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21 November 2009

Answer to Question #2801 Submitted to "Ask the Experts"

Category: Nuclear Power — Nuclear Energy

The following question was answered by an expert in the appropriate field:

If a worker has to work near or on heavy water pipes (CANDU Reactors), is it safe to work without lead shields over pipes? What is the safe clearance? Should extremity chips be worn? What are the levels before it is not safe (in millirems)?

A CANDU (Canada Deuterium Uranium) Reactor has moderator and heat transport (HT) systems that contain heavy water. Heavy water in a moderator system thermalizes neutrons. The HT system heavy water is a coolant. The moderator water has high tritium concentrations of about 10 Ci/kg compared to about 1 Ci/kg in the HT system. Tritium in the system does not contribute to gamma hazard as it is beta emitter only. The gamma dose rate depends upon the reactor power conditions. Let's address the gamma hazard during shut condition. The majority of the moderator and HT system pipes and equipment are inside the containment areas which are not readily accessible. During shut down, personnel work in the containment. The lead blankets or shielding are installed as recommended by health physics. Not all equipment and pipes are shielded. Any power entries are of short duration. The dose rate depends upon the nature of components and it varies a lot from location to location.

As a general rule within a system, extremity TLDs are required when working on an open HT system. The need for extremity TLDs are specified on the exposure permit for the job. The exposure permit is prepared by ALARA section staff.

To reduce dose exposure to staff, pipes and equipment are shielded by lead blankets on a case-by-case basis. This is specified by the ALARA health physicist. There are situations where personnel can walk around equipment and pipes that don't have lead blankets.

Jag K. Mohindra, CHP

Answer posted on 21 November 2003. The information and material posted on this Web site is intended as general reference information only. Specific facts and circumstances may alter the concepts and applications of materials and information described herein. The information provided is not a substitute for professional advice and should not be relied upon in the absence of such professional advice specific to whatever facts and circumstances are presented in any given situation. Answers are correct at the time they are posted on the Web site. Be advised that over time, some requirements could change, new data could be made available, or Internet links could change. For answers that have been posted for several months or longer, please check the current status of the posted information prior to using the responses for specific applications.