Heavy water concentration measurement in air

The heavy water in PHWRs flows at high temperature and pressure, hence leaks in the heat transport system are not uncommon. The loss of heavy water due to such leaks can lead to spreading of radioactivity and it also contributes to operating cost of the nuclear reactor. It is advantageous to detect small leaks, because if remains undetected, they may develop into a severe leak, which may lead to reactor shutdown. None of the sensors which are currently in use can meet all the requirement of high sensitivity, and real time measurement which is free from interference from other gamma emitters. A trace HDO detector has been developed using OA-ICOS technique which is based on laser absorption spectroscopy. In this technique high sensitivity is achieved by trapping the light in an optical cavity. Here, the mode noise associated with the optical cavity is the major source of noise. This has been successfully overcome by using cavity length dithering. The demonstrated setup has achieved the sensitivity of 45 ppbv in air with averaging. This sensitivity in HDO concentration measurement in air is sufficient for the application in PHWR heavy water leak detection system. (author)