NUREG/CR-6624

Recommendations for 

Revision of 

Regulatory Guide 1.7ý 

Pacific Northwest National Laboratory 

U.S. Nuclear Regulatory Commission 

Office of Nuclear Regulatory Research 

Washington, DC 20555-0001

)
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NUREG/CR-6624

Recommendations for 

Revision of 

Regulatory Guide 1.78 

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 W6308
Abstract

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.

iii NUREG/CR-6624
Table of Contents 

Page 

A b stra c t ................................................................................................................................ iii 

T ab le o f C o n te n ts ................................................................................................................. v 

A b b rev iatio n s ......................................................................................................................... v ii 

1.0 In tro d u c tio n ................................................................................................................. 1 

2.0 Evaluation of NIOSH IDLH Values for Assessing Control Room Habitability ....................... 2 

2 .1 B ac k g ro u n d ............................................................................................................. 2 

2.2 Current NIOSH Use of IDLH Standards .................................................................... 2 

2.3 NIOSH Documentation Process ............................................................................ . 3 

2.4 Other Existing Emergency Exposure Guidelines and Standards .................................. .. 4 

2.5 Odor Thresholds ........................................................................................ ........ . 5 

2.6 Discussion and Recommendations ........................................................................ . 5 

2,7 Conclusions ..................................................................................................... 7 

3.0 Evaluation of Frequent Shipment Screening Criteria ......................................................... ... 8 

3.1 Introduction and Summary .................................................................................. 8 

3.2 Review of Transportation Accident Data ................................................................. 8 

3.2.1 W ASH-1238 ...................................................................................... 8 

3.2.2 NUREG-0170 ..................................................................................... . 9 

3.2.3 SLA-74-0001 .................................................................................. ..... 10 

3.2.4 NUREG/CR-4829 ................................................................................. 10 

3.2.5 PNL Risk Studies ............................................................................... 11 

3.2.6 FEM A/DOT/EPA Handbook ...................................................................... 12 

3.2.7 Department of Transportation-Bureau of Transportation Statistics ................ . 13 

3.3 Definition of Frequent Shipments ............................................................................ 15 

3.4 Recommended Revisions to Regulatory Guide 1.78 ..................................................... 17 

4.0 Revisions of M eteorological and Flow M odel ..................................................................... 18 

4.1 Introduction ...................................................................................................... 18 

4.2 Description of Revised M odels ............................................................................. 18 

4.3 Recommended Revisions to Regulatory Guide 1.78 ................................................ . 18 

5.0 Value/Impact Assessment for Proposed Changes to Regulatory Guidel.78 .............................. 23 

5.1 Description of Regulatory Action and Alternatives ..................................................... 23 

5.2 Identification of Attributes ................................................................................... 23 

5.3 Quantification of Attributes ................................................................................. 24 

5.3.1 Public Health (Accident) ........................................................................ 24 

5.3.2 Occupational Health (Accident) ............................................................... 26 

5.3.3 Offsite Property ................................................................................... 27 

5.3.4 Onsite Property .................................................................................... 27

V NUREG/CR-6624
5.3.5 Industry Implementation ........................................................................... 27 

5.3.6 Industry O peration .................................................................................. 28 

5.3.7 NRC Implementation ............................................................................. 28 

5.3.8 N R C O peration .................................................................................... 29 

5.3.9 Improvements in Knowledge ................................................................... 29 

5.3.10 Regulatory Efficiency .............................................................................. 29 

6.0 Recommended Changes to Regulatory Guide 1.78 ............................................................. 30 

6.1 Use of IDLH in Regulatory Guide 1.78 .................................................................... 30 

6.1.1 T able C -1 .................................................................................. . . . ......... 30 

6.2 Recommended Transportation Accident Revisions to Regulatory Guide 1.78 .................... 30 

6.2.1 T able C -2 .............................................................................................. 30 

6.3 C om puter C odes ................................................................................................. 31 

7 .0 R e fe re n c e s ..................................................................................................................... 3 3 

Appendix A: Revised IDLH Values Established By NIOSH ......................................................... 36 

Appendix B: AIHA Odor Detection Thresholds and NIOSH IDLH Values ....................................... 48 

Figures 

3.1 Motor Vehicle Accident Rate Trend From 1990 To 1996 .............................................. 14 

3.2 Trend In Rail Accident Rates, 1970 To 1996 .............................................................. 14 

Tables 

2.1 Comparison of ERPGs and IDLHs for Selected Chemicals ..................................... .... 6 

3.1 Summary of Transportation Accident Statistics ........................................................ 9 

3.2 Summary of FSAR Application of Regulatory Guide 1.78 

Shipm ent Frequency C riteria ............................................................................... 16 

5.1 Identification of Affected Decision Attributes ........................................................... 24 

5.2 Summary of Value-Impact Assessment ..................................................................... 25

NUREG/CR-6624 vi
Abbreviations

ACGIH 

AEC 

AIHA 

CDF 

CHEM 

DOT 

EEGL 

EPA 

ERPG 

EXTRAN 

FEMA 

FHA 

FRA 

HABIT 

IDLH 

LEL 

m 

mg 

NAS 

NIOSH 

NRC 

NUREG/CR 

OSHA 

PEL 

PNNL 

ppm 

RY 

REL 

SAR 

SCP 

SCBA 

TLV 

VIA

NUREG/CR-6624

American Conference of Governmental Industrial Hygienist 

Atomic Energy Commission 

American Industrial Hygiene Association 

core damage frequency 

Computer code of toxic chemicals in control room 

Department of Transportation 

Emergency Exposure Guideline Level 

Environmental Protection Agency 

Emergency Response Planning Guideline 

Computer codes for estimating concentrations of substances at control room air intakes 

Federal Emergency Management Agency 

Federal Highway Administration 

Federal Railroad Administration 

Computer codes for evaluation of control room habitability 

Immediately dangerous to life or health 

lower explosive limit 

meter 

milligram 

National Academy of Sciences 

National Institute of Occupational Safety and Health 

Nuclear Regulatory Commission 

Nuclear Regulatory Commission Contractor Report 

Occupational Safety and Health Administration 

permissible exposure level 

Pacific Northwest National Laboratory 

parts per million 

reactor year 

recommended exposure limit 

Safety Analysis Report 

Standards Completion Program 

self-contained breathing apparatus 

threshold limit values 

value-impact assessment

vii
1.0 Introduction 

Criterion 4 of Appendix A, "General Design Criteria for Nuclear Power Plants," to 10 CRF Part 50, "Licensing of 

Production and Utilization Pacilities" requires that ". . . structures, systems and components important to safety be 

designed to accommodate the effects of and to be compatible with the environmental conditions associated with 

operation maintenance, testing and postulated accidents." Criterion 19, "Control Room," requires that a control 

room be provided from which actions can be taken to operate the nuclear power unit safely under normal conditions, 

and to either maintain or shut down the reactor safely under accident conditions. 

Control room operators could be exposed to high levels of hazardous chemicals that may be discharged as a result of 

equipment failure, operator errors, or events external to plant operation. Under these circumstances, measures must 

be in place to allow them to continue to safely operate the plant. It is expected that trained operators could don 

protective apparel within 2 minutes. Thus, protective emergency limits should be based on levels that will allow 

operators to function during a 2-minute period while they put on respirators and protective clothing. 

Regulatory Guide 1.78, "Assumptions for Evaluating the Habitability of a Nuclear Power Plant Control Room 

During a Postulated Hazardous Chemical Release" AEC, 1974) and Regulatory Guide 1.95, "Protection of Nuclear 

Plant Operators Against and Accidental Chlorine Release" (NRC, 1977) identified 27 commonly encountered 

hazardous chemicals including chlorine and set 2-minute exposure concentration limits for these chemicals. The 2

minute limits in the Regulatory Guides are outdated for some hazardous chemicals. In addition, some NRC 

licensees have requested use of Immediately Dangerous to Life of Health (IDLH) concentrations set by the National 

Institute of Occupational Safety and Health (NIOSH) for the 2-minute limits. 

Much of the guidance in Regulatory Guide 1.78 and Regulatory Guide 1.95 are similar with regard to the 

requirements of nuclear power plants to operate safely under normal conditions and accident conditions. Combining 

these two documents in a revised Regulatory Guide 1.78 will likely result in a reduced burden to licensees without 

compromizing the safety of control room operators. 

The purpose of the work described in this report is to provide the technical basis to revise Regulatory Guide 1.78. 

This includes an update of toxicity limits, clarification of the definition of "frequent shipment", and a revision to 

atmospheric dispersion modeling. The report addresses the results of the four major tasks of the project as described 

below. 

1) The purpose of the first task (Section 2) was to update the toxicity limits of hazardous chemicals using 

IDLH values in the NIOSH "Pocket Guide to Chemical Hazards" and to evaluate the appropriateness of 

IDLH concentrations to replace existing toxicity limits in Regulatory Guide 1.78. 

2) The second task (Section 3) re-examined the basis for hazardous chemical shipment frequencies. 

Specifically, the objective was to examine more recent transportation accident statistics in order to 

provide a clarification of the term "frequent shipment". 

3) The objective of the third task (Section 4) was to provide a technical basis for recommending revisions of 

the meteorological and control room ventilation flow models for use in revising Regulatory Guide 1.78. 

4) Finally, a value/impact assessment (Section 5) was performed to provide technical evaluations of the 

benefits (values) and costs (impacts) of the proposed revision to Regulatory Guide 1.78. 

A summary of recommended revisions to Regulatory Guide 1.78 is provided is Section 6 based on the findings 

outlined in the task sections.

I NUREG/CR-6624
2.0 Evaluation of NIOSH IDLH Values for Assessing Control 

Room Habitability 

2.1 Background 

To ensure safe operation of commercial nuclear power plants, control room personnel (operators) must be protected 

from dangers arising from possible exposure to hazardous chemicals. It is imperative that exposures be less than 

those that would prevent them from safely operating the plant. It is expected that trained operators could put on 

protective apparel within 2 minutes. Thus, protective emergency limits should be based on exposure levels that will 

allow operators to function during a 2-minute period while respirators and protective clothing are donned. 

In Regulatory Guide 1.78 and Regulatory Guide 1.95, 27 hazardous chemicals were identified and 2-minute 

exposure concentration limits set for control rooms. Also, NUREG/CR-5669 provided 2-minute limits for five 

hazardous materials of special interest. The 2-minute limits in the Regulatory Guides are outdated for some 

hazardous chemicals. In addition, some Nuclear Regulatory Commission (NRC) licensees have requested use of the 

Immediately Dangerous to Life or Health (IDLH) concentrations established by the National Institute of 

Occupational Safety and Health (NIOSH) for the 2-minute limit. 

One objective of this task is to evaluate the revised NIOSH IDLH concentrations (Ludwig et al., 1994) for the 

purpose of using the IDLH to replace the toxicity limits in Regulatory Guide 1.78 (AEC, 1974). Another objective is to provide an updated list of toxicity limits of chemicals listed in NIOSH's "Documentation for Immediately 

Dangerous to Life or Health Concentrations (IDLH)" (Ludwig et al., 1994) and published in NIOSH's Pocket Guide 

to Chemical Hazards (NIOSH, 1997) for revising the current Regulatory Guide 1.78 (AEC, 1974). 

IDLH values, published regularly since 1981 by NIOSH in the updated versions "Pocket Guide to Chemical 

Hazards," (NIOSH, 1997) were originally determined for the purpose of respirator selection criteria as part of 

Standards Completion Program (SCP). This became the original basis for the NIOSH/Occupational Safety and 

Health Administration (OSHA) Occupational Health Guidelines for Chemical Hazards (NIOSH/OSHA 1981). The 

IDLH values were based on effects that might result from a 30-minute exposure, although this was not to imply that 

the worker should remain in an adverse work environment any longer than necessary. NIOSH originally defined 

IDLH concentrations as ".... the maximum concentration from which, in the event of respirator failure, one could 

escape within 30 minutes without a respirator and without experiencing any escape-impairing (e.g., severe eye 

irritation) or irreversible health effects" (NIOSH, 1997). The SCP IDLH concentrations were developed in the 

mid- 1970s and first published by NIOSH in 1981. 

There were several limitations to these values as originally developed. The IDLH values were developed from 

reviews of secondary literature without review of the original reports, documentation was not published for the 

individual IDLH values selected, and peer reviews of the work were never performed. The adequacy of SCP IDLH to 

protect the worker from acute exposure to some toxic compounds was seriously questioned (Alexeeff, et al., 1989). 

Because of these limitations and criticisms, NIOSH revised the SCP IDLH values and published them in the recent 

editions of the NIOSH Pocket Guide to Chemical Hazards (NIOSH, 1997). In addition, NIOSH published the 

documentation for the development of original SCP IDLH and the revised IDLH values (Ludwig et al., 1994). 

2.2 Current NIOSH Use of IDLH Standards 

Current NIOSH definition for an IDLH is any condition ".... that poses a threat of exposure to airborne 

contaminations when that exposure is likely to cause death or immediate or delayed permanent adverse health effects 

or prevent escape from such an environment" (Ludwig et al., 1994; NIOSH, 1997). The purpose for an IDLH is to 

ensure that a worker can escape from a given contaminated environment in the event of failure of the respiratory 

protection equipment. For entry into IDLH atmospheres, it is the intention of NIOSH that a self-contained breathing 

apparatus equipped with full face piece and operated in a pressure-demand or other positive-pressure mode would be 

selected. 

IDLH values influence the selection of respirators for non-emergency controlled conditions, when the occupational 

exposure limit is close to the IDLH. Under these conditions, selection of respiratory protection with lower levels of

NUREG/CR-6624 2
protection may be used, especially if monitoring data is available. However, respiratory protection that gives the 

highest level of protection must be used when the IDLH value is reached or exceeded. Respiratory protection which 

offers the highest level of protection is a full face piece with a positive pressure or pressure demand self-contained 

breathing apparatus (SCBA) or a supplied-air respirator with a full face piece operated either in a pressure demand or 

other pressure mode in combination with an auxiliary SCBA. 

NIOSH is currently evaluating more recent toxicological data for use in future IDLH recommendations. Thus the 

currently revised IDLH values could undergo further revision as this evaluation is completed. 

2.3 NIOSH Documentation Process 

Revised NIOSH IDLH documentation (Ludwig et al., 1994) used to support the revised IDLH values were evaluated 

for their appropriateness to replace and add to those in the current Regulatory Guide 1.78 for the 2-minute time 

required to don protective breathing apparatus in reactor control rooms. 

The NIOSH documentation for IDLH represents a compilation of the sources of information and the rationale for 

IDLH used by NIOSH in the original Standards Completion Program (SCP) in the early 1970s. This document 

also includes a review and revision of the original SCP IDLH for each chemical evaluated. Although the 

documentation for the IDLH is now published and the IDLH values have been revised, in many cases little change 

was made to the original SCP documentation on which the revised values were based. In order to evaluate the 

applicability of the revised values for the needs of the NRC, it is necessary to understand the purpose for which the 

IDLH were developed and the process and the criteria used by NIOSH to derive the revised IDLH. 

IDLH were originally established and are still intended by NIOSH to be used as one of several selection criteria in 

the NIOSH Respirator Decision Logic for the selection of respiratory protection equipment. The purpose of IDLH as 

stated by NIOSH is ". . . to ensure that workers can escape from a contaminated environment should respiratory 

protection equipment fail." At the onset, this definition more closely meets the NRC's intended use better than 

criteria used in the development of other exposure limits such as threshold limit values (TLV), permissible exposure 

level (PEL), Emergency Response Planning Guidelines (ERPG), Emergency Exposure Guideline Level (EEGL), etc. 

The toxicity criteria for determining the revised IDLH used the original SCP IDLH and applied newer methodology 

outlined by NIOSH. This approach followed a hierarchy such that acute human data, if available, were considered 

first, followed by acute animal inhalation data, then finally acute animal oral toxicity data. When acute data were 

lacking, chronic data, although not directly applicable to emergency exposures, were used. In some cases, the IDLH 

concentrations were based on surrogate chemicals. Secondary references were the primary source of the toxicological 

data. After a revised IDLH value was developed, the value was compared to other existing guidelines or exposure 

limits for consensus. Other criteria were that the revised IDLH could not be set greater than 2000 times the NIOSH 

Recommended Exposure Limit (REL) or OSHA PEL, and they not be greater than the original SCP IDLH. Finally, 

making this documentation available to the scientific community gives more validity to the IDLH values. 

Some of the same criticisms pertain to the process for establishing the revised IDLH values as for the original SCP 

process. The actual documentation supporting the revised IDLH was performed with less rigor than, for example, the 

documentation required for developing other short-term exposure guidelines, such as the American Industrial 

Hygiene Association's (AIHA). NIOSH relied heavily on secondary references rather than including primary 

references. The number of supporting documents for many of the NIOSH IDLH is typically small and typically fairly 

old, indicating that a recent, exhaustive literature search may not have been performed in all cases. 

Not all IDLH values were derived directly from the toxicological data, as NIOSH developed a "preliminary" IDLH 

and then, in some cases, applied a safety factor of at least 10 to derive the revised IDLH. Thus NIOSH used criteria 

other than toxicity to derive a portion of the IDLH. As an example, NIOSH a-priori made the decision to limit the 

revised IDLH for flammable gases to no greater than 10 percent of the lower explosive limit (LEL), typically a 

percentage concentration in air. 

One of the factors potentially used to modify the preliminary IDLH was an indicator of severe respiratory irritation. 

While this may be an important factor for escape, it may result in a revised IDLH not representative of a life 

threatening situation.

3 3NUREG/CR-6624
The revised IDLH are typically much more conservative than the original SCP IDLH. This conservatism was 

obviously intentional on NIOSH's part and is the result of several factors: 1) the lack of acute toxicity data in 

humans, 2) the use of safety factors (typically a factor of 10), 3) the use of criteria other than toxicity (such as data 

from surrogate chemicals), and 4) a more conservative interpretation of the toxicity data. In particular, the revised 

IDLH for the flammable liquids and gases are dramatically lower than those values for the SCP IDLH. The 

industrial standard for restricted entry into an "explosive" atmosphere was, for decades, 25 percent of the LEL. 

Because of a new OSHA regulation, the revised NIOSH IDLH are set at 10 percent of the LEL (typically thousands 

of ppm in air) (Ludwig et al., 1994; NIOSH, 1997). 

