Analysis of tritium retention in beryllium pebbles in EXOTIC, PBA and HIDOBE-01 experiments

Abstract Tritium desorption studies were performed on beryllium pebbles irradiated in three different irradiation campaigns (i.e., EXOTIC-8, PBA and HIDOBE-01), which were carried out in the High Flux Reactor in Petten. Irradiation doses corresponded to the following helium production levels: 100 appm in EXOTIC-8/3, 284 appm in EXOTIC-8/7, 260 appm in EXOTIC-8/8, 400–500 appm in PBA, and 3090 appm in HIDOBE-01. The beryllium pebbles used in this study were manufactured by melt spray and rotating electrode methods. Measured tritium inventories were compared with the MCNP/FISPACT calculations, and it was concluded that for the irradiation temperatures below 900 K the tritium retention is nearly 100% of tritium production. At the irradiation temperatures above 1000 K, tritium escaped from beryllium during irradiation and the retention fraction, in case of the HIDOBE-01 experiment, was about 5% or less. Analysis of the release spectra revealed a number of characteristic peaks measured as gas (HT) and water (HTO). Below 500 K, tritium release takes place in the HTO form and can be ascribed to the exchange processes taking place at the beryllium surface and tritium dissociation from small clusters. At higher temperatures, tritium release takes place in the HT form and in most spectra represented as one peak centered between 1100 and 1200 K. Optical and scanning electron microscopy studies demonstrated that between 1073 and 1173 K, helium bubbles form an extended network of interconnected channels, which are apparently responsible for the observed tritium release.