Environ. Sci. Technol. 2001, 35, 4174-4179
Determination of Uranium Isotopes in Soil Core Samples Collected on the JCO Grounds after the Criticality Accident SATOSHI YOSHIDA,* YASUYUKI MURAMATSU, AND KEIKO TAGAMI Environmental and Toxicological Sciences Research Group, National Institute of Radiological Sciences, Anagawa 4-9-1, Inage-ku, Chiba-shi, Chiba 263-8555, Japan
To evaluate the impact of the 1999 criticality accident in Tokai-mura on the U isotope composition in soils, U isotopes (235U and 238U) were determined with inductively coupled plasma-mass spectrometry (ICP-MS) for soil core samples collected on the JCO grounds after the accident. The 235U/ 238U ratios were higher than the natural ratio in most samples. The highest ratio observed was 0.0262. Although vertical profiles of the 235U/238U ratio differed among the soil cores, the ratios tended to be high at the surface and decreased with depth. The U concentration also changed with depth. The percentages of 235U in the excess U, estimated from the positive correlation between U concentration and the 235U/238U ratio in soil samples, were less than 4% by mass (mostly 1-3%) and were much lower than the enrichment of the U used in the uranium conversion building at the time of the criticality accident (18.8%). These findings indicate that enriched U had been released before the criticality accident during the U processing at JCO in connection with the reconversion of light water reactor fuel. Since the range of the U concentrations found was comparable to the range of uncontaminated Japanese surface soils, the amount of U added to the soil was judged negligible from a radiation protection viewpoint.
Introduction Determination of isotopic composition gives useful information on the source term and the behavior of radionuclides in the environment (e.g. refs 1-3). Naturally occurring 235U and 238U are long-lived isotopes (t1/2 ) 7.04 × 108 y and 4.47 × 109 y, respectively) formed in a primary stellar nuclear synthesis process. The minor difference in mass between 235U and 238U precludes significant isotopic fractionation effects by natural processes, and the 235U/238U abundance ratio is the same everywhere, 0.00725 in atom ratio or 0.045 in alpha activity ratio, regardless of sampling location, rock type, and degree of weathering (4). Hence any deviation from the natural ratio attests to a soil affected by human activity (2), with the well-known exception of the Oklo reactor. Alpha spectrometry is frequently used for the determination of U isotopes (e.g. refs 1 and 2). This method is, however, time-consuming for the 235U and 238U measurements, since these nuclides are extremely long-lived. Mass * Corresponding author phone: +81-43-206-3255; fax: +81-43206-3267; e-mail:
[email protected]. 4174
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spectrometric techniques are suitable for the ultratrace isotopic analysis of U. In recent years, quadrupole inductively coupled plasma-mass spectrometry (Q-ICP-MS) (e.g. refs 5 and 6) and high-resolution ICP-MS (HR-ICP-MS) (e.g. ref 7) have been increasingly applied for the analyses of long-lived radionuclides including U isotopes. These methods are relatively low cost and practical enough for U isotope measurements in environmental samples, although the precision of the isotope ratios is poorer compared with thermal ionization mass spectrometry and multicollector ICPMS (8). Since enriched U (18.8% of 235U by mass) was used in the uranium conversion building of JCO Co., Ltd., Tokai-mura, Ibaraki Prefecture, Japan, on the day of the criticality accident (September 30, 1999), determination of U isotopes (235U and 238U) should be useful in assessing the possible contamination of U. There are several possible sources of enriched U on the JCO grounds and enrichment of U differs depending on the intended user and time of use. The main product of JCO was reconverted light water reactor fuel for conventional nuclear power reactors (both BWR and PWR). For this purpose, a total of 715 ton/year of enriched U (
