Determination Of The Specific Activity Of Organically Bound Tritium In Environmental Samples

Nowadays, under the pressure of growing demand for energy and environmental protection, nuclear power, as a kind of clean energy, has been developed rapidly worldwide. However, it can also involve a series of problems about radioactive contamination. The production, release and protection of tritium is one of the issues. Especially with the development of Heavy Water Reactor(HWR) and the developing Molten Salt Reactor(MSR), the problems about tritium is more serious. Recent years, with the deepening research of environmental tritium,the measurement of organically bound tritium(OBT) and its behavior in environment have become the focus and difficulty for the research on tritium. The main work of this research is to develop a combustion apparatus successfully. And taking the OBT in soil around Qinshan Nuclear Power Plant as the research object, the distribution characteristic of OBT in soil was preliminary determined, in order to provide support and reference for the research of OBT formation mechanism and the transfer-transformation behavior of tritium in soil.Currently existing OBT combustion apparatus on the market are not suitable for the measurement of low-level OBT due to the vigorous burning of sample and low recovery. To measure the concentration of OBT more accurately we made three improvements on our new combustion apparatus: 1) In combustion region, a gas flow direction controller was designed to prevent the incomplete combustion product from flowing back. 2) The catalytic region was developed using a tube-in-tube structure. This new design makes it possible to allow gas inlet from two ends and also makes it convenient to use a spherical noble metal catalyst, which lead to a higher catalytic oxidation efficiency. 3)A cold trap was used to collect the HTO produced by combustion, making the apparatus more suitable for the measurement of low-level OBT.Using the successfully developed OBT combustion apparatus, the heating curves for OBT in lettuce, pork, milk and fish samples were determined. The method involved measuring the quick pyrolysis temperature range by thermogravimetric analysis(TG), and then choosing the optimized heating curves through different heating rates experiments. The experiments results indicated that the recoveries of OBT in lettuce, pork, milk and fish were 94.5±1.1%, 93.2±1.5%, 93.0%±1.3% and 92.9%±1.1%, respectively. Through the analysis of ashes and exhaust produced by the combustion of lettuce and pork, it was found that 2.1% and <0.2% of the hydrogen in the lettuce ashes and pork ashes were not oxidized to HTO, respectively. There was small amount of methane in the exhaust gases both in the lettuce and pork combustion experiments.Moreover a small amount of other organic gas was also found, but their concentration should be much lower than methane. According to the calculation, there was <0.1% and <0.2% of hydrogen in lettuce exhaust and pork exhaust, respectively.Taking the soil around Qinshan Nuclear Power Plant as example, the concentrations of OBT in the soil with particle size of >250μm, 53-250μm and 20-53μm at depth of 0-25 cm were measured. The experiment results showed that the OBT activities in soil ranged from 6.554±4.407Bq/L to 119.403±26.194Bq/L and the OBT in soil showed particle size, depth and space distribution characteristic. In most of the cases, the OBT concentrations in 53-250μm were the highest in deep soil(5-25cm), and the OBT concentrations in surface soil(0-5cm) were significantly lower than that in other soil layers. By comparing the OBT concentrations in soil samples collected from different sites, we found that the operation of NPP could affect the space distribution of OBT in some degree.Based on the correlation analysis between the OBT activity and organic content and the OBT/HTO ratios, present research preliminarily analyzed the source of OBT in soil. The results demonstrated that there was no direct relationship between OBT and total organic content. The OBT/HTO ratios were similar in the soil with particle size of>53-250μm and 20-53μm, with values of 1.36 ± 0.19 and 1.40 ± 0.14, respectively, which were significantly higher than the ratio in >250μm soil, with a value of 1.17 ± 0.18. OBT/HTO could imply the ability of organism to convert HTO to OBT in some degree, so the results suggested the existing of some living organisms in the soil with particle size of 53-250μm and 20-53μm. Since various microorganisms mostly exist in the soil with particle size of 53-250μm and 20-53μm, it can be preliminarily deduced that the difference of OBT/HTO ratio in different particle size soils may be correlated with the distribution of microorganisms.