Advanced Neutron Multipliers Research Articles

Development of advanced blanket materials for a solid breeder blanket of a fusion reactor

The design of an advanced solid breeding blanket in a DEMO reactor requires a tritium breeder and a neutron multiplier that can withstand high temperatures and high neutron fluences, and the development of such advanced blanket materials has been carried out by collaboration between JAERI, universities and industries in Japan. The Li2TiO3 pebble fabricated by a wet process is a reference material as a tritium breeder, but its stability at high temperatures has to be improved for its application in a DEMO blanket. One of these improved materials, TiO2-doped Li2TiO3 pebbles, was successfully fabricated and studied. For the advanced neutron multiplier, beryllides that have a high melting point and good chemical stability have been studied. Some characterization of Be12Ti was conducted, and it became clear that it had lower swelling and tritium inventory than beryllium metal. Pebble fabrication study for Be12Ti was also performed and Be12Ti pebbles were successfully fabricated. These activities have shown that there is a bright prospect in realizing a DEMO blanket by the application of TiO2-doped Li2TiO3 and beryllides.

Elementary development for beryllide pebble fabrication by rotating electrode method

Beryllides (Be 12Ti, Be 12V, etc.) are expected to be promising candidates for advanced neutron multipliers for the DEMO blanket. However, it has been difficult to fabricate beryllide pebbles by rotating electrode method (REM) that is the standard fabrication process for the beryllium (Be) pebbles, because of the brittleness of beryllide electrode rod. In this study, the chemical composition of the materials and the fabrication process of the electrode were surveyed. Then the small-scale electrode was fabricated and preliminary trials of pebbles production were conducted by REM. The electrode of the improved material withstood the thermal stress under arc heating and a certain amount of pebbles were obtained. These pebbles by REM were dense and had fine structure. The feasibility of the production of beryllide pebbles by REM was confirmed.

Thermal conductivity of neutron irradiated Be 12Ti

Beryllides are expected to be used as the advanced neutron multiplier for the DEMO blanket owing to their peculiar properties like higher melting point, high oxidation resistance, etc. which can improve the blanket design. In this study, thermal conductivity of neutron irradiated Be 12Ti was measured. The Be 12Ti specimens were fabricated by HIP process using beryllium and titanium powder, and were irradiated in the JMTR up to a fast neutron fluence ( E>1 MeV) of 4×10 20 n/cm 2 at temperatures of 330, 400 and 500 °C. Thermal diffusivity and specific heat of un-irradiated and irradiated specimens were measured up to 1000 °C by laser flash method. Then, the thermal conductivity was evaluated from these data. The effective thermal conductivity in the pebble bed of fusion blanket was also estimated and interesting prospects for blanket design were obtained.

Compatibility test between Be 12Ti and Li 2TiO 3

Beryllides have many interesting properties at high temperature and are candidate materials as advanced neutron multiplier for DEMO Reactor blanket developed by JAERI. A new blanket design with mixed pebble packing of neutron multiplier and tritium breeder has been proposed to improve tritium breeding ratio (TBR). Therefore, it is necessary to evaluate the compatibility between the neutron multiplier and the tritium breeder. In this study, the compatibility between Be 12Ti and Li 2TiO 3 was investigated. The compatibility between Be and Li 2TiO 3 was also investigated for the comparison with the case of Be 12Ti. When the Be 12Ti specimen was placed in contact with Li 2TiO 3, neither reaction products nor diffusion of Li in beryllide were found after annealing during 1000 h at 800 °C. In the case of Be specimen in contact with Li 2TiO 3 no reaction products were observed, however, a diffusion of Li into Be was identified after annealing for more than 300 h at 800 °C. These results show that the compatibility of Be 12Ti with Li 2TiO 3 was higher than that of Be, therefore, from the point of view of compatibility mixed pebble beds of neutron multiplier Be 12Ti and tritium breeder Li 2TiO 3 pebbles can be used in a blanket.