Measurement and Calculation of Helium Generation in Beryllium Pebbles Irradiated in EBR-II

Abstract

Beryllium pebbles are being evaluated for their potential use as a neutron multiplier in fusion power reactors. Commercially available pebbles were irradiated in the Experimental Breeder Reactor II to study property changes and gas production. Helium production rates from fast neutron irradiation are significantly higher in beryllium than in other metals due to the low neutron energy threshold and large (n,2n)2α nuclear cross section. Furthermore, the 9Be(n,α)6He(β) reaction breeds 6Li, which has a high thermal neutron cross section for producing helium and tritium. Calculations of helium and tritium production were performed based on neutron dosimetry measurements. Stepped-anneal and total helium release measurements were conducted on single and multiple beryllium pebbles, from two different suppliers, Brush Wellman and Nippon Gashi Co.-Japan. The measurements were conducted to determine the total helium content for comparison with predictions, and to determine the helium release characteristics as a function of time and temperature. These data are required for fusion reactor design. Sequential helium analyses were conducted over a temperature range from 500°C to 1100°C in approximately 100°C steps. Observed helium release in both beryllium lots was nonlinear with time at each temperature interval, with each step being characterized by a rapid initial release rate, followed by a gradual slowing of the rate over time. Fractional helium release as a function of temperature was observed to be substantially different for the two lots. Comparisons of the measured and calculated total helium production in the beryllium agree within ±5%. This agreement validates our understanding of the reaction mechanisms, nuclear cross sections, and neutron dosimetry results, thereby demonstrating that we can confidently predict helium generation and release characteristics for the design of fusion reactors.