[en] New graphite-moderated critical experiments are in progress at the PROTEUS facility of the Paul Scherrer Institute as part of an International Atomic Energy Agency cooperative research program on the validation of safety-related physics calculations for low-enriched, gas-cooled reactors. The experiments, which began in July 1992, use 16.7% enriched uranium, pebble-type fuel originally intended for use in the Arbeitsgemeinschaft Versuchsreaktor in Juelich, Germany. Accurate measurement of changes in k_eff and neutron balance components caused by water ingress into undermoderated high-temperature gas-cooled reactors (HTRs or HTGRs) is one of the primary objectives of the PROTEUS experimental program. Several different geometric arrangements of the pebbles (deterministic as well as the usually employed stochastic) are planned to provide data on changes caused by different void fractions and void shapes. The deterministic arrangements also serve to provide better experimental access to the interior of the core for the measurement of reaction rates and reactivity worths