Experimental measurements of isothermal reactivity coefficient and temperature-dependent reactivity changes with associated uncertainty evaluations

The low power Reactor Critical Facility (RCF) at Rensselaer Polytechnic Institute has recently been utilized to design and perform low-power temperature dependent experiments. The experiments include determination of isothermal excess reactivity and reactivity coefficient at different temperatures and measurements of time-dependent reactivity change with system temperature change. The above-mentioned experiments have produced significant quantities of data, and numerous post-processing steps are needed in order to make the data useful for computational model validation. One important step toward providing the data as an accurate and high-fidelity experiment benchmark for modern code validation is to develop in-depth experimental uncertainty quantification. In this paper, we analyze in particular two types of reactor physics experiments driven by the change in system temperature within a small temperature range. Uncertainty quantification includes separately derived experimental errors for both independent (temperature) and dependent (reactivity) variables, and an iterative weighted least squares approach is used to combine them into the equivalent uncertainty in reactivity. The results of the experiments show the negative effects of the system temperature on reactor reactivity due to combined effects of the Doppler broadening in the fuel, the S(α,β) thermal scattering physics and the decrease in water density. The low uncertainty values obtained for the reactor isothermal excess reactivity (<0.10 cents), the isothermal reactivity coefficient (<0.01 cents/°C), and the time-dependent reactivity change with system temperature increase (<0.16 cents) show that these experiments can be reliably used as benchmarks for code validation. The iterative method adapted in this work for combining temperature and reactivity uncertainties can be used to analyze similar temperature-dependent reactivity benchmark experiments.