An approach to assess reactivity changes due to core thermal expansion

The presented method shows a way to approach estimation of reactivity effects due to the thermal expansion of the core structures in the case when actual pin, subassembly and core mechanical behaviour is not known. The set of reactivity coefficients obtained above was not complete due to difficulties to represent some basis changes within our MCNP model. However, in future work one can build a MCNP model taking into account from the beginning the necessity to represent certain geometrical changes, and in this case a more complete set of basis changes can be modeled. The example use of obtained coefficients has shown a formal statistical agreement with the exact result. However, the cumulative reactivity change of about 25 pcm for considered scenario is quite small with respect to the statistical error of the exact result, therefore this example does not provide any confidence on the method's precision. This exercise, however, can be considered as an illustration of the use of obtained reactivity coefficients. (orig.)