Chiaki Sagawa

Distributions of radionuclides on and in spent nuclear fuel claddings of pressurized water reactors

Abstract Radioactivities in Zircaloy-4 cladding of PWR spent nuclear fuel have been examined as a function of fuel burnup in the region of about 7000–40000 MWd/t. The fission products 137 Cs and 106 Ru in the cladding increased linearly with fuel burnup, whereas the radionuclides 134 Cs and 154 Eu, which were produced by neutron capture of fission products, increased with the second power of fuel burnup. Tritium and the activation products of alloy constituents and impurities increased in proportion with fuel burnup. The gross activities of α emitters on the inner and outer surfaces of the cladding increased with 3.1 and 1.3 powers of fuel burnup, respectively. The distribution of radionuclides in the cladding has been examined by radiochemical analysis combined with stepwise etching from the inner surface of the cladding. More than 98% of fission products were present within a 10 μm depth from the inner surface of the cladding, while activation products such as 60 Co were distributed homogeneously in the interior of the cladding except on the outer surface. The distribution of tritium tended to become heterogeneous with increasing fuel burnup.

Chemical reactions induced by 40 keV He+ ions in Fe3O4 and α-Fe2O3

The chemical reactions induced by 40 keV He+ ions in α-Fe2O3 and Fe3O4 were investigated by the conversion electron Mössbauer spectroscopy(CEMS). Magnetite(Fe3O4) was formed upon the bombardment of α-Fe2O3, whereas no change was observed in Fe3O4. The initial G value for Fe3O4 formation is estimated to be 3.5×10−4 for 100 nm depth from the surface. The application of CEMS and sputtering technique to ion bombardment chemistry is discussed.

Separation of Curium from Neutron Irradiated Am-241

The conditions under which curium can be separated from irradiated 241Am target were elucidated. The isolation process consists of three steps: In the first step, Am(III) is oxidized to pentavalent state in a dilute nitric acid solution, and then plutonium and curium are extracted from the irradiated target by solvent extraction with HDEHP. Curium in the organic phase is back-extracted with 1 N nitric acid, and thereafter plutonium with a reducing solution containing ferrous sulfamate. The curium is finally purified by cation exchange, using α-hydroxy isobutyrate as the eluting solution. About 0.4 μg of 242Cm and 4×10−3 μg of 243Cm were found in the curium fraction, which had been separated from 1 mg of irradiated 241Am sample.

Ion-beam-induced reduction of iron oxide supported on porous glass

Abstract When ferric oxide supported on porous glass was irradiated by 40 keV He + ions, the reduction of Fe 3+ to Fe 2+ species occurred. The yield of Fe 2+ species, which was monitored by the Mossbauer spectroscopic measurement, increased with increasing total dose, and went up to ca. 80%. This unexpectedly large yield meant that the effect was exerted beyond the range of the incident He + ions. Clear dose rate dependence of the Fe 2+ yield was also observed. We examined the possibility that the reduction of the Fe 3+ species was caused by some type of gaseous reductant produced by radiolysis of surface chemical species such as physisorbed water molecules and surface hydroxyls.

A new system of time differential emission Mössbauer spectroscopy with event-by-event mode

A new system for time differential Mössbauer spectroscopy was developed. This system applied an event-by-event technique consisting of 2 ADC modules, which enabled flexible handling of the data. The spectra of57Co labelled cobalt tellurate were measured using this apparatus. No time dependence for chemical after-effects was observed in the compound at room temperature.

Crud Behaviors in High Temperature Water, (II): Characterization of Corrosion Layer on Type 304 Stainless Steel and Zircaloy-2

Sample specimens of type 304 stainless steel and Zircaloy-2 were exposed to the OWL-1 loop water at 280°C for 23d. After the exposure, a number of crystallites are distributed either separately or sometimes in agglomerate on the surfaces. The X-ray diffraction analysis and XMA analysis of the type 304 stainless steel surface revealed that more than 95% of the crystallites have a M2O3-type of structure, and some are enriched in Cr. In the sub-sequent radiochemical analysis of the fractions detached by the various cleaning processes (ultrasonic, electrostripping and electropolishing), it is noticed that the 60Co/Co ratio decreases with the depth of the corrosion layer from the value close to that of insoluble crud in the water (crud-P) to that of soluble crud (crud-F).

Tracer diffusion of 60Co into SUS-304

Abstract A piece of stainless steel 304 was thermal annealed either in a 60 Co solution at 280°C, or in a He atmosphere at 300 or 600°C after being smeared with a 60 Co solution. The depth profile of 60 Co in SUS was determined with these samples, together with those sampled from the water cleaning system in the Japan Power Demonstration Reactor and those exposed to a high-temperature water ( ~ 280°C) circulating OWL-loop installed in the Japan Material Testing Reactor, in order to determine the apparent values of the volume and grain boundary diffusion constants. The results have been compared with those of the previous measurements on Ni, Cr, and Fe above 500°C. A consistency has been noticed between both results at 600°C, while the present values at 280 and 300°C are much larger than those expected from the extrapolation of the previous measurements. Although this inconsistency is ascribed to the contribution of the dislocation pipe diffusion at such a low temperature, an attempt has been made to apply the present values to estimate how deep 60 Co would penetrate SUS—one of the main constructing materials of the primary cooling system—in the life time of a power reactor.