Development of an analytical method for radioactive cesium in water containing suspended solids using gellant

After the accident at the Fukushima Daiichi Nuclear Power Station on 2011 March 11, the levels of radioactive cesium isotopes in environment media have been widely monitored. For determining the levels of radioactive cesium (134Cs and 137Cs) in water samples, they are generally put in a 2 L Marinelli beaker and analyzed by a germanium semiconductor detector (abbreviated as "direct method" below). However, when the concentration of suspended solids in water is high, there is a possibility that any sedimentation of the suspended solids may change the geometry of the water in the 2 L Marinelli beaker during the measurement time; therefore, an exact analysis value may not be provided. In this research, we developed an analytical method for determining radioactive cesium in water containing suspended solids using gellant in order to overcome any geometry problem. We first evaluated the effect of any sedimentation of suspended solids on measurements of radioactive cesium isotopes in water by a "direct method". After that, we examined the applicability of the "gelling method" developed in this study, and a traditional "filtration method" widely used as an analytical method to compare with the results obtained by the direct method. Firstly, in the direct method, when the concentrations of suspended solid in water was over 100 mg L-1, the results of the measured concentrations of each radioactive cesium were more than 120% of the calculated concentration. From this result, it was confirmed that the measured concentrations of radioactive cesium isotopes in this method tend to be overestimated. In the gelling method, the errors of the measured concentrations of radioactive cesium isotopes in highly suspended solids (100 - 10000 mg L-1) were less than 15%. On the other hand, in the filtration method, the concentration of suspended solids was applicable about 100 mg L-1; however, the result will be applicable only to the concentrations of radioactive cesium isotopes (particulate form). This result indicated that the gelling method is very suitable for measuring the concentrations of radioactive cesium isotopes in suspended water. (author)