K. Kuramoto

Rock-Like Oxide Fuels and Their Burning in LWRs

Research on the plutonium rock-like oxide (ROX) fuels and their once-through burning in light water reactors has been performed to establish an option for utilizing and disposing effectively the excess plutonium. The ROX fuel is a sort of the inert matrix fuels and consists of mineral-like compounds such as yttria stabilized zirconia, spinel and corundum. A particle-dispersed fuel was devised to reduce damage by heavy fission fragments. Some preliminary results on swelling, fractional gas release and microstructure change for five ROX fuels were obtained from the irradiation test and successive post-irradiation examinations. Inherent disadvantages of the Pu-ROX fuel cores could be improved by adding 238U or 232Th as resonant materials, and all improved cores showed a nearly the same characteristics as the conventional UO2 core during transient conditions. The threshold enthalpy of the ROX fuel rod failure was found to be comparable to the fresh UO2 rod by pulse-irradiation tests simulating reactivity initiated accident conditions.

In-pile irradiation of rock-like oxide fuels

Abstract Five kinds of rock-like oxide fuels were prepared and irradiated using 20% enriched U instead of Pu. The microstructure analyses for irradiated fuel pellets were carried out by ceramography and electron probe microanalysis (EPMA). For MgAl 2 O 4 (spinel)-based fuel, decomposition of the spinel and vaporization of MgO (magnesia) was observed. For Al 2 O 3 (corundum)-based fuels, significant appearance changes were not observed under the irradiation condition of sufficiently high fuel temperature. Fission product (FP) distributions were also analyzed by EPMA. The distributions of Nd and Ba were similar with those of U and Zr. Mo and Pd were found as fine inclusions in the yttria-stabilized zirconia (YSZ) phase. A part of Cs and Xe migrated to the periphery of YSZ particles and to pores in YSZ. The FP distributions of spinel-based fuel were almost similar to corundum-based fuel.

Irradiation behavior of rock-like oxide fuels

Two irradiation tests were performed on the rock-like oxide (ROX) fuels in order to clarify in-pile irradiation stabilities. In the first test small disk-shape fuel targets were irradiated in the JRR-3 in JAERI. In the second test pellet-type fuels were employed. Irradiation behaviors such as swelling, fractional fission gas release (FGR) and phase change were examined by puncture test, profilometry and ceramography. Swelling and FGR behavior of the pellet-type fuels improved considerably compared with the disk-type fuels. Yttria stabilized zirconia (YSZ) single-phase fuel showed an excellent irradiation behavior, i.e. low FGR (<3%), negligible swelling and no appreciable restructuring. The particle dispersed fuels showed lower swelling and higher FGR than those of mechanically blended fuels. Spinel decomposition and subsequence restructuring in the spinel matrix fuels was observed for the first time in the present investigation. It would be possible to reduce the FGR of the spinel matrix fuels to that of the corundum ones, if the maximum fuels temperature is limited below 1700 K where neither spinel decomposition nor restructuring was observed. Damaged area of spinel matrix due to fission fragment irradiation seemed to be confined to thin layers around the surface of YSZ particles as expected.

Development of ceramic waste forms for actinide-rich waste : Radiation stability of perovskite and phase and chemical stabilities of Zr- and Al-based ceramics

Abstract Ceramics are considered as most promising materials for conditioning of long-lived radionuclides because of their outstanding durability for long term. The Japan Atomic Energy Research Institute (JAERI) has developed ceramic waste forms, e.g. Synroc and zirconia-based ceramics, for the actinide-rich wastes arising from partitioning and transmutation processes. In the present study, α-decay damage effects on the density and leaching behavior of perovskite (one of three main minerals forming Synroc) were investigated by an accelerated experiment using the actinide doping technique. A decrease in density of Cm-doped perovskite reached 1.3 % at a dose of 9 × 10 17 α-decays·g −1 . The leach rates (MCC-1 leach test inpH ∼ 2 solution at 90°C for 2 months) of perovskite specimens with accumulated doses of 1.6 × 10 17 , 4.0 × 10 17 and 8.3 × 10 17 α-decays·g −1 were 1.7, 2.3 and 3.0 μ·m −2 ·day −1 , respectively. Application of zirconia- and alumina-based ceramics for incorporating actinides was also investigated by the experiments using non-radioactive elements (Ce and Nd) with an emphasis on crystallographic phase stability and chemical durability. The yttria-stabilized zirconia was stable crystallographically in the wide ranges of Ce and/ or Nd content and had excellent chemical durability.

Current status of researches on the plutonium rock-like oxide fuel and its burning in light water reactors

Abstract Intention of the ROX-LWR system research is to provide an option for utilization or disposition of surplus plutonium. Researches on inert matrix materials and irradiation performance shows that the most favorable candidate for the ROX fuel is a particle dispersed fuel where small particles consisted of yttria stabilized zirconia, PuO 2 and some additives are homogeneously dispersed in spinel matrix. Reactor safety analyses show that the ROX fueled PWR core has nearly the same performability as the existing UO 2 fueled PWR under both reactivity initiated accidents and loss of coolant accidents.