Despite the conservatism of the revised IDLH, most of the values are many times greater than typical occupational 

exposure limits or other short-term exposure guidelines. In addition, the revised IDLH are still intended to represent 

a hazardous atmosphere from which escape is possible within 30 minutes. Consequently, they represent reasonable 

limits to use for providing adequate time to don protective apparel because they will provide conservative protection 

yet ample flexibility. Given that a control room operator is expected to don a respirator within 2 minutes, the 

revised IDLH are conservative for the NRC's needs. This is particularly the case for the flammable gases and liquids, 

for which the IDLH values are set at 10% of the explosive concentration, far below their expected life-threatening 

toxicity level. Finally, using the IDLH values as a response criteria for donning respirators by reactor control room 

personnel will meet the intent of the original IDLH and will provide an adequate margin of safety for protecting the 

operators while providing greater flexibility. A list of the revised LDLH values are presented in Appendix A. 

The relationship between the current NRC toxicity limits (Reg. Guide 1.98) and the NIOSH IDLH's is difficult to 

access. The toxicity limits were adapted from SAX (1968), a secondary reference, but documentation describing the 

process used to establish these values are not available. It appears, however, that primary references were not used. 

Many values are whole number multiples of the PEL (2, 4, 5, times the PEL for example). These 2-minute limits 

are also obviously outdated. The toxicity limits are based on a 2-minute exposure, whereas the IDLH values are 

based on a 30-minute exposure. Another difference is that the toxicity limits were defined in terms of time required 

to don respirators in an emergency, whereas the IDLH's were defined in terms of time required to exit a contaminated 

area in the event of a respirator failure. This 30-minute limit does not give license, however, to remain in an adverse 

work environment any longer than necessary to escape. 

2.4 Other Existing Emergency Exposure Guidelines and Standards 

A number of occupational emergency exposure guidelines and standards have been developed by government 

agencies and private associations to address health and safety issues in and outside the workplace. Some, such as the 

TLV (developed by the American Conference of Governmental Industrial Hygienist [ACGIH]), the Workplace 

Environmental Exposure Levels (developed by AIHA), the PEL (developed by OSHA), and the REL (developed by 

the NIOSH) are inappropriate for emergency response. These values were developed for the protection of workers and 

are based on repeated daily exposures in the workplace over a lifetime. These guidelines provide administrative 

protection against acute and chronic health effects usually over an 8-hour workday. PEL's or REL's have not been 

established for known human carcinogens and OSHA and NIOSH recommend that engineering controls such as 

respirators provide protection in the workplace for these chemicals. 

Several guidelines have been developed for use in emergency situations involving a single exposure to substances in 

occupational or community environments. The National Research Council has established emergency guidelines for 

approximately 40 chemicals for the military (NAS, 1983-88). More recently, AIHA introduced the concept of ERPG 

for potential releases of chemicals in the community (AIHA, 1988-93) These guidelines are useful primarily for 

emergency planning and response. ERPG-3 is the worst-case guideline and is defined as ". . . the maximum 

airborne concentration below which it is believed that nearly all individuals could be exposed for up to one hour 

without experiencing or developing life-threatening health effects." ERPG-2 is set at a level where irreversible or 

serious health effects would not be expected from a 1-hour exposure and ERPG-I identifies a concentration that does 

not pose a health risk to members of the community. 

Compared to the IDLH, the ERPG were developed with more vigor in terms of documentation, peer review and use 

of current primary references. They were also derived directly from the toxicological data without the explicit use of 

safety factors. Unfortunately, ERPG have been established for only a few chemicals. These guidelines were 

established to provide a level of safety for a period of 1 hour for emergency situations where the use of protective 

equipment would not be anticipated. Table 2.1 compares ERPG-3 values with the revised IDLH concentrations and

NUREG/CR-6624 4
Regulatory Guide toxicity limits in those cases where ERPG levels have been established. This comparison 

demonstrates the conservativeness of the IDLH concentrations for these compounds and suggests that they would 

provide ample protection if used as toxicity limits in the revised Regulatory Guide. 

2.5 Odor Thresholds 

The detection of chemicals by smell is a significant warning for people to protect themselves from chemical 

contaminants. The ability of the general population to detect specific chemical odors is influenced by the wide 

variability of different persons' olfactory capability, to some degree their previous experience with the chemical, and 

their degree of awareness or consciousness of their surroundings at the time (Amoore and Hautala, 1983.) Odor 

thresholds have been established for chemicals having experimental data that meets the evaluation criteria developed 

by AIHA and for which occupational health standards have been established (AIHA, 1989). In general, the lower the 

odor threshold compared to the IDLH, the greater the likelihood of the chemical being detected before an IDLH level 

is reached. Comparing AIHA odor threshold levels and NIOSH IDLH values (for the over 90 chemicals for which 

both are established) shows in all cases, except for acetonitrile, carbon tetrachloride and 1,1,2-trichloroethane, that 

the IDLH values are greater than the odor threshold levels (Appendix B). This means that in most cases a person 

would be warned of adverse atmospheres by smell before the IDLH concentration was reached. However, several 

chemicals not listed by AIHA for a number of reasons, including a lack of data, are not readily detected by odor. 

Examples include carbon dioxide, carbon monoxide, chlorine dioxide, formic acid, methycyclohexanol, and 

propane. 

Odor thresholds have limited use for triggering the donning of respirators for the intended use of Regulatory Guide 

1.78. Odor thresholds are not highly correlated with toxicity of chemicals, vary widely by different individuals for 

the same chemical and do not always lend themselves to chemical identification. In addition individuals vary in 

their sensitivity to different odorants. AIHA states that caution should be taken in relying on the use of odor alone as 

a warning of potentially hazardous exposures. However, respirators may provide relief from odor and irritation to the 

nose and eyes when exposure data are lacking. 

2.6 Discussion and Recommendations 

The revised IDLH concentrations are recommended for use in the updated Regulatory Guide 1.78. The IDLH values 

were developed for respirator selection for a large number of chemicals, whereas other exposure guidelines have been 

developed for occupational or public emergency purposes without regard to respirators. In addition, other standards 

are developed for only a limited number of chemicals. The revised IDLH values are generally conservative enough to 

provide an adequate margin of safety. There may be a few specific cases where a value other than the IDLH would be 

more appropriate, but overall the IDLH is the most appropriate guideline for this purpose. The revised IDLH values 

are listed in Appendix A. Several special cases are also considered below. 

It is recommended, however, that respirator donning time of 2 minutes be retained. The 30-minute escape time for 

the IDLH concentrations are maximium times and it is intended that every effort be made to exit immediately. A 2

minute limit would provide an extra margin of safety in the use of IDLHs in the updated Regulatory Guide 1.78. 

IDLH values should be used in the context of a trigger point for the donning of respirators for control room 

operators. These levels should in no way be used to imply an upper limit of safety to replace occupational standards 

set by OSHA. 

Four of the revised IDLH are lower than the corresponding limits in the current Regulatory Guide 1.78 (Table 

1): Chlorine, ethyl chloride (based on 10% LEL), ethylene dichloride (based on one human study), and hydrogen 

sulfide (based on a person's ability to become desensitized to chemical odor or "olfactory fatigue" at 100 ppm). In 

addition, there is no IDLH set for vinyl chloride (also in Regulatory 1.78) because NIOSH considers this compound 

a known human carcinogen and has not established a REL. NIOSH recommends that respirators be worn at any 

detectable concentration when there is no REL and therefore has not established an IDLH. 

An evaluation of exposure limits for ammonia, chlorine, Halon, and sulfur dioxide for nuclear reactor control room 

habitability was recently performed and published as NUREG/CR-5669 (Malhum and Sasser, 1991). This 

evaluation specifically addressed the limits for Regulatory Guide 1.78, and rationale and documentation were 

developed to support the recommendations. The exposure limits recommended in NUREG/CR-5669 for ammonia

5 NUREG/CR-6624
Table 2.1. Comparison of ERPGs with IDLHs and Regulatory Guide 1.78 Toxicity Limits for Selected Chemicals 

Chemical Regulatory Guide 1.78 ERPG-3 IDLH 

Acetaldehyde 200 ppm 2000 ppm 

Acetone 2000 ppm 2500 ppm 

Acrolein - 3 ppm 2 ppm 

Acrylonitrile 40 ppm 85 ppm 

Allyl chloride - 300 ppm 250 ppm 

Ammonia 100 (300)* ppm 1000 ppm 300 ppm 

Aniline 10 ppm 100 ppm 

Benzene 50 ppm 500 ppm 

Benzyl chloride 25 ppm 10 ppm 

Bromine - 5 ppm 3 ppm 

1,3-Butadiene 1000 ppm 5000 ppm 2000 ppm 

Carbon dioxide 10000 ppm - 40000 ppm 

Carbon disulfide 5000 ppm 500 ppm 

Carbon monoxide 1000 ppm - 1200 ppm 

Carbon tetrachloride 750 ppm 200 ppm 

Chlorine 15 (30)* ppm 20 ppm 10 ppm 

Chlorine trifloride 10 ppm 20 ppm 

Chloropicrin 0.2 ppm 3 ppm 2 ppm 

Crotonaldehyde 10 ppm 10 ppm 50 ppm 

Dimethylamine 100 ppm 500 ppm 500 ppm 

Epichlorohydrin 20 ppm 100 ppm 75 ppm 

Ethyl chloride 10000 ppm 3800 ppm 

Ethyl ether 800 ppm 1900 ppm 

Ethylene dichloride 100 ppm 50 ppm 

Ethylene oxide 200 ppm 500 ppm 800 ppm 

Fluorine 2 ppm 25 ppm 

Formaldehyde 10 ppm 25 ppm 20 ppm 

Hydrogen chloride 100 ppm 50 ppm 

Hydrogen cyanide 20 ppm 25 ppm 50 ppm 

Hydrogen fluoride 50 ppm 30 ppm 

Hydrogen sulfide 500 ppm 100 ppm 100 ppm 

Methyl alcohol 400 ppm 5000 ppm 6000 ppm 

Methyl iodide - 125 ppm 100 ppm 

Methyl mercaptan - 100 ppm 150 ppm 

Phenol - 200 ppm 250 ppm 

Phosgene - I ppm 2 ppm 

Sodium oxide 2 mg/m3 

Sulfur dioxide 5 ppm 15 ppm 100 ppm 

Sulfuric acid 2 mg/m 3 30 mg/m3 15 mg/m3 

Vinyl chloride 1000 ppm 

Xylene 400 ppm 900 ppm 

* NUREG/CR-5669 recommended limit in parenthesis

NUREG/CR-6624 6
and sulfur dioxide are the same as the revised IDLH concentrations. The chlorine and Halon 1301 values differ 

between the two evaluations. The revised IDLH for chlorine is 10 ppm compared to the NUREG/CR-5669 exposure limit of 30 ppm. For Halon 1301 the IDLH is 4% whereas the NUREG/CR-5669 exposure limit is 5%. We 

recommend using the NUREG/CR-5669 limit for chlorine and Halon 1301, since the NUREG/CR-5669 supporting 

documentation is based on recent peer-review documentation (Malhum and Sasser, 1991) performed for the NRC. 

However, for consistency, the IDLH values could be used for these two chemicals as they are more-conservative than 

the NUREG/CR-5669 values. IDLH concentration is not listed for Halon 1211, therefore it is recommended that the 

value recommended in NUREG/CR-5669 be retained. 

A recommended toxicity limit was not established for 15 carcinogens (12 listed in Appendix B of NIOSH pocket 

guide) because the IDLH have never been developed by NIOSH. Instead of setting exposure limits (PEL, REL, 

IDLH, etc.), government agencies require the use of engineering controls, work practices, and personal protective equipment to provide protection against these chemicals. No other existing standard is available which would be 

appropriate to use as toxicity limits for Regulatory Guide 1.78. 

IDLH are not available for three additional chemicals (octachloronaphthalene, pentachloro- naphthalene, and 

trichloronaphthalene) listed in NIOSH documentation (AEC, 1974) because sufficient acute toxicity data or 

appropriate data for surrogate chemicals were not available. In view of this absence of data, no attempt was made to 

develop toxicity limits for these chemicals. 

The alternative to using the revised IDLH for providing respiratory donning trigger levels would be to develop more 

toxicologically based limits using a more rigorous process . This would be a very time-consuming, resource

intensive process given the large number of revised IDLH and is not recommended. 

2.7 Conclusions 

The NIOSH "Documentation for Immediately Dangerous to Life or Health Concentrations," was evaluated in order 

to provide a basis for a recommendation that the revised NIOSH IDLH replace the chemical limits in the Regulatory 

Guide 1.78, "Assumptions for Evaluating the Habitability of a Nuclear Power Plant Control Room During a 

Postulated Hazardous Chemical Release," as control room habitability limits. A discussion of the NIOSH document 

and conclusions and recommendation are included. In general, the revised IDLH values are recommended for 

replacing the toxicity limits in Regulatory Guide 1.78. The IDLH concentrations are conservative and should 

provide ample protection if used as toxicity limits in the revised Regulatory Guide.

7 NUREG/CR-6624
3.0 Evaluation of Frequent Shipment 

Screening Criteria 

3.1 Introduction and Summary 

The objective of this section is to re-examine the basis for hazardous chemical shipment frequencies referenced in 

Regulatory Guide 1.78, Assumptions for Evaluating the Habitability of a Nuclear Power Plant Control Room 

During a Postulated Hazardous Chemical Release (AEC 1974). Specifically, the objective is to examine 

transportation accident statistics referenced in Regulatory Guide 1.78 to determine if the definitions of "frequent 

shipments" are still valid. Based on the guidance in Regulatory Guide 1.78, licensees perform a two-level screening 

analysis to identify postulated transportation accidents that could affect control room habitability. The first screen 

eliminates hazardous chemicals that are not shipped within 5 mi. of the control room. The second screening is 

performed based on shipment frequency. If there are shipments passing within 5 mi. of the control room but they 

are not frequent, as defined in the Regulatory Guide, no calculations are necessary. The definitions of frequent 

shipments given in the Regulatory Guide were derived using the truck, rail, and barge accident statistics presented in 

WASH-1238 (AEC 1972). This section examines the definitions of frequent shipments in light of more recent 

transportation accident rate data and provides clarification of the definitions. 

Reviews were conducted of accident rate data in WASH- 1238 (AEC 1972) and several recent studies to determine if 

an update of Regulatory Guide 1.78 would be recommended. In summary, 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 language will 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 the following sections. 

3.2 Review of Transportation Accident Data 

A review of readily-available transportation accident data was conducted, the results of which are summarized in the 

following sections. The accident rates and conditional spill probabilities identified during the review are presented 

in Table 3.1. Note that some of the reports reviewed here addressed accident rates and conditional probabilities of 

accidents involving radioactive materials. A few reports are described that address hazardous chemical 

transportation accidents. It was necessary to make assumptions to apply the accident data on radioactive material 

shipments to hazardous chemical shipments. These assumptions are specified where necessary in the following 

sections. 

3.2.1 WASH-1238 

Accident Rates 

The WASH-1238 (AEC 1972) accident rates for truck travel were taken from 1969 accident statistics published by 

the Federal Highway Administration (FHA). The truck accident rates were based on accidents defined as 

"reportable" by the FHA. This included accidents that resulted in fatalities, injuries, and property damage in excess 

of $250. The data for truck accidents involved "for-hire" carriers only and were indicated to include only "large 

motor carriers." For hazardous materials, the accident rate was 1.69 accidents per million vehicle miles. 

Federal Railroad Administration (FRA) accident statistics were used to calculate rail accident rates. For the FRA, 

reportable accidents were defined as those involving fatalities, injuries, and property damage in excess of $750. The 

rail accident rates were determined by dividing the total number of reportable rail accidents by the total number of 

train-miles traveled in 1969. This was converted to the rate per railcar-mile by dividing the rate per train-mile by an 

average of 70 cars per train. The result was an overall accident rate of 0.14 accidents per million car miles. Of this 

overall accident rate, the rate for other than grade-crossing accidents was about 0.08 accidents per million car miles. 

Since an average of 10 cars are involved in each non-grade-crossing accident, the overall accident rate for other than 

grade-crossing accidents was estimated to be 0.8 accidents per million car miles.

NUREG/CR-6624 8
Table 3.1. Summary of Transportation Accident Statistics

WASH-1238 NUREG- SLA-74- I NUREG/CR PNL Risk Handbook 

0170 0001 -4829 Studies 

Accident Rate (accidents/mi.) 

Truck 1.69E-06 1.69E-06 2.5E-06 6.4E-06 2.5E-06 2E-06 

Rail aT 8.OE-07 1.5E-06 1.5E-06 1.7E-06 1.4E-06 6E-07 

Barge 1.8E-06 9.8E-06 Not given Not given Not given 1.5E-06 

Conditional Spill Probability 

Truck 0.18 0.09 0.023 0.0383 0.012 - 0.025 0.08 

Rail 0.10 0.2 0.10 0.0537 0.008 - 0.067 0.075 

Barge 0.025 0.023 Not given Not given Not given 0.03 - 0.1 

(a) Accidents per railcar-mi 

(b) Rate for collisions and groundings on lakes, rivers, and intercoastal waterways 

Barge accident rates given in WASH-1238 were based on statistics compiled by the U.S. Coast Guard for the year 

1970. The accident rate given was 1.8 accidents per million barge-miles. They appear to represent collision and 

grounding incidents on inland waterways. 

Conditional Release Probabilities 

The accident rates given above include all severities of accidents, ranging from minor collisions that do not threaten 

the integrity of the cargo container to extreme accidents that demolish the vehicle and release all of its contents. Of 

interest to Regulatory Guide 1.78 are accidents that result in a substantial release of contents that could threaten the 

plant operators and safe shutdown of the plant. WASH-1238 divided the accidents into five severity categories, 

including minor, moderate, severe, extra severe, and extreme. Conditional probabilities of each severity category 

were developed based on previous studies and analyses of the accident data to characterize the potential effects of 

impact, puncture, and fire conditions produced in the accidents. 

In WASH-1238, the frequencies of encountering the five severity categories were given for truck, rail, and barge 

accidents. The study also gave the general accident rates for these three transportation modes. Therefore, the 

conditional probabilities of the five severity categories can be calculated by dividing the severity category 

frequencies by the general accident rate. Of interest here are the severity categories that could result in a significant release. For conservatism, this is assumed to be all severity categories except minor accidents. Following the 

calculation procedure given above, the conditional probabilities of significant releases were calculated as follows: 

"* Truck: 0.18 (18% of truck accidents involve conditions defined as moderate, severe, extra severe, and extreme 

that could potentially result in a significant release of cargo). 

"* Rail: 0.10 

"* Barge: 0.025 

3.2.2 NUREG-0170 

This report provides accident rates and conditional probabilities of the severities of truck and rail accidents 

involving radioactive materials. 

Accident Rates 

The overall accident rate for motor carriers transporting radioactive materials used in NUREG-0 170 was 1.06E-06 

accidents/km (1.71E-06 accidents/mi), nearly the same as the truck accident rate given in WASH- 1238. The rail 

accident rate was given as 9.3E-07 railcar accidents per railcar-km (1.5E-06 railcar accidents per railcar-mi). For 

barge accidents, the rate given was 6.06E-06 accidents/km (9.8E-06 accidents/mi).

