Development of U-Mo research reactor fuel in KAERI

In connection with the back-end option as well as the need for upgrading the performance in the HANARO reactor core, KAERI has developed a rod type U-Mo fuel with a high U density, up to 6 g-U/cc, since the middle of 1990s. Because U-Mo alloy has a ductile nature, the alloy cannot be easily converted to powder by a comminution process. The centrifugal atomization process has been applied to fabricate U-Mo fuel powders. The qualification experiments, the KOMO-1,2, and 3 irradiation tests, for both atomized U-Mo dispersion fuels with a U-loading of up to 6 g-U/cc and monolithic fuels with U-loadings of 6 - 16 g-U/cc, in the HANARO research reactor were carried out. The first irradiation test for U-Mo dispersion rod fuels revealed the fuels with U-loading of 6.0 g-U/cc not to be acceptable due to a complete interaction between the U-Mo fuel particles and the Al matrix at low burn-ups(∼10 at% BU). In the 2nd irradiation test (KOMO-2), rod-type U-Mo dispersion fuels with 4 g-U/cc and 4.5 g-U/cc were irradiated up to 60 at% BU in HANARO. PIE results showed that a whole range of the fuel particles were interacted with the Al matrix at the central region of the fuel meat. The latest KOMO-3 irradiation test has been prepared by reflecting the KOMO-2 test results. In order to alleviate the severe interaction problem, several approaches have been adapted. Remedies for a dispersion fuel include 1) a use of large-sized U-Mo fuel particles to reduce the total interfacial area between the U-Mo and Al matrix, 2) a modification of U-Mo by adding a small amount of Zr into the U-Mo alloy, and 3) an addition of Si into the matrix Al. From the current PIE analysis on the KOMO-3 test fuels, the U-7Mo/Al dispersion fuel (4.5 g-U/cc) contained large-sized U-Mo particles(210-300 μm) which appeared to exhibit a sound irradiation performance. The fuels modified with an alloying (Zr and Si) also showed a considerably lower IL thickness than U-7Mo/Al. The centerline temperature (∼204degC) of the large-sized U-7Mo/Al particle fuel was much lower than that of a similar fuel with smaller fuel particles irradiated in the KOMO-2 test(∼480degC), demonstrating the advantage of using large-sized U-Mo particles. This supports that a use of a large-sized particle is one of the promising methods worth pursuing further in the future tests combining a alloy modification to deal with the interaction growth problem in a U-Mo/Al dispersion fuel. (author)