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).