Regulatory Guide 1.78

Evaluating the Habitability of a Nuclear Power Plant Control Room During a Postulated Hazardous Chemical Release

ML21253A071
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Issue date:	12/07/2021
From:	Casper Sun NRC/RES/DSA/SPB
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DG-1387 RG-1.078, Rev 2
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U.S. NUCLEAR REGULATORY COMMISSION

REGULATORY GUIDE 1.78, REVISION 2

Issue Date: December 2021 Technical Lead: Casper Sun EVALUATING THE HABITABILITY OF A

NUCLEAR POWER PLANT CONTROL ROOM DURING

A POSTULATED HAZARDOUS CHEMICAL RELEASE

A. INTRODUCTION

Purpose

This regulatory guide (RG) describes approaches and technical b ases that are acceptable to the staff of the U.S. Nuclear Regulatory Commission (NRC) to meet r egulatory requirements for evaluating the habitability of a nuclear pow er plant (NPP) control room (CR) during a postulated hazardous chemical release. Releases of hazardous chemicals,1 on site and off site, can result in the nearby CR becoming uninhabitable. The driver of this RG is Title 10 of the Code of Federal Regulations (10 CFR) Part 50,

Domestic Licensing of Production and Utilization Facilities, Appendix A, General Design Criteria for Nuclear Power Plants, General Design Criterion (GDC) 19, Cont rol Room, (Ref. 1). GDC 19 requires operating reactor licensees to provide a CR from which actions can be taken to maintain the nuclear power unit in a safe condition under accident conditions, including loss-of-coolant accidents.

This RG contains technical bases and guidelines that are accept able to the NRC staff for use in assessing the habitability of a CR during and after a postulated external release of hazardous chemicals (e.g., vapor and gaseous) from a stationary source on site and multiple mobile sources off site, based on the immediately dangerous to life or health (IDLH) values (Ref. 2).

Applicability

This guidance applies to applicants and reactor licensees under 10 CFR Part 50 and

10 CFR Part 52, Licenses, Certifications, and Approvals for Nu clear Power Plants (Ref. 3). Although this RG is meant for NPP appli cations, the technical basis and analytical methods described for chemical

1. As defined by the Occupational Safety and Health Administrations (OSHAs) Hazard Communication Standard, https://www.osha.gov/hazcom, a hazardous chemical is any chemical that can cause a physical or health hazard.

Written suggestions regarding th is guide or development of new guides may be submitted through the NRCs public Web site in t he NRC

Library at https://nrcweb.nrc.gov/reading-rm/doc-collections/reg-guides/, under Document Collections, in Regulatory Guides, at https://nrcweb.nrc.gov/reading-rm/doc-collections/reg-guides/co ntactus.html.

Electronic copies of this RG, previous versions of RGs, and oth er recently issued guides are also available through the NRCs public Web site in the NRC Library at https://nrcweb.nrc.gov/reading-rm/doc-collections/reg-guides/, under Document Collections, in Regulatory Guide

s. This RG

is also available through the NRCs Agencywide Documents Access and Management System (ADAMS) at http://www.nrc.gov/reading- rm/adams.html, under ADAMS Accession Number (No.) ML21253A071. The regulator y analysis may be found in ADAMS under Accession No. ML21119A159. The associated draft guide DG-1387 may be found in ADAMS under Accession No. ML21 119A157, and the staff responses to the public comments on DG-1387 may be found under ADAMS Acce ssion No ML21253A074.

safety could also be implemented for nonreactor and advanced no n-light-water reactor facilities to address habitability concerns involving use or storage of hazardous or toxic chemicals.

Applicable Regulations

• The GDC in Appendix A to 10 CFR Part 50 establish minimum requi rements for the principal design criteria for water-cooled nuclear power plants.

o GDC 19 requires that a CR be provi ded from which actions can be taken to operate the nuclear power unit safely under normal conditions and to mainta in it in a safe condition under accident conditions.

o GDC 4, Environmental and Dynamic Effects Design Bases, requir es, in part, that, like the CR, structures, systems, and components important to safety be designed to accommodate the effects of and to be compatible with normal operation, maintenance, testing, and postulated accidents.

• 10 CFR Part 50 provides regulations for licensing production an d utilization facilities.

o 10 CFR 50.34(3)(i) requires that an applicant for a water-coole d nuclear power plant establish the minimum principal design criteria as specified in the GDC i n Appendix A of

10 CFR Part 50.

• 10 CFR Part 52 governs the issu ance of early site permits, standard design certifications, combined licenses, standard design approvals, and manufacturing licenses for nuclear power facilities. The guidance in this RG is intended for standard de sign certifications and combined license applicants under 10 CFR Part 52.

o Section 52.47(a)(3)(i) requires an applicant for a design certification to include the facilitys principal design criteria, the minimum requirements for which a re in Appendix A of

10 CFR Part 50.

o Section 52.79(a)(4)(i) requires an applicant for a combined lic ense to include the facilitys principal design criteria, the minimum requirements for which a re in Appendix A of

10 CFR Part 50.

• 10 CFR Part 20, Standards for Protection against Radiation, S ubpart H, Respiratory Protection and Controls to Restrict Internal Exposure in Restricted Areas (Ref. 4) establishes requirements to mitigate the intake of chemicals and radionuclides during ro utine or emergency operations. For example, Part 20, Subpart H and Appendix A contain safety requirements that are applicable to applicants and licensees in the evaluation of controlled chemic al release to the CR.

Related Guidance

• RG 1.91, Evaluations of Explos ions Postulated To Occur on Tran sportation Routes Near Nuclear Power Plants, (Ref. 5) , describes methods for determining the risk of damage caused by an explosion (including from li quids, cryogenically liquefied h ydrocarbons, vapor clouds, etc.) at a nearby facility or on a transportation route.

