Inversion point of the isothermal reactivity coefficient of the IPEN/MB-01 reactor - 1: Experimental procedure

A new experimental quantity is presented to serve as a benchmark to verify the adequacy of the newly released ²³⁵U thermal and subthermal cross sections for the determination of the reactivity coefficients of light water reactors. Such a quantity is denominated the inversion point, and by definition it is the temperature for which the isothermal reactivity coefficient of a reactor system becomes positive. The experimental bases for its determination are discussed. The experiment has been performed in the IPEN/MB-01 reactor facility. Instead of heating the reactor system as usual in experiments considering temperature variations, the reactor system is cooled to approximately 8.5 °C. By means of a heating/cooling system, the temperature is allowed to increase slowly in a stepwise manner. For each step, the control bank critical position is recorded, and by analyzing its behavior as a function of temperature, the inversion point is inferred. The inversion point has been found to be an adequate experimental quantity to validate the thermal and sub-thermal ²³⁵U cross section because it does not require any sort of calculated correction factors or any quantity that comes either from the calculational methodologies or from another experiment. In addition, the inversion point is an experimental quantity that can be measured with an excellent level of accuracy due mainly to the very precise characteristics of the control bank system of the IPEN/MB-01 reactor. The final value obtained for the IPEN/MB-01 reactor is 14.99±0.15 °C.