Development of a method for measuring the MTC by noise analysis and its experimental verification in Ringhals-2

Paper in proceeding, 2002

This paper deals with the estimation of the Moderator Temperature Coefficient of reactivity (MTC) by noise analysis. Previous experimental investigations showed that the MTC was systematically underestimated by a factor of two to five compared to its design-predicted value. In these measurements, the MTC was always determined by cross-correlating the neutron noise provided by a single in-core neutron detector with the local temperature noise given by a single core-exit thermocouple located at the top of the same fuel assembly, or of a neighbouring fuel assembly. It is shown in this paper via a noise measurement performed at the Swedish Ringhals-2 Pressurised Water Reactor (PWR) that the moderator temperature noise is radially strongly heterogeneous. Such a non-homogeneous temperature noise is proven theoretically to explain why the MTC was always underestimated in the previous experimental work when only the local temperature was used. A new MTC noise estimator, relying on the core-averaged moderator temperature noise, is thus proposed. This new estimator is demonstrated to provide an accurate MTC evaluation, as long as the radial structure of the moderator temperature noise can be measured. In the case of Ringhals-2, such in-core temperature measurements are carried out by Gamma-Thermometers (GTs), which in the frequency range of interest for the MTC investigation by noise analysis are working as ordinary thermocouples. This method, which is non-intrusive and free of calibration, can therefore be applied to monitor the MTC throughout the cycle.