RG 1.78, Revision 2, Page 2

• RG 1.174, An Approach for Using Probabilistic Risk Assessment in Risk-Informed Decisions on Plant-Specific Changes to the Licensing Basis, (Ref. 6), de scribes an approach and guidance on analyzing the risk from proposed changes in plant design and operation.

• RG 1.189, Fire Protection for Nuclear Power Plants, (Ref. 7), describes an approach and the associated requirements to manage a NPPs fire protection program.

• RG 1.200, An Approach for Deter mining the Technical Adequacy o f Probabilistic Risk Assessment Results for Risk-Informed Activities, (Ref. 8), describes an approach acceptable for determining whether a base proba bilistic risk assessment (PRA), in total or in the portions that are used to support an application, is sufficient to provide confid ence in the result

s. such that the PRA

can be used in regulatory decision making for light-water react ors.

Purpose of Regulatory Guides

The NRC issues RGs to describe methods that are acceptable to the staff for implementing specific parts of the agencys regulations, to explain techniqu es that the staff uses in evaluating specific issues or postulated events, and to describe information that will assist the staff with its review of applications for permits and li censes. Regulatory guides are not NRC regulations and compliance with them is not mandatory. Methods and solutions that differ from t hose set forth in RGs are acceptable if supported by a basis for the issu ance or continuance of a permi t or license by the Commission.

Paperwork Reduction Act

This RG provides voluntary guidance for implementing the mandat ory information collections in

10 CFR Parts 20, 50 and 52 that are subject to the Paperwork Re duction Act of 1995 (44 U.S.C. 3501 et.

seq.). These information collections were approved by the Offic e of Management and Budget (OMB),

under control numbers 3150-0014, 3150-0011 and 3150-0151, respe ctively. Send comments regarding this information collection to the FOIA, Library, and Informati on Collections Branch ((T6-A10M), U.S.

Nuclear Regulatory Commission, Washington, DC 20555 0001, or by e-mail to Infocollects.Resource@nrc.gov, and to the Desk Officer, Office o f Information and Regulatory Affairs, NEOB-10202 (3150-0014, 3150-0011 and 3150-0151) Office of Manag ement and Budget, Washington, DC, 20503.

Public Protection Notification

The NRC may not conduct or sponsor , and a person is not require d to respond to, a collection of information unless the document re questing or requiring the col lection displays a currently valid OMB

control number.

RG 1.78, Revision 2, Page 3

B. DISCUSSION

Reason for Revision

The revision of this guide (Revision 2) presents up-to-date and defense-in-depth guidance using the latest scientific methods and the updated, NRC-endorsed com puter code for CR habitability evaluation called HABIT. HABIT is an integrated set of computer codes that the NRC uses to evaluate CR habitability and estimate the control room personnels expos ure to a chemical release. Revision 1 of RG 1.78 endorsed an earlier version of HABIT, which is describe d in NUREG/CR-6210, Supplement 1, Computer Codes for Evaluation of Control Room Habitability (HA BIT V1.1), issued October 1998 (Ref. 9). More recently, the NRC staff endorsed a newer version of HABIT in NUREG-2244, HABIT

2.2: Description of Models and Me thods, issued May 2021 (Ref. 10). This latest version of HABIT is available at the Radiation Protection Computer Code Analysis an d Maintenance Program Web site, https://ramp.nrc-gateway.gov/ .

Background

GDC 19 requires operating reactor licensees to provide a CR fro m which actions can be taken to maintain the nuclear power unit in a safe condition under accident conditions including protecting the CR

from hazardous chemicals that may be discharged as a result of equipment failures, human errors, or events and conditions outside the control of the NPP. Based on NUREG/CR-6624, Recommendations for Revision of Regulatory Guide 1.78, (Ref. 11), the NRC issu ed RG 1.78, Revision 1 in 2001. It updated the two guidance tables (i.e., Table C-1 and Table C-2) with the latest IDLH values and established the connection of CR habitability and hazardous chemicals from mobile (e.g., tank trucks, railroad cars, and barges) and stationary (e.g., storage tanks, pipelines, fire-fighting equipment) sources which in turn provided the segue for further validating the cri teria and for developing the procedures used in CR habitability evaluations.

Further, NUREG/CR-6624 also affirmed that all nuclear reactor C R operators should be trained and expected to don personal pro tection equipment (PPE) such as respirators and protective clothing within 2 minutes, so that they will not be subjected to risk from prolonged exposure more than two minutes at the chemicals IDLH value. Table 1, Selected IDLH V alues for Twenty-Nine Hazardous Chemicals, has the same IDLH values from Revision 1 of RG 1.78 .

Promulgated by OSHA, the IDLH c oncept was established originally for use in assigning respiratory and face-mask equipment as part of the Standards Co mpletion Program, a joint project with the National Institute for Occupational Safety Health (NIOSH) during the mid-1970s. The IDLH values define the levels of chemical concentration that are likely to cause death or immediate or delayed permanent adverse health effects if no PPE is afforded within 3 0 minutes. The IDLH values are used to:

(1) ensure that the worker can identify and escape from a given contaminated environment in the event of failure of the respiratory protection equipment; and (2) determine the required minimum air-purifying factor (APF) for a PPE to provide sufficient protection consistent with the criterion of Appendix A,

Assigned Protection Factors for Respirators, to 10 CFR Part 2 0.

Further, Table 2, Minimum Chemical Weights That Require Consid eration in CR Habitability Evaluation, of this RG illustrates the importance of distance between the release source and the CR to determine the mass (i.e., weight) of chemicals, regardless of w hat kind of toxic chemi cals are identified.

The frequency of shipments from a mobile source, the quantity a nd duration of a release, the toxicity of released chemicals, meteorological conditions (for dispersion calculations), and the rate of air infiltration into the CR are also documented from NUREG/CR-6624. Further, RG 1.78, Revision 1, covered both toxic and asphyxiating chemicals and recognized that the asphyx iating chemicals should only be

RG 1.78, Revision 2, Page 4 considered in CR habitability determinations if their release c ould result in displacement of a significant fraction of the CR air and result in an oxygen-deficient atmosp here.

