Hiromi Yamazawa

Preliminary Estimation of Release Amounts of 131I and 137Cs Accidentally Discharged from the Fukushima Daiichi Nuclear Power Plant into the Atmosphere

(2011). Preliminary Estimation of Release Amounts of 131I and 137Cs Accidentally Discharged from the Fukushima Daiichi Nuclear Power Plant into the Atmosphere. Journal of Nuclear Science and Technology: Vol. 48, No. 7, pp. 1129-1134.

Aerodynamic roughness over an inhomogeneous ground surface

The aerodynamic roughness parameter z0 over inhomogeneous ground surfaces, such as cities, rural towns and so on, is determined by analyzing the wind data at AMeDAS observatories in the Tohoku and Kanto districts of Japan, by making use of Rossby number similarity theory. It is found that the aerodynamic roughness parameter is proportional to the average size of the roughness elements.A practical method of estimating the aerodynamic roughness parameter over an extensive area with various inhomogeneities is developed. In this method, the Digital National Land Information data bank is employed. As an example, the roughness parameter distribution around Tsukuba Academic City is presented.

Bulk transfer coefficient over a snow surface

The drag coefficient CDand the bulk transfer coefficient for sensible heat CHover a flat snow surface were determined experimentally. Theoretical considerations reveal that CDdepends on the friction velocity u* as well as on the geometrical roughness h of the snow surface. It is found that CDincreases with increasing u* and/or h. The dependency of CHon u* and h is so small that it is possible to consider CHas a constant for practical purposes: CH, 1= 2.0 × 10−3 for a reference height of 1 m. The bulk transfer coefficient for water vapor is estimated at CE, 1= 2.1 × 10−3 for a reference height of 1 m.

Atmospheric transport modelling in support of CTBT verification—overview and basic concepts

Abstract Under the provisions of the Comprehensive Nuclear-Test-Ban Treaty (CTBT), a global monitoring system comprising different verification technologies is currently being set up. The network will include 80 radionuclide (RN) stations distributed all over the globe that measure treaty-relevant radioactive species. While the seismic subsystem cannot distinguish between chemical and nuclear explosions, RN monitoring would provide the “smoking gun” of a possible treaty violation. Atmospheric transport modelling (ATM) will be an integral part of CTBT verification, since it provides a geo-temporal location capability for the RN technology. In this paper, the basic concept for the future ATM software system to be installed at the International Data Centre is laid out. The system is based on the operational computation of multi-dimensional source–receptor sensitivity fields for all RN samples by means of adjoint tracer transport modelling. While the source–receptor matrix methodology has already been applied in the past, the system that we suggest will be unique and unprecedented, since it is global, real-time and aims at uncovering source scenarios that are compatible with measurements. Furthermore, it has to deal with source dilution ratios that are by orders of magnitude larger than in typical transport model applications. This new verification software will need continuous scientific attention, and may well provide a prototype system for future applications in areas of environmental monitoring, emergency response and verification of other international agreements and treaties.

A one-dimensional dynamical soil–atmosphere tritiated water transport model

Abstract A one-dimensional numerical model for simulating transport of heat, water and tritiated water (hereafter referred to as HTO) in unsaturated surface soil and their soil–atmosphere exchange has been developed. This model consists of five prognostic equations for soil temperature, soil water content, soil air humidity and HTO concentrations in liquid and gas phases. The model is applied to a hypothetical and simplified scenario of HTO contamination of an unsaturated soil layer with actually observed meteorological conditions in Japan to analyze the dependencies of the HTO transport and evaporation on the meteorological conditions, hydrological conditions and soil properties. Results obtained indicate that the infiltration of water and the soil moisture conditions largely affect the transport and evaporation. Therefore, precipitation and soil moisture characteristics seem to be the most essential factors. Other meteorological factors are found to have substantial effects through two pathways, one being direct and fast by changing the atmospheric conditions and availability of vapor at the ground surface, and the other being indirect and slow by affecting the infiltration and water content. It can also be demonstrated that the usage of time-averaged meteorological and hydrological conditions reduce the estimated amounts of HTO transport and evaporation considerably, and that high-concentration episodes virtually determine the annual mean HTO concentration and evaporation.

Measurement of snow surface emissivity

Measurements of the snow surface emissivity for infrared radiation have been made with an infrared radiation thermometer. The mean value of the emissivity is estimated at ɛ = 0.970 ± 0.008.

Estimation of release rate of iodine-131 and cesium-137 from the Fukushima Daiichi nuclear power plant

The atmospheric release rates of I-131 and Cs-137 from the Fukushima Daiichi nuclear power plant in March 2011 were estimated by comparing environmental monitoring data of air concentration and deposition rate on a regional scale with calculated values from an atmospheric dispersion model. Although the release rates were not estimated for all days after 11 March, because of lack of monitoring data, temporal changes in the release rates were reasonably estimated with estimated uncertainties in a factor of 3.3 and 2.9 for I-131 and Cs-137, respectively. A large release was estimated from the night of 14 March to at least the afternoon of 15 March, with maximum values of 7.2 × 1015 Bq h−1 for I-131 and 1.5 × 1014 Bq h−1 for Cs-137. The release rates during other periods were estimated at one- to two-orders of magnitude smaller than the largest release rate on 15 March. Uncertainty in the estimated release rate for 15 and 20 March was larger than for other periods. The significant release during 14 and 15 March and the trend of the release rate by the end of March were consistent with previous reports. This agreement, despite using different datasets, shows robustness of the temporal changes estimated in the studies.

Estimation of global radon exhalation rate distribution

A radon exhalation rate distribution model was developed, which considers dependency on soil water saturation, soil temperature and soil texture. The global averages of the radon exhalation rates were calculated to be 17.5 and 18.0 mBq⋅m−2⋅s−1 by the model with different input soil moisture data. These values were slightly smaller than the widely‐accepted value. Although the calculated radon exhalation rates were smaller than the measured ones in the East Asia, the estimated latitudinal distributions of the radon exhalation rate agreed fairly well with that reported by Conen et al. (2002).

Atmospheric deposition of 7Be, 40K, 137Cs and 210Pb during 1993-2001 at Tokai-mura, Japan

To evaluate the radionuclide migration from the atmosphere to the ground surface, atmospheric deposition samples were collected from 1993 to 2001 with a basin set up at Tokai-mura, Japan. Monthly samples were evaporated to dryness to obtain residual samples and measured with a Ge detector for 7Be, 40K, 137Cs and 210Pb. According to the analysis, clear seasonal variations with spring peaks of deposition weight and deposition amounts of all the radionuclides were found. The analysis also showed that these radinuclides can be divided into two groups, each having different carrier particle sizes and, hence, different deposition processes.

Suppression of Radon Exhalation from Soil by Covering with Clay-mixed Soil

The effectiveness of bentonite-mixed soil as a barrier to radon exhalation from soil is investigated experimentally at a closed uranium mine in Japan. Continuous observations of radon flux from a clay-covered soil surface and a bare soil surface were conducted for a period of 80 days. The mean radon flux from the clay-covered plot was one fifth of that from the bare plot, demonstrating that the clay-mixed soil is effective as a boundary suppressing radon exhalation from the contaminated soil. Numerical analyses using a one-dimensional radon transport model for an unsaturated soil for the dry weather conditions confirmed the effectiveness of the clay-mixed soil cover under a range of weather conditions.