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Behaviour of 3 BWR segments FK-1, FK-2 & FK-3 under RIA test conditions in NSRR.
Boiling water reactor (BWR) fuel rods with bumps of 41 to 45 GWd/tU were pulse-irradiated in the Nuclear Safety Research Reactor (NSRR) to investigate the fuel behaviour during a reactivity initiated accident (RIA) at cold startup. BWR fuel segment rods of 8x8BJ (STEP I) type from the Fukushima Daiichi Nuclear Power Station Unit 3 were refabricated into short test rods, and they were subjected to prompt enthalpy insertion from 293 to 607 J/g (70 to 145 cal/g) within about 20 ms. The fuel cladding had enough ductility against the prompt deformation due to pellet cladding mechanical interaction. The plastic hoop strain reached 1.5% at the peak location. The cladding surface temperature locally reached about 600 deg.C. Recovery of irradiation defects in the cladding due to high temperature during the pulse irradiation was indicated via X-ray diffractometry. The amount of fission gas released during the pulse irradiation was from 3.1% to 8.2% of total inventory, depending on the peak fuel enthalpy and the normal operation conditions.
The rods FK-1, FK-2 and FK-3 were refabricated from irradiated segment fuel rods of BWR 8x8BJ (STEP I) design. The segments were irradiated to an assembly average burn-up of 30.4 MWd/kgU in Fukushima Daiichi Nuclear Power Station Unit 3. An irradiation history has been prepared for each test section which can be found in the attached files.
Before refabricating, the whole fuel rod was examined by:
Eddy current testing
oxide thickness measurement
fission gas sampling
Each test was conducted on sections of fuel stack ~106 mm long, chosen to have a flat axial burn-up profile. An iron core was placed in the top end fitting to measure fuel stack elongation and an internal pressure sensor was built into the bottom fitting. Hafnium disks were placed at both ends of the fuel column to prevent power peaking and the rods sealed with 0.3 MPa helium gas corresponding to the original filling conditions. Prior to the test, each rod was subjected to the following examination:
Helium leak test
Eddy current testing
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Keywords: BWR reactors, boiling water reactor, fuel behaviour, reactivity initiated accident.