Optimization on producibility improvement and the recycling process of neutron multipliers for fusion applications

Abstract

Beryllium and beryllium intermetallic compound (beryllide) pebbles have been regarded as a neutron multiplier in an international thermonuclear experimental reactor (ITER), as well as a demonstration (DEMO) fusion reactor. A novel fabrication process of the beryllide pebbles has been successfully established by combining the plasma sintering and rotating electrode processes. However, owing to the brittleness of beryllides, their granulation yield is approximately 70%, which does not generally satisfy the requirement, whereas the fragments (designated to be not spherical) with 30% are generated as by-products. To improve the granulation yield and in considering a new recycling process, a novel step on fundamental experiments was adopted to confirm feasibility on the recycling process using the same plasma sintering and rotating electrode processes. Because the formation of oxidized surface and neutron-induced defects in these materials is anticipated, these defects should be eliminated during the recycling process. The plasma sintering process is known to remove impurities on powder surfaces by applying a pulse current, whereas the rotating electrode process (REP) is a granulation process that uses arc melting at a temperature higher than the melting point. Hence, a feasibility test on the recycling process was performed by applying this process with the fragments for the pebbles simulated as the used pebbles. In the case of mixture ratios of 1:1 and 2:1 for the mixed powders and fragments, respectively, and the powders pulverized by 100% fragments, the rods produced through plasma sintering were successfully fabricated even if several areas with low density are identified. Not all rods were broken during the REP, indicating granulation results with similar size distribution and yield. Regarding the oxygen contents of as-received pebbles, fragments, and rod and pebbles produced with 100% fragments, the plasma sintering effect on impurity cleaning is therefore not significant, whereas the REP evidently leads to remarkable reduction of oxygen as impurity.