Consistent with risk-informed regulatory decision making, this RG revision encourages licensees to make greater use of risk insights in submitting applications for plant-specific changes to the licensing basis, using the guidance provided in RG 1.174. Further, this R G revision continues to provide flexibility for licensees to use traditional engineering approaches. Also, consistent with the intent of SECY-00-0191, High-Level Guidelines for Performance-Based Activities, dated September 1, 2000 (Ref. 12), on performance-based initiatives, this RG revision provides performance-based guidance rather than traditional, prescriptive guidance.

Consideration of International Standards

The International Atomic Energy Agency (IAEA) works with member states and other partners to promote the safe, secure, and peaceful use of nuclear technologies. The IAEA develops Safety Requirements and Safety Guides for protecting people and the en vironment from harmful effects of ionizing radiation. This system of safety fundamentals, safety requirements, safety guides, and other relevant reports, reflects an international perspective on what constitutes a high level of safety. To inform its development of this RG, the NRC considered IAEA Safety Requ irements and Safety Guides pursuant to the Commissions International Policy Statement (Ref. 13) an d Management Directive and Handbook

6.6, Regulatory Guides (Ref. 14).

The following IAEA Specific Safety Guide (SSG) documents were considered in the development/update of this RG:

• IAEA SSG-3, Development and App lication of Level 1 Probabilist ic Safety Assessment for Nuclear Power Plants, issued 2010 (Ref. 15)

• IAEA SSG-54, Accident Management Programmes for Nuclear Power Plants, issued 2019 (Ref. 16)

In addition, the following Intern ational Organization for Standardization (ISO) standard was also considered in the development/update of this RG:

• ISO 17873: 2004 Nuclear facilities Criteria for the design a nd operation of ventilation systems for nuclear installations other than nuclear reactors (Ref. 17 )

This RG incorporates similar design and performance guidelines as provided in the IAEA

documents and ISO standard and is consistent with the safety principles provided in these publications.

Documents Discussed in Staff Regulatory Guidance

This RG endorses the use of one or more codes and standards dev eloped by external organizations as third-party gui dance documents. These codes, standards and third-party guidance documents may contain references to other codes, standards or third-party guidance documents (secondary references). If a secondary reference has itself been incorporated by reference into NRC

regulations as a requirement, then licensees and applicants mus t comply with that standard as set forth in the regulation. If the secondary reference has been endorsed in a RG as an acceptable approach for meeting an NRC requirement, then the standard constitutes a met hod acceptable to the NRC staff for meeting that regulatory requirement as described in the specific RG. If the secondary reference has neither been incorporated by refe rence into NRC regulations nor endorsed in a RG, then the secondary

RG 1.78, Revision 2, Page 5 reference is neither a legally-binding requirement nor a gener ic NRC approved acceptable approach for meeting an NRC requirement. However, licensees and applicants m ay consider and use the information in the secondary reference, if appropriately justified, consistent with current regulatory practice, and consistent with applicable NRC requirements.

RG 1.78, Revision 2, Page 6 C. STAFF REGULATORY GUIDANCE

This section includes the staff regulatory guidance for evaluat ing the habitability of a NPP CR

during a postulated hazardous chemical release. Any hazardous c hemical stored on site within a half (1/2)

kilometer (km) [1,640 feet (ft)] of the CR in a quantity greate r than 45 kilograms (kg) [(100 pounds (lb)]

should be considered for CR hab itability evaluation. Hazardous chemicals should not be stored within 0.1 km (330 ft) of a CR or its fresh air inlets, including ventilat ion system intakes and locations of possible infiltration such as penetrations. Licensees are encouraged to conduct periodic surveys of stationary and mobile sources of hazardous chemi cals near their plant sites to keep the site-specific inventories up to date. However, this RG also provides essential assumptions and criteria for screening out release events that need not be considered in the evaluation of CR habitabilit y. The following criteria identify the release events that need not be considered further for CR habitability evaluation.

1. Hazard Screening

Whether a chemical source (sta tionary or mobile) constitutes a hazard that requires a CR

habitability evaluation depends on prevailing meteorological co nditions, the inleakage characteristics of the CR, and the air concentration in the CR as compared to the applicable toxicity concentrations shown in Table 1 and the combination of the weight quantity of chemical and the distance from the plant shown in Table 2.

1.1 Exemption Criteria for Stationary Sources

Chemicals stored or situated at distances greater than 5 miles from the plant need not be considered because, if a release occurs at such a distance, atmospheric dispersion will dilute and disperse the incoming plume to such a degree that either toxic limits wi ll never be reached or there would be sufficient time for the CR operators to take appropriate action . In addition, small quantities (i.e., less than

10 kg) for laboratory use in the plant can be exempt.

In addition, the maximum allowable inventory in a single contai ner should be stored at specified distances beyond 0.1 km from the CR (e.g., its fresh air inlet) and varies according to the distance and the CR type, as specified by CR air change per hour (ACH) rates in Table 2. If there are several chemical containers, the evaluation norma lly considers only the failure of the largest container unless the containers are interconnected in such a manner that failure of a single co ntainer could cause a release from several containers.

1.2 Screening Criteria for Mobile Source Chemicals

For the chemicals in Table 1, known or projected to be present in either stationary form or in mobile form by rail, water, or ro ad routes within an 8 km radiu s of a NPP, a CR habitability evaluation may be considered based on both Table 1 and Table 2 screening values. The Table 2 variables were established under Category F Pasquill stability class 2 and at a fixed 50 mg/m3 concentration value. They are adjustable parameters needed for determining the total quan tity (i.e., the minimum chemicals weight)

of the mobile sources and the seven tiers of incremental distan ce described in Table 2. The first column of Table 2 contains radii between 0.3 and 5 miles from the CR, and the three columns to the right list the calculated weights for three ACH values.