9 NUREG/CR-6624
Conditional Release Probabilities

In NUREG-0 170, eight categories were established and associated conditional probabilities were developed to 

characterize the severities of accidents. It is assumed here that the severity categories I and 11 represent minor 

accidents that would not threaten the integrity of a hazardous material shipping container and thus would not result 

in a significant release of cargo. If these two severity categories are excluded, the conditional probabilities of 

significant releases, given an accident occurs, are as follows: 

"* Truck: 0.09 

"* Rail: 0.20 

"* Barge: 0.023 

3.2.3 SLA-74-0001 

This document (Clarke et al. 1976) was prepared in the late 1970s to describe the severities and conditional 

probabilities of air, highway, and rail accidents involving small Type B packages transporting radioactive materials.' 

Analytical models were developed and statistical analyses were performed to describe potential accident 

environments that a package may be subjected to during an accident and the probabilities of encountering these 

environments. 

Accident Rates 

The accident rates given in the report are as follows: 

"* Truck: 2.5E-06 accidents/mi. 

"* Rail: 1.5E-06 accidents per car mile. 

Conditional Release Probabilities 

The probabilities of exceeding five severity categories, including minor, moderate, severe, extra severe, and extreme 

accident environments, were given in SLA-74-0001. For this study, it was assumed that minor accidents would not 

result in a significant release of a hazardous cargo. The conditional probabilities of the other four categories were 

summed to develop the conditional release probabilities given below: 

"* Truck: 0.023 

"* Rail: 0.10 

3.2.4 NUREG/CR-4829 

This report (Fischer et al. 1987) was prepared to evaluate the responses of commercial spent nuclear fuel shipping 

casks to severe truck and rail accidents and assess the level of safety to the public during the shipment of this 

material. Water transport was not addressed. 

Accident Rates 

The "Modal Study" (NUREG/CR-4829; Fischer et al. 1987) used accident rates developed from data published by 

the American Petroleum Institute (API) for heavy trucks (tractor/semi-trailers). Fischer et al. (1987) also reviewed 

FHA data for the years 1973 to 1981. This represents an improvement over the WASH-1238 truck accident data 

because it incorporates the imposition of the national speed limit in 1973 and also included accident and mileage 

data for private carriers (i.e., companies transporting their own goods in their own, or leased, vehicles). However, 

B packages must be designed to withstand STypeimpact, puncture, fire, and immersion accident conditions. See 

10 CFR 71 for complete definition of Type B packaging standards.

NUREG/CR-6624 10
because the API data was for vehicles most closely-resembling the size, weight, and operating characteristics of 

trucks that would be transporting spent fuel (the focus of the study), Fischer et al. (1987) opted to use the API data. 

The accident rate developed by Fischer et al. (1987) from the API data was 6.4E-06/mile. 

The rail data in Fischer et al. (1987) were taken from FRA accident statistics compiled for the years 1975 to 1982. 

Reporting thresholds (dollar damage) were increased by the FRA to account for inflation beginning in 1975. Fischer 

et al. (1987) differed from WASH-1238 in that it did not convert the rates to a per-railcar-mile basis but rather left 

them as per train-mile. This assumption overstates the accident rate by implying that, if an accident occurs, it will 

involve the railcar of interest (in this case it would involve a spent fuel shipping container). It does not account for 

the many train accidents that do not result in damage to all of the cars on the train. The accident rate developed by 

Fischer et al. (1987) was 1.7E-06/rail-car-mile after conversion assuming an average of 70 cars per train and 10 cars 

are involved in each accident. 

Conditional Release Probabilities 

One of the focuses of Fischer et al. (1987) was to develop conditional probabilities of a range of accident severities 

for truck and rail transport of commercial spent nuclear fuel. To do so, the analysts reviewed accident reports, 

police reports, etc., and developed analytical models of package response to a range of accident conditions suggested 

in the reports. They then performed detailed statistical analyses to characterize the probabilities of encountering this 

range of accident environments. Based on event trees developed by Fischer et al. (1987) and the event outcomes 

that were labeled as "significant" events, the following conditional probabilities of significant releases were 

calculated: 

"* Truck: 0.0383 

"* Rail: 0.0537 

These probabilities are based on qualitative descriptions given by Fischer et al. (1987). More complete descriptions 

of the outcomes of the "significant" events are needed to accurately characterize the conditional release probabilities. 

However, they appear to line up relatively well with the probabilities taken from other studies. 

3.2.5 PNL Risk Studies 

In the late 1970s and early 1980s, the Pacific Northwest Laboratory (PNL) developed risk models and quantified the 

risks of transporting various hazardous cargoes, including propane (Geffen et al. 1980), gasoline (Rhoads et al. 

1978), chlorine (Andrews et al. 1980), and several radioactive materials. The analyses included development of 

failure thresholds for the shipping containers and assessments of the probabilities of encountering accident 

conditions that exceeded these thresholds. The accident rates and conditional probabilities given in the PNL studies 

of the hazardous chemical cargoes are given below. 

Accident Rates 

The overall accident rates given in the PNL studies were not cargo-dependent. Therefore, the same accident rates 

were used in all three hazardous chemical transportation risk assessments. These are: 

"* Truck: 2.5E-06 accidents/mi 

"* Rail: 1.4E-06 accidents/railcar-mi 

The actual rail accident rate used in these assessments was 6.2E-06 train accidents/train-km. Assuming there are, on 

average, 70 cars per train and that 10 railcars are involved in each accident, as was done in NUREG-0 170 and other 

studies, the rail accident rate shown above was calculated.

I1I NUREG/CR-6624
Conditional Release Probabilities

The conditional probabilities of "significant" or "substantial" releases given in the PNL risk studies were assumed to 

be representative of the probabilities of accidental releases that could affect control room habitability. The 

calculations leading to the conditional probabilities of substantial or significant releases are presented below: 

"* Rail transport of chlorine: Predicted 1.8 substantial release accidents/yr and 150 total accidents/yr. Conditional 

probability = 1.8/150 = 0.012. 

"* Truck transport of propane: Predicted 14 significant release accidents/yr and 570 total accidents/yr. 

Conditional probability = 14/570 = 0.025. 

"* Rail transport of propane: Predicted 0.5 significant release accidents/yr (1 every 2 yrs) and 60 total 

accidents/yr. Conditional probability = 0.5/60 = 0.0083. 

"* Truck transport of gasoline: Predicted one in 15 accidents will involve a significant release. Conditional 

probability = 1/15 = 0.067. 

Note that the cargo-dependent conditional probabilities arise from the different types of cargo containers that are 

used to transport the three types of hazardous chemicals. In addition, the conditional probabilities are dependent on 

the cargo's response to mechanical and thermal accident environments. 

3.2.6 FEMA/DOT/EPA Handbook 

A Handbook has been developed by the Federal Emergency Management Agency (FEMA), U.S. Department of 

Transportation (DOT), and U.S. Environmental Protection Agency (EPA), that provides methods for evaluating the 

frequencies and consequences of potential releases of hazardous materials from fixed facilities and transportation 

systems. The transportation accident rate data and conditional spill probabilities from that document are 

summarized in this section. 

Accident Rates 

For the truck accident rates, FEMA/DOT/EPA (1988) reviewed a number of publications and recommended that 

rates be used that were derived from FHA data for trucks carrying bulk quantities of hazardous materials. The 

Handbook gave suggested accident rates for truck shipments but also indicated that local data, if available, would be 

more representative of the actual transport conditions of interest. 

The FEMA/DOT/EPA Handbook used FRA data as the basis for their rail accident rates. One advantage of the 

Handbook-derived rail accident rates is that it converts the overall accident rate (per train-mile) to the accident rate 

per railcar-mile by multiplying the overall rate by a 20% factor that accounts for the average fraction of railcars in a 

train that are damaged in an accident. This more accurately accounts for multiple-railcar accidents and would be 

more representative of the hazards to control room personnel from railcar accidents. Furthermore, the accident rates 

in the Handbook were derived from more recent statistics than both Fischer et al. (1987) and AEC (1972) and would 

therefore reflect the effects of improved truck and rail safety equipment, braking systems, computerized switching, 

and other safety improvements. 

The barge accident rate suggested in the Handbook was derived from reviewing a number of sources but were 

predominantly based on U.S. Coast Guard data. Reports from as recent as 1983 were referenced. Therefore, the 

Handbook represents more recent, and probably more representative, accident data than WASH-1238. The 

Handbook data is more inclusive than the WASH-1238 data as it includes collisions, groundings, and ramming 

incidents. This is different than WASH-1238 which included only the time when the barges were moving, thus 

excluding accidents while the vessels were moored or docked and may also exclude collisions and groundings that 

occur in harbors and bays.

NUREG/CR-6624 12
Conditional Release Probabilities

The Handbook gives conditional release probabilities for truck, rail, and marine vessel accidents and spill size 

distributions for pipeline accidents. The conditional probabilities for truck, rail, and barge accidents were derived 

from reviews of accident data and other sources. The results are summarized below: 

" Truck: Conditional spill probability 0.2 

Spill size distribution - 0.6 for 10% loss of cargo 

0.2 for 30% of cargo 

0.2 for 100% of cargo 

Assuming that spills that amount to less than 10% of the cargo would not represent a significant hazard to 

control room operators, the conditional probability of spill large enough to threaten safe shutdown of a nuclear 

power plant is 0.2 x 0.4 = 0.08. 

" Rail: Conditional spill probability 0.15 

Spill size distribution - 0.5 for 10% loss of cargo 

0.2 for 30% of cargo 

0.3 for 100% of cargo 

Assuming that spills that amount to less than 10% of the cargo would not represent a significant hazard to 

control room operators, the conditional probability of spill large enough to threaten safe shutdown of a nuclear 

power plant is 0.15 x 0.5 = 0.075. 

"* Marine: Conditional spill probability 0.15 if using one rate regardless of vessel 

0.05 for double-hulled/double-bottomed vessels 

Spill size distribution 0.35 for 10% loss of 1 tank or 

compartment 

0.35 for 30% loss 

0.30 for 100% loss 

Again assuming that spills that amount to less than 10% of a tank or compartment pose no significant threat to 

nuclear plant control room operators, the conditional probability of an accident large enough to threaten safe 

shutdown of a nuclear power plant is 0.15 x 0.65 = 0.098 (round to 0.1) if using one rate regardless of the vessel 

type or 0.05 x 0.65 = 0.03 for double-hulled/double-bottomed vessels. 

3.2.7 Department of Transportation - Bureau of Transportation Statistics 

Two reports from the Department of Transportation - Bureau of Transportation Statistics (DOT-BTS) were reviewed 

to identify trends in accident rates that could affect the conclusions of this study. The BTS annually prepares a 

report to the U.S. Congress on the state of the U.S. transportation system, including transportation accident, fatality, 

injury, and property damage statistics. This report is Transportation Statistics Annual Report, 1998 (DOT 1998a). 

The basis for the statistical data presented in this report are contained in National Transportation Statistics, 1998 

(DOT 1998b). 

The data presented in these two documents is not directly comparable to the accident rate data presented in Table 

3.1. This is because the data in Table 3.1 focus on heavy combination truck accident statistics whereas the DOT 

statistics referred to above include all motor vehicles (including passenger cars, delivery vans, motorcycles, etc. in 

addition to large trucks). However, it may be used to illustrate trends in accident rates that could affect the 

conclusions regarding the accident rate data presented in WASH- 1238. Highway accident rates given in DOT 

(1998b) are plotted as a function of time in Figure 3.1. Figure 3.2 presents a similar illustration for rail accident 

rates. It can be seen that both motor vehicle and rail accident rates are generally declining over time. However, the 

downward trend in motor vehicle accident rates shown in Figure 3.1 is illustrative of a general downward trend in 

truck accident rates.

13 NUREG/CR-6624
Figure 3.1. Motor Vehicle Accident Rate Trend From 1990 to 1996

Figure 3.2. Trend in Rail Accident Rates, 1970 to 1996.

NUREG/CR-6624

S305o 

300 

. 295 Trendline 

290 (linear regression) Rate 

~280 o285 •--

275

270 

~265 •260 

1 90 I 1I I I 

1990 1991 1992 1993 1994 1995 1996 1997

Year

1400 

1200 Accident Rate 

1000 

800- • 

Trendline 

(linear regression) 

S200 

1970 1975 1980 1985 1990 1995 2000 

Year

14
Water transportation accident rates are not provided in DOT (1998a and 1998b) so a plot similar to Figures 3.1 and 

3.2 could not be prepared. However, the number of fatalities and accidents involving water transport are provided in 

DOT (1998a). A downward trend in the number of fatalities and number of accidents by year is also evident from 

the water transportation accident statistics. 

3.3 Definition of Frequent Shipments 

Regulatory Guide 1.78 provides two levels of screening to determine the hazardous chemicals that must be 

considered in the evaluation of control room habitability. The first level is the distance from the nearest highway, 

railway, or waterway over which hazardous chemicals are transported. If there are no shipments of hazardous 

materials passing within 5 mi. of the control room, no calculations are necessary. The second set of screening 

criteria is related to shipment frequencies. If there are shipments passing within 5 mi. of the control room but they 

are not frequent, as defined in Regulatory Guide 1.78, no calculations are necessary. However, if there are 

shipments passing within 5 mi. of the control room and they are frequent, then calculations of toxic chemical 

concentrations in the control room are necessary to demonstrate that the operators are protected. This section 

examines the definitions of frequent shipments in light of more recent accident data, and provides farther 

clarification of the definitions. 

Regulatory Guide 1.78 contains a definition of frequent shipments that must be considered in the evaluation of 

control room habitability (Regulatory Position 2). These are 10 per year for truck traffic, 30 per year for rail, and 50 

per year for barges. Hazardous materials that are shipped at these frequencies or greater (within 5 miles of the plant) 

must be considered. The Regulatory Guide indicates these frequencies reflect WASH-1238. 

The shipment frequency screening criteria were derived to provide a relatively simple method of screening out non

credible hazardous chemical transportation accidents from control room habitability considerations. The screening 

criteria were developed, in part, so that fluctuations in accident rates over time would not make it necessary to revise 

them. As can be seen from the accident rate data given in Table 3.1, the accident rates varied little from one study to 

another. Truck and rail accident rates were found to be within a factor of three from highest to lowest and 

barge/marine vessel accident rates were less than a factor of two apart. Furthermore, a review of accident data given 

in DOT (1998a and 1998b) indicates general downward trends in motor vehicle, rail, and water transportation 

accidents. Use of current accident rate data would most likely increase the definition of frequent shipments, 

resulting in a less-conservative screening analysis. 

Sixteen reactor Safety Analysis Reports (SARs) were reviewed to determine how the shipment frequency screening 

criteria were used in the past. Insights from this review were used to determine if shipment frequencies are effective 

screening criteria and if the screening methodology needs clarification. The analysis of transportation accidents 

appears in Section 2.2.3 of the SARs and is sometimes summarized in Section 6.4 ("Habitability Systems"). A 

summary of the information relevant to the use of the shipment frequency criteria is provided in Table 3.2. The 

results of the analyses are used to define design-basis accidents that the plants must withstand. 

All of the SARs reviewed provided information on the distances between the site and the nearest roads, rail lines, 

and navigable waterways. The most frequently-used argument against incorporating transportation accidents 

involving releases of specific hazardous materials into the design basis was the distance criteria. All of the SARs 

reviewed argued that the distances exceeded NRC criteria for at least one mode, barge being the most commonly 

screened mode based on separation distance. Thus, in most SARs, the shipment frequency criteria are applied to 

truck and rail shipments of hazardous chemicals. 

Eleven of the sixteen SAPs indicated analyses were performed to determine the frequencies of movements of 

hazardous materials. Often the information presented in the SAR consisted of a summary of a survey performed to 

identify hazardous chemicals shipped to, from, or near the site but the actual analyses were not incorporated into the 

SARs. Based on these analyses, which included comparisons of the frequencies of movements as well as the 

quantities transported to the Regulatory Guide criteria, the licensees selected one or more specific materials to 

evaluate. They then performed analyses to determine the concentrations of the chemicals that could reach the

15 NUREG/CR-6624
Table 3.2. Summary of FSAR Applications of Regulatory Guide 1.78 Shipment Frequency Criteria 

Plant Name Summary of Transportation Accident Screening Analyses 

Seabrook Hazardous material transport data not available; surveyed industrial facilities 

within 5 mi. of plant to characterize hazardous material shipments. Probabilistic 

analysis concluded that accidents involving nearby transportation facilities would 

not affect safe operation of the plant and their frequencies are <1 E-07/yr. 

Byron Surveyed industries within 10 mi. of the plant to establish type, quantities, and 

shipment frequencies for hazardous materials. Screened offsite shipments of toxic 

chemicals based on shipment frequency. Chlorine detectors provided due to 

onsite chlorine storage. 

Limerick Surveyed industrial facilities within 5 mi. of site. Evaluated 153 chemicals. 

Shipment frequency screening eliminated all but 6 hazardous chemicals. Provided 

detection and control room isolation capabilities for all 6 chemicals. 

Vogtle Screening analysis of shipment frequencies performed; concluded that 

transportation accidents involving hazardous chemicals within 5 mi. of the site 

have acceptably low probabilities. Several potential chemical releases analyzed 

and the calculated toxic concentrations did not exceed limits. 

Perry Performed screening based on frequency and quantity. Eliminated all but two 

hazardous chemicals shipped within 5 mi. of the plant. 

Clinton Eliminated all truck shipments of hazardous materials based on frequency. Survey 

found 19 chemicals transported by rail that exceeded 30 shipments/yr. Further 

screening performed based on low vapor pressures, low toxicity, eliminating 

asphyxiants. 

Fermi No screening performed because no roads, rail lines, or navigable waterways 

within 5 mi. of site (except site access road and rail spur). 

Hope Creek Survey determined that two chemicals transported by barge exceeded shipment 

frequency criteria. No other toxic or hazardous chemicals are regularly stored, 

used, or transported within 5 mi. of the site. 

Millstone 3 Screened out highway shipments based on separation distance. For rail, screened 

out all chemicals except two, one of which (chlorine) is stored onsite in rail tank 

cars for water treatment. 

Watts Bar Screened truck shipments based on frequency. No rail line within 5 mi. of the site 

except the site access line. Barge accidents involving toxic chemicals screened 

out, except for smoke. 

River Bend Screened out all offsite shipments except for 2 hazardous materials; performed 

toxic concentration calculations for those 2 chemicals. 

Beaver Valley No screening based on shipment frequency was apparent. Survey identified 341 

hazardous chemicals transported within 5 mi. of site; eliminated all but 119 based 

on low vapor pressure, solid physical form. Remaining chemicals screened based 

on probabilities of accidents less than 1E-06/yr. 

