A rapid and simple method to measure the concentration of 90Sr in water by measuring β-rays from 90Y was presented. Under the situation that 90Sr/90Y, 134Cs and 137Cs are the main radionuclides included in the water sample, only β-rays from 90Y can transmit through 1.5-mm-thick polyethylene. From this fact, it is possible to measure β-rays from 90Y using a β-ray detector, such as the GM-counter, set beneath the 1.5-mm-thick bottom of the water bottle containing the sample with 90Sr/90Y. The acrylic resin collimator having 0 cm, 1.00 cm, 1.50 cm or 3.00 cm diameter was made to detect β-rays at the fixed region of the GM-counter used. Contributions from bremsstrahlung produced by β-rays and γ-rays from radionuclides such as 134Cs and 137Cs/137mBa are removed by subtracting the count rate measured with a 1.00 cm acrylic resin collimator without a hole as the background count rate. The developed method was studied using the bottle routinely used at the Fukushima Daiichi Nuclear Power Station. It was confirmed that the developed method can be applied to measure the 90Sr concentration in water to the order of several Bq/cm3 if 134Cs and 137Cs concentrations are less than or equal to the 90Sr/90Y concentration.
Strontium-90/Y-90 are major radionuclides observed in the water samples tested recently at the site of the Fukushima Daiichi Nuclear Power Station of Tokyo Electric Power Company. A simple method of evaluating Sr-90 concentration in these water samples by measuring β rays from Y-90 with a GM-detector setup was developed. By applying the precipitation method, Sr-90 and Y-90 were separated and quantitatively collected with a filter. β rays from Y-90 in the filter were measured two times at appropriate intervals by inserting a polyethylene plate of 2 mm thickness as a β-ray absorber. The contribution of γ rays from Cs-134 and Cs-137 to the Y-90 count rates was quantitatively evaluated using a 10-mm-thick acrylic resin plate. From the parent-daughter relationship between Sr-90 and Y-90, the Sr-90 concentration was evaluated using the conversion coefficient of Y-90 count rate (cps) to Sr-90 concentration (Bq/cm3). It was verified that Sr-90 concentration of below 0.01 Bq/cm3 in water samples can be correctly measured by this simple method.
Comprehensive understanding of FBR plants is required to realize safe and well-balanced plant design. Difficulties are related to their complex systems and multi-physics phenomena, which have mutual interaction among various professional fields. To overcome these problems, a total simulation program was developed to evaluate plant characteristics under steady state conditions and to survey allowable specifications among trade-off parameters. This program was proved to be effective for better understanding of FBR plants through unitization for course exercises at the graduate school of the University of Tokyo.
Regarding the long-term storage of spent Cs adsorption vessels containing zeolites in the Fukushima Daiichi nuclear power station, corrosion of the material of the spent Cs adsorption vessels is one of the important issues. We performed electrochemical tests of stainless steel (SUS316L) in the zeolites containing artificial seawater under gamma-ray irradiation. The spontaneous potential (ESP) and critical pitting potential (VC) of SUS316L were measured to understand the corrosion resistance of the stainless steel in this study. The rest potential of the stainless steel increased with time after gamma-ray irradiation. ESP, defined as the steady rest potential, increased with increasing dose rate; this increase in ESP was suppressed by the contact of SUS316L with the zeolites. The concentration of H2O2 in bulk water increased with increasing dose rate. This concentration increase was suppressed by the contact of SUS316L with the zeolites due to decomposition of H2O2. There was good relationship between ESP and the concentration of H2O2. The VC of SUS316L contacted with the zeolites decreased with increasing Cl－ ion concentration and is slightly smaller than the VC in bulk water. The contact of SUS316L with the zeolites suppressed the increase in ESP under irradiation. The contact with the zeolites can reduce the probability of the localized corrosion of SUS316L.
A study on reduction of potential radiotoxicity for spent fuel by using high-temperature gas-cooled reactors (HTGRs) has been performed. Unlike partitioning and transmutation (P&T), the reactor concept is investigated from the viewpoint of reduction of radiotoxicity generation itself. To reduce radiotoxicity, 238U, which generates Pu, Am and Cm, should be excluded. Therefore, we proposed HTGR fueled by new-concept fuels with alternative fuel matrixes instead of 238U. Those are yttria-stabilized zirconia (YSZ) and thorium, and the fissile material is highly enriched uranium (HEU) with an enrichment of 93%. With HEU, the radiotoxicity can be significantly reduced, and the cooling time to decay to a natural uranium level can be shortened to approximately 800 years. Fuel integrity and proliferation resistance can be maintained by dilution using YSZ, and the neutronic characteristics of self-regulation are maintained by the loading of erbium. The fuel can generate the same amount of heat as ordinary uranium fuel. The electricity generation cost is as cheap as GTHTR300. It is concluded that the proposed reactor concept can reduce the cooling time by less than 1% from 100 thousand years to 800 years without additional development of innovative technology.
We have been developing a new partitioning method of high-level radioactive waste by the single-cycle extraction process. This process is composed of the extraction of actinides (An) and fission products (FP, e.g., Pd, Ru, Mo and Tc), and mutual separation by reverse extraction. The extractant employed in this process is required to extract soft, hard acid metals and oxonium anions simultaneously. The NTAamide (N,N,N’,N’,N’’,N’’-hexaoctyl-nitrilotriacetamide) is one of the candidate extractants. After the extraction of An and FP, the mutual separation by reverse-extraction should be set up. Distribution ratios of Pd and Ru, which are obtained by NTAamide extraction, can be suppressed by masking agents, thiourea, systeine, diethylenetriamine, and trisaminoethylamine. The masking of Mo can be performed using methylimino-N,N’-diethylacetamide (MIDEA), NTAamide(C2) and iminodimethylphosphoric acid, and Re can be stripped using an aqueous phase with high pH. The information on extraction and masking for these metals will be utilized in the development of the single-cycle process.