HELIUM CONTENT AND INDUCED SWELLING OF NEUTRON-IRRADIATED BERYLLIUM

The effects of neutron irradiation on beryllium in terms of helium gen eration and induced swelling have been studied. Beryllium samples were irradiated in the Belgian materials testing reactor BR2 at low temper ature (40-50 degrees C) up to 20 000 appm He (for a fission neutron fl uence of 5.3 x 10(22) n cm(-2)). Post-irradiation annealing was perfor med to study the effect of temperature on the helium behaviour and the induced swelling. Macroscopic examinations of the gas content reveale d a linear correlation of He-4 content with fast neutron fluence. He-4 contents were found to be about 10% higher than their expected format ion as calculated from the various formation schemes. The He-3 formati on was also measured. The 3He calculations have to take into account t he continuous transition between He-3 and H-3 according to the scheme [GRAPHICS] the latter reaction occurring only when the reactor is in o peration. Macroscopic examinations of the swelling by density measurem ents revealed a linear correlation of low temperature irradiation-indu ced swelling with He content. This swelling was found to be strongly e nhanced after post-irradiation annealing at temperatures above 250 deg rees C. The enhancement of the swelling could be correlated with (He c ontent)(2) and with (annealing temperature)(4). Microscopic examinatio ns on the samples have been started to elucidate the behaviour of He i n beryllium. Preliminary results are given.