Development of post accident radiation survey instrument

Based on the existing reactor safety study scenarios, associated source term releases and the three nuclear accidents that occurred in the past (Windscale, TMI and Chernobyl), 131I and 137Cs have been identified as the radionuclides of major concern with respect to potential dose. Rapid measurement techniques, that use indigenous instruments for assessing deposited activity during an accidental release, have been evolved to facilitate quick decisions on subsequent protective measures. However, conventional radiation survey instruments give only the gross activity/dose rate without any energy, discrimination possible. It would be more appropriate and meaningful while planning the counter-measures, if the contributions due to 131I and 137Cs can separately be measured. With this objective in mind, a novel portable radiation survey instrument has been developed for the first time in India with the additional features such as inclusion of energy discrimination. This paper reports the development of such an instrument. The instrument uses conventional PM tube along with low power hybrid circuits based miniature discriminator. The versatility of the post accident monitor has further been improved by providing five ranges of energy discrimination possible. With this instrument, it is possible to measure gross activity below and above 500 keV as well as individual discriminator widows for 131I, 137Cs and 60Co. The system has a simple analog meter display. The use of miniature electronics has ensured low power consumption while enhancing the reliability of the system. (author)