CEA, Fontenay-aux-Roses France

Irradiation of cooled target
A very elegant in-pile irradiation system has been designed for CEA in Fontenay-aux-Roses (France). Simultaneous refrigeration for hydrogen and nitrogen recondensation is provided by a special version of a Stirling Cryogenics two stage cryogenerator.

The system, which is used in a swimming pool type reactor consists of a frame-mounted manoeuvrable cryostat. The loop is filled with liquid hydrogen or neon and the thermal shroud is cooled with liquid nitrogen. The flux intensity to which the target is exposed may be varied since the target can be repositioned with respect to the reactor core. The platform, indeed, on which the cryogenerator and the cryostat are mounted, can be displaced in two directions. In this way the target can cover a surface of 30 x 30 cm. After irradiation, the target is lifted and, by a 180° rotation of the top of the cryostat, placed in a small receptacle for further measurements.

The loop contains a liquid hydrogen environment surrounded by a liquid nitrogen jacket. Both liquids are at atmospheric pressure, the temperatures being approximately 20K and 77K respectively. Due to the heat input, thermal and other losses, partial evaporation of the liquid takes place. In order to recover these boil-off vapours, use has been made of a modified version of a two stage cryogenerator generating simultaneous cooling power at two temperature levels.