The first major upgrade of Joyo to the 100 MWt MK-II irradiation-test-bed was successfully operated from 1982 to 2000. Work began in 2000 on the 140 MWt MK-III program, which was the second major upgrade to improve the irradiation capability of Joyo. A start-up test of the MK-III core was conducted from June to October 2003. The rated power operational cycle of MK-III core was started in May 2004. An irradiation test of the self-actuated shutdown system (SASS) and others were conducted as scheduled.

After completion of the second duty cycle in 2004, a fuel failure simulation test was carried out to verify plant operational procedures in the event of fuel failure. Since the third duty cycle, oxide dispersion-strengthened ferritic steel (ODS) and MOX fuel with 5% americium, and with both neptunium and americium added, have been irradiated.

In 2003, an upgrade programme increased the irradiation capability of Joyo and its reactor power to 140MWt. Compared to the previous MK-II core, the MK-III core has approximately four times as much irradiation capability. In addition, it now has a high-performance core, improved plant availability factor and an upgraded irradiation technique. Major irradiation tests in Joyo MK-III core are as follows:

  • neutron flux distribution measurement
  • irradiation test with material testing rig with temperature control (MARICO-2)
  • irradiation test for MA-MOX fuel
  • sodium leak detection technology using laser resonance ionization mass spectrometry

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