Development of monitor for multiple beta-ray nuclides in liquid radioactive waste

Recently, the safety management of radioactive isotopes has been emphasized by the increase of radioactive isotope application in the area of medical, industrial and scientific research along with industrialization. Most radionuclides used in the medical diagnosis and scientific research are beta-ray emitting nuclides. Not only the careful handing of the radionuclides in the nuclear facilities but also the management of radioactive liquid waste from medical utility is very importance in a viewpoint of the conservation of environment. The currently used method to monitor the radioactive waste, however, is impractical in the aspect of continuous monitoring the waste matter as well as the preparation of measuring sample is very annoying. In addition, when sample is mixed with several beta-ray nuclides individual nuclide should be separated chemically and then analyzed using different analysis procedure. Accordingly, the development of a new monitoring system to overcome these disadvantages is necessary. A new liquid scintillator system for measurement of multiple beta-labeled mixtures was developed and its characteristic was investigated. The signal processing system consisted of two photomultiplier tubes and the coincidence count circuit which showed 99% background rejection rate. An algorithm using least square method was developed for simultaneous radioassay of multiple beta-ray nuclides. The characteristic of the system was analyzed using 4 beta-labeled samples(3H, 14C, 36Cl and 90Sr). The analytical values were in good agreement with the reference values within 7% relative error. The detection limits achieved for all those nuclide exception for 90Sr was below the detection level of government regulation with a few minutes measurement. For 90Sr, the detection limits was satisfied by the elongation of measurement time rather long 20 minutes as well as binning method. An automatic sample mixer system to mix the liquid radioactive waste with the liquid scintillator was designed and manufactured to realize continuous monitoring. The mechanical systems were consisted of the following 4 stages: automatic sample transfer, capping and uncapping the sample bottle, sample preparation system and its attachment to the detector system. This monitoring system could be on- line operated through RS-232 communication protocol and the integrated operating software installed in main control computer