South Texas No screening based on shipment frequency was apparent. Evaluated all hazardous 

chemicals shipped to/from and stored at the plant. 

Palo Verde Survey performed to identify hazardous chemical shipments on rail line near site. 

Chlorine shipments screened based on frequency; others screened based on 

quantities being below Regulatory Guide 1.78 allowable weights for releases 4.2 

mi. from the plant. All highway shipments screened out based on distance 

between plant and nearest highway where hazardous materials would be 

transported. 

Shearon Harris Screened hazardous chemical truck and rail shipments based on Regulatory Guide 

1.78 shipment frequency criteria. Determined that frequencies of rail and truck 

accidents involving significant hazardous chemical releases below 1E-07/yr. 

Nine Mile Point Surveyed industries within 6.2 mi. of plant to identify potential hazardous 

chemicals. Used shipment frequency screening criteria then identified several 

chemicals for dispersion analysis.

NUREG/CR-6624 16
control room intake (or other air intake). Five of the SAPs concluded that the distances and frequencies of 

movements were such that Regulatory Guide 1.78 criteria were met for all hazardous chemical shipments. Six 

additional SARs screened out either truck or rail shipments based on the shipment frequency criteria. The SARs 

also perform analyses of explosions associated with transport of explosives, flammable gases, etc., as required by Regulatory Guide 1.91 (NRC 1978). Analyses of chlorine releases were also performed as required by Regulatory Guide 1.95 (NRC 1977). For the SARs in which the distance criteria were not met for one or more transport modes, hazardous material shipment quantities for the highways, rail lines, or waterways within 5 miles of the site were 

tabulated. They then used the criteria in Regulatory Guide 1.78 to identify the hazardous chemical releases that 

could affect control room habitability. 

The review determined that most SARs contained some application of the shipment screening methodology to 

determine which hazardous chemicals should be included in their control room habitability evaluations. All of the 

SARs that applied shipment screening criteria acknowledged that obtaining hazardous chemical shipment frequencies was difficult but comprehensive surveys of shippers, manufacturing facilities, ports, and other likely hazardous chemical users were performed to establish a reasonable and defensible baseline. Based on these surveys, the licensees performed credible shipment screening assessments and appear to have appropriately identified which, 

if any, hazardous chemical shipments should be the subject of control room habitability evaluations. 

3.4 Recommended Revisions To Regulatory Guide 1.78 

The accident rate data that were used to derive the definitions of frequent shipments in Regulatory Guide 1.78 were 

examined relative to recent accident rate data to determine if an update to the Regulatory Guide is warranted. It was 

observed that the accident rates given in the recent studies are relatively close to each other and to the accident rates 

used in Regulatory Guide 1.78. The spread between the highest and lowest accident rates shown in Table 3.1 is 

small. A general downward trend in accident rates for motor vehicle, rail, and water transportation was also 

observed over time. Based on this review, it is recommended that the screening criteria based on distance from the 

control room and frequency of hazardous chemical shipments within 5 mi of the control room be retained in 

Regulatory Guide 1.78. The definitions of frequent shipments given in Regulatory Guide 1.78 are recommended to 

be retained in their present form. If revised, the definitions, which were derived from accident rate data given in 

WASH-1238, would most likely be increased to higher levels of traffic, due to generally decreasing trends in highway, rail, and water transportation accident rates. However, clarification of the definition of frequent shipments is recommended to be added to Regulatory Position C.2. The clarification should indicate that the total shipment frequency (i.e., the sum of the frequencies of all hazardous chemical shipments by transport mode) regardless of the 

type of chemical should not exceed the specified number given for each transport mode.

17 NUREG/CR-6624
4.0 Revision of Meteorological And Flow Models 

4.1 Introduction 

Significant progress has been made in modeling capabilities since the publication of Regulatory Guide 1.78. The 

purpose of this task is to provide the technical basis for recommending revisions to Regulatory Guide 1.78 with 

regard to the meteorological and control room ventilation flow models. 

4.2 Description of Revised Models 

The meteorological and ventilation flow models used in Regulatory Guide 1.78 are outdated and do not reflect the 

current technology that had been developed since it's publication. The NRC has sponsored a number of research 

and development programs to improve the capability to model exposures of nuclear power plant control room 

personnel to radioactive material and toxic chemicals. This research has resulted in development of a software 

package named HABIT (Stage 1996, Ramsdell and Stage 1998). HABIT is an integrated set of computer codes 

designed for control room habitability assessments. The major modules within HABIT include the HABIT main 

window, which controls the execution of the other modules, and EXTRAN, CHEM, TACT5, FPFP 2, and 

CONHAB. Each module calculates a specific component required to estimate radiological doses and toxic chemical 

exposures in the control room. 

Of most relevance to Regulatory Guide 1.78 is the EXTRAN component of the HABIT software package. 

EXTRAN (Ramsdell 1991), for EXternal TRANsport of toxic chemicals, calculates atmospheric chemical 

concentrations that would result from a release of a toxic chemical. The present atmospheric dispersion model 

described in Appendix B of Regulatory Guide 1.78 does not predict the variations in concentrations in building 

wakes associated with changes in meteorological conditions. EXTRAN represents an improvement in technology 

relative to the Regulatory Guide 1.78 atmospheric dispersion models as it combines procedures for estimating the 

amount of airborne material, a Gaussian puff model, and the most recent of the building-wake diffusion coefficient 

algorithms (Ramsdell 1995). 

4.3 Recommended Revision to Regulatory Guide 1.78 

It is recommended that Regulatory Positions C.5 and C.6 be revised to read as outlined below. It is also 

recommended that Appendix B be replaced with a descriptions of the models and algorithms used by the EXTRAN 

computer code to model atmospheric dispersion and predict the concentrations of radioactive materials or toxic 

chemicals at the control room intake. The revised Appendix B is described following Regulatory Positions C.5 and 

C.6. 

Regulatory Position C.5. Chemical Release Amounts 

The EXTRAN and CHEM portions of the HABIT computer codes (Stage, 1996 and Ramsdell, 1991) may be used 

to estimate the rates of release, atmospheric dispersion, and subsequent concentrations of toxic chemicals at the 

control room intake. If another computer program is used, it should consider physical processes similar to those 

considered in CHEM and EXTRAN. 

Two types of industrial accidents should be considered for each source of hazardous chemicals: maximum 

concentration chemical accidents and maximum concentration-duration chemical accidents. 

a. For a maximum concentration accident the quantity of the chemical to be considered is the instantaneous 

release of the total contents of one of the following (1) the largest storage container falling within the 

guidelines of Table C-2 and located at a nearby facility, (2) the largest shipping container (or for multiple 

containers of equal size, the failure of only one container unless the failure of that container could lead to 

successive failures) falling within the guidelines of Table C-2 and frequently transported near the site, or (3)

NUREG/CR-6624 18
the largest container stored onsite (normally the total release from this container unless the containers are 

interconnected in such a manner that a single failure could cause a release from several containers). 

For chemicals that are not gases at 100 F and normal atmospheric pressure but are liquids with vapor 

pressures in excess of 10 torr, consideration should be given to the rate of flashing and boiloff to determine 

the rate of release to the atmosphere and the appropriate time duration of the release. In situations where 

liquid pools may form on the ground or other surfaces, evaporation from such pools should also be 

considered. 

b. For a maximum concentration-duration accident, the continuous release of hazardous chemicals from the 

largest safety relief valve in a stationary, mobile, or onsite source falling within the guidelines of Table C-2 

should be considered. Guidance on the atmospheric diffusion model is presented in Regulatory Guide 1.3, 

Assumptions Used for Evaluating the Potential Radiological Consequences of a Loss-of-Coolant Accident for 

Boiling Water Reactors," and Regulatory Guide 1.4, Assumptions Used for Evaluating the Potential 

Radiological Consequences of a Loss-of-Coolant Accident for Pressurized Water Reactors 

Regulatory Position C.6. Atmospheric Dispersion 

The atmospheric diffusion model to be used in the evaluation should be the same as or similar to the model 

presented in Chapter 6 of NUREG/CR-62 10, Computer Codes for Evaluation of Control Room Habitability 

(HABIT) (Stage 1996, Ramsdell and Stage 1998) and presented in Appendix B of this guide. The model in the 

appendix allows for dispersion in the vertical direction when the distance between the release point and the control 

room is small. The model assumes uniform mixing between the ground and the elevation of the fresh air inlet (a 15

m elevation from ground level is assumed). 

The value of the atmospheric dilution factor between the release point and the control room that is used in the 

analysis should be the value that is exceeded only 5% of the time. Techniques for determining this value may be 

found in Ramsdell (1995, 1997). 

When the boiloff or a slow leak is analyzed, the effects of density on vertical diffusion may be considered if 

adequately substantiated by reference to data from experiments. Density effects of heavier-than-air gases should not 

be considered for releases of a violent nature or for release material that becomes entrained in the turbulent air near 

buildings. 

In evaluating dispersion, formulas should be used that give a good representation of data for low wind cases 

(Ramsdell, 1994). 

Additional credit due to building wake or other dispersive phenomena may be allowed, depending on the properties 

of the released gas, the method of release, and the intervening topology or structures. 

Appendix B 

The following was extracted from the software documentation for the Control Room Habitability Package (HABIT) 

(Stage 1996) and HABIT V1.1 (Ramsdell and Stage 1998). 

Transport and Diffusion 

EXTRAN, one of the components of the HABIT software package, models dispersion of toxic chemicals in the 

environment. EXTRAN includes a Gaussian puff dispersion model. This approach was selected because puff 

models permit more realistic treatment of temporal variations in release terms and concentrations. It is consistent 

with the Gaussian plume models used by the NRC for other licensing applications and the puff models used for 

emergency response applications.

19 NUREG/CR-6624
Puff Model 

The derivation of Gaussian plume model starts with a specific solution to the one-dimensional diffusion equation. 

A three-dimensional puff diffusion model is then produced by superposition of solutions to the one-dimensional 

equation. If it is assumed that diffusion proceeds independently in the longitudinal, lateral and vertical directions 

and that the center of the puff is at position x., y., zo then, in the absence of boundaries, the concentration at position 

x, y, z is given by 

Cxyz) Q e 0.5(x- x0 )2 1 -0.5(__y-yo)2 1 [-0.5(z- z0 )2 [(2,T)' 12 cxcry'l pL 2 [ C rxp 1 2(B) 

where 

C(x,y,z) = the concentration at x,y,z 

Q = the mass of material in the puff 

yx, a%, az = diffusion coefficients in the longitudinal, lateral and vertical directions. 

The diffusion coefficients are characteristic dimensions of the puff. They are functions of the distance (or time) 

from the release point, the atmospheric stability, and the surface roughness. 

Next, a Cartesian coordinate system is defined that has its origin at the ground directly below the release point with 

the x-axis parallel to the wind vector, the y-axis directed cross wind, and the z-axis vertical. With this definition, the 

center of the puff can now be allowed to move with the wind. At any moment t following the release, the 

coordinates of the center of the puff are x. = Ut, y.= 0, z7= h where U is the wind speed and h is the height of 

release. This results in 

Q ep-.5(x-Ut)21 F0.5(y 2yo)2 1 - 0.5(z- h)2 ) C(x, y, z) =.5(p-2 2 exp 23.2) 

. [[(2,T) cr, L_ Y Lry 

The final step in derivation of the plume model is integration of Equation (B.2) from t = 0 to t = c. This 

step makes the plume model a steady-state model. The time delay between the source and the receptor does not 

appear explicitly in the model, and temporal variations in the source can only be modeled by assuming a sequence 

of steady-state releases. 

The puff model alternative chosen for EXTRAN stops at Equation (B.2). Using the puff model, a plume is 

approximated by releasing a sequence of puffs at small time intervals. The concentration at a point in the plume is 

then calculated by summation of the concentrations at the point resulting from all puffs. In essence, the integration 

that leads to the plume model is replaced by 

C(x, y,z) = Z (B.3) 

where C(x,y,z) is the concentration at x, y, z and the Ci are the contributions to the total concentration of the 

individual puffs given by Equation (3.2). It is common to assume that a, and ay are equal and to substitute ay for 

a,. Equations (B.2) and (B.3) retain the ability to model the temporal variation in concentrations at an air intake 

realistically because the concentration will not increase until a puff approaches the receptor, and the values of Q 

may be varied as a function of time. 

The accuracy of the puff approximation can be checked by modeling a steady-state release. It is a function of the 

distance between puffs. The approximation can be made as accurate as desired by reducing this distance. Ramsdell, 

et al. (1983) show that if the distance between adjacent puffs is less than ay, concentrations estimated by the puff

NUREG/CR-6624 20
model are within one or two percent of those estimated by a plume model. Puff release rates in EXTRAN are 

adjusted to maintain this accuracy. 

Equation (B.2) assumes that the diffusion takes place without the interference of boundaries. That assumption is not 

tenable for releases at or near ground level. It is common to assume that the ground acts as a reflecting surface. 

This assumption is incorporated into puff and plume models by assuming an imaginary second source of equal 

strength located at or below ground at a level equal to the negative of the release height. Concentrations are then 

computed by adding the contributions from the real and imaginary sources. Mathematically this is accomplished by 

replacing the term 

exp 0.5(z - h)2]' 

in Equation (B.2) with 

expj0.5(z-h)2]+exp -0.5(z +h)2 - ] 

Source Term 

Concentrations in the puffs are directly proportional to the mass that is included in the puff. In the EXTRAN code, 

puffs are released at regular intervals of length dt where dt is determined by the distance between the release point 

and the air intake, the wind speed, and the atmospheric stability. The mass in a puff released at time t is the mass 

entering the atmosphere in the period between t and t + dt. 

If the toxic substance is a liquefied gas and both flashing and evaporation are occurring, two puffs will be released 

simultaneously. One of these puffs will have the mass of the liquid that has flashed, and the other will have the 

mass that has evaporated. Formulae for calculating the masses flashed to liquid and masses evaporated are given in 

Stage (1996). Otherwise only one puff will be released, and the mass in the puff will be determined using the 

formula for the mass that has evaporated. 

Diffusion Coefficients 

Equation (3.2) shows that the decrease in concentrations in puffs as they move downwind is due only to increases in 

the magnitudes of the diffusion coefficients. Relationships describing the increase in these coefficients in flat 

terrain under normal atmospheric conditions are readily available in the literature. The coefficients increase with 

increasing distance and generally decrease as the atmosphere becomes more stable. 

These standard relationships do not adequately describe the growth of diffusion coefficients in the wakes of 

structures. The effect of wakes is to increase the rate of diffusion, but the effect is limited to the vicinity of the 

structure. As a result, composite diffusion coefficients that include both normal diffusion and wake effects are used 

in EXTRAN. These coefficients are computed using 

ac> = (C' + a'? + afl (B.4) 

where 

c = the composite diffusion coefficient 

cr = a normal diffusion coefficient

21 NUREG/CR-6624
ca = a meander diffusion coefficient 

a 2 = a wake diffusion coefficient. 

Normal diffusion coefficients are computed with the "Eimutis and Konicek" (1972) relationships used in the NRC 

PAVAN (Bander 1982) and XOQDOQ (Sagendorf, et al. 1982) codes. In these relationships the diffusion 

coefficients are functions of distance and atmospheric stability. The diffusion coefficients a, and a2 are computed 

using equations derived by Ramsdell following an analysis of data from building-wake diffusion experiments 

(Ramsdell 1995). Derivations of the equations are presented in Ramsdell (1988, 1990ab, 1995). 

The puff diffusion equation was derived for point-source releases. The point source equations are reasonable as 

long as the distance between the release point and the receptor is large. In some EXTRAN applications the point 

source assumption may lead to unrealistically high concentrations at the source. Consequently, an adjustment is 

made to the diffusion coefficients to account for the size of the source. The diffusion coefficients are given initial 

values that result in concentrations at the center of the puff that are no greater than the concentration the pure vapor 

would have at the atmospheric conditions. These dimensions are related to the density of the vapor and the area of 

the pool. If a wake is a factor, the adjustment is made to the wake diffusion coefficients. Otherwise, the adjustment 

is made to the normal coefficients. 

Transport 

The transport of material is completely defined during model input. Puffs are assumed to move with the wind 

directly from the release point to the air intake. The time required for material to arrive at the intake is determined 

by the wind speed and the growth of the puffs. It is somewhat less than the time estimated by x/U where x is the 

distance to the intake and U is the wind speed.

NUREG/CR-6624 22
5.0 Value-Impact Assessment For Proposed Changes To 

Regulatory Guide 1.78 

5.1 Description of Regulatory Action and Alternatives 

This value-impact assessment (VIA) provides an evaluation of the benefits (values) and costs (impacts) associated 

with the proposed revisions to Regulatory Guide 1.78. The proposed revisions have the potential to affect the 

values and impacts associated with nuclear power plant operations. The purpose of this chapter is to provide the 

information needed for the NRC staff and Commission to determine whether the proposed revisions to Regulatory 

Guide 1.78 are justified. 

Values and impacts are expressed in terms of the effects on various "attributes" associated with the proposed 

revisions. The potential value attributes of the proposed revisions include reductions in public health risks from 

accidents and routine operations at nuclear power plants, changes in nuclear power plant worker risks from 

accidents, and reductions in offsite and onsite property damage costs from a lower likelihood of accidents. Impact 

attributes include the increased costs to the industry of operating and maintaining nuclear power plants as well as 

the costs of developing, implementing, and periodic monitoring of implementation by the NRC. In some cases, 

other attributes are affected, including increases in routine occupational exposures, impacts on other government 

agencies, etc. To determine if the proposed revisions are justified, the values and impacts associated with the 

proposed revisions to Regulatory Guide 1.78 are compared to the no-action alternative of making no changes to 

Regulatory Guide 1.78. All changes in values and impacts associated with the proposed revisions to Regulatory 

Guide 1.78 are measured against the no-action alternative baseline, which for the purposes of this analysis are 

considered as "zeroes." 

5.2 Identification of Attributes 

Attributes are standardized categories of values and impacts associated with the implementation of the proposed 

regulatory action. Table 5.1 provides a checklist of attributes taken from NUREG/BR-0058, Rev. 2, Regulatory 

Analysis Guidelines of the US. Nuclear Regulatory Commission (NRC 1995) and NUREG/BR-0 184, Regulatory 

Analysis Technical Evaluation Handbook (NRC 1997). Their relevancy to the proposed revision of Regulatory 

Guide 1.78 is also discussed in the table. The table describes the basis for dismissing some standardized attributes 

from further consideration. Only the attributes that are affected by the revisions to Regulatory Guide 1.78 are 

discussed further. Detailed discussions of the attributes affected by the proposed revision are provided in the next 

section. 

