Nuclear reactor reload pattern optimization by application of heuristic search and perturbation theoretical methods

Applied Science

Assessment of reactivity effects due to localized perturbations in BWR lattices

Optimization criteria for the representability of numerical models for the estimation of relative reactivity changes, due to localized perturbations in boiling water reactor (BWR) lattices, have been theoretically developed and tested. The validity of the derived theoretical expressions has been assessed for the case of a reactivity perturbation corresponding to the removal of an individual fuel pin from a nominal BWR assembly, thus effectively substituting the pin by water. Such reactivity effects are of importance in the context of evaluating advanced fuel element designs, e.g., those employing part-length rods. Two different geometry models have been implemented for the LWR-PROTEUS critical research facility [full core (FC) and a smaller, reduced geometry (RG)], using the light water reactor assembly code BOXER, and calculations have been performed for the nominal cases (all pins present in the central test assembly) and the perturbed cases (individual pins removed). The FC results have been compared with the results of the RG model with two different boundary conditions (reflective and critical albedo). The comparisons have shown that the results of critical albedo calculations feature superior representability. Differences in relative reactivity effects, with respect to results of the FC calculation, are found to be within the range 1 to 4%

Quantification of the transferability of reactivity effect investigations in large multiregion systems

A methodology has been developed for the accurate assessment of localised reactivity perturbations in a BWR lattice embedded in a larger multiplying system, based on a full-system, unperturbed calculation, and on perturbed calculations on reduced-geometry models with reflective boundary conditions (typically, reflected-assembly calculations). Reflective reduced-geometry calculations are to be followed by a fast transferability correction for making the results representative of what full system computations would have produced. In this way, one can avoid the problem of having insufficient accuracy in the results (in spite of extremely lengthy iterations), particularly for cases of small reactivity effects. Furthermore, the factorization of reactivity effect transferability, a key feature of the developed methodology, provides valuable insight into the different effects contributing to a particular integral transferability factor, along with a quantification of the relative importance of these effects for each individually considered case. The initial investment, needed for realizing the relatively low required computational effort involved in the postcorrection procedure, is to obtain a limited number of adjoint equation solutions defined for the reference state at full system level. Application results are reported for the numerical analysis of fuel pin removal reactivity effects in LWR-PROTEUS. The latter is a programme of integral experiments, employing essentially a central LWR test zone driven critical by surrounding driver and buffer region

Development and application of a decomposition methodology for interpretation of reactivity effect discrepancies

With reactivity being the most important integral reactor physics quantity - and simultaneously the one that can be measured with the highest accuracy there is a great interest in understanding how possible space- and energy-dependent data and/or modeling discrepancies may propagate into a calculated reactivity change, and with which magnitude this occurs. In the context of pin removal reactivity effects in a light water reactor assembly, for example, it is illustrative to carry out, for any arbitrary localized material composition perturbation, a decomposition of the total effect into individual space- and energy-dependent contributions of the different unit cells in the assembly. If this decomposition is normalized to +100% in the case of a positive reactivity effect and to -100% in the case of a negative reactivity effect, an importance map is established that indicates the relative contribution (in percent) of each individual contributing cell to the total reactivity effect caused by the localized material composition change. Such an importance map can be interpreted as a sensitivity matrix that quantifies the final discrepancy in a calculated reactivity effect, with respect to its reference value, as a weighted sum of the complete collection of cell-wise data and/or modeling discrepancies. The current paper outlines the basic theory and gives certain practical applications of the proposed decomposition methodology. Thus, it is found that the developed methodology offers in-depth, quantitative explanations for calculational discrepancies observed in the analysis of fuel pin removal experiments conducted in the framework of the LWR-PROTEUS program at the Paul Scherrer Institut