Canadian ceramic breeder technology: recent results

Abstract

Abstract Pebble bed ceramic breeders have been under development in Canada for over ten years. The goal is to fabricate and characterize these materials for use in engineering test reactors and in subsequent fusion power reactors. The program emphasis is on 1.2 mm diameter Li2ZrO3 and Li2TiO3 pebbles. Practical use of these pebbles requires a mass-production fabrication process, and characterization of the pebble beds with respect to bed behaviour and irradiation effects. This paper summarizes the relevant work within Canada since 1991. The fabrication process presently used is suitable for mass production, and is in the process of being transferred to industry. Thermal cycling tests have been conducted on zirconate and titanate pebbles under both laboratory and “engineering” conditions. Cycling reduces the pebble strength, although there are indications that different fabrication conditions produce more robust pebbles. This is an active area of work. Single-size lithium zirconate pebbles have been well-characterized in terms of the bed thermal conductivity and purge gas pressure drop. Recent results include measurement of thermal conductivity from 100 to 1200°C (and 0–2 bar), and of purge gas pressure drop as a function of porosity. Binary beds have also been studied, using steel or lithium zirconate smaller pebbles. Extensive irradiation testing of the as-fabricated ceramic is a critical factor in their acceptance. Lithium zirconate has been characterized under several European irradiation tests, and 1.2 mm lithium zirconate pebbles have been tested to 5.2% lithium atom burnup and over 250–1000°C in the BEATRIX-II and CRITIC-2 purged-capsule experiments. Tritium release is rapid even at low temperatures, with no effects of burnup seen. The pebble bed temperature has been consistent with model predictions, and stable under irradiation. Post-irradiation anneal tests of lithium titanate show good tritium release. Post-irradiation examination of the BEATRIX-II lithium zirconate pebbles is just beginning. Reference blanket designs have been developed based on breeder-in-tube geometry. Engineering-oriented tests have been carried out on large-volume (41) and long-pin (3 m) geometries, to characterize the behaviour of the pebble beds under more realistic conditions. The results of the work described here, and related tests within the world fusion community, continue to support the use of these ceramic breeder pebbles in fusion reactor blankets.