The values and impacts associated with the revision of Regulatory Guide 1.78 have been addressed in a primarily 

qualitative manner. The results are summarized in Table 5.2. The risk and safety benefits appear small based on a 

semi-quantitative evaluation. The effects on onsite and offsite property were determined to be small. The industry 

implementation costs are estimated to be $2.8 million. The industry operation costs are estimated to be small and a 

potential for cost savings could result if there is a decrease in the requirements for control room habitability systems 

as a result of increased toxicity limits. In addition, there is a potential for a larger cost savings due to fewer plant 

shutdowns and increased plant capacity that could result from the higher toxicity limits for some chemicals. These 

cost savings are difficult to estimate but could be substantial. NRC implementation costs are estimated at $400,000. 

NRC operation costs are small. The proposed revision also represents an improvement of knowledge as the revision 

incorporates more precise toxicity limits and a more comprehensive list of hazardous chemicals. An increase in 

regulatory efficiency was not quantified but is an important attribute for this proposed regulatory action.

23 NUREG/CR-6624
Table 5.1. Identification of Affected Decision Attributes(")

Attribute Affected? Explanation 

Public Health (Accident) YES Addressed in this VIA. 

Public Health (Routine) NO Routine releases are not in the scope of the existing or 

proposed revision to Regulatory Guide 1.78. 

Occupational Health (Accident) YES Addressed in this VIA. 

Occupational Health (Routine) NO Routine exposures to radiation are not in the scope of the 

existing or proposed revision to Regulatory Guide 1.78. 

Offsite Property YES Addressed in this VIA. 

Onsite Property YES Addressed in this VIA. 

Industry Implementation YES Addressed in this VIA. 

Industry Operation YES Addressed in this VIA. 

NRC Implementation YES Addressed in this VIA. 

NRC Operation YES Addressed in this VIA. 

Other Government NO No actions from federal government agencies other than 

the NRC or Agreement States were identified as being 

required by the existing or proposed revision to Regulatory 

Guide 1.78. No changes to offsite emergency capabilities 

or offsite services identified. 

General Public NO No "out-of-pocket" expenses to be paid by the general 

public were identified as resulting from the existing or 

proposed revision to Regulatory Guide 1.78. 

Improvements in Knowledge YES Addressed in this VIA. 

Regulatory Efficiency YES Addressed in this VIA. 

Antitrust Considerations NO No potential violations of antitrust laws identified. 

Safeguards and Security NO Neither the existing nor the proposed revision to 

Considerations Regulatory Guide 1.78 affects, or is affected by, 

safeguards and security considerations at nuclear power 

plants. 

Environmental Considerations NO The existing and proposed revisions to Regulatory Guide 

1.78 were judged to be covered within existing generic and 

site-specific environmental documentation. 

Other Considerations NO No other attributes were identified.

(a) Attributes are described in NUREG/BR-0058, Rev. 2, Regulatory Analysis Guiaelnes 0j the u.n. vuc~ear 

Regulatory Commission, and NUREG/BR-0 184, Regulatory Analysis Technical Evaluation Handbook

5.3 Quantification of Attributes 

Each of the affected attributes identified in the proceeding section is discussed either quantitatively or qualitatively 

in the following subsections. 

5.3.1 Public Health (Accident) 

Implementation of the proposed revisions to Regulatory Guide 1.78 could potentially change the frequencies or 

consequences of core damage accidents (risk is the product of core damage frequency and consequences) at nuclear 

power plants. This attribute measures the impacts on (or changes in) public risks from accidents that could result 

from implementation of the proposed revisions. Based on the discussion provided below, it is believed that no 

appreciable change will result from this revision.

NUREG/CR-6624 24
Table 5.2. Summary of Value-Impact Assessment Results

Affected Attribute Assessment of Value or Impact 

Public Health (accident) Small effect; could be an overall increase or decrease depending on 

revised toxicity limits. 

Occupational Health (accident) Same as "Public Health" attribute. 

Offsite Property Same as "Public Health" attribute. 

Onsite Property Same as "Public Health" attribute. 

Industry Implementation Approximately $2.8 million. 

Industry Operation Potential cost savings due to fewer shutdowns and less plant downtime. 

NRC Implementation Approximately $420,000. 

NRC Operation Small; not quantified. 

Improvements in Knowledge Not quantified; favors implementation of revised Regulatory Guide 

1.78. 

Regulatory Efficiency Not quantified; favors implementation of revised Regulatory Guide 

1.78. 

The changes in the public risk as a result of the proposed revision to Regulatory Guide 1.78 are believed to be small. 

A release of hazardous chemical impacting control room habitability will not by itself result in a severe reactor 

accident. The low likelihood of simultaneous occurrence of a hazardous chemical release and core damage accident 

initiator (such as a loss of coolant accident or loss of offsite power) indicates that public health impacts are small. 

Two examples were developed to illustrate that the effects on public risks are small. The first example is for a 

transportation accident that releases a hazardous chemical. The frequency of a transportation accident on a nearby 

highway was calculated using the traffic accident rate referenced in the current regulatory guide. This was 1.69E-06 

accidents/mile. Of these accidents, 18% were postulated to result in a spill of a hazardous chemical. For this 

example, it was assumed that there are 10 miles of highway in the vicinity of a nuclear power plant and 10 

shipments of this chemical are transported per year. The overall frequency of a significant spill is the product of 

these values, or about 3.OE-05/yr. 

To result in an effect on a nuclear power plant control room operator, the hazardous chemical would have to be 

transported from the accident scene to the control room air intake. Since atmospheric transport is the only credible 

transport mechanism, the wind would have to be blowing in the direction from the accident scene to the control 

room. In addition, the weather would need to be relatively calm or the release chemicals would be dispersed 

quickly and not reach concentrations that could affect an operator's ability to perform required actions. As 

discussed above, the transportation accident itself would not lead to core damage. Therefore, this scenario would 

also require simultaneous occurrence of an internally-induced core damage initiating event followed by failure of 

automatic safety systems. These factors would reduce the frequency of this affected core damage sequence by at 

least two orders of magnitude and most likely more. Assuming a conditional probability of 0.05 for adverse 

weather conditions, the magnitude of the affected CDF is on the order of 1.5E-06/reactor-yr (RY). The adverse 

weather probability is based on use of site-specific relative concentrations, or /Q values, that are exceeded only 5% 

of the time on an overall basis, regardless of wind direction. This is consistent with guidance given in Regulatory 

Guide 1.146, Atmospheric Dispersion Models for Potential Accident Consequence Assessments at Nuclear Power 

Plants. Weather conditions that are more frequent (i.e., have a higher probability of occurrence) result in greater 

dispersion and lower relative concentrations (i.e., smaller /Q value) of the toxic chemical at the control room 

intake. This conservatively ignores the probability that the wind is blowing in the direction from the accident 

towards the control room intake. The affected CDF would be even smaller than the value given above when 

simultaneous occurrence of core damage initiating events are considered and failure probabilities of automatic 

safety systems are included in the potential core damage sequences. The changes in public risk from transportation 

accident induced core damage brought about by the proposed revisions to the regulatory guide would therefore be 

small.

25 NUREG/CR-6624
The second example is for a hazardous chemical storage tank on a nuclear power plant site. In this scenario, an 

onsite storage tank of a hazardous chemical is postulated to rupture and release its contents. The frequency of such 

an event was estimated to be I E-06/yr for double-walled storage tanks and 1 E-04/yr 

for single-walled storage tanks and pressure vessels (see the Handbook of Chemical Hazard Analysis Procedures, 

FEMA/DOT/EPA 1988). The Handbook also recommends using a spill size equivalent to a 1-inch hole 90% of the 

time and the entire tank contents 10% of the time. Combining these leads to a significant release frequency on the 

order of 1 E-04 to 1 E-05/yr. As with the transportation accident scenario, weather factors would have to be suitable 

to transport the released chemical to the control room intake in concentrations that could affect operators and an 

internal initiating event would need to simultaneously occur. Consequently, the affected CDF from a storage tank 

rupture scenario would be small. 

The proposed changes to the regulatory guide may have a mixed effect on public health. The proposed list of 

hazardous chemicals is much more comprehensive than that in the original regulatory guide. If additional chemicals 

are recognized as hazards by specific reactors, the public health risk will be lowered. Note that many of the 

proposed toxicity limits are higher than those originally in Regulatory Guide 1.78. A comparison of Regulatory 

Guide 1.78 Table C-l and Section 2 indicates that the proposed toxicity limits are higher for 24 chemicals and lower 

for only 3 chemicals (ethyl ether, ethylene dichloride, and hydrogen sulfide). Incorporating the new toxicity limits 

may result in a relaxation of control room habitability requirements and a small increase in public risk relative to 

current practices. However, the proposed revisions have a superior technical basis to the toxicity limits in the 

original Regulatory Guide. Significant research has been completed over the 25 yrs since the original Regulatory 

Guide was published. A comprehensive listing of IDLH values was not available 25 yrs ago. Also, even tough the 

proposed toxicity limits are higher than those in the original Regulatory Guide, it is not intended to expose the 

operators to the concentrations for 30 minutes, part of the definition of IDLH. Rather, it is still intended that 

operators don appropriate protective equipment within 2 minutes after toxic gas concentrations are detected. As a 

result, control room operators are not likely to be overcome or incapacitated even if toxic gases or aerosols reach the 

control room at IDLH concentrations. Furthermore, with respect to timing, a hazardous chemical spill is most likely 

to occur at some distance from the control room. It takes time for the released material to reach the control room 

intake and additional time for the concentration of toxic gases to build to a dangerous level. Since detection limits 

for the toxic gases would be set lower than the IDLH values, operators would have additional time to don protective 

equipment before a dangerous concentration builds in the control room. Consequently, operators would not be 

exposed to IDLH concentrations during the 2 minutes it takes to don protective equipment. When all this is taken 

into consideration, the proposed revisions to the toxicity limits are believed to provide adequate protection to 

control room operators and by extension, to the public. The increased risk associated with higher toxicity limits, if 

any, would be small. 

5.3.2 Occupational Health (Accident) 

The occupational health (accident) attribute measures the change in risk to plant workers that would result from 

changes in accident frequencies and consequences brought about by the proposed revisions to the regulatory guide. 

Based upon the above discussion of public health, changes to a plant's overall core damage frequency that result 

from the revisions to Regulatory Guide 1.78 are small. The radiological consequences of core damage accidents are 

not affected by the revisions. Therefore, the occupational health radiological impacts from the proposed regulatory 

action are small. 

The control room habitability systems also serve an important role in protecting control room personnel from toxic 

chemicals. Consideration of the additional chemicals in the proposed regulatory action may result in decreased 

occupational health risk. Incorporating higher toxicity limits for many of the chemicals may result in relaxation of 

requirements and a slightly higher occupational risk. As with public health risks, this would at least partially offset 

the reduced occupational risks associated with enhanced protection of control room operators. Occupational health 

impacts for the proposed regulatory action do not appear to be significant.

NUREG/CR-6624 26
5.3.3 ffsite Property 

This attribute measures the monetary effect on offsite property and is typically calculated as the product of the 

change in core damage frequency and the property consequences as a result of a core damage accident (e.g., 

interdiction, cleanup, and evacuation costs). Based upon the above discussion of public health, the change in core 

damage frequency that would result from the revisions to Regulatory Guide 1.78 is small and therefore the impacts 

of the proposed revision to Regulatory Guide 1.78 on offsite property are small. 

5.3.4 Onsite Property 

This attribute measures the monetary effect on onsite property, including replacement power, decontamination, and 

refurbishment costs. As with offsite property, this attribute is typically calculated as the product of the change in 

core damage frequency and the onsite property consequences as a result of a core damage accident. Based upon the 

above discussion of public health, the change in core damage frequency is small. Therefore, the impacts of the 

proposed revision to Regulatory Guide 1.78 on onsite property are small. 

5.3.5 Industry Implementation 

This attribute measures the expected economic effects on licensees to implement the changes required by the 

proposed regulatory guide revision. Examples include administrative, equipment, labor, and materials costs as well 

as replacement power costs where the proposed regulatory action plant shutdowns or outage extensions. In this 

case, implementation costs would consist of labor costs to update the plant's Safety Analysis Report (SAR) where 

necessary. 

The proposed revision to Regulatory Guide 1.78 adds a substantial number of hazardous chemicals to be considered 

in evaluating control room habitability. However, Table C- 1 in Regulatory Guide 1.78 was clearly described as not 

all-inclusive. If the original hazardous chemical survey was comprehensive, little cost impact is expected. Since the 

proposed toxicity limits for many chemicals seem to typically be higher than those in Table C-l of Regulatory 

Guide 1.78, little cost impact will result from new control room habitability requirements. In fact, a decrease in 

requirements could result. It is difficult to quantify industry implementation costs. For illustrative purposes, the 

following case is quantified: 

1. Reactor review of potential impacts from revised listing of chemicals and toxicity limits: 

$5,000 per reactor times 111 reactors equals $555,000 

Basis: Labor requirements to review the revised Regulatory Guide, evaluate the impacts on the plant's 

licensing basis, and document the results of the evaluations were estimated at about 2 man-wks/plant. Each 

plant would be required to perform this activity. The industry labor rate used in this calculation was 

$62.50/staff-hr. The total number of plants was taken from Appendix B of NUREG/BR-0184 (NRC 1997). 

2. 50% require no additional action; 50% require SAR update: 

$40,000 per reactor times 56 reactors equals $2,240,000 

Basis: Eight of the 16 plants listed in Table 3.2 were judged to require an update of their SAR. The other 8 

plants were judged not to require a SAR update because they had either screened out all toxic chemical 

shipments on the basis of distance from transport routes or frequent shipment screening criteria. If this is 16 

plant sample is representative of the entire population of nuclear power plants, 50% of 111 plants, or about 56 

plants, will require a SAR update. Sciacca (1989) estimated labor requirements for a complicated technical 

specification change to be 16 staff-wks of technical, legal, and management effort. This was assumed to be 

approximately the same amount of resources needed for a complicated update of a SAR. This was judged to be 

a complicated update because it requires application of computer codes. The industry labor rate used here was 

$62.50/staff-hr.

27 NUREG/CR-6624
3. 0% of these require additional personal protection or detection requirements: 

$0 

Basis: Based on the review of plant SARs and the general increase in toxicity limits associated with the 

revisions to Regulatory Guide 1.78, it was judged that no plants would require additional protection for control 

room operators from external sources of toxic chemicals. 

The total industry implementation costs are therefore estimated at about $2.8 million. 

5.3.6 Industry Operation 

This attribute measures the economic impacts on the licensees of routine or recurring activities that result from the 

new requirements. As discussed above, minimal new requirements are assumed and there may be a potential for a 

decrease in the requirements for control room habitability systems as a result of increased toxicity limits. It is 

possible, although unlikely, that a re-analysis could be used to justify relaxation of technical specifications, 

surveillance and maintenance programs, and other requirements associated with operation of chemical detection 

systems, operator training, etc., that support control room habitability programs. Increases in control room toxic 

chemical limits could result in using less sensitive detectors, requiring lower air exchange rates, no isolation valves, 

smaller filters, and smaller air ducts. Smaller and less complex systems generally require fewer resources to inspect 

and maintain. However, the cost savings on existing control rooms is likely to be small because the ventilation and 

chemical detection systems are already in place. 

The greatest cost savings potential results from reducing plant shutdowns from spurious actuation of chemical 

detection systems, including malfunctions of toxic chemical detection systems that lead to actuation of the control 

room emergency ventilation system. A preliminary review of Licensee Event Reports (LERs) indicated there were 

numerous occurrences of control room emergency ventilation system actuations but few of them actually lead to 

plant shutdown. Most LERs indicated that the plants entered a Technical Specification action statement and were 

able to correct the problems before plant shutdown was required. The most common component malfunctions were 

radiation monitors on the control room intake supply system, which are not in the scope of Regulatory Guide 1.78. 

Chlorine gas detectors were the next most frequently mentioned components. 

These cost savings are difficult to estimate but could be substantial. To illustrate this, assume that increasing the 

allowable toxic chemical concentrations will increase plant capacity by one day per year for the entire population of 

nuclear power plants. Assuming replacement power costs are $480,000/day (NRC 1997) and the average remaining 

lifetimes of the plants is about 20 yr (approximated from information in Appendix B of NRC 1997), this savings 

could be about $10 million over the next 20 yr. This conservative estimate would more than offset the increased 

industry costs to implement the revisions to Regulatory Guide 1.78. 

5.3.7 NRC Implementation 

NRC implementation is an attribute that measures the economic impacts of the NRC to place the proposed revision 

to Regulatory Guide 1.78 into action. NRC implementations costs consist of the costs to issue revised Regulatory 

Guide 1.78 and the costs associated with reviewing the required initial licensee analyses described in the Industry 

Implementation section. 

The Regulatory Analysis Technical Evaluation Handbook (NRC 1997) provides some approximate costs for NRC 

Implementation. For a non-controversial amendment to an existing rule or regulation, one professional NRC staff 

person-year at a cost of $122,000 is estimated to be required. A cost of $122,000 is assumed required to complete 

and issue the revision of Regulatory Guide 1.78. 

As discussed in the Industry Implementation section, 50% of the plants or about 56 plants are estimated to require 

SAR updates that must be reviewed and approved by the NRC. It is estimated that 2 NRC staff-wks of labor are 

required to review and approve each plant's submittal. Therefore, about 112 staff-wks of NRC labor are needed. At

NUREG/CR-6624 28
a NRC labor rate of $67.50/staff-hr (NRC 1997), the total NRC costs to review and approve these analyses are 

estimated at about $300,000. The total NRC implementation cost is therefore about $420,000. 

5.3.8 NRC Operation 

NRC operation is an attribute that measures the economic effects on the NRC of routine or recurring activities (e.g., 

additional inspection and enforcement activities) necessary to monitor licensee implementation of the proposed 

revisions to the regulatory guide. 

Currently, the NRC monitors implementation of the existing Regulatory Guide. Periodic reviews and inspections 

and other recurring activities by NRC staff are anticipated to change little with respect to the current efforts to 

monitor implementation of the Regulatory Guide. Therefore, the incremental costs would be small with respect to 

the costs of current activities so they were not quantified. 

5.3.9 Improvements in Knowledge 

This attribute accounts for the potential value of new information on the safety of licensee activities. The 

recommendation to replace the chemicals and limits in Regulatory Guide 1.78 with the more recent IDLH values 

represents an improvement in knowledge. The more recent NIOSH IDLH values represent many years of testing 

and toxicity modeling that were not available when the current version of Regulatory Guide 1.78 was issued. The 

IDLH values have a sound technical basis and the list of hazardous chemicals for which IDLH values exist is more 

comprehensive than reflected in the Regulatory Guide. Furthermore, improvements to the meteorological and 

control room ventilation flow models represent improvements in the ability to estimate toxic chemical 

concentrations in control rooms and improvements in the accuracy of the results. Consequently, this attribute 

favors the proposed revisions to Regulatory Guide 1.78. 

