Fig. 8-1 Thermal expansion data for Tc metal and a Tc-Ru alloy This figure shows the results of thermal expansion measurements in the longitudinal direction of the prepared rod. These data show that the thermal expansion of Tc metal is larger than that of Ru metal, the thermal expansion of the Tc-Ru alloy decreases with increasing Ru concentration, and thermal expansions of Tc metal and Tc-Ru alloy are smaller than those of candidate materials for cladding tubes, 316-stainless steel and Cr-Mo steel. Through these measurements, it was determined that there should not be any mechanical constraint between a Tc metal target and its cladding tube.

Fig. 8-2 The results of an examination for reactivity of CuI both with a candidate material for its cladding tube and with Cu These figures are the results of examination for reactivity of a CuI pellet (upper left) both with a candidate material for its cladding tube, Cr-Mo steel, and with Cu. The examination were carried out at near irradiation temperatures. It is found that the Cr-Mo steel plate (right) contacted with the CuI pellet is partly corroded, but that Cu plate (lower left) is not corroded. From these results, it was established that the corrosion of cladding tube material by CuI could be prevented by putting a Cu metal liner between the CuI and the cladding tube.

Long-lived fission products such as 99Tc (half-life: 2.111 × 105 years) and 129I (half-life: 1.57 × 107 years) are contained in the spent nuclear fuel from nuclear power plants. If such long-lived fission products are disposed of as radioactive wastes in deep underground repositories, surveillance of the environment will have to be continued for a long time in order not to leak radioactive substances into the environment. On the other hand, if it is possible to partition 99Tc and 129I from spent fuels and to transmute 99Tc into stable 100Ru and 129I into stable 130Xe, the environmental burden of geological disposal could be reduced. In order to realize transmutation of long-lived fission products into stable nuclides, it is necessary to carry out a selection of targets for transmutation, some investigation of the thermal properties of the target, a determination of the compatibility of the cladding tube with the target, etc. The most promising chemical form for a target for transmutation of 99Tc is a metal. However there were no available data with which to estimate the behavior of the Tc metal target during transmutation, and some investigation of thermal properties of the target was needed. Therefore Tc metal and Tc-Ru alloy samples were prepared in a glove box, and the fundamental properties of thermal expansion (Fig.8-1), heat capacity, and thermal diffusivity have been measured. The temperature variation and Ru concentration dependence in Tc-Ru alloys of these thermal properties have also been established. The Ru concentration in Tc-Ru alloys increases with transmutation of Tc. In contrast with the case of Tc, it is necessary to select suitable chemical forms for the 129I transmutation target, which is unstable and reactive. Of chemical forms which are highly favorable for fabrication and handling of the target, CuI is one of just a few choices which are stable in air. But CuI has the property of reacting with the cladding tube and corroding it. Therefore a CuI pellet (Fig. 8-2) was prepared, and reactivity studies of CuI with a candidate material for the cladding tube (Cr-Mo steel) and with Cu metal were carried out. From the results of the reactivity tests, it was concluded that the corrosion of the cladding tube by the CuI could be avoided by putting a Cu metal liner between them. Thus, a target configuration for the transmutation of 129I, which consists of CuI, a Cu metal liner, and a cladding tube, was proposed for the first time ever. Investigation of the thermal properties of Tc metal and Tc-Ru alloys, combined with a new proposal for the target configuration for transmutation of I, has led to significant progress in the research and development of targets for transmutation of 99Tc and 129I into stable nuclides.

Reference Y. Shirasu et al., Thermal Expansions of Technetium-Ruthenium Alloys, J. Alloy. Com., 335, 224 (2002).

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