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The physical properties and irradiation behavior of Np-and Am-bearing MOX were evaluated for the development of advanced fast reactor fuels. The physical properties, lattice parameter, melting temperature, thermal conductivity, and oxygen potential of the fuels were described as functions of Np content, Am content, and oxygen-to-metal (O=M) ratio, and the effect of a few percent Np and Am addition into MOX on those properties was found to be negligibly small. The irradiation tests of fuel pellets having O=M ratios of 1.98 or 1.96 were carried out at a high linear heat rate of about 430 W/cm for 24 h. The redistributions of actinide element and oxygen were analyzed by using the physical properties for Np-and Am-bearing MOX, and the maximum temperatures of the pellets were estimated. The maximum temperatures of the pellets of O=M ¼ 1:96 and 1.98 were estimated to be 2,701 and 2,510 K, respectively. The analysis result showed that the higher maximum temperature in the low-O=M pellet was caused by the low thermal conductivity in the outer region of the pellets, and the maximum temperatures of the pellets did not exceed their melting temperatures. In the results of postirradiation examination, no trace of melting was observed.

Options for fuel cycle technology improvement have strongly regained attention lately with the revival of nuclear energy production interests and plans for next generation nuclear systems. Various fuel forms, geometries and production paths are being looked at. Within the FUJI collaboration program among Japan Atomic Energy Agency (JAEA, former JNC), Paul Scherrer Institute (PSI, Switzerland) and Nuclear Research and Consultancy Group (NRG, the Netherlands) the production paths of plutonium and neptunium mixed oxide-(sphere-pac-and vipac-) particle fuels (20 wt% Pu, 5 wt% Np and 75 wt% U) are tested as well as the initial sintering and power-to-melt behavior under simulated Fast Breeder Reactor (FBR) conditions. The various fuel forms were produced at PSI under the support of JNC, the irradiations were accomplished at High Flux Reactor (HFR) in Petten, the post irradiation examinations are being achieved mainly at NRG and the fuel modelling being performed at JNC and PSI. The present paper reviews mainly the project planning, fuel behaviour-pre-calculations and the fuel-and fuel segment-production. A short overview of the irradiation conditions and ceramographic post irradiation examination analyses is also given.