5.3.10 Regulatory Efficiency 

This attribute attempts to measure regulatory and compliance improvements. The revision of Regulatory Guide 

1.78 to include a comprehensive list of chemicals and toxicity limits generated by NIOSH will result in more 

consistent and comprehensive control room habitability evaluations. Improvements in atmospheric dispersion and 

control room ventilation flow models, although not expected to enhance regulatory efficiency, increase confidence 

in the results. Implementation of the revisions is also anticipated to reduce licensee burden by incorporating new 

data that is more appropriate and more accurate than the original basis for Regulatory Guide 1.78. In addition, 

clarification that the definition of "frequent shipments" includes all hazardous chemical shipments will increase 

regulatory efficiency by reducing each licensee's need to interpret the old guidance and the NRCs need to review 

and approve the possible varying interpretations. Therefore, this attribute favors the proposed revisions to the 

regulatory guide.

29 NUREG/CR-6624
6.0 Recommended Changes to Regulatory Guide 1.78 

6.1 Use of IDLH in Regulatory Guide 1.78 

We reviewed the IDLH concentrations for the purpose of using them to replace the toxicity limits and expand the 

number of chemicals in Regulatory Guide 1.78. We found that the IDLH concentrations represent reasonable limits 

to provide adequate time to don protective apparel and determined that they would provide an adequate margin of 

safety for protecting the operators. Therefore, we recommend that the IDLH values replace the chemical toxicity 

limits listed in Regulatory Guide 1.78. IDLH values of most chemicals not listed in the Regulatory Guide 1.78 are 

also appropriate to use as toxicity limits if this list requires expanding. 

6.1.1 Table C-1 

We recommend that the NIOSH Pocket Guide (NIOSH, 1997) be referenced in the revised Regulatory Guide 1.78. 

This document is widely used in industry and is a readily available resource. One option is to replaceTable C-1 in 

Regulatory Guide 1.78 with Appendix A of this report. Alternatively, NRC may wish to continue to provide a list of 

"example" chemicals with their toxicity limits (Table C-I). In this case, Table C-I would not necessarily need to be 

extensively revised. However, selection of the chemicals listed could include those used in nuclear power plants and 

some of the more common chemicals transported in large quantities. In addition, any chemical not listed in the 

"Pocket Guide" or any for which a value other than the IDLH is being used as an exposure limit should be included 

(could include chlorine and Halon). 

6.2 Recommended Transport Accident Revisions to Regulatory Guide 1.78 

The recent accident rate data used to derive the definitions of frequent shipments in Regulatory Guide 1.78 were 

examined to determine if an update to the Regulatory Guide is warranted. It was observed that the accident rates 

given in the recent studies are relatively close to each other and to the accident rates used in Regulatory Guide 1.78. 

The spread between the highest and lowest accident rates is small. A general downward trend in accident rates for 

motor vehicle, rail, and water transportation was also observed over time. Based on this review, it is recommended 

that the screening criteria based on distance from the control room and frequency of hazardous chemical shipments 

within 5 miles of the control room be retained in Regulatory Guide 1.78. The definitions of frequent accidents given 

in Regulatory Guide 1.78 are recommended to be retained in their present form. It is also recommended that 

clarification be added to the Regulatory Guide to indicate that the frequent shipment criteria refer to total shipments 

irrespective of the nature of the chemicals. 

6.2.1 Table C-2 

Table C-2, which provides an example of the allowable weights of hazardous chemicals as a function of distance, 

would need to be revised to reflect application of the recommended meteorological and ventilation flow models to 

be incorporated in the revised regulatory guide. In addition, it is recommended that the last sentence from footnote 

(4) on page 1.78-2 be deleted, as there is no need to retain the reference to the "Guide for Emergency Services for 

Hazardous Materials." 

If the computer codes (see section 4) are used in the revision of Regulatory Guide 1.78, Table C-2 would need to be 

revised to reflect distance/weight relationships calculated by the codes. Appendix A could probably be deleted as 

the codes would have to be implemented to adjust the distance/weight relationships for specific chemical toxicities, 

control room airflow rates, and Pasquil stability. Appendix B would have to be revised to incorporate the 

recommendation to change from the hand calculation approach in the current Regulatory Guide to the computer 

codes described below.

NUREG/CR-6624 30
6.3 Computer Codes

It is recommended that the current Appendix B be replaced in its entirety by a brief description of the 

meteorological and control room ventilation flow models (EXTRAN and CHEM code models), which are 

components of the Control Room Habitability software package, HABIT. Minor revisions to Regulatory Positions 

C.5 and C.6 are also recommended as outlined below. 

Regulatory Position C.5. Chemical Release Amounts 

The EXTRAN and CHEM portions of the HABIT computer codes (Stage, 1995 and Ramsdell, 1991) may be used 

to estimate the rates of release, atmospheric dispersion, and subsequent control room concentrations of toxic 

chemicals. If another computer program is used, it should consider physical processes similar to those considered in 

CHEM and EXTRAN. 

Two types of industrial accidents should be considered for each source of hazardous chemicals: maximum 

concentration chemical accidents and maximum concentration-duration chemical accidents. 

a. For a maximum concentration accident the quantity of the chemical to be considered is the 

instantaneous release of the total contents of one of the following (1) the largest storage container 

falling within the guidelines of Table C-2 and located at a nearby facility, (2) the largest shipping 

container (or for multiple containers of equal size, the failure of only one container unless the failure 

of that container could lead to successive failures) falling within the guidelines of Table C-2 and 

frequently transported near the site, or (3) the largest container stored onsite (normally the total release 

from this container unless the containers are interconnected so that a single failure could cause a 

release from several containers.) 

For chemicals that are not gases at I 00°F and normal atmospheric pressure but are liquids with vapor 

pressures in excess of 10 torr, consideration should be given to the rate of flashing and boiloff to 

determine the rate of release to the atmosphere and the appropriate time duration of the release. In 

situations where liquid pools may form on the ground or other surfaces, evaporation from such pools 

should also be considered. 

b. For a maximum concentration-duration accident, the continuous release of hazardous chemicals from 

the largest safety relief valve in a stationary, mobile, or onsite source falling within the guidelines of 

Table C-2 should be considered. 

Regulatory Position C.6. Atmospheric Dispersion 

The atmospheric diffusion model to be used in the evaluation should be the same as or similar to the model 

presented in Chapter 6 of NUREG/CR-6210, Computer Codes for Evaluation of Control Room Habitability 

(HABIT) (Stage 1995, Ramsdell and Stage 1998) and presented in Appendix B of this guide. The model in the 

appendix allows for dispersion in the vertical direction when the distance between the release point and the control 

room is small. The model assumes uniform mixing between the ground and the elevation of the fresh air inlet (a 15

m elevation from ground level is assumed). 

The value of the atmospheric dilution factor between the release point and the control room that is used in the 

analysis should be the value that is exceeded only 5% of the time. Techniques for determining this value may be 

found in Ramsdell (1995, 1997). 

When the boiloff or a slow leak is analyzed, the effects of density on vertical diffusion may be considered if 

adequately substantiated by reference to data from experiments. Density effects of heavier-than-air gases should not

31 NUREG/CR-6624
be considered for releases of a violent nature or for release material that becomes entrained in the turbulent air near 

buildings. 

In evaluating dispersion, formulas should be used that give a good representation of data for low wind cases 

(Ramsdell, 1994). 

Additional credit due to building wake or other dispersive phenomena may be allowed, depending on the properties 

of the released gas, the method of release, and the intervening topology or structures. 

Appendix B: 

It is recommended that this Appendix be replaced in its entirety to reflect improvements made meteorological and 

ventilation flow models since publication of Regulatory Guide 1.78 in 1974. The entire revised Appendix B was 

presented in Chapter 4 and will not be repeated here.

NUREG/CR-6624 32
7.0 References

AEC (U.S. Atomic Energy Commission). Environmental Survey of Transportation of Radioactive Materials To 

and From Nuclear Power Plants. USAEC, Washington, D.C. (1972). 

AEC (U.S. Atomic Energy Commission ). Regulatory Guide 1.78: Assumptions for Evaluating the Habitability of 

a Nuclear Power Plant Control Room During a Postulated Hazardous Chemical Release. USAEC, 

Washington, D.C. (1994). 

AIHA (American Industrial Hygiene Association). Emergency Response Planning Guidelines. AIHA Akron, OH. 

(1988-1993). 

AIHA (American Industrial Hygiene Association). Odor thresholds for chemicals with established occupational health standards. AIHA Akron, OH. (1989). 

Alexeeff, G. V., M. J. Lipsett, and K. W. Kizer. Problems Associated with the Use of Immediately Dangerous to 

Life and Health (IDLH) Values for Estimating the Hazard of Accidental Chemical Releases. Am. Ind Hyg. 

Assoc. J. 50:598-605 (1989). 

Amoore, J. E., and E. Hautala. Odor as an aid to chemical safety: odor thresholds compared with threshold limit 

values and volatilities for 214 industrial chemicals in air and water dilution. J. Appl. Toxicol. 3:272-290 

(1983). 

Andrews, W. B. et al. An Assessment of the Risk of Transporting Liquid Chlorine by Rail. PNL-3376. Pacific 

Northwest Laboratory, Richland, Washington. (1980). 

Bander, T. J. PA VAN.- An atmospheric Dispersion Program for Evaluating Design Basis Accidental Releases of Radioactive Materials from Nuclear Power Stations. NUREG/CR-2858, PNL-4413, prepared by Pacific 

Northwest Laboratory for the U. S. Nuclear Regulatory Commission, Washington, D.C. (1982). 

Clarke, R. K., J. T. Foley, W. F. Hartman, and D. W. Larson. Severities of Transportation Accidents. 

SLA-74-0001. Sandia National Laboratory, Albuquerque, New Mexico. (1976). 

DOT (U.S. Department of Transportation). Transportation Statistics Annual Report, 1998. BTS98-S-01. Bureau 

of Transportation Statistics, Washington, D.C. (1998a). (available electronically at URL 

http://www.bts.gov/ programs/transtultsar.html) 

DOT (U.S. Department of Transportation). National Transportation Statistics, 1998. Bureau of Transportation Statistics, Washington, D.C. (1998b). (available electronically at URL http://www.bts.gov/programs/ 

btsprod/nts) 

Eimutis, E. C., and M. G. Konicek. Deprivations of continuous functions for the lateral and vertical atmospheric 

dispersion coefficients. Atmos. Environ. 6:859-863 (1972). 

FEMA/DOT/EPA (Federal Emergency Management Agency, U.S. Department of Transportation, and 

Environmental Protection Agency).Handbook of Chemical Hazard Analysis Procedures. Washington, D.C. 

(1988). 

Fischer, L.F. et al. Shipping Container Response to Severe Highway and Railway Accident Conditions. 

NUREG/CR-4829. Lawrence Livermore National Laboratory, Livermore, California. (1987). 

Geffen, C. A. et al. An Assessment of the Risk of Transporting Propane by Truck and Train. PNL-3308. Pacific 

Northwest Laboratory, Richland, Washington. (1980). 

Ludwig, H.R., S.G. Cairelli, and J.J. Whalen. Documentation for Immediately Dangerous to Life or Health 

Concentrations (IDLH). National Institute for Occupational Safety and Health. Cincinnati, OH. (1994).

33 NUREG/CR-6624
Room Operators. NUREG/CR-5669. U. S. Nuclear Regulatory Commission, Washington, DC. (1991). 

NAS (National Academy of Sciences), National Research Council. Emergency and Continuous Exposure Limits for 

Selected Airborne Contaminants. Vol. 1-8. Washington, DC. National Academy Press. (1983-1988). 

NIOSH (National Institute for Occupational Safety and Health), U.S. Department of Health Services, Public Health 

Service, Center for Disease Control. NIOSH Pocket Guide to Chemical Hazards. U.S. Government Printing 

Office, Washington, DC. (1997). 

NRC (U.S. Nuclear Regulatory Commission). Assumptions for Evaluating the Habitability of a Nuclear Power 

Plant Control Room During a Postulated Hazardous Chemical Release. Regulatory Guide 1.78. 

Washington, D.C. (1974). 

NRC (U. S. Nuclear Regulatory Commission). Final Environmental Statement on the Transportation of 

Radioactive Material by Air and Other Modes. NUREG-0170. Washington, D.C. (1977a). 

NRC (U. S. Nuclear Regulatory Commission). Protection of Nuclear Power Plant Control Room Operators 

Against An Accidental Chlorine Release. Regulatory Guide 1.95. Washington, D.C. (1977b). 

NRC (U. S. Nuclear Regulatory Commission). Evaluation of Explosions Postulated to Occur on Transportation 

Routes Near Nuclear Power Plant Sites. Regulatory Guide 1.91. Washington, D.C. (1978). 

NRC (U.S. Nuclear Regulatory Commission). Regulatory Analysis Guidelines of the U.S. Nuclear Regulatory 

Commission. NUREG/BR-0058, Rev. 2. Washington, D.C. (1995). 

NRC (U.S. Nuclear Regulatory Commission). Regulatory Analysis Technical Evaluation Handbook 

NUREG/BR-0184. Washington, D.C. (1997). 

Ramsdell, J. V. Atmospheric Diffusion for Control Room Habitablilty Assessments. NUREG/CR-5055, PNL

6391, prepared by Pacific Northwest Laboratory for the U. S. Nuclear Regulatory Commission, Washington, 

D.C. (1988). 

Ramsdell, J. V. Alternatives to Current Procedures Used to Estimate Concentrations in Building Wakes. PNL

SA-17941. Presented at the 21st DOE/NRC Nuclear Air Cleaning Conference, August 13-16, 1990, San 

Diego, CA. (1990a). 

Ramsdell, J. V. Diffusion in Building Wakes for Ground-Level Releases. Atmos. Environ. 24B (3):377-388. 

(1990b). 

Ramsdell, J. V. EXTRAN: A Computer Code for Estimating Concentrations of Toxic Substances at Control 

Room Air Intakes. NUREG/CR-5656, prepared by Pacific Northwest Laboratory for the U. S. Nuclear 

Regulatory Commission, Washington, D.C. (1991). 

Ramsdell, J. V. Atmospheric Dispersion Estimates in the Vicinity of Buildings. PNL-10286, prepared by Pacific 

Northwest Laboratory for the U. S. Nuclear Regulatory Commission, Washington, D.C. (1994). 

Ramsdell, J. V. Jr., C. A. Simonen and S. B. Smyth. Atmospheric Relative Concentrations in Building Wakes. 

NUREG/CR-633 1. U.S. Nuclear Regulatory Commission, Washington, D.C. (1995). 

Ramsdell, J. V. Jr. and C. A. Simonen. Atmospheric Relative Concentrations in Building Wakes. NUREG/CR

6331 Rev. 1. U.S. Nuclear Regulatory Commission, Washington, D.C. (1997). 

Ramsdell, J.V., and S. A. Stage. 1998. Computer Codes for Evaluation of Control Room Habitability (HABIT 

V1.1). NUREG/CR-6210, Supp. 1, PNNL-10496. Pacific Northwest National Laboratory, Richland, 

Washington. 

Rhoads, R. E. et al. An Assessment of the Risk of Transporting Gasoline by Truck. PNL-2133. Pacific Northwest 

Laboratory, Richland, Washington. (1978).

NUREG/CR-6624 34
Rhoads, R. E. et al. An Assessment of the Risk of Transporting Gasoline by Truck. PNL-2133. Pacific Northwest 

Laboratory, Richland, Washington. (1978). 

Sagendorf, J. F., B. L. Goll. And W. F. Sandusky. XOQDOQ: Computer Program for the Meteorological 

Evaluation of Routine Effluent Releases at Nuclear Power Stations. PNL-43 80. Prepared by Pacific Northwest 

Laboratory for the U. S. Nuclear Regulatory Commission, Washington, D.C. (1983). 

Sax, N. I. Dangerous Properties of Industrial Materials. Third Edition, Reinhold Book Corp., New York, NY 

(1968). 

Stage, S. A. Computer Codes for Evaluation of Control Room Habitability (HABIT). NUREG/CR-6210, 

prepared by Pacific Northwest Laboratory for the U. S. Nuclear Regulatory Commission, Washington, D.C. 

(1996).

