shim procedures. The experiments confirmed previous predictions that Hf02 shim coating reduces the effective reactivity worth of hydrogen relative to an unshimmed configuration more so than a TaC shim. With a $1.50 shim content distributed uniformly throughout the core, the average support stem hydrogen is reduced by approximately 14% with Hf02 but only by 9% with TaC. Because TaC does provide a greater dry shim worth for a given thickness than Hf02 and results in a smaller reduction of stem hydrogen, TaC was tentatively selected as the prime shim coating for the R-1 reference design.
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