Answer to Question #1202 Submitted to "Ask the Experts"

Q

1. What are the relevant pathways of exposure which are to be considered while evaluating the radiation dose to the occupants of the control room under a design basis accident in a nuclear power plant? 2. Is there any permissible limit of the exposures under such situations? 3. Is there any ready-made computer code available for this?

A

Answer to Question 1: There are several: (1) exposure due to inhalation of contaminated air inside the control room due to infiltration/ventilation from the plume or from adjacent buildings, (2) external exposure due to shine from contaminated air inside control room due to infiltration/ventilation from the plume or from adjacent buildings, (3) radiation shine from release cloud outside of the control room envelope, (4) radiation shine from radioactive material in the reactor containment, and (5) radiation shine from radioactive materials in systems or components inside or outside the control room envelope, for example, recirculation filters. The significance of the different pathways depends greatly on the plant design and operation, for example, amount of infiltration, location of control room relative to other buildings. (See Regulatory Guide 1.183 section 4.2 and NUREG-0800 Section 6.4.)

Answer to Question 2: The term "permissible exposure" is inappropriate. There are criteria that apply to the design of the control room. The facility is designed to withstand certain design basis accidents. The radiation exposure of personnel during the accident are controlled under the occupational worker limits of 10 CFR 20. That said, 10 CFR 20 does not limit actions that may be necessary to protect health and safety. Licensees are required to include provisions for emergency worker exposures during emergencies. These are required to be based on the Environmental Protection Agency Emergency Worker and Livesaving Activity Protective Action Guides. It should be noted that the Nuclear Regulatory Commission (NRC) credits any such exposures against the individual's occupational exposure history. 10 CFR 50 Appendix A General Design Criterion 19 provides: "Adequate radiation protection shall be provided to permit access and occupancy of the control room under accident conditions without personnel receiving radiation exposures in excess of 5 rem whole body, or its equivalent to any part of the body, for the duration of the accident." NUREG-0800 Section 6.4 interprets "equivalent" as 30 rem thyroid and 30 rem skin. For facilities that have implemented an alternative source term under 10 CFR 50.67, the dose criterion is 5 rem total effective dose equivalent.
Answer to Question 3: The NRC-sponsored RADTRAD code (NUREG/CR-6604) and HABIT codes (NUREG/CR-6210) can be used to assess the dose from exposure to contaminated air inside the control room. Modules within the SCALE software suite (NUREG/CR-0200) can be used for the radiation shine doses. There are other proprietary codes used by licensees and vendors to make these assessments.

Alan Roecklein
Nuclear Regulatory Commission