35 NUREG/CR-6624
Appendix A. Revised IDLH Values Established by NIOSH

Chemical Name CAS Numbert IDLH Values

1 

2 

3 

4 

5 

6 

7 

8 

9 

10 

11 

12 

13 

14 

15 

16 

17 

18 

19 

20 

21 

22 

23 

24 

25 

26 

27 

28 

29 

30 

31 

32 

33

Acetaldehyde 

Acetic acid 

Acetic anhydride 

Acetone 

Acetonitrile 

2-Acetylaminofluorene 

Acetylene tetrabromide 

Acrolein 

Acrylamide 

Acrylonitrile 

Aldrin 

Allyl alcohol 

Allyl chloride 

Allyl glycidyl ether 

4-Aminodiphenyl 

2-Aminopyridine 

Ammonia 

Ammonium sulfamate 

n-Amyl acetate 

sec-Amyl acetate 

Aniline and homologs 

Anisidine (o-, p-isomers) 

Antimony and compounds 

ANTU (alpha-naphthyl-thiourea) 

Arsenic (inorganic compounds) 

Arsine 

Asbestos 

Azinphos-methyl 

Barium compounds (Ba(N03)2) 

Barium compounds (BaCI2) 

Benzene 

Benzidine 

Benzoyl peroxide

10022-31-8 

10361-37-2 

71-43-2 

92-87-5 

94-36-0

75-07-0 

64-19-7 

108-24-7 

67-64-1 

75-05-8 

53-96-3 

79-27-6 

107-02-8 

79-06-1 

107-13-1 

309-00-2 

107-18-6 

107-05-1 

106-92-3 

92-67-1 

504-29-0 

7664-41-7 

7773 -06-0 

628-63-7 

626-38-0 

62-53-3 

o: 90-04-0; p: 104-94-9 

7440-36-0 

86-88-4 

7740-38-2 

7784-42-1 

1332-21-4 

86-50-0

NUREG/CR-6624

2000 ppm 

50 ppm 

200 ppm 

2500 ppm* 

500 ppm 

Ca 

8 ppm 

2 ppm 

60 mg/m3 

85 ppm 

25 mg/m3 

20 ppm 

250 ppm 

50 ppm 

Ca 

5 ppm 

300 ppm 

1500 mg/m3 

1000 ppm 

1000 ppm 

100 ppm 

50 mg/m3 

50 mg Sb/m3 

100 mg/m3 

5 mg As/m 3 

3 ppm 

Ca 

10 mg/m 3 

50 mg Ba/m3 

50 mg Ba/m3 

500 ppm 

Ca 

1500 mg/m3

36
34 

35 

36 

37 

38 

39 

40 

41 

42 

43 

44 

45 

46 

47 

48 

49 

50 

51 

52 

53 

54 

55 

56 

57 

58 

59 

60 

61 

62 

63 

64 

65 

66 

67 

68 

69 

70 

71

Benzyl chloride 

Beryllium (and compounds) 

Boronoxide 

Boron trifluoride 

Bromine 

Bromoform 

1,3-Butadiene 

2-Butanone 

2-Butoxyethanol 

n-Butyl acetate 

sec-Butyl acetate 

tert-Butyl acetate 

n-Butyl alcohol 

sec-Butyl alcohol 

tert-Butyl alcohol 

Butylamine 

tert-Butyl chromate 

n-Butyl glycidyl ether 

Butyl mercaptan 

p-tert-Butyltoluene 

Cadmium dust 

Cadmium fume 

Calcium arsenate 

Calcium oxide 

Camphor (synthetic) 

Carbaryl (Sevin®) 

Carbon black 

Carbon dioxide 

Carbon disulfide 

Carbon monoxide 

Carbon tetrachloride 

Chlordane 

Chlorinated camphene 

Chlorinated diphenyl oxide 

Chlorine 

Chlorine dioxide 

Chlorine trifluoride 

Chloroacetaldehyde

100-44-7 

7440-41-7 

1303-86-2 

7637-07-2 

7726-95-6 

75-25-2 

106-99-0 

78-93-3 

111-76-2 

123-86-4 

105-46-4 

540-88-5 

71-36-3 

78-92-2 

75-65-0 

109-73-9 

1189-85-1 

2426-08-6 

109-79-5 

98-51-1 

7440-43-9 

1306-19-0 

7778-44-1 

1305-78-8 

76-22-2 

63-25-2 

1333-86-4 

124-38-9 

75-15-0 

630-08-0 

56-23 -5 

57-74-9 

8001-35-2 

55720-99-5 

7782-50-5 

10049-04-4 

7790-91-2 

107-20-0

NUREG/CR-6624

10 ppm 

4 mg Be/m3 

2000 mg/m3 

25 ppm 

3 ppm 

850 ppm 

2000 ppm* 

3000 ppm 

700 ppm 

1700 ppm * 

1700 ppm * 

1500 ppm* 

1400 ppm* 

2000 ppm 

1600 ppm 

300 ppm 

15 mg Cr(VI) /m3 

250 ppm 

500 ppm 

100 ppm 

9 mg Cd/m3 

9 mg Cd/m3 

5 mg As/m3 

25 mg/m3 

200 mg/m3 

100 mg/m3 

1750 mg/m3 

40000 ppm 

500 ppm 

1200 ppm 

200 ppm 

100 mg/m 3 

200 mg/m3 

5 mg/m3 

10 ppm 

5 ppm 

20 ppm 

45 ppm

37
72 

73 

74 

75 

76 

77 

78 

79 

80 

81 

82 

83 

84 

85 

86 

87 

88 

89 

90 

91 

92 

93 

94 

95 

96 

97 

98 

99 

100 

101 

102 

103 

104 

105 

106 

107 

108 

109

alpha-Chloroacetophenone 

Chlorobenzene 

o-Chlorobenzylidine malononitrile 

Chlorobromomethane 

Chlorodiphenyl (42% Cl) 

Chlorodiphenyl (54% Cl) 

Chloroform 

bis-Chloromethyl ether 

Chloromethyl methyl ether 

1-Chloro-l-nitropropane 

Chloropicrin 

beta-Chloroprene 

Chromic acid and chromates 

Chromium metal 

Chromium (11) compounds 

Chromium (III) compounds 

Coal tar pitch volatiles 

Cobalt metal, dust, and fume 

Copper dusts and mists 

Copper fume (Cu) 

Copper fume (CuO) 

Cotton dust (raw) 

Crag® herbicide 

Cresol (all isomers) 

Crotonaldehyde (trans-isomer) 

Cumene 

Cyanides (KCN) 

Cyanides (NaCN) 

Cyclohexane 

Cyclohexanol 

Cyclohexanone 

Cyclohexene 

Cyclopentadiene 

2,4-D (2-4-dichlorophenoxyacetic acid) 

DDT (divhlorodiphenyltrichloroethane) 

Decaborane 

Demeton 

Diacetone alcohol

532-27-4 

108-90-7 

269841-1 

74-97-5 

53469-21-9 

11097-69-1 

67-66-3 

542-88-1 

107-30-2 

600-25-9 

76-06-2 

126-99-8 

7738-94-5 

7440-47-3 

varies 

varies 

65996-93-2 

7440484 

7440-50-8 

7440-50-8 

1317-38-0 

none 

136-78-7 

1319-77-3 

123-73-9 

98-82-8 

151-50-8 

143-33-9 

110-82-7 

108-93-0 

108-94-1 

110-83-8 

542-92-7 

94-75-7 

50-29-3 

1770241-9 

8065-48-3 

12342-2

NUREG/CR-6624

15 mg/m3 

1000 ppm 

2 mg/m3 

2000 ppm 

5 mg/m3 

5 mg/m3 

500 ppm 

Ca 

Ca 

100 ppm 

2 ppm 

300 ppm 

15 mg Cr(VI)/m3 

250 mg Cr/m3 

250 mg Cr(II)/m3 

25 mg Cr(III)/m3 

80 mg/m3 

20 mg Co/m3 

100 mg Cu/m 3 

100 mg Cu/m 3 

100 mg Cu/m 3 

500 mg/m3 

500 mg/m3 

250 ppm 

50 ppm 

900 ppm* 

25 mg/m3(as CN) 

25 mg/m3 (as CN) 

1300 ppm * 

400 ppm 

700 ppm 

2000 ppm 

750 ppm 

100 mg/m3 

500 mg/m3 

15 mg/m3 

10 mg/m3 

1800 ppm *

38
110 

Ill 

112 

113 

114 

115 

116 

117 

118 

119 

120 

121 

122 

123 

124 

125 

126 

127 

128 

129 

130 

131 

132 

133 

134 

135 

136 

137 

138 

139 

140 

141 

142 

143 

144 

145 

146 

147

NUREG/CR-6624

Diazomethane 

Diborane 

1,2-Dibromo-3-chloropropane 

Dibutyl phosphate 

Dibutylphthalate 

0-Dichlorobenzene 

p-Dichlorobenzene 

3,3'-Dichlorobenzidine 

Dichlorodifluoromethane 

1,3-Dichloro-5,5- dimethylhydantoin 

1,1-Dichloroethane 

1,2-Dichloroethylene 

Dichloroethyl ether 

Dichloromonofluoromethane 

1,1 -Dichloro- 1 -nitroethane 

Dichlorotetrafluoroethane 

Dichlorvos 

Dieldrin 

Diethylarnine 

2-Diethylaminoethanol 

Difluorodibromomethane (Halon 1202) 

Diglycidyl ether 

Diisobutyl ketone 

Diisopropylamine 

Dimethyl acetamide 

Dimethylamine 

4-Dimethylarninoazobenzene 

Dimethylaniline 

Dimethyl- 1, 2-dibromo-2, 

2-dichlorethyl phosphate 

Dimethylformamide 

1,1 -Dimethylhydrazine 

Dimethylphthalate 

Dimethyl sulfate 

Dinitrobenzene (all isomers) 

Dinitro-o-cresol 

Dinitrotoluene 

Di-sec octyl phthalate 

Dioxane

334-88-3 

19287-45-7 

96-12-8 

107-66-4 

84-74-2 

95-50-1 

106-46-7 

91-94-1 

75-71-8 

118-52-5 

75-34-3 

540-59-0 

111-44-4 

75-43-4 

594-72-9 

76-14-2 

62-73-7 

60-57-1 

109-89-7 

100-37-8 

75-61-6 

2238-07-5 

108-83-8 

108-18-9 

127-19-5 

124-40-3 

60-11-7 

121-69-7 

300-76-5 

68-12-2 

57-14-7 

131-11-3 

77-78-1 

100-25-4 

534-52-1 

25321-14-6 

117-81-7 

123-91-1

2 ppm 

15 ppm 

Ca 

30 ppm 

4000 mg/m3 

200 ppm 

150 ppm 

Ca 

15000 ppm 

5 mg/m3 

3000 ppm 

1000 ppm 

100 ppm 

5000 ppm 

25 ppm 

15000 ppm 

100 mg/m3 

50 mg/m3 

200 ppm 

100 ppm 

2000 ppm 

10 ppm 

500.ppm 

200 ppm 

300 ppm 

500 ppm 

Ca 

100 ppm 

200 mg/m3 

500 ppm 

15 ppm 

2000 mg/m3 

7 ppm 

50 mg/m3 

5 mg/m3 

50 mg/m3 

5000 mg/m3 

500 ppm

39
148 Diphenyl 92-52-4 100 mg/m3 

149 Dipropylene glycol methyl ether 34590-94-8 600 ppm 

150 Endrin 72-20-8 2 mg/m3 

151 Epichlorohydrin 106-89-8 75 ppm 

152 EPN 2104-64-5 5 mg/m3 

(ethyl p-nitrophenyl thionobenzene phosphonate) 

153 Ethanolamine 141-43-5 30 ppm 

154 2-Ethoxyethanol 110-80-5 500 ppm 

155 2-Ethoxyethyl acetate 111-15-9 500 ppm 

156 Ethyl acetate 141-78-6 2000 ppm* 

157 Ethyl acrylate 140-88-5 300 pm 

158 Ethylamine 75-04-7 600 ppm 

159 Ethylbenzene 100-41-4 800 ppm* 

160 Ethyl bromide 74-96-4 2000 ppm 

161 Ethyl butyl ketone 106-35-4 1000 ppm 

162 Ethyl chloride 75-00-3 3800 ppm * 

163 Ethylene chlorohydrin 107-07-3 7 ppm 

164 Ethylenediamine 107-15-3 1000 ppm 

165 Ethylene dibromide 106-93-4 100 ppm 

166 Ethylene dichloride 107-06-2 50 ppm 

167 Ethylene glycol dinitrate 628-96-6 75 mg/m3 

168 Ethyleneimine 151-56-4 100 ppm 

169 Ethylene oxide 75-21-8 800 ppm 

170 Ethyl ether 60-29-7 1900 ppm* 

171 Ethyl formate 109-94-4 1500 ppm 

172 Ethyl mercaptan 75-08-1 500 ppm 

173 N-Ethylmorpholine 100-74-3 100 ppm 

174 Ethyl silicate 78-10-4 700 ppm 

175 Ferbam 14484-64-1 800 mg/m3 

176 Ferrovanadium dust 12604-58-9 500 mg/m3 

177 Fluorides (NaF) 7681-49-4 250 mg F/m3 

178 Fluorides (Na3A1F6) 15096-52-3 250 mg F/m 3 

179 Fluorine 7782-41-4 25 ppm 

180 Fluorotrichloromethane 75-69-4 2000 ppm 

181 Formaldehyde 50-00-0 20 ppm 

182 Formic acid 64-18-6 30 ppm 

183 Furfural 98-01-1 100 ppm

NUREG/CR-6624 40
184 Furfuryl alcohol

185 Glycidol 

186 Graphite

Hafnium and compounds 

Halon 1211 

Halon 1301 (Trifluorobromomethane) 

Heptachlor 

n-Heptane 

Hexachloroethane 

Hexachloronaphthalene 

n-Hexane 

2-Hexanone 

Hexone 

sec-Hexyl acetate 

Hydrazine 

Hydrogen bromide 

Hydrogen chloride 

Hydrogen cyanide 

Hydrogen fluoride 

Hydrogen peroxide 

Hydrogen selenide 

Hydrogen sulfide 

Hydroquinone

187 

188 

189 

190 

191 

192 

193 

194 

195 

196 

197 

198 

199 

200 

201 

202 

203 

204 

205 

206 

207 

208 

209 

210 

211 

212 

213 

214 

215 

216 

217 

218 

219

556-52-5 

7782-42-5 

7440-58-6 

353-59-3 

75-63-8 

76-44-8 

142-82-5 

67-72-1 

1335-87-1 

110-54-3 

591-78-6 

108-10-1 

108-84-9 

302-01-2 

10035-10-6 

7647-01-0 

74-90-8 

7664-39-3 

7722-84-1 

7783-07-5 

7783-06-4 

123-31-9 

7553-56-2 

1309-37-1 

123-92-2 

123-51-3 

528-75-4 

110-19-0 

78-83-1 

78-59-1 

108-21-4 

67-63-0 

75-31-0 

108-20-3 

4016-14-2

150 ppm 

1250 mg/m 3

50 mg Hf/m3 

none (20000 ppm)** 

40000 ppm 

35 mg/m3 

750 ppm 

300 ppm 

2 mg/m3 

1100 ppm* 

1600 ppm 

500 ppm 

500 ppm 

50 ppm 

30 ppm 

50 ppm 

50 ppm 

30 ppm 

75 ppm 

1 ppm 

100 ppm 

50 mg/m3

2 ppm 

2500 mg Fe/m3 

1000 ppm 

500 ppm 

500 ppm 

1300 ppm* 

1600 ppm 

200 ppm 

1800 ppm 

2000 ppm* 

750 ppm 

1400 ppm* 

400 ppm

NUREG/CR-6624

Iodine 

Iron oxide dust and fume 

Isoamyl acetate 

Isoamyl alcohol (primary) 

Isoamyl alcohol (secondary) 

Isobutyl acetate 

Isobutyl alcohol 

Isophorone 

Isopropyl acetate 

Isopropyl alcohol 

Isopropylamine 

Isopropyl ether 

Isopropyl glycidyl ether

98-00-0 75 ppmn

41
220 Ketene

221 

222 

223 

224 

225 

226 

227 

228 

229 

230 

231 

232 

233 

234 

235 

236 

237 

238 

239 

240 

241 

242 

243 

244 

245 

246 

247 

248 

249 

250 

251 

252 

253 

254 

255 

256

Lead 

Lindane 

Lithium hydride 

L.P.G. (Liquified petroleum gas) 

Magnesium oxide fume 

Malathion 

Maleic anhydride 

Manganese compounds 

Mercury vapor and compounds 

Mercury (organo) alkyl compounds 

Mesityl oxide 

Methoxychlor 

Methyl acetate 

Methyl acetylene 

Methyl acetylene-propadiene mixture 

Methyl acrylate 

Methylal 

Methyl alcohol 

Methylamine 

Methyl (n-amyl) ketone 

Methyl bromide 

Methyl Cellosolve® 

Methyl Cellosolve® acetate 

Methyl chloride 

Methyl chloroform 

Methylcyclohexane 

Methylcyclohexanol 

o-Methylcyclohexanone 

Methylene bisphenyl isocyanate 

Methylene chloride 

Methyl formate 

5-Methyl-3-heptanone 

Methyl hydrazine 

Methyl iodide 

Methyl isobutyl carbinol 

Methyl isocyanate

7439-92-1 

58-89-9 

7580-67-8 

68476-85-7 

1309-48-4 

121-75-5 

108-31-6 

7439-96-5 

7439-97-6 

varies 

141-79-7 

72-43-5 

79-20-9 

74-99-7 

59355-75-8 

96-33-3 

109-87-5 

67-56-1 

74-89-5 

110-43-0 

74-83 -9 

109-86-4 

110-49-6 

74-87-3 

71-55-6 

108-87-2 

25639-42-3 

583-60-8 

101-68-8 

75-09-2 

107-31-3 

541-85-5 

60-34-4 

74-88-4 

108-11-2 

624-83-9

NUREG/CR-6624

100 mg Pb/m3 

50 mg/m3 

0.5 mg/m3 

2000 ppm * 

750 mg/m3 

250 mg/m3 

10 mg/m3 

500 mg Mn/m3 

10 mg Hg/m3 

2 mg Hg/m3 

1400 ppm * 

5000 mg/m3 

3100 ppm 

1700 ppm* 

3400 ppm* 

250 ppm 

2200 ppm* 

6000 ppm 

100 ppm 

800 ppm 

250 ppm 

200 ppm 

200 ppm 

2000 ppm 

700 pprn 

1200 ppm* 

500 ppm 

600 ppm 

75 mg/m3 

2300 ppm 

4500 pprn 

100 ppm 

20 ppm 

100 ppm 

400 ppm 

3 ppm

463-51-4 5 ppm

42
Methyl mercaptan 

Methyl methacrylate 

alpha-Methyl styrene 

Mica (containing <1% quartz) 

Molybdenum (soluble compounds) 

Molybdenum (insoluble compounds) 

Monomethyl aniline 

Morpholine

257 

258 

259 

260 

261 

262 

263 

264 

265 

266 

267 

268 

269 

270 

271 

272 

273 

274 

275 

276 

277 

278 

279 

280 

281 

282 

283 

284 

285 

286 

287 

288 

289 

290 

291 

292 

293

Octachloronaphthalene 

Octane 

Oil mist (mineral) 

Osmium tetroxide 

Oxalic acid 

Oxygen difluoride 

Ozone

74-93-1 

80-62-6 

98-83-9 

12001-26-2 

7439-93-7 

7439-93-7 

100-61-8 

110-91-8

8030-30-6 

91-20-3 

134-32-7 

91-59-8 

13463-39-3 

7440-02-0 

54-11-5 

7697-37-2 

10102-43-9 

100-01-6 

98-95-3 

92-93-3 

100-00-5 

79-24-3 

10102-44-0 

7783-54-2 

55-63-0 

75-52-5 

108-03-2 

79-46-9 

62-75-9 

88-72-2;99-08-1 ;99-99-0

2234-13-1 

111-65-9 

8012-95-1 

20816-12-0 

144-62-7 

7783-41-7 

10028-15-6

150 ppm 

1000 ppm 

700 ppm 

1500 mg/m3 

1000 mg Mo/m 3 

5000 mg Mo/m 3 

100 ppm 

1400 ppm*

Naphtha (coal tar) 

