Conversion Preliminary Safety Analysis Report for the NIST Research Reactor

Abstract

heavy water moderated and cooled reactor operating at 20 MW. It is fueled with high-enriched uranium (HEU) fuel elements. A Global Threat Reduction Initiative (GTRI) program is underway to convert the reactor to low-enriched uranium (LEU) fuel. This program includes the qualification of the proposed fuel, uranium and molybdenum alloy foil clad in an aluminum alloy, and the development of the fabrication techniques. This report is a preliminary version of the Safety Analysis Report (SAR) that would be submitted to the U.S. Nuclear Regulatory Commission (NRC) for approval prior to conversion. The report follows the recommended format and content from the NRC codified in NUREG-1537, "Guidelines for Preparing and Reviewing Applications for the Licensing of Non-power Reactors," Chapter 18, "Highly Enriched to Low-Enriched Uranium Conversions." The emphasis in any conversion SAR is to explain the differences between the LEU and HEU cores and to show the acceptability of the new design; there is no need to repeat information regarding the current reactor that will not change upon conversion. Hence, as seen in the report, the bulk of the SAR is devoted to Chapter 4, Reactor Description, and Chapter 13, Safety Analysis.

Review of the Technical Basis for Properties and Fuel Performance Data Used in HEU to LEU Conversion Analysis for U-10Mo Monolithic Alloy Fuel

“…However, because these fits are all based on a single set of measurements, it is strongly recommended, and considered imperative, to collect a set of density values on samples with comparable porosity to prototypic as-fabricated foils to establish the accurate values. Lastly, NBSR [24], MURR [4], and MITR [25] do not explicitly address the temperature-dependence of density in their safety analyses because they neglect the thermal expansion of the fuel. Therefore, to conserve mass, temperature dependence of the density is also neglected in the analyses.…”

“…It is recommended to use the combined correlation in the following section that includes the impact of both temperature and irradiation on thermal conductivity of U-10Mo. A report for HFIR [3] predicts the thermal conductivity with a linear fit of data from Matsui [40] and Burkes [14,18] A report for NBSR [24] cites two sources for the thermal conductivity as a function of temperature, the UMo Handbook [43] and Burkes [14]. From the calculation notebooks [5,35], the UMo Handbook formulation was utilized (equations 2.19 to 2.22).…”

“…A report for HFIR [3] Analyses for NBSR [24,35] do not consider the reduction in thermal conductivity with irradiation in safety analysis because of the negligible impact on the results for the limiting LOCA. Instead, NBSR utilizes the fresh fuel thermal conductivity, regardless of the fuel burnup.…”

“…Additionally, thermal conductivity measurements for unirradiated U-10Mo for temperatures ranging from room temperature to more than 600 o C have been collected, although the measurements conducted by Burkes and used to prepare equation 2.26 were for material that is not prototypic. Figure 2.18 shows the range of available thermal conductivity data (blue boxes in the figure) alongside the results of the calculated peak U-10Mo fuel temperature from accident analyses for MITR [52], MURR [4], and NBSR [24]. It can be seen that, for a significant number of cases for these reactors, the peak fuel temperature occurs at fuel burnups that are outside the range of data which have been collected to date for U-10Mo.…”

“…Analyses for MURR [4] utilized equation 4.10, as discussed above, and applied it to fission densities up to 3.4x10 21 fissions/cm 3 , which is the maximum fission density of the MURR LEU design for prototypic operations. Analyses for MITR [6] and NBSR [24] both utilized a constant (not dependent upon burnup) fuel temperature safety limit. The fuel temperature limit was set to 350°C up to a fission density of 5.0x10 21 fissions/cm 3 for MITR analyses and 380°C up to a fission density of 7.2x10 21 fissions/cm 3 for NBSR analyses.…”