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-PROTEU