2. Pasquill stability class is a meteorological c lassification method for categorizing atmosphere stability and is defined by, among other things, the regional conditions of wind speed, solar radiation during the day, and cloud cover during the night. See https://www.ready.noaa.gov/READYtools.php for more information.

RG 1.78, Revision 2, Page 7 Table 1. Selected IDLH Values fo r Twenty-Nine Hazardous Chemicals Chemical ppm(a) mg/m3 (b) Chemical ppm mg/m3 Acetaldehyde 2,000 3,600 Fluorine 25 50

Acetone 2,500 6,000 Formaldehyde 20 24 Acrylonitrile 85 149 Halon 1211 20,000

Anhydrous ammonia 300 210 Halon 1301 50,000

Aniline 100 380 Helium asphyxiant Benzene 500 1,600 Hydrogen cyanide 50 55 Butadiene 2,000 4,400 Hydrogen sulfide 100 150

Butene asphyxiant Methyl alcohol 6,000 7,800

Carbon dioxide 40,000 7,360 Nitrogen (liquid or Carbon monoxide 1,200 1,320 compressed) asphyxiant Chlorine 10 30 Sodium oxide 2 Ethyl chlorine 3,800 9.880 Sulfur dioxide 100 520

Ethyl ether 1,900 5,700 Sulfuric acid 15 Ethylene dichloride 50 200 Vinyl chloride 1,000 2,600

Ethylene oxide 800 720 Xylene 900 3,915 (a). Parts of vapor or gas per million parts of air by volume at 25 °Celsius and 760 torr (standard temperature and pressure).

(b). Approximate milligrams of chemicals per cubic meter (mg/m3) of air, at standard temperature and pressure, based on listed ppm values. To convert ppm to mg/m3, multiply the ppm value with the chemicals molecular weight (i.e., gram/mole) and divide by the universal standard temperature and pressure gas constant, 24.45.

Table 2. Minimum Chemical Weights That Require Consideration in CR Habitability Evaluation (a)

Distance from CR ACH ACH

in Mile (km)(b) 0.015(c) ACH 0.06 1.2

0.3 (0.5) to 0.5 (0.8) 4.1 (d) 1.0 0.050

0.5 (0.8) to 0.7 (1.1) 16 4.0 0.20

0.7 (1.1) to 1.0 (1.6) 55 14 0.68

1 (1.6) to 2 (3.2) 123 31 1.5

2 (3.2) to 3 (4.8) 590 150 7.4

3 (4.8) to 4 (6.5) 1,680 420 21

4 (6.5) to 5 (8.0) 4,000 1,000 50

(a) The table is adapted from RG 1.78, Rev. 1 (2001) and added with SI units.

(b) Values in parenthesis are in unit kilometer (km).

(c) An ACH of 0.015 (i.e., 0.015 of the control room air by volume is replaced by atmospheric ambient air in one hour) is considered representative of a tight CR that has very low leakage construction features and automatic isolation capabilities. ACH of 0.06 is considered representative of a CR that has normal leakage construction features and automatic isolation capabilities, whereas an ACH of 1.2 is considered representative of the CR with construction features that are not as efficient for leakage control and without automatic isolation capabilities.

(d) Storage weights, in unit of metric ton (i.e., 2,205 lb) are obtained based on a 50 mg/m3 concentration and Category F Pasquill Stability Class.

The evaluation of CR habitability should consider estimates of the frequencies for shipments that are within 8 km radius of a NPP. The NRC considers shipments to be frequent if there are 10 total shipments per year for truck traffic, 30 total shipments per ye ar for rail traffic, or 50 total shipments per year for barge traffic. These frequencies are based on transpor tation accident statistics, conditional spill

RG 1.78, Revision 2, Page 8 probability given an accident, and a limiting criterion for the number of spills from NUREG/CR-6624.

Therefore, the technical basis for Table 2 in this RG is the same as that of RG 1.78, Revision 1.

Therefore, mobile sources need not be considered further if the total shipment frequency for all hazardous chemicals, i.e., all hazardous chemicals considered as a singular cargo category without further distinction of the nature of these chemicals, does not exceed the specified number by traffic type.

Frequent shipments, i.e., shipments exceeding the specified number by traffic type, need not be considered in the analysis if the quantity of hazardous chemica ls is less than the quantity shown in Table

2 (adjusted for the appropriate toxicity limit, meteorology, and ACH in the CR).

2. Risk Evaluation

Releases of hazardous chemicals from stationary sources or from frequently shipped mobile sources in quantities that do not meet the screening criteria i n the Sections C.1.1 or C.1.2 above should undergo detailed analyses for CR habitability. Licensees may pr ovide risk information to demonstrate that the radiological risk to the public from such toxic chemical re leases is small, consistent with the Commissions Safety Goal Policy Statement, SECY-00-0077, Modif ications to the Reactor Safety Goal Policy Statement, dated March 30, 2000 (Ref. 18). Releases of toxic chemicals that could potentially result in a significant concentration in the CR need not be con sidered for further detailed evaluation if the releases occur at a frequency of 1x10-6 per year or less because the NRC considers these resultant low levels of radiological risk to be acceptable.

To facilitate risk-informed license amendments, risk information should be provided in accordance with the guidance set forth in RG 1.174. As explaine d in RG 1.174, one key principle in risk- informed regulation is that proposed increases in risk are smal l and consistent with the intent of the Commissions Safety Goal Policy Statement. The safety goals and associated quantitative health objectives (QHOs) define acceptable level of risk as a small fraction (0.1%) of other risks to which the public is exposed. Procedures outlin ed in the Framework for Risk-Informed Changes to the Technical Requirements of 10 CFR Part 50, an attachment to SECY-00-0198 (Ref. 19), may also be used as guidelines for quantifying risks. If the level of risk associat ed with the release of a toxic chemical is not acceptable, then a detailed CR habitability evaluation should be performed. A method acceptable to the NRC staff for evaluating the CR habitability is described in Section C.3 below.

