Safety margin characterization for high power fueled experiments

The updated safety basis of the Advanced Test Reactor (ATR) has specific criteria for experiments that are cooled by the primary coolant. It requires that during a Condition 2 transient (where flow coastdown is typically the most limiting), the Critical Heat Flux ratio (CHFR) is greater than 2.0 after consideration of several power multipliers. This safety criteria is often overly conservative for fueled experiments and restricts the acceptance of experiments. In the updated safety basis, a provision for a stochastic approach is included. In this study, coupled Dakota-RELAP5 and RAVEN-RELAP5 are used to propagate uncertainties. The objective of this study is to provide a quantitative basis for utilizing the provisions in the ATR experiment safety criteria. Three different fueled experiments, MP-1, FSP-1, and AFIP-6 are considered. RELAP5 models for each experiment are used to simulate the flow coastdown transient. Several thermal-hydraulic correlations are assessed to determine the Onset of Significant Voiding (OSV), Onset of Flow Instability (OFI), and Critical Heat Flux (CHF). The OSV and OFI correlations considered indicate that there is a large safety margin. The CHF correlations also indicate that there is a moderate to large safety margin. The high mass flux of the experiments exceed the Look-up Table (LUT) correlation range. This results in an artificially restricted prediction of CHF value. A high mass flux modifier was implemented to improve LUT applicability. Due to the specifications of the experiments considered, the correlations must be applicable to higher mass flux ranges. It was also found that, often, the conventional CHFR >2.0 criterion is not met. However, the statistical CHFR margin ($\mu -3\sigma >1.2$) is met. This result suggests that new criteria, while maintaining adequate safety margins, can be used to expand the operating envelope of high power fueled experiments at ATR. The Pearson ranking indicates that the power level and mass flux have the largest impact on the CHFR and OFI predictions. If a reduction of uncertainty in predictions is sought, the factors affecting fuel experiment fission power should be prioritized for scrutiny, followed by the mass flow rate.