Abstract: In a gas cooled nuclear reactor, cesium metal release into the coolant gas is dependent on the coolant channel surface temperature in the core. The present invention relates to a filter for removing cesium from the coolant gas before it passes to the heat exchanger. The filter comprises a block or blocks of graphite having holes therein with a hydraulic diameter between 1 mm and 6 mm and a ratio of length to equivalent hydraulic diameter between 170 and 1000. This filter is arranged at one end of the reactor core so that the gases pass straight from the core into the filter block.

Abstract: Particle fuel, such as is used in high temperature nuclear reactors, and having the fuel material coated with a pyrolytic carbon coating impermeable to gaseous fission products and embedded in a matrix of graphite, incorporates radium 226 which may be included in the binder for the grains of the fuel. During the manufacturing process, the integrity of the pyrolytic carbon coating can be checked before irradiation in a reactor by circulating an inert gas such as argon over the particles at a temperature in excess of 800.degree.C. This may be carried out in the final carbonisation step during manufacture or immediately before vacuum degassing. The inert gas after passing over the particles is checked for the presence of radon 222, for example by passing the gas into a decay chamber having a filter in its outlet to collect radio active solid daughters of radon and counting the alpha particles emitted by the material collected by the filter.