3. Control Room Habitability Evaluation

When performing a detailed evaluation of CR habitability during a hazardous chemical release using this guidance, the metri c applicants and licensees should use for each chemical is the IDLH that can be tolerated without physical incapacitation of a CR operator. In deriving the toxicity level in the CR, the detailed calculations should consider several factors, such as accident type; release characterization (e.g., release rate, duration); atmospheric dispersion characteristics, including prevailing meteorological conditions at the site; and the air exchange rate of the CR. The checklist for the determinations of the toxicity level (i.e., concentration) in the CR, based on the to xic chemical and CR air quality parameter values, is as follows: (1) name of the most hazardous chemical, (2) type of source (stationary or mobile)

during the accidental release; (3) maximum quantity or concentr ation measured (if available); (4) IDLH

values (i.e., ppm or mg/m3); (5) average continuous release rate of hazardous chemical; (6) vapor pressure (torr) of hazardous chemical (at lo cal ambient plant temperature); (7) fraction of chemical flashed and rate of boiloff when spilling occurs; (8) total plume travel distanc e between the CR and the chemicals; and

(9) local meteorological data.

For determining the air quality in the CR for habitability eval uation, the NRC recommends the following 7 considerations: (1) the design height of air intake windows; (2) the volume size of CR; (3)

RG 1.78, Revision 2, Page 9 the air-exchange rate of CR; (4) the flow rate as cubic feet pe r minute of the CR; (5) the unfiltered makeup or inleakage air for the CR; (6) the filtered makeup and recirculated air under normal and emergency operations; and (7) th e use of a filtered nuclear air-cleaning system or personal breathing-air supplying device during an emergency.

3.1 IDLH Concentrations

Table 1 presents the IDLH values as maximum toxic concentration s for the selected

29 chemicals. This table lists commonly encountered chemicals, but the list is not all-inclusive. A more complete list of chemicals is in NUREG/CR-6624. An unprotected operator should not stay in a CR with chemical concentrations exceeding those in Table 1 for longer t han 2 minutes.

3.2 Accident Types and Release Characteristics

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

maximum concentration chemical accidents (MCAs) and average con centration-duration chemical accidents (ACAs).

MCAs result in a short-term puff or instantaneous release of a large quantity of hazardous chemicals. An example of this type of accident would be the fai lure of a manhole cover on the chemical container or the outright failure of the container itself. Such a failure could occur during transport of a container from a handling mishap or from naturally or accidenta lly produced environments such as earthquakes, flooding, fire, explosive overpressure, or missiles. A significant inventory could be released right away, with the balance releasing over an extended period. Under MCAs, the analysis should consider: (1) the largest storage container within the guidelin es of Table 2 located at a nearby stationary facility; (2) the largest shipping container within the guidelines of Table 2 that is frequently transported near the site; or (3) the largest container stored on site. For multiple shipping containers of equal size, the evaluation should consider failure of only one container unless the failure of that container could lead to successive failures. For the largest container stored on site, the evaluation should consider 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.

ACAs result in a long-term, low-leakage-rate, continuous releas e. Most onsite chlorine releases experienced to date within NPPs have been ACAs, involving leaka ge from valves or fittings and resulting in a long-term release with a leakage rate from near zero to le ss than 1 pound of chlorine per second.

Given warning, the CR operator needs only a breathing apparatus to be protected from ACAs. However, because such a release might continue unabated for many hours, a self-contained breathing apparatus, a tank source of air with manifold outlets, or equivalent protection capable of operation for an extended period should be available. For e xample, the continuous release of hazardous chemicals from the largest safety relief valve on a stationary, mobile, or onsite source w ithin the guidelines of Table 2 should be considered.

For both types of accidents, MCAs and ACAs, the evaluation should consider release of contents during an earthquake, tornado, or flood for chemical container facilities that are not designed to withstand these natural events. In the evaluation of CR habitability, it may also be appropriate to consider hazardous chemical releases coincident with the radiological consequences (e.g., a design-basis loss-of-coolant accident for plants that are vulne rable to both events simultaneously) and demonstrate that such coincident events do not produce an unacceptable level of risk.

RG 1.78, Revision 2, Page 10

3.3 Atmospheric Dispersion

NUREG/CR-6210 documented that HABIT has two basic Fortran modul es, i.e., EXTRAN and CHEM. The EXTRAN module is formula ted for a Gaussian plume or puff dispersion model and longitudinal, lateral, and vertical dispersions between the poi nt of release to the intake of the CR. The CHEM module is calculated for the chemical concentration and ex posure in the CR based on the ventilation system and associated air-cleaning installations. T he EXTRAN also allows for the effect of building wakes and for additional dispersion in the vertical di rection when the distance between the release point and the CR is small. When boiloff or a slow leak is analyzed, the effects of density on vertical diffusion may be consid ered if adequately substantiated by reference to data from experiments.

For chemicals that are not gases at 100 degrees Fahrenheit at n ormal atmospheric pressure but are liquids with vapor pressures in excess of 10 torr, applicants and licensees should consider the rate of flashing and boiloff to determine the rate of release to the at mosphere and the appropriate time duration of the release. For gases that are heavier than air, the buoyancy effect should be considered for many parameters, such as density of th e plume and roughness of the ground surface, in determining the dispersion characteristics. NUREG-2244, HABIT 2.2: Description of Models and Methods, incorporates both the U.S. Environmental Protection Agencys DE nse GAs DISpersion Model (DEGADIS) code (Ref. 20) and the U.S. Department of Energys atmospheric dispersion model SLAB

code (Ref. 21) for denser-than-air releases codes for dense gas transport phenomena.

