Completion of long term high temperature operation with HTTR (High Temperature Engineering Test Reactor), 3; Behaviors of HTTR fuel

Ueta, Shohei   ; Aihara, Jun ; Inoi, Hiroyuki; Umeda, Masayuki; Sawa, Kazuhiro

Behavior of HTTR fuel during long term high temperature operation has been examined, which was evaluated by periodical measurements of radioactivities of fission products (FPs) by primary coolant sampling. Fractional release of Kr during the operation became 10, how is 10 times lower than the design limit. The most dominant reason for very good retention against FP in HTTR fuel was to accomplish very low initial through-coatings failure fraction at commercial fabrication, about 210, how is 10 times lower than the design limit. Technical developments such as optimizations of fuel coating and compaction processes made the fuel failure fraction very low. Also, experimental results well agreed with the evaluation by FP gas release model. It was revealed that releases of FP gases from fuel change from recoil to diffusion by increasing fuel temperature as reactor power increases, and release from uranium contamination becomes dominant during this long term high temperature operation.