Naphthalene 

alpha-Naphthylamine 

beta-Naphthylamine 

Nickel Carbonyl 

Nickel metal and compounds 

Nicotine 

Nitric acid 

Nitric oxide 

p-Nitroaniline 

Nitrobenzene 

4-Nitrobiphenyl 

p-Nitrochlorobenzene 

Nitroethane 

Nitrogen dioxide 

Nitrogen trifluoride 

Nitroglycerine 

Nitromethane 

1-Nitropropane 

2-Nitropropane 

N-Nitrosodimethylamine 

p-Nitrotoluene

NUREG/CR-6624

1000 ppm* 

250 ppm 

Ca 

Ca 

2 ppm 

10 mg Ni/m3 

5 mg/m3 

25 ppm 

100 ppm 

300 mg/m3 

200 ppm 

Ca 

100 mg/m3 

1000 ppm 

20 ppm 

1000 ppm 

75 mg/m3 

750 ppm 

1000 ppm 

100 ppm 

Ca 

200 ppm 

unknown 

1000 ppm* 

2500 mg/m3 

1 mg Os/m 3 

500 mg/m3 

0.5 ppm 

5 ppm

43
294 

295 

296 

297 

298 

299 

300 

301 

302 

303 

304 

305 

306 

307 

308 

309 

310 

311 

312 

313 

314 

315 

316 

317 

318 

319 

320 

321 

322 

323 

324 

325 

326 

327 

328 

329 

330 

331 

332

NUREG/CR-6624

Paraquat 

Parathion 

Pentaborane 

Pentachloronaphthalene 

Pentachlorophenol 

n-Pentane 

2-Pentanone 

Perchloromethyl mercaptan 

Perchloryl fluoride 

Petroleum distillates (naphtha) 

Phenol 

p-Phenylene diamine 

Phenyl ether (vapor) 

Phenyl ether-biphenyl mixture (vapor) 

Phenyl glycidyl ether 

Phenyihydrazine 

Phosdrin® 

Phosgene 

Phosphine 

Phosphoric acid 

Phosphorus (yellow) 

Phosphorus pentachloride 

Phosphorus pentasulfide 

Phosphorus trichloride 

Phthalic anhydride 

Picricacid 

Pindone 

Platinum (soluble salts) 

Portland cement 

Propane 

beta-Propiolactone 

n-Propyl acetate 

n-Propyl alcohol 

Propylene dichloride 

Propylene imine 

Propylene oxide 

n-Propyl nitrate 

Pyrethrum 

Pyridine

1910-42-5 

56-38-2 

19624-22-7 

1321-64-8 

87-86-5 

109-66-0 

107-87-9 

594-42-3 

7616-94-6 

8002-05-9 

108-95-2 

106-50-3 

101-84-8 

8004-13-5 

122-60-1 

100-63-0 

7786-34-7 

7544-5 

7803-51-2 

7664-38-2 

7723-14-0 

10026-13-8 

1314-80-3 

7719-12-2 

8544-9 

88-89-1 

83-26-1 

varies 

65997-15-1 

74-98-6 

57-57-8 

109-60-4 

71-23-8 

78-87-5 

75-55-8 

75-56-9 

627-13-4 

8003-34-7 

110-86-1

I mg/m 3 

10 mg/m3 

I ppm 

unknown 

2.5 mg/m 3 

1500 ppm* 

1500 ppm 

10 ppm 

100 ppm 

1100 ppm* 

250 ppm 

25 mg/m 3 

100 ppm 

10 ppm 

100 ppm 

15 ppm 

4 ppm 

2 ppm 

50 ppm 

1000 mg/m3 

5 mg/m3 

70 mg/m3 

250 mg/m3 

25 ppm 

60 mg/m3 

75 mg/m3 

100 mg/m3 

4 mg Pt/m3 

5000 mg/m3 

2100 ppm* 

Ca 

1700 ppm 

800 ppm 

400 ppm 

100 ppm 

400 ppm 

500 ppm 

5000 mg/m3 

1000 ppm

44
333 Quinone

334 Rhodium (metal fume &.insoluble) 

335 Rhodium (soluble compounds) 

336 Ronnel 

337 Rotenone 

338 Selenium compounds 

339 Selenium hexafluoride 

340 Silica, amorphous 

341 Silica, crystalline dust 

(cristobalite,tridymite) 

342 Silica, crystalline dust (quartz, tripoli) 

343 Silver (metal dust and soluble 

compounds) 

344 Soapstone (with <1% quartz) 

345 Sodium fluoroacetate 

346 Sodium hydroxide 

347 Stibine 

348 Stoddard solvent

349 

350 

351 

352 

353 

354 

355

Strychnine 

Styrene 

Sulfur dioxide 

Sulfuric acid 

Sulfur monochloride 

Sulfur pentafluoride 

Sulfuryl fluoride

356 2,4,5-T (2,4,5 Trichlorophenoxyacetic 

acid) 

357 Talc (with no asbestos and <1% quartz) 

358 Tantalum (metal and oxide dust) 

359 TEDP (tetraethyl dithionopyrophosphate) 

360 Tellurium and compounds 

361 Tellurium hexafluoride 

362 TEPP (tetraethyl pyrophosphate) 

363 Terphenyls 

364 1,1,2,2-Tetrachloro-l,2- difluoroethane 

365 1,1,1,2-Tetrachloro-2,2- difluoroethane

7440-16-6 

7440-16-6 

299-84-3 

83-79-4 

7782-49-2 

7783-79-1 

7631-86-9 

14808-60-7 

14808-60-7 

7440-22-4 

none 

62-74-8 

1310-73-2 

7803-52-3 

8052-41-3 

57-24-9 

100-42-5 

7446-09-5 

7664-93-9 

10025-67-9 

5714-22-7 

2699-79-8

93-76-5

14807-96-6 

7440-25-7 

3689-24-5 

13494-80-9 

7783-80-4 

107-49-3 

26140-60-3 

76-12-0 

76-11-9

100 mg Rh/m3 

2 mg Rh/m3 

300 mg/m3 

2500 mg/m3 

1 mg Se/m3 

2 ppm 

3000 mg/m3 

25 mg/m3 

50 mg/m3 

10 mg Ag/m3 

3000 mg/m3 

2.5 mg/m3 

10 mg/m3 

5 ppm 

20000 mg/m3 

3 mg/m3 

700 ppm 

100 ppm* 

15 mg/m3 

5 ppm 

1 ppm 

200 ppm

250 mg/m3

1000 mg/m3 

2500 mg Ta/m3 

10 mg/m3 

25 mg Te/m 3 

1 ppm 

5 mg/m3 

500 mg/m3 

2000 ppm 

2000 ppm

NUREG/CR-6624

100 mg/m3 106-51-4

45
366 1,1,2,2-Tetrachloroethane 

367 Tetrachloroethylene 

368 Tetrachloronaphthalene 

369 Tetraethyl lead 

370 Tetrahydrofuran 

371 Tetramethyl lead 

372 Tetramethyl succinonitrile 

373 Tetranitromethane 

374 Tetryl 

375 Thallium (soluble compounds) 

376 Thiram 

377 Tin (inorganic compounds except oxides) 

378 Tin (organic compounds) 

379 Titanium dioxide 

380 Toluene 

381 Toluene-2,4-diisocyanate 

382 o-Toluidine 

383 Tributyl phosphate 

384 1,1,2-Trichloroethane 

385 Trichloroethylene 

386 Trichloronaphthalene 

387 1,2,3-Trichloropropane 

388 1,1,2-Trichloro-1,2,2-trifluoroethane 

389 Triethylamine 

390 2,4,6-Trinitrotoluene 

391 Triorthocresyl phosphate 

392 Triphenyl phosphate 

393 Turpentine 

394 Uranium (insoluble compounds) 

395 Uranium (soluble compounds) 

396 Vanadium pentoxide (dust) 

397 Vanadium pentoxide (fume) 

398 Vinyl chloride 

399 Vinyl toluene 

400 Warfarin

79-34-5 

127-18-4 

1335-88-2 

78-00-2 

109-99-9 

75-74-1 

3333-52-6 

509-14-8 

479-45-8 

7440-28-0 

137-26-8 

7440-31-5 

7440-31-5 

13463-67-7 

108-88-3 

584-84-9 

95-53-4 

126-73-8 

79-00-5 

79-01-6 

1321-65-9 

96-18-4 

76-13-1 

121-44-8 

118-96-7 

78-30-8 

115-86-6 

8006-64-2 

7440-61-1 

7440-61-1 

1314-62-1 

1314-62-1 

75-01-4 

25013-15-4 

81-81-2

100 ppm 

150 ppm 

unknown 

40 mg Pb/m3 

2000 ppm* 

40 mg Pb/m 3 

5 ppm 

4 ppm 

750 mg/m3 

15 mg Ti/m3 

100 mg/m3 

100 mg Sn/m3 

25 mg Sn/m3 

5000 mg/m3 

500 ppm 

2.5 ppm 

50 ppm 

30 ppm 

100 ppm 

1000 ppm 

(unknown) 

100 ppm 

2000 ppm 

200 ppm 

500 mg/m3 

40 mg/m3 

1000 mg/m 3 

800 ppm 

10 mg U/m3 

10 mg U/m3 

35 mg V/m3 

35 mg V/m3 

Ca 

400 ppm 

100 mg/m3

NUREG/CR-6624 46
401 Xylenes (o-, m-, and p- isomers) 

402 Xylidine

403 Yttrium compounds 

404 Zinc chloride fume 

405 Zinc oxide fume 

406 Zirconium compounds

1330-20-7 

1300-73-8 

7440-65-5 

7646-85-7 

1314-13-2 

7440-67-7

900 ppm 

50 ppm

500 mg Y/m 3 

50 mg/m3 

500 mg/in 3 

50 mg Zr/m3

tCAS - Chemical Abstract Service registy number 

Ca - Carginogen, no IDLH established. 

* IDLH based on 10% of the LEL 

** Toxicity limit based on Nureg/Cr-5669

47 NUREG/CR-6624
Appendix B. AIHA Odor Detection Thresholds and NIOSH IDLH Values 

Chemical Name CAS Numbert Odor Threshold IDLH Values 

ppm ppm 

Acetaldehyde 75-07-0 0.067 2000 

Acetic acid 64-19-7 0.074 50 

Acetic anhydride 108-24-7 <0.14 200 

Acetone 67-64-1 62 2500 

Acetonitrile 75-05-8 1160 500 

Acrolein 107-02-8 1.8 2 

Acrylonitrile 107-13-1 1.6 85 

Allyl alcohol 107-18-6 1.7 20 

Ammonia 7664-41-7 17 300 

Aniline and homologs 62-53-3 2.4 100 

Benzene 71-43-2 61 500 

Benzyl chloride 100-44-7 0.041 10 

1,3-Butadiene 106-99-0 0.45 2000 

2-Butanone 78-93-3 16 3000 

2-Butoxyethanol 111-76-2 0.1 700 

n-Butyl acetate 123-86-4 0.31 1700 

n-Butyl alcohol 71-36-3 1.2 1400 

sec-Butyl alcohol 78-92-2 3.2 2000 

tert-Butyl alcohol 75-65-0 960 1600 

Butylamine 109-73-9 0.08 300 

Butyl mercaptan 109-79-5 0.001 500 

Carbon tetrachloride 56-23-5 252 200 

Chlorine 7782-50-5 0.08 10 

Chlorobenzene 108-90-7 1.3 1000 

Chloroform 67-66-3 192 500 

Cresol (all isomers) 1319-77-3 0.0006 250 

Crotonaldehyde (trans-isomer) 123-73-9 0.11 50 

Cumene 98-82-8 0.032 900 

Cyclohexane 110-82-7 780 1300 

Cyclohexanol 108-93-0 0.16 400 

Cyclohexanone 108-94-1 3.5 700 

Diacetone alcohol 123-42-2 0.27 1800 

0-Dichlorobenzene 95-50-1 0.7 200 

p-Dichlorobenzene 106-46-7 0.12 150 

Diethylamine 109-89-7 0.053 200 

2-Diethylaminoethanol 100-37-8 0.11 100 

Diisobutyl ketone 108-83-8 2.8 500 

Diisopropylamine 108-18-9 0.13 200 

1,1 -Dimethylhydrazine 57-14-7 9.2 15 

Dioxane 123-91-1 12 500 

2-Ethoxyethanol 110-80-5 2.7 500 

2-Ethoxyethyl acetate 111-15-9 0.06 500 

Ethyl acetate 141-78-6 18 2000 

Ethyl acrylate 140-88-5 0.00024 300 

Ethylamine 75-04-7 0.27 600 

Ethylene dichloride 107-06-2 26 50 

Ethylene oxide 75-21-8 420 800

NUREG/CR-6624 48
Ethyl formate 109-94-4 0.66 1500 

Ethyl mercaptan 75-08-1 0.00035 500 

N-Ethylmorpholine 100-74-3 0.085 100 

Ethyl silicate 78-10-4 3.6 700 

Furfuryl alcohol 98-00-0 8 75 

n-Heptane 142-82-5 230 750 

Hydrazine 302-01-2 3.7 50 

Hydrogen sulfide 7783-06-4 0.0094 100 

Isoamyl acetate 123-92-2 0.22 1000 

Isobutyl acetate 110-19-0 1.1 1300 

Isobutyl alcohol 78-83-1 3.6 1600 

Isophorone 78-59-1 0.19 200 

Isopropyl acetate 108-21-4 4.1 1800 

Isopropyl alcohol 67-63-0 43 2000 

Isopropylamine 75-31-0 0.21 750 

Isopropyl ether 108-20-3 0.017 1400 

Mesityl oxide 141-79-7 0.17 1400 

Methyl acetate 79-20-9 180 3100 

Methyl alcohol 67-56-1 160 6000 

Methylamine 74-89-5 4.7 100 

Methyl chloroform 71-55-6 390 700 

Methylene chloride 75-09-2 160 2300 

Methyl formate 107-31-3 2000 4500 

Methyl mercaptan 74-93-1 0.00054 150 

Methyl methacrylate 80-62-6 0.049 1000 

Morpholine 110-91-8 0.011 1400 

Naphthalene 91-20-3 0.038 250 

Nitrobenzene 98-95-3 0.37 200 

1-Nitropropane 79-46-9 140 1000 

Octane 108-03-2 150 1000 

2-Pentanone 107-87-9 7.7 1500 

Phenol 108-95-2 0.06 250 

Phosphine 7803-51-2 0.14 50 

n-Propyl acetate 109-60-4 0.18 1700 

n-Propyl alcohol 71-23-8 5.3 800 

Propylene dichloride 78-87-5 0.26 400 

Propylene oxide 75-56-9 45 400 

Pyridine 110-86-1 0.66 1000 

Styrene 100-42-5 0.14 700 

Sulfur dioxide 7446-09-5 2.7 100 

1,1,2,2-Tetrachloroethane 79-34-5 7.3 100 

Tetrachloroethylene 127-18-4 47 150 

Tetrahydrofuran 109-99-9 31 2000 

Toluene 108-88-3 1.6 500 

1,1,2-Trichloroethane 79-00-5 390 100

49 NUREG/CR-6624
Trichloroethylene 

Triethylamine 

Xylene

79-01-6 

121-44-8 

1330-20-7

tCAS - Chemical Abstract Service registy number

NUREG/CR-6624

82 

0.25

20

1000 

200

900

50
NRC FORM 335 U.S. NUCLEAR REGULATORY COMMISSION 1. REPORT NUMBER 

(2-89) (Assigned by NRC, Add Vol., Supp., Rev., 

NRCM 1102, and Addendum Numbers, If any.) 

3201, 3202 BIBUOGALPHIC DATA SHEET 

(See ins.d on Vw reverse) NUREG/CR-6624 

2. TITLE AND SUBTITLE 

Recommendations for Revision of Regulatory Guide 1.78 3. DATE REPORT PUBLISHED 

MONTH YEAR 

November 1999 

4. FIN OR GRANT NUMBER 

W6308 

5. AUTHOR(S) 6. TYPE OF REPORT 

L.B. Sasser, P.M. Daling, P. Pelto, M. Yurconic Technical 

7. PERIOD COVERED (Inclusive Dates) 

September 1998-September 1999 

8. PERFORMING ORGANIZATION - NAME AND ADDRESS (f NRC, provide Division, Ofce or Region, U.S. Nucleer Regulatory Commission, and mailing address; ,fconfrctu, 

provide nrme and mailing address.) 

Pacific Northwest National Laboratory 

P.O. Box 999 

Richland, WA 99352

9. SPONSORING ORGANIZATION - NAME AND ADDRESS (f NRC, type Same as above' if confractor, provide NRC Division, Oftfce or Region, U.S. Nucleer Regulabtry Commission, 

and mailing address.)

Division of Systems Analysis and Regulatory Efectiveness 

Office of Nuclear Regulatory Research 

U.S. Nuclear Regulatory Commission 

Washington, DC 20555-0001

10. SUPPLEMENTARY NOTES 

S. Basu, NRC Project Manager

11. ABSTRACT (2w wrdsv or less) 

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. Regulatory Guide 1.78 provides guidance in assessing the control room habitability of the control room during and 

after a postulated external release of hazardous chemicals from mobile or stationary sources, offsite or onsite. This report provides 

recommendations for revising the Regulatory Guide 1.78 in two areas, namely, control room ventilation flow modeling and toxicity 

limit. Additionally, the report provides a value and impact analysis associated with the revision of Regulatory Guide 1.78. 

In the area of ventilation flow modeling, the report recommends the use of the HABIT code, in particular, the EXTRAN module of 

the code. EXTRAN represents an improvement in atmospheric dispersion modeling. In the area of toxicity limits, the report 

recommends the use of National Institute for Occupational Safety and Health (NIOSH) Immediately Dangerous to Life and Health 

(IDLH) concentration values. The IDLH values, based on a 30-minute exposure level, is defined as one that is likely to cause death 

or immediate delayed permanent adverse health effects if no protection is afforded within 30 minutes. Control room operators are 

expected to use protective measures within 2 minutes after the detection of hazardous chemicals so that they will not be subjected 

to prolonged exposure at the IDLH concentration levels. Thus, the IDLH limits represent reasonable values to provide adequate 

margin of safety in protecting control room operators.

12. KEY WORDS/DESCRIPTORS (List words or phrases that will assist resarchers in localing tMe report.) 13. AVAILABILITY STATEMENT 

Regulatory Guide 1.78, Regulatory Guide 1.95, control room habitability, hazardous chemical release, unlimited 

toxic chemical release, chlorine, toxicity limits, immediately dangerous to life and health (IDLH) odor 14 SECUR[TY CLASSIFICATION 

threshold, control room ventilation flow, air dispersion modeling, Gaussian plume model, meteorology, (This Page) 

frequent shipment criterion, transportation accident statistics, value/impact assessment, regulatory unclassified 

efficiency (ThMis Report) 

unclassified 

15. NUMBER OF PAGES 

16. PRICE

NRC FORM 335 (2-89)
Federal Recycling Program
NUREG/CR-b624 KtAJMMlI~lNIJA I b10 1'4 ~JI VM FV lkl.JIN V)t AXAL~714 Ltl IlkADI% I %Y%) X"LU A. /~ 0 S1R % # .Ji-*JGS.sAj - -

UNITED STATES 

NUCLEAR REGULATORY COMMISSION 

WASHINGTON, DC 20555-0001

FIRST CLASS MAIL 

POSTAGE AND FEES PAID 

USNRC 

PERMIT NO. G-67

OFFICIAL BUSINESS 

PENALTY FOR PRIVATE USE, $300