High Temperature Test Facility sensitivity and calibration studies to inform OECD-NEA benchmark calculations

Abstract

The High Temperature Test Facility (HTTF) is a thermal hydraulics test facility for block-type high-temperature gas-cooled reactors (HTGRs) built to provide data to validate HTGR modeling tools. Several experiments were conducted at HTTF in the spring of 2019 that serve as the basis for an upcoming Organization for Economic Cooperation and Development – Nuclear Energy Agency HTGR thermal hydraulics benchmark. We present sensitivity studies of steady state and both the pressurized and depressurized conduction cooldown (PCC and DCC) transients in HTTF in this paper. These studies identified that uncertainties in the thermal conductivity of the ceramic blocks making up the HTTF core are significant drivers of peak temperatures in steady state, PCC, and DCC. The emissivity of the heater rods and core blocks also provide significant control over heater rod temperatures. We also performed calibration studies based on HTTF Experiment PG-26, which was intended to serve as the basis for the DCC benchmark problem. We found that no calibration was able to reproduce PG-26 peak block temperatures within an average of 30%, but by taking the best-estimate PG-26 calibration and applying it to a similar experiment—PG-29—we were able to reproduce the peak block temperatures within an average of 6.2% error. Based on this nearly fivefold improvement in peak block temperature predictions from PG-26 to PG-29 and confounding factors in PG-26 that contributed to previous difficulties modeling the experiment in RELAP5-3D and SAM we conclude that PG-26 data are insufficient for developing a DCC benchmark and recommend using PG-29 as the benchmark experiment instead.