3.4 Control Room Air Flow

The evaluation should consider t he air flows for infiltration, makeup, and recirculation for both normal and accident conditions. It should also consider the volume of the CR and all other rooms, including the ventilation systems, that share the same ventilating air during both normal and accident conditions.

The CR envelope should be construc ted and equipped with a low-l eakage ventilation system to stop or reduce inleakage. For example, low-leakage dampers, low -leakage shut-off valves and other low- leakage Heating, Ventilation and Air Conditioning (HVAC) components should be installed on the upstream side of recirculation fa ns or at locations where negative pressure exists (e.g., fan shaft seals).

The inleakage characteristics of the CR envelope during a hazardous chemical challenge should be determined by testing. A comprehensive test of the CR ventil ation systems will identify the total inleakage associated within the CR envelope but will not necessary identify all inleakage sources. An effective and NRC staff-accepted method to test CR envelope inl eakage is American Society of Testing and Materials (ASTM) Standard E741, Standard Test Method for D etermining Air Change in a Single Zone by Means of a Tracer Gas Dilution (Ref. 22). Further, if credit has been taken in the evaluation for the removal of hazardous chemicals by filtration, adsorption, o r other means, the applicant or licensee should provide a technical basis for the dynamic removal capabi lity of the removal system considered.

4. Protection Measures

For adequate safety and protection of the CR operators against the types of accidental releases discussed in Section 3.2 above, the plant design should include features to: (1) provide capability to detect such releases; (2) isolate the CR if there is a release; (3) make the CR sufficiently leak tight; and

(4) provide equipment and procedures for ensuring that the CR o perators have access to breathable air, proper PPE, or both. Provisions tha t are adequate for the large, instantaneous release should also provide protection against the low-leakage-rate release. Section 3.4 pr ovides the guidance related to making the CR sufficiently leak tight. The implementation of chemical safe ty and protection measures may be

RG 1.78, Revision 2, Page 11 excluded if the detailed evaluation of CR habitability shows th at the highest concentration predicted in the CR is below the IDLH value. Otherwise, licensees may select and implement specific protection measures based on the design features of their facilities.

4.1 Detection System

The detection system should be able to detect and signal a conc entration level that is significantly lower than the IDLH value, for example, a concentration level of 5 ppm for chlorine with an IDLH value of 10 ppm. The detection system should be qualified for all exp ected environments, including severe environments. The system should also be designated as seismic Category I and be qualified as such in accordance with the guidance in the second paragraph of Section 4.2 to address this issue. The installation of the detectors should ensure that they are protected from adv erse temperature effects. The manufacturers guideline for maintenance, testing, and calibration, as well as adjustment to such guideline made by licensees, are acceptable provided they follow sound en gineering practices and are compatible with the proposed application.

If neither toxic information nor d etection instruments are available, human detection, such as unpleasant smell, burning odor, i rritated eyes, and choking, may be useful as a warning of a dangerous condition and a signal to don PPE.

Quick-response detectors should be placed in the fresh air inlets (both normal and emergency air intakes). Depending on the design, it may also be appropriate t o have separate channels of detectors for fresh air inlets and to have detectors in the CR envelope venti lation system recirculation lines. The system response time, which incorporates the detection response time, the valve closure time, and associated instrument delays, should be less than or equal to the required isolation time based on the IDLH value.

Remote detectors may be located at storage and unloading locati ons. These detectors may be placed, and the detector trip poi nts adjusted, to ensure detection of either a leak or a container rupture. A

detector trip signal should isolate the CR before toxic chemica l concentration within the CR exceeds the chemicals IDLH value. The detector trip signal should also set off an alarm and provide a readout in the CR. An alternative to the installation of remote detectors woul d be an isolation system that uses local detectors with a very short isolation time.

4.2 Isolation System

The evaluation should consider the capability to close the CR a ir ducts with dampers and thus isolate the CR. For onsite storage, measures should be in place to manually isolate the CR. Upon detection of a toxic chemical, a detector should initiate compl ete closure of isolation dampers to the CR

with minimal delay. The isolation time is a function of the CR design, in particular, the inleakage characteristics. If the detectors are upstream from the isolation dampers, then credit will be allowed for the travel time between the detectors and the dampers.

The isolation system and its components, the recirculating filt er system, and the air conditioning system should meet Institute of Electrical and Electronics Engi neers (IEEE) Standard 603-2018, IEEE

Standard Criteria for Safety Systems for Nuclear Power Generating Stations (Ref. 23), since these systems are needed to maintain a habitable environment in the C R during a design-basis accident.

For plants that isolate CRs, steps should be taken to ensure th at the isolated exchange rate is not inadvertently increased by design or operating error. Ventilation equipment for the CR and for the adjacent zones should be reviewed to ensure that enhanced air exchange between the isolated CR and the outside will not occur. All doors leading to the CR should be k ept closed when not in use.

RG 1.78, Revision 2, Page 12

4.3 Protection System

If the evaluation of possible accidents for any hazardous chemi cal indicates that the applicable toxicity limits may be exceeded in the CR, measures should be i n place to provide adequate protection to CR operators. The evaluation should consider the use of full-fa ce, self-contained, pressure-demand-type breathing apparatus (or the equivalent) and protective clothing . Adequate air capacity for the breathing apparatus (at least 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br />) shoul d be readily available on site to ensure that at least 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> is available to transport additional bottled air from offsite locations. This o ffsite supply should be capable of delivering several hundred hours of bottled air. The units of breathing ap paratus should be enough for the emergency crew or staff working in the CR.

Storage provisions for breathing apparatus and procedures for t heir use should be such that operators can begin using the apparatus within 2 minutes after detection of a hazardous release. Breathing apparatus, air supply equipment, and protective clothing should meet the criterion that a single toxic gas event would not render nonfuncti onal the total inventory of suc h protective equipment.

