Recommendations for Revision of Regulatory Guide 1.78 (NUREG/CR-6624)

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Publication Information

Manuscript Completed: October 1999
Date Published: November 1999

Prepared by:
L.B. Sasser, P.M. Daling, P. Pelto, M.Yurconic

Pacific Northwest National Laboratory
Richland, WA 99352

S. Basu, NRC Project Manager

Prepared for:
Division of Systems Analysis and Regulatory Effectiveness
Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001

NRC Job Code W6246

Availability Notice


To ensure safe operation of commercial nuclear power plants, control room operators must be protected from dangers arising from possible exposure to hazardous chemicals that may be discharged as a result of equipment failure, operator errors, or events external to plant operation. Conditions must exist where accidental exposure to such materials still allows the operators to operate the plant safely. Protective emergency limits should be based on levels that will allow operators to function while fresh-air mask and protective clothing are donned (two-minute limit), and for up to eight hours afterward if the toxic material is not eliminated. Regulatory Guide 1.78 provides toxicity limits for 27 example chemicals used in or near reactor control rooms. This document needs to be updated and expanded to include more chemicals. This project was initiated to provide updated 2-minute limits based on the Immediately Dangerous to Life or Health (IDLH) values established by the National Institute for Occupational Safety and Health (NIOSH) as operator response limits in for Regulatory Guide 1.78.

A review of the 1994 revised NIOSH IDLH concentrations was conducted for the purpose of using the IDLH to replace and expand the toxicity limits in NRC Regulatory Guide 1.78. A list of IDLH values was provided for chemicals listed in the 1994 draft of the (NIOSH) documentation for IDLH concentrations. It was concluded that the IDLH values represent reasonable limits to provide adequate time to don protective apparel and will provide an adequate margin of safety for protecting the operators. In general, the revised NIOSH IDLH values are recommended for replacing the chemical toxicity limits in Regulatory Guide 1.78. Where these values were determined to be inadequate, values from other sources were recommended for some chemicals.

A review of more recent transportation accident statistics was conducted to determine if the definitions of frequent shipments in Regulatory Guide 1.78 are still valid. It is recommended that the current definitions of frequent shipments be retained as screening criteria to determine the hazardous chemicals that must be considered in evaluating the habitability of control rooms during postulated hazardous chemical releases. However, a clarification in the Regulatory Guide 1.78 language should be provided to indicate that the criteria refer to total shipments irrespective of the nature of chemicals. The technical basis for this conclusion is described in this report.

A significant amount of research has been conducted that improves the meteorological and ventilation flow models presented in Regulatory Guide 1.78. This research has resulted in development of a modular control room habitability evaluation software package name HABIT. Of most interest to Regulatory Guide 1.78 is a HABIT module called EXTRAN that calculates atmospheric concentrations of radioactive and toxic chemical materials that would result from a release event. EXTRAN represents an improvement in technology relative to the Regulatory Guide 1.78 atmospheric dispersion model as it combines procedures for estimating the amount of airbome material, a Gaussian puff model, and the most recent building wake diffusion coefficient algorithms. Consequently, it is recommended that Regulatory Positions C.5 and C.6 as well as Appendix B of Regulatory Guide 1.78 be revised to incorporate these improvements in meteorological and ventilation flow models.

The values and impacts associated with the revision of Regulatory Guide 1.78 have been addressed in a primarily qualitative manner. Any increase in industry costs associated with the revisions are estimated to be offset by potential for cost savings that could result from a decrease in the requirements for control room habitability systems as a result of revising the toxicity limits. The proposed revision also represents an improvement of knowledge as the revision incorporates updated toxicity limits and a more comprehensive list of hazardous chemicals. An increase in regulatory efficiency is an important attribute for this proposed regulatory action. It is as such noted but not quantified.

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