Chemical Durability of Yttria-Stabilized Zirconia for Highly Concentrated TRU Wastes

Neptunium has an extremely long half-life and will be the main toxic element in the later stage of disposal. Yttria-Stabilized Zirconia (YSZ), with a fluorite structure, samples doped with Np-237 in high concentration (20, 30, 40 mol %) were fabricated (sintered in Air or Ar, at 1773 K, for 80 hours), and their leaching test (in deionized water, at 423 K, for 84 days) was carried out. The results indicated that the obtained leaching rates were much smaller than those of the Synroc and glass waste form, and that the increase in Np content did not cause any drastic changes in the leaching rates of Zr, Y, or Np. They were also compared to the results of YSZ doped with Ce or Nd used as surrogates for actinides. The work showed that YSZ doped with Np in high concentration has an excellent chemical durability

Durability test on irradiated rock-like oxide fuels

Abstract For a profitable use of Pu, Japan Atomic Energy Research Institute has been promoting researches for once-through type fuels. The strategy consists of stable rock-like oxide fuel fabrication in conventional fuel facilities followed by almost complete Pu burning in LWR and disposal of chemically stable spent fuel without further processing. Because leach rates of hazardous nuclides, such as TRU and β-emitters, that have long half-lives, are very important for the evaluation of geological safety, leaching tests in deionized water at 363 K were performed with reference to the MCC-1 method. Five irradiated fuel pellets, a single phase fuel of a yttria-stabilized zirconia (YSZ) containing UO 2 (U-YSZ), two fuels of U-YSZ particle dispersed in MgAl 2 O 4 (SPI) or Al 2 O 3 (COR) matrix, two homogeneous-blended fuels of U-YSZ and SPI or COR powders, were submitted to the tests. Stainless steel containers with Au coating and ethylene propylene diene monomer were used as leaching vessels and packing, respectively. The evaluated normalized leach rates of Zr, U and Pu were obviously lower than those of the other important elements and nuclides. Americium, Np and especially Y showed unexpectedly high evaluated normalized leach rates. The volatile elements, Cs and I, showed enhanced leaching within particle-dispersed type fuels because of crack formation around the particle.

Post-irradiation examination of uranium-doped rock-like oxide fuels

Post-irradiation examinations of rock-like oxide fuels were performed in JAERI to evaluate irradiation behavior and geochemical stability. Five kinds of fuels were prepared using 20% enriched U instead of Pu; a single-phase fuel of an yttria-stabilized zirconia containing UO 2 (U-YSZ), two particle-dispersed type fuels of U-YSZ particles + MgAl 2 O 4 /Al 2 O 3 powder, two homogeneously blended type fuels of U-YSZ powder + MgAl 2 O 4 /Al 2 O 3 powder. The fuels were irradiated in JRR-3 for about 100 days and estimated irradiation conditions were as follows; linear power was 15 kW m -1 , thermal neutron fluence was 7 x 10 24 m -2 and fuel temperatures at the surface were 740-1130 K. From the results of non-destructive examinations, the stainless steel cladding surfaces were partially discolored by oxidation and no remarkable deformation of the pins was observed. Significant pellet fragmentation was not observed in spite of the crack formation as observed in irradiated LWR UO 2 fuels. Nonvolatile FPs were observed only in pellets but volatile Cs moved partly to the plenum. From these examinations, no significant difference in macroscopic irradiation behavior was distinguished among 5 fuels.

Re-evaluation of the phase relationship between plutonium and zirconium dioxides

The phase relationship between ZrO 2 and PuO 2 was examined in a low PuO 2 content region, from 3.1 to 11.2 mol% PuO 2 , at temperatures between 1273 K and 1473 K, by high temperature X-ray diffractometry. The measurements were carried out in air. At 1273 K, the samples in this composition range consisted of two phases, monoclinic and cubic. Another phase, tetragonal, was observed at 1373 K. The low temperature monoclinic phase disappeared at 1473 K. It was confirmed that the monoclinic phase disappears around 1463 K; the disappearance temperature does not depend on the composition of the sample. It was, thus, inferred that there should be a eutectoid line in the phase diagram. Though the eutectoid point is not clear, the PuO 2 content at the point should be less than 3.1 mol%.

Phase Relations Between a Fluorite and a Pyrochlore Structure in the System of Actinides and Zirconium Oxides

Cubic stabilized zirconia (CSZ) exhibits the fluorite structure and zirconium pyrochlores form a special structure. Because of slight difference in crystallographic symmetry, local structures are formed in the pyrochlores: a distorted coordination of eight oxygen around a large dopant cation, and a symmetric coordination of six oxygen around Zr+. A similar local structure is confirmed in the Np-doped yttria stabilized zirconia (YSZ) solid solutions by means of 237Np Mössbauer spectroscopy. The Mössbauer study shows that small amounts of Np5+ are formed in the solid solution, and are distributed statistically on the available cation sites. Based o n these findings, a mean cation ionic radii model is developed to evaluate the lattice parameter change with composition in the Pu- and Np-doped YSZ solid solutions. An effect of the oxygen vacancy is treated as reducing the oxygen coordination number arouid Zr4+ from eight to seven in the model. The model represents successfully the observed lattice parameters. A phase transformation from the fluorite to the pyrochlore structure is considered to results in at average Zr coordination number of seven. Suggestions for further studies in this field are also provided.