Minoru Takeishi

Monitoring of Low-Level Radioactive Liquid Effluent in Tokai Reprocessing Plant

The Tokai reprocessing plant (TRP), the first reprocessing plant in Japan, has discharged low-level liquid wastes to the Pacific Ocean since the start of its operation in 1977. We have performed liquid effluent monitoring to realize an appropriate radioactive discharge control. Comparing simple and rapid analytical methods with labor-intensive radiochemical analyses demonstrated that the gross-alpha and gross-beta activities agreed well with the total activities of plutonium isotopes (238Pu and 239+240Pu) and major beta emitters (e.g., 90Sr and 137Cs), respectively. The records of the radioactive liquid discharge from the TRP showed that the normalized discharges of all nuclides, except for 3H, were three or four orders of magnitude lower than those from the Sellafield and La Hague reprocessing plants. This was probably due to the installation of multistage evaporators in the liquid waste treatment process in 1980. The annual public doses for a hypothetical person were estimated to be less than 0.2 μSv y−1 from the aquatic pathway. Plutonium radioactivity ratios (238Pu/239+240Pu) of liquid effluents were determined to be 1.3–3.7, while those of the seabed sediment samples collected around the discharge point were 0.003–0.059, indicating no remarkable accumulation of plutonium in the regional aquatic environment. Thus, we concluded that there were no significant radiological effects on the public and the aquatic environment during the past 30-year operation of the TRP.

Monitoring Methodologies and Chronology of Radioactive Airborne Releases from Tokai Reprocessing Plant

Tokai reprocessing plant (TRP) has released radionuclides such as 3H, 14C, 85Kr and 129I into the atmosphere since the start of operation in 1977. We have established the monitoring methodologies for these nuclides, to realize an appropriate and continuous radioactive discharge control. The methodologies having various special technical considerations for matching the monitoring of reprocessing off-gas, were summarized in this paper. Briefly, 3H was collected by a cold-trap technique and the concentration was evaluated being independent of the water collection efficiency; 14C was collected by a monoethanolamine bubbler and then measured by liquid scintillation counting without any interferences from 3H and 85Kr; 85Kr was continuously measured by combination of two kinds of detectors to cover very wide range of the concentration; and 129I was collected by a charcoal filter and a charcoal cartridge in series with a relatively high collecting performance. The monitoring records over the period from the start of operation to fiscal 2006, the end of contract-based reprocessing, certainly ensured that the releases of these nuclides from the TRP have never exceeded the authorized discharge limits.

Coupling the advection-dispersion equation with fully kinetic reversible/irreversible sorption terms to model radiocesium soil profiles in Fukushima Prefecture

Radiocesium is an important environmental contaminant in fallout from nuclear reactor accidents and atomic weapons testing. A modified Diffusion-Sorption-Fixation (mDSF) model, based on the advection-dispersion equation, is proposed to describe the vertical migration of radiocesium in soils following fallout. The model introduces kinetics for the reversible binding of radiocesium. We test the model by comparing its results to depth profiles measured in Fukushima Prefecture, Japan, since 2011. The results from the mDSF model are a better fit to the measurement data (as quantified by R2) than results from a simple diffusion model and the original DSF model. The introduction of reversible sorption kinetics means that the exponential-shape depth distribution can be reproduced immediately following fallout. The initial relaxation mass depth of the distribution is determined by the diffusion length, which depends on the distribution coefficient, sorption rate and dispersion coefficient. The mDSF model captures the long tails of the radiocesium distribution at large depths, which are caused by different rates for kinetic sorption and desorption. The mDSF model indicates that depth distributions displaying a peak in activity below the surface are possible for soils with high organic matter content at the surface. The mDSF equations thus offers a physical basis for various types of radiocesium depth profiles observed in contaminated environments.