Appraisal of Core Analysis Methods against Zero-Power Experiments on Full-Scale BWR Fuel Assemblies

The present doctoral research aims at the appraisal of nodal core simulators used for the calculation of commercial boiling water reactor (BWR) cores, against measurements carried out at the Paul Scherrer Institute under the LWR-PROTEUS experimental programme. The research focuses mainly on the prediction of radial and axial total-fission rate (i.e. power) distributions in the vicinity of core heterogeneities, caused by features such as the presence of control blades, enrichment boundaries or partial length rods. As such, this thesis complements previous validation work performed for LWR calculational methods on the basis of integral experiments in zero-power research reactors, the latter corresponding largely to the validation of two-dimensional, reflected fuel-assembly calculations. Two commercial code systems have been investigated currently: HELIOS/PRESTO-2, presently used for core monitoring at the Leibstadt Nuclear Power Plant (Switzerland), and CASMO-5/SIMULATE-5, which represents the most recent generation of the widely applied CASMO/SIMULATE system. Six different LWR-PROTEUS configurations have been modelled, for which the nodal reconstructed total-fission rate distributions have been compared against experimental results, as well as against reference, three-dimensional whole-reactor Monte Carlo calculations using the MCNPX code. To start with, a methodology has been developed and tested for appropriate representation of the multi-zone experimental configurations, as employed in the LWR-PROTEUS programme, by means of reduced-geometry models set up using the investigated nodal code systems. The approach adopted has been to apply, in each case, case-dependent three-dimensional boundary conditions to the nodal code's modelling of the 3x3 array of BWR assemblies constituting the LWR-PROTEUS test zone. This has been done in terms of so-called Partial Current Ratios (PCRs), which describe the relation between the incoming and outgoing neutron currents across the test-zone boundary. These PCRs have been derived from the fore-mentioned MCNPX calculation of the multi-zone PROTEUS reactor, thus allowing for the adequate description of the three-dimensional effects associated with the interaction between the test zone and its surroundings. The results obtained for the reference LWR-PROTEUS configuration, with a test zone consisting of an unperturbed regular array of identical, axially uniform BWR fuel assemblies (of type SVEA-96+), showed that both the investigated nodal methodologies reproduce the experimental total-fission rate distribution with very good accuracy, equivalent to that achievable with high-order transport calculations. This has provided adequate indication that the developed methodology of using three-dimensional PCRs, calculated by means of an appropriate whole-reactor MCNPX model, offers a reliable platform for the desired validation. The full insertion of a L-shaped hafnium control blade, especially fabricated for the LWR-PROTEUS programme, has permitted the study of the behaviour of calculated total-fission rate distributions in the presence of strong radial and azimuthal flux gradients. In this case, the use of reflective boundary conditions in the lattice calculations, combined with the azimuthal non-uniformity of the currents that couple the test zone with the outer zones of the reactor, produce systematic deviations which, together, lead to some loss of accuracy in comparison with the regular, uncontrolled case. The three-dimensional effects of the changes in enrichment and gadolinium content between two different axial sections of the fuel assembly have also been studied. In this case, the calculations of the axial total-fission rate distributions are seen to generally reproduce the experimental data with good accuracy, although certain systematic effects are observed. In particular, at local level, and very near the strong axial heterogeneity caused by the enrichment boundary, relatively large deviations have been found to occur. However, due to their very local character, these deviations are not considered to represent a relevant issue in connection with power reactor monitoring and design. Also for the demanding case of the LWR-PROTEUS configuration with a partially inserted control blade, the results obtained in this thesis show a very good behaviour. As in the boundary enrichment case, however, some very localized deviations occur in the vicinity of the strong axial flux gradients. In a separate phase of the LWR-PROTEUS programme, BWR assemblies with partial length rods (PLRs), viz. of type SVEA-96 Optima2, were studied in the central test zone. The results of the comparisons made currently for the corresponding LWR-PROTEUS configurations have shown that, also for fuel assemblies with axial heterogeneities of this type, the performance of both the investigated nodal methodologies is very satisfactory for predicting the global distribution of the total-fission rate. The local three-dimensional effects caused by the PLRs are also predicted with good accuracy, similar, in fact, to that obtained with high-order three-dimensional transport calculations. The present research shows that the two nodal code systems investigated – HELIOS/PRESTO-2 and CASMO-5/SIMULATE-5 – reproduce the LWR-PROTEUS experimental results with high accuracy, both for the radial and the three-dimensional (i.e. at pellet level) comparisons. Significant deviations occur almost exclusively within a short distance from the nodal interface, in cases featuring strong gradients across the core midplane. The axial and radial shapes of the total-fission rate distributions are well predicted in most cases, which represents an important observation concerning the monitoring of operational limits in power reactor cores. Thus, overall – within the range of applicability of the experimental conditions studied in the LWR-PROTEUS programme – the results of the comparisons performed in this thesis confirm that nodal methodologies with pin-power reconstruction have a very high level of performance. In fact, the accuracy that they can achieve for the assembly-internal total-fission rate distribution is, in general, higher than that practically achievable in an operating nuclear power plant. This is due to unavoidable, additional uncertainties in the characterisation of a BWR core under power reactor conditions. Finally, the present research has provided quantitative insights into the applicability of integral data, produced in critical facilities such as PROTEUS, for validating the calculation of power-reactor core heterogeneities. One has thus been able to demonstrate that the experimental evidence from the LWR-PROTEUS programme indeed constitutes a very valuable basis for the validation and appraisal of three-dimensional nodal methodologies with pin-power reconstruction.