4.4 PPE Training

CR operators should train and have the ability to don a respira tor and associated PPE within

2 minutes. The interpretation of IDLH value is considered appro priate since it provides an adequate margin of safety as long as CR operators use protective measures within 2 minutes after detection of hazardous chemicals.

5. Emergency Planning

The licensee should initiate CR emergency procedures as describ ed in NUREG-0696, Functional Criteria for Emergency Response Facilities, Office of Inspection and Enforcement, issued February 1981 (Ref. 24), if a hazardous chemical release occurs within or near the plant. These procedures should address both ACA and MCA and should identify the most probable chemical releases at the station. The procedures should discuss methods of detecting the event by sta tion personnel, both during normal workday operation and during minimum staffing periods (e.g., la te night and weekend shift staffing).

Special instrumentation provided for the detection of hazardous chemical releases should be described, including the action initiated by the detecting instrument and the level at which this action is initiated. The emergency procedures should describe the isolation of the CR, t he use of protective breathing apparatus or other protective measures, and maintenance of the plant in a safe condition, including the capability for an orderly shutdown or scram. F inally, the procedure should des cribe criteria and procedures for evacuating nonessential personnel from the station.

Emergency planning should include training emergency planning personnel on the use of instruments. It should also inc lude periodic drills on the proc edures.

Arrangements should be made with Federal, State, and local agencies or other cognizant organizations for the prompt no tification to the NPP when accidents involving hazardous chemicals have occurred within 5 miles of the plant.

RG 1.78, Revision 2, Page 13

D. IMPLEMENTATION

The NRC staff may use this RG as a reference in its regulatory processes, such as licensing, inspection, or enforcement. However, the NRC staff does not intend to use the guidance in this RG to support NRC staff actions in a manner that would constitute bac kfitting as that term is defined in

10 CFR 50.109, Backfitting, and as described in NRC Management Directive 8.4, Management of Backfitting, Forward Fitting, Issue Finality, and Information R equests, (Ref. 25), nor does the NRC staff intend to use the guidance to affect the issue finality of an approval under 10 CFR Part 52. The staff also does not intend to use the guidance to support NRC staff action s in a manner that constitutes forward fitting as that term is defined and described in Management Dir ective 8.4. If a licensee believes that the NRC is using this RG in a manner inconsistent with the discussion in this Implementation section, then the licensee may file a backfitting or forward fitting appeal with the NRC in accordance with the process in Management Directive 8.4.

RG 1.78, Revision 2, Page 14 REFERENCES3

1. U.S. Code of Federal Regulations (CFR), Domestic Licensing of Production and Utilization Facilities, Part 50, Chapter 1, Title 10, Energy.

2. National Institute of Occupational Safety and Health, NIOSH Pocket Guide to Chemical Hazards, U.S. Department of Health and Human Services Publication Number 2005-149, September 2007 (https://www.cdc.gov/niosh/docs/2005-149/default.html ).

3. CFR, Licenses, Certifications, and Approvals for Nuclear Power Plants, Part 50, Chapter 1, Title 10, Energy.

4. CFR, Standards for Protection against Radiation, Part 20, Cha pter 1, Title 10, Energy.

5. U.S. Nuclear Regulatory Commission (NRC), Regulatory Guide (RG) 1.91, Evaluations of Explosions Postulated to Occur on Transportation Routes Near Nu clear Power Plants.

Revision 2, Washington, DC.

6. NRC, RG 1.174, An Approach for Using Probabilistic Risk Assess ment in Risk-Informed Decisions on Plant-Specific Changes to the Licensing Basis, Wa shington, DC.

7. NRC, RG 1.189, Fire Protection for Nuclear Power Plants, Wash ington, DC.

8. NRC, RG 1.200, Acceptability of Probabilistic Risk Assessment Results for Risk-Informed Activities. Washington, DC.

9. Stage, S.A., Computer Codes for Evaluation of Control Room Hab itability (HABIT).

NUREG/CR-6210, Pacific Northwest National Laboratory, prepared for the NRC, Washington, DC, June 1996. (ADAMS Accession No. ML063480558).

10. Tomon, J.J., Sun, L.C., Haider , S.I., Spicer, T.O., HABIT 2.2: Description of Models and Methods, NUREG-2244, prepared fo r the NRC, Washington, DC, May 2021. (ADAMS

Accession No. ML21120A069).

11. Sasser, L.B., P.M. Daling, P. Pelto, M. Yurconic, Recommendati ons for Revision of Regulatory Guide 1.78, NUREG/CR-6624, Pacifi c Northwest National Laboratory, prepared for the NRC,

Washington, DC, November 1999. (ADAMS Accession No. ML003726870 ).

12. NRC, SECY-00-0191, High-Level Guidelines for Performance-Based Activities, Washington, DC, September 2000. (ADAMS Accession No. ML003742883).

13. NRC, Nuclear Regulatory Commiss ion International Policy Statement, Federal Register, Vol.

79, No. 132, July 10, 2014, pp. 39415-39418.

3 Publicly available NRC published documents are available electronically through the NRC Library on the NRCs public Web site at http://www.nrc.gov/reading-rm/doc-collections/ and through the NRCs Agencywide Documents Access and Management System (ADAMS) at http://www.nrc.gov/reading-rm/adams.html.The documents can also be viewed online or printed for a f ee in the NRCs Public Document Room (PDR) at 11555 Rockville Pike, Rockville, MD. For problems with ADAMS, contact the PDR staff at 301-415-4737 or (800) 397-4209; fax (301) 415-3548; or e- mail pdr.resource@nrc.gov.

RG 1.78, Revision 2, Page 15

14. NRC, Management Directive (MD) 6.6, Regulatory Guides, Washington, DC, May 2, 2016 (ADAMS Accession No. ML18073A170).

