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JAEA successfully conducted a test simulating core-melt accident of FBRs jointly with Kazakhstan.

Apr. 27, 2006

The Japan Atomic Energy Agency (President Yuichi Tonozuka) (herein after referred to as JAEA) had successfully carried out an in-pile test simulating core-melt accident of FBRs, revealing movement of molten fuel in the sodium environment. The test was conducted under a joint research program, which started in FY2000, with National Nuclear Center of Republic of Kazakhstan (NNC/KZ). The Japan Atomic Power Company (JAPC) also cooperated to this program.
In the scenario of core-melt accidents of FBRs, behavior of high-temperature molten fuel plays an important role dominating the consequences. Simulation tests of such accidents with certain scale in dimension had not been executed before, as it required a high-performance test facility and advanced testing techniques that were previously unavailable. Therefore, in the conventional safety assessments of FBRs, appropriateness of safety measures has been confirmed based on quite conservative assumptions.
In the test at NNC/KZ, a condition of fuel melting was realized in a double containers placed in the central experimental channel of IGR (Impulse Graphite Reactor), allowing observation of molten-fuel behavior in the sodium environment (Fig.1). Such a test required techniques to heat the test fuel pins up to the required temperature and measure the movement of molten fuel while maintaining the fuel and sodium in the container. The final-stage test was approached in a step-by-step way (Fig.2) to check the required techniques in advance.
The first one of the final-stage tests was conducted on March 6 using about 8kg of test fuel (17%-enriched uranium dioxide) and about 9kg of sodium. The fuel movement within the sodium was successfully observed from changes in signals of more than 100 different sensors installed in the test device, including neutron detectors (Fig.3 and Fig.4).
The obtained test data, together with other data obtained through past step-by-step tests, will be used to confirm how the molten fuel flows out from the core, especially into the lower space, and then mixes with sodium settling into a coolable state.

[Accompanying materials]
•Fig.1. IGR and the image of the test section
•Fig.2. Three steps in the test program
•Fig.3. Signal from the neutron detectors
•Fig.4. Example of experimental data related to fuel motion

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