Both ECN and IRI take part in the benchmark of start-up core physics of the High Temperature Engineering Test Reactor (HTTR), which is described in detail in Ref. 1, and is part of the IAEA Co-ordinated Program “Evaluation of HTGR Performance The IRI results obtained with Monte Carlo techniques for the problems in this benchmark are described elsewhere [2]. Because the configuration of the core is quite complicated with in total 12 different uranium enrichments, ECN and IRI decided to analyse a simpler configuration also, and compare cross sections and the core model. In this simpler configuration only 5.2 % enriched uranium is used for all fuel blocks in the reactor (the fuel block in layer 5, zone 2, see ref. 1). For the comparison of cross sections, it was agreed to condense the energy range into two groups with the boundary at 2.1 eV. This report first describes the generation of cross sections for both Monte Carlo calculations and for calculations with deterministic codes. Subsequently, results of the analysis of the simple core configuration with the multi-group Monte Carlo code KENO and the diffusion theory codes BOLD VENTURE and PANTHER are presented. Then results obtained with BOLD VENTURE are given for the fully loaded core which has a variety on enrichments in the fuel assemblies. This includes the generation time and the effective fraction of delayed neutrons at critical. Finally, results for the fully loaded core with PANTHER are presented.@en

In the frame of the European contract HTR-N, a work package is devoted to the code validation and method improvements as far as the high temperature gas-cooled reactor (HTGR) core modelling is concerned. Institutions from three countries are involved in this work package: FZJ in Germany, NRG and IRI in the Netherlands, and CEA in France. The present work is based on a benchmark problem proposed by JAERI through the IAEA. It concerns the HTTR’s start-up core physics experiments that were a good opportunity for the European partners to validate their calculational tools and methods. The number of fuel columns necessary to achieve the first criticality and the excess reactivity for 18, 24, and 30 fuel columns in the core had to be evaluated. Pre-test and post-test calculational results, obtained by the partners, are compared with each other and with the experiment. Parts of the discrepancies between experiment and pre-test predictions are analysed and tackled by different treatments. In the case of the Monte Carlo code TRIPOLI4, used by CEA, the discrepancy between measurement and calculation at the first criticality is reduced to Δk/k∼0.85%, when considering the revised data of the HTTR benchmark [Fujimoto, private communication]. In the case of the diffusion codes, this discrepancy is reduced to Δk/k∼0.8% (FZJ) and 2.7 or 1.8% (CEA).@en