Development of reflector element with beryllium pebbles packing in JMTR

Takemoto, Noriyuki ; Kitagishi, Shigeru  ; Tsuchiya, Kunihiko ; Nagao, Yoshiharu  ; Ishihara, Masahiro ; Kawamura, Hiroshi

The JMTR is being refurbished and the upgraded reactor will restart from FY 2011. To conduct attractive irradiation tests after refurbished, the new reflector element which has a housing structure with beryllium or aluminum pebbles packing was developed to advance the neutron irradiation control. As results of neutronic calculation, it was found that continuously variable control of thermal neutron flux or neutron spectrum is possible within control capability limits between the beryllium reflector element and the aluminum one by changing of packing fraction of pebbles. As a result of investigation about specification of beryllium pebbles, filling method, housing structure and so on, it was confirmed that the new reflector element is possible to fabricate. Therefore, it would be possible to fine-grained control of neutron irradiation condition by applying the new reflector element.