On June 23, 2012, Rajasthan-5 was under planned biennial shutdown. Work was in progress to weld a piping connection for making alternate provision for water addition to the moderator system. During the work, tritium concentration in the work area increased when an opening was made in the moderator cover gas line. Two of the contract workers involved in the job got internal exposure of tritium equivalent to radiation dose of 24.32 mSv and 16.54 mSv. The radiation dose to the workers though less than the statutory annual dose limit of 30 mSv for workers, exceeded the dose constraints.
Rajasthan-5 is Pressurized Heavy Water Reactor (PHWR) based nuclear power plant. PHWRs use Heavy Water as coolant and moderator. Tritium activity in Heavy Water systems increases over a period of time. Tritium is a soft beta emitter and can cause internal exposure once it enters the body. Tritium gets removed from the body through natural processes.