15. International Atomic Energy Agency (IAEA), Development and App lication of Level 1 Probabilistic Safety Assessment for Nuclear Power Plants, IAEA Specific Safety Guide SSG-3, Vienna, Austria, 2010.4

16. IAEA, Accident Management Programmes for Nuclear Power Plants, IAEA Specific Safety Guide SSG-54, Vienna, Austria, 2019.

17. International Organization for Standardization (ISO) 17873: 2004 Nuclear facilities Criteria for the design and operation of ventilation systems for nuclear installations other than nuclear reactors 2004. 5

18. NRC, SECY-00-0077, Modifications to the Reactor Safety Goal Policy Statement, Washington, DC, dated March 30, 2000. (ADAMS Accession No. ML003684288).

19. NRC, SECY-00-0198, Status Report on Study of Risk-Informed Cha nges to the Technical Requirements of 10 CFR Part 50 (Option 3) and Recommendations o n Risk-Informed Changes to

10 CFR 50.44 (Combustible Gas Control), Washington, DC, Septem ber 2000. (ADAMS

Accession No. ML003747699).

20. EPA-450/4-89-019, User's Guide f or the DEGADIS 2.1 Dense Gas D ispersion Model, U.S.

Environmental Protection Agency, Research Triangle Park, NC, No vember 1989.

21. Ermak, D. L., Users Manual for SLAB: An Atmospheric Dispersio n Model for Denser-Than-Air Releases, UCRL-MA-105607, Lawrence Livermore N ational Laboratory, Livermore, CA, June 1990.

22. American Society for Testing and Materials (ASTM), E741, Revisi on 17, Standard Test Method for Determining Air Change in a Single Zone by Means of a Trace r Gas Dilution, Conshohocken, PA, 2017 6.

23. Institute of Electrical and Electronics Engineers (IEEE) , Standard 603, Criteria for Safety Systems for Nuclear Power Generating Stations, IEEE Power and Energy Society, Piscataway, NJ, 2018.7

24. NRC, NUREG-0696, Functional Crite ria for Emergency Response Facilities, Office of Inspection and Enforcement, Washington, DC, February 1981. (ADA MS Accession No. ML051390358).

4 Copies of IAEA documents may be obtained through the IAEA Web site, http://www.IAEA.org, or by writing the International Atomic Energy Agen cy, P.O. Box 10 Wagramer Strasse 5, A-1400, Vienna, Austria.

5 Copies of ISO documents can be purchased from the ISO Custom er Care: customerservice@iso.org

6 Copies of ASTM documents may be obtained by writing ASTM Headquarters, 100 Barr Harbor Drive, P.O. Box C700,

West Conshohocken, PA 19428-2959, or via email at service@astm.org

7 Copies of IEEE documents may be obtained from the IEEE Servi ce Center, 445 Hoes Lane, Piscataway, NJ

08855-1331.

RG 1.78, Revision 2, Page 16

25. NRC, Management Directive 8.4, Management of Backfitting, Forw ard Fitting, Issue Finality, and

=

, Washington, DC, September 20, 2019. (ADAMS Accession No. ML18093B087).

RG 1.78, Revision 2, Page 17 APPENDIX A

PROCEDURE FOR CALCULATING WEIGHTS OF HAZARDOUS CHEMICALS

NECESSITATING THEIR CONSIDERATION IN HABITABILITY EVALUATIONS

This appendix describes a simplified multiplication or division procedure to adjust the distance/weight relationships for specific chemical toxicities (i.e., IDLH value), CR airflow rates, and for varying Pasquill stability classes, assuming that the transport of material is moving with the wind directly from the release point to the air intake.

The weights presented in Table 2 o f this RG were generated from the EXTRAN computer code without the wake-effect correction, based on the following assu mptions:

• An IDLH value of 50 milligrams per cubic meter (mg/m 3)

• CR air exchange hourly rates (i.e., ACH) of 0.015, 0.06, and 1. 2

• Category F Pasquill stability class

If the IDLH value, air exchange rate, or meteorological conditions differ from the assumptions used in Table 2, simplified relationships can be used to determ ine the new weights guidance of hazardous chemicals that are to be consider ed for the CR habitability evaluation using Table 2 directly.

Varying IDLH Concentration

The weights presented in Table 2 are directly proportional to t he toxicity concentration; that is, the total chemical weights increase when IDLH value increase. If a chemical had an IDLH of 500 mg/m 3, then the allotment of weights in Table 2 (based on 50 mg/m 3) should increase by a factor of 10.

Varying Air Exchange Rate

The weights in Table 2 are inversely proportional to the ACH; t hat is, the total chemical weights decrease when the ACH increases. If a CR has an ACH of 2.4, then the weights from Table 2 (based on an ACH of 1.2 per hour) decrease by a factor of two. In other words, the weights are appropriately adjusted for the actual fresh-air exchange rate. CRs with automatic isolation capabilities may have leakage characteristics different from those listed in Table 2. Again, appropriate adjustments of weight should be made based on the actual air exchange rate. The use o f an ACH less than 0.06 should have a periodic test to validate the low leakage rate.

Varying Metrology Stability Category

Varying meteorology stability category is not a linear extrapol ation like the examples above.

Three weighting factors are provided in Table A-1. If the meteo rology was out of the Category F

condition, for better (i.e., Category E) or for worse (i.e., Category G), then the tabulated values 2.5 and

0.4 could be used for adjusting the new weight limiting value f or Table 2.If there is no change from Category F condition, then the multiplication factor is a unity . Note that in RG 1.78, Revision 1,the Category F Pasquill stability class did represent the worst 5th -percentile meteorology observed at the majority of the NPP sites.

RG 1.78, Revision 2, Appendix A, Page A-1 Table A-1. Factors for Varying Meteorology Category Pasquill Stability Category Weighting Factor A --

B --

C --

D --

E 2.5 F 1 G 0.4

There are no relevant constant or variable factors for Categori es from A to D. Please consult with the local meteorologist if desired.

RG 1.78, Revision 2, Appendix A, Page A-2