Formation and accumulation of radiation-induced defects and radiolysis products in modified lithium orthosilicate pebbles with additions of titanium dioxide

Abstract Lithium orthosilicate (Li 4 SiO 4 ) pebbles with 2.5 wt.% excess of silicon dioxide (SiO 2 ) are the European Union's designated reference tritium breeding ceramics for the Helium Cooled Pebble Bed (HCPB) Test Blanket Module (TBM). However, the latest irradiation experiments showed that the reference Li 4 SiO 4 pebbles may crack and form fragments under operation conditions as expected in the HCPB TBM. Therefore, it has been suggested to change the chemical composition of the reference Li 4 SiO 4 pebbles and to add titanium dioxide (TiO 2 ), to obtain lithium metatitanate (Li 2 TiO 3 ) as a second phase. The aim of this research was to investigate the formation and accumulation of radiation-induced defects (RD) and radiolysis products (RP) in the modified Li 4 SiO 4 pebbles with different contents of TiO 2 for the first time, in order to estimate and compare radiation stability. The reference and the modified Li 4 SiO 4 pebbles were irradiated with accelerated electrons (E = 5 MeV) up to 5000 MGy absorbed dose at 300–990 K in a dry argon atmosphere. By using electron spin resonance (ESR) spectroscopy it was determined that in the modified Li 4 SiO 4 pebbles, several paramagnetic RD and RP are formed and accumulated, like, E' centres (SiO 3 3− /TiO 3 3− ), HC 2 centres (SiO 4 3− /TiO 3 − ) etc. On the basis of the obtained results, it is concluded that the modified Li 4 SiO 4 pebbles with TiO 2 additions have comparable radiation stability with the reference pebbles.