Irradiation tests of a BWR advanced Zr alloy (HiFi alloy) and Zircaloy-2 (Zry-2) were carried out in a Japanese commercial reactor and the irradiation performances of the materials were investigated. HiFi alloy and Zry-2 showed excellent resistance to corrosion up to 70 GWd/t, and furthermore, HiFi kept lower hydrogen pickup compared with Zry-2. TEM observation showed that the Fe/(Fe+Cr) ratio of Zr(Fe,Cr) 2 type second phase particles (SPPs) for HiFi alloy and Zry-2 tended to decrease as fast neutron fluence increased and to saturate at high fluence. Zr-Fe-Cr SPPs did not completely disappear even for 6 cycles for the irradiated HiFi alloy and Zry-2. In order to clarify the mechanism of hydrogen absorption, an electrochemical technique was used for the oxide film of both materials as part of the out-ofpile test. The relation between the oxide surface potential and the hydrogen pickup fraction was estimated suggesting that the potential difference over the oxide film suppressed hydrogen (proton) diffusion in the oxide film.

Japan Atomic Energy Agency (JAEA) has developed fabrication technology for oxidation-resistant fuel elements to improve the safety of high-temperature gas-cooled reactors in severe oxidation accidents on the basis of its previous research. Simulated fuel particles (alumina particles) were coated with a mixed powder of Si, C and a small amount of resin to form over-coated particles, which were molded and sintered by hot-pressing to form simulated oxidation-resistant fuel elements with a SiC/C mixed matrix, where the SiC was formed by reaction bonding. Simulated oxidation-resistant fuel elements with a matrix whose Si/C mole ratio (about 0.551) was three times as large as that in previous research were fabricated. No Si peak was detected by X-ray diffraction of the matrix. A monoaxial compressive fracture test was carried out, and the fracture stress was found to be more than three times as large as the standard for fuel compacts of High Temperature Engineering Test Reactor (HTTR). Better oxidation resistance than that of an ordinary fuel compact with a ordinary graphite matrix was confirmed by an oxidation test in 20% O 2 at 1673 K. All simulated coated fuel particles remained in specimen after 10 h oxidation.

Irradiation tests of a BWR advanced Zr alloy (HiFi alloy) and Zircaloy-2 (Zry-2) were carried out in a Japanese commercial reactor and the irradiation performances of the materials were investigated. HiFi alloy and Zry-2 showed excellent resistance to corrosion up to 70 GWd/t, and furthermore, HiFi kept lower hydrogen pickup compared with Zry-2. TEM observation showed that the Fe/(Fe+Cr) ratio of Zr(Fe,Cr) 2 type second phase particles (SPPs) for HiFi alloy and Zry-2 tended to decrease as fast neutron fluence increased and to saturate at high fluence. Zr-Fe-Cr SPPs did not completely disappear even for 6 cycles for the irradiated HiFi alloy and Zry-2. In order to clarify the mechanism of hydrogen absorption, an electrochemical technique was used for the oxide film of both materials as part of the out-ofpile test. The relation between the oxide surface potential and the hydrogen pickup fraction was estimated suggesting that the potential difference over the oxide film suppressed hydrogen (proton) diffusion in the oxide film.

The development of fabrication and inspection technologies for an oxidation-resistant fuel element was conducted referring to previous research to improve the safety of high-temperature gas-cooled reactors (HTGRs) during a severe oxidation accident. Herein, simulated coated fuel particles (CFPs) , alumina particles, were coated with a mixture of Si and C powders and a small amount of resin to form "over-coated" particles that were subsequently molded and hot-pressed to sinter the simulated oxidation-resistant fuel elements with a SiC/C mixed matrix. SiC was formed by reaction bonding. Simulated oxidation-resistant fuel elements containing a matrix with a Si/C mole ratio of 1.00 were fabricated. Elemental Si and C peaks were not detected in X-ray diffraction of the matrix. The failure fraction of CFPs in fuel elements is a very important HTGR fuel inspection subject; it is essential that CFPs are extracted from fuel elements without additional failure. Herein, a method of extracting CFPs was developed. The dissolution of SiC using KOH or by pressurized acidolysis should be applied to extract CFPs. However, the outer high-density pyrolytic carbon layer should remain in spite of its transformation to SiC during sintering by reaction bonding with Si in the mixed powder.

In order to develop 3S-TRISO fuel for Pu-burner High Temperature Gas Reactor (HTGR), we conducted lab. scale experiments such as preparation test of simulated fuel kernel; CeO2-YSZ particle, and coating pre-test with simulated kernel. In the preparation test, based on the actual achievement of manufacturing fuel for High Temperature Engineering Test Reactor (HTTR)[1], we tried to fabricate some CeO2-YSZ particles through external gelation process. As a result, we successfully obtained the manufacturing parameters that can prepare good particles. In addition, we carried out some parametric coating test with fluidized-bed equipment and ZrO2 particle as simulated ZrC coated fuel kernel, and obtained the prospect of the possibility to coat the layer having desired thickness.