Neutronics Benchmark of CEFR Start-Up Tests: Temperature Coefficient, Sodium Void Worth, and Swap Reactivity

The China Institute of Atomic Energy (CIAE) proposed some of the China Experimental Fast Reactor (CEFR) neutronics start-up test data for the IAEA benchmark within the scope of the IAEA’s coordinated research activity. The coordinated research project (CRP) on “Neutronics Benchmark of CEFR Start-Up Tests” was launched in 2018. This benchmark aims to perform validation and verification of the physical models and the neutronics simulation codes by comparing calculation results against collected experimental data. Twenty-nine participating research organizations finished performing independent blind calculations and are refining their calculation results by referring to measurement data. The main objective of this benchmark is to improve understanding of the start-up of SFRs and validate the state-of-the-art fast reactor analysis computer codes against the recent experimental data obtained at the modern experimental SFR.
This paper introduces the following three kinds of reactivity measurements in the CEFR start-up test and presents the results by participants: temperature coefficient, sodium void reactivity, and swap reactivity. The measurements were done at the basic core in operation loading with fuel assemblies. First, for the measurement of temperature coefficients, ten sets of data were obtained by increasing and decreasing the temperature. For each temperature, the control rod position is changed to maintain the reactor as a critical. Second, sodium void reactivity is measured by replacing a fuel SA by vacuum-sealed SA and searching for the critical position of control rods. Third, for the measurement of the swap reactivity, fuel subassembly is replaced by SS subassembly, and SS subassembly is switched with one fuel subassembly. Swap reactivities are measured in two ways, with more than two control rods moving to find the criticality of the core in ‘Multiple Rods’ case, and only one control rod moving in the ‘Single Rod’ case. All three reactivities are obtained by a combination of control rod worth for changed rod position and criticality difference. Therefore, accuracy on control rod worth is important for benchmark calculation.
The comparison shows that uncertainty of calculations, modeling errors, and inaccurately determined control assembly worths make it challenging to calculate the temperature coefficient precisely. Depending on how modeling a vacuum-sealed fuel SA, the sodium void reactivity results from participants show large deviations. For swap reactivity calculation, the reactivity induced by the assembly swap is relatively larger than the temperature coefficient measurement. The swap reactivity calculation results from participants have similar trends and show good agreement with measurement.