Regulatory Guide 1.95

Protection of Nuclear Power Plant Control Room Operators Against an Accidental Chlorine Release

ML12298A134
Person / Time
Issue date:	01/31/1977
From:	Office of Nuclear Regulatory Research, NRC/OSD
To:
References
RG-1.095, Rev 1
Download: ML12298A134 (7)

U.S. NUCLEAR REGULATORY COMMISSION

REGULATORYGUIDE

OFFICE OF STANDARDS DEVELOPMENT

REGULATORY GUIDE 1.95 PROTECTION OF NUCLEAR POWER PLANT CONTROL ROOM

OPERATORS AGAINST AN ACCIDENTAL CHLORINE RELEASE

Revision 1 January 1977

A. INTRODUCTION

Criterion 4, "Environmental and missile design bases," of Appendix A, "General Design Criteria for Nuclear Power Plants," to 10 CFR Part 50, "Licens- ing of Production and Utilization Facilities," re- quires, in part, that structures, systems, and compo- nents important to safety be designed to accom- modate 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 maintain it in a safe condition un- der accident conditions.

The release of chlorine could potentially result in the control room operators becoming incapacitated.

This guide describes design features and procedures that are acceptable to the NRC staff for the protec- tion of nuclear plant control room operators against an accidental chlorine release. The Advisory Com- mittee on Reactor Safeguards has been consulted concerning this guide and has concurred in the regulatory position.

B. DISCUSSION

Many nuclear power plants use chlorine for water treatment in the circulating water system and in other auxiliary systems. Chlorine is normally stored on the site as liquified gas in one-ton tanks or large railroad cars (typically 16 to 55 tons).

• Lines indicate substantive changes from previous issue.

Regulatory Guide 1.78, "Assumptions for Evaluating the Habitability of a Nuclear Power Plant Control Room During a Postulated Hazardous Chemical Release," identifies chlorine as a hazardous chemical which, if present in the control room at- mosphere in sufficient quantity, could result in the control room becoming uninhabitable. It is the pur- pose of this guide to describe specific design features and procedures that are acceptable to mitigate hazards to control room operators from an acciden- tal chlorine release. Although this guide was developed to provide protection from an onsite chlorine release, the protection provisions described here are also expected to be sufficient for an offsite chlorine release. The positions stated in this guide are based on the specific physical properties and physiological effects of chlorine.

Two basic accident types can be postulated: a long- term, low-leakage-rate release and a short-term puff release. The majority of chlorine releases experienced to date have been of the first type, involving leakage from valves or fittings and resulting in a long-term release with a leakage rate from near zero to less than one pound of chlorine per second. Given warning, only breathing apparatus is necessary to protect the control room operator from this kind of release.

However, because such a release might continue un- abated for many hours, self-contained breathing ap- paratus, a t sourc a..

i.. mafo- outlets, or equivalent protection capable of operation for an ex- tended period of time should be available.

A less probable but more severe accident would be the failure of a manhole cover on the chlorine con- tainer or the outright failure of the container itself.

USNRC REGULATORY GUIDES

Comments should be sent to the Secretary of the Commission, U.S. Nuclear Regulatory Guides are issued to describe and make available to the public Regulatory Commission. Washington, DC. 20555, Attention: Docketing and methods acceptable to the NRC staff of implementing specific parts of the Service Section.

Commission's regulations, to delineate techniques used by the staff in evalu- The guides are issued in the following ten broad divisions:

ating specific problems or postulated accidents, or to provide guidance to appli cants. Regulatory Guides are not substitutes for regulations, and compliance

1. Power Reactors

6. Products with them is not required. Maethods and solutions different from those set out in

2. Research and Test Reactors

7. Transportation the guides will be acceptable if they provide a basis for the findings requisite to

3. Fuels and Materials Facilities

8. Occupational Health the issuance or continuance of a permit or license by the Commission.

4. Environmental and Siting

9. Antitrust Review Comments and suggestions for improvements in these guides are encouraged

5. Materials and Plant Protection

10. General at all times, and guides will be revised, as appropriate, to accommodate com- ments and to reflect new information or experience. This guide was revised as a Copies of published guides may be obtained by written request indicating the result of substantive comments received from the public and additional staff divisions desired to the U.S. Nuclear Regulatory Commission, Washington, D.C.

review.

20555, Attention: Director. Office of Standards Development.

K

Such failure could occur during the transportation of a container as a result of a handling mishap or could be due to naturally or accidently produced environ- ments such as earthquakes, flooding, fire, explosive overpressure, or missiles. A failure of this type could result in a puff release of about 25% of the chlorine.

The balance of the chlorine would be vaporized and released over an extended period of time. As a result of the cloud formed by the release from such an acci- dent, the chlorine concentration inside the control room might increase rapidly. In the absence of special design measures to limit the buildup within the con- trol room, the operators might be incapacitated before they are able to don breathing apparatus.

Adequate protection of the control room operators against the types of accidental chlorine release discus- sed above will be achieved if features are included in the plant design to (1) automatically isolate the con- trol room if there is a release, (2) make the control room sufficiently leak tight, and (3) provide equip- ment and procedures for ensuring the use of breathing apparatus by the control room operators.

Protection provisions adequate for the large instan- taneous release will also provide protection against the low-leakage-rate release. Staff analysis of control room designs and the degree of protection afforded by each design has resulted in criteria for acceptance, as will be discussed in the next section. These criteria are based on the limitation (given in Regulatory Guide 1.78) that the chlorine concentration within the control room should not exceed 15 ppm by volume (45 mg/m 3) within two minutes after the operators are made aware of the presence of chlorine.' This concentration, the toxicity limit, is the maximum concentration that can be tolerated for two minutes without physical incapacitation of an average human (i.e., severe coughing, eye burn, or severe skin irritation).

Table 1 gives the maximum allowable weight of a single container of chlorine as a function of distance from the control room for various control room types. It is based on an instantaneous release of 25%

of the contents of the chlorine container and an al- lowable chlorine concentration in the control room of 45 mg/m 3, the toxicity limit, for two minutes. The initial cloud dimensions assume expansion of the chlorine gas into a spherical cloud having. a Gaussian concentration gradient. Dispersion of the cloud was calculated using the instantaneous release diffusion model appearing in Appendix B of Regulatory Guide 1.78. For those cases where the control room was located a short distance from the release point and the amount of chlorine release was small, the model was adjusted to allow for additional dispersion in the vertical direction by assuming uniform mixing between the ground and the elevation of the fresh air Two minutes is considered sufficient time for a trained operator to put a self-contained breathing apparatus into operation, if these are to be used.

inlet (a 15-meter elevation from ground level was used). The maximum allowable chlorine weights were determined by using worst case conditions for calculating the control room concentrations (signifi- cant parameters being wind speed, cloud dimensions, normal air exchange rate, time to isolate, and isolated air exchange rate). For certain control room characteristics and high wind speed, the maximum operator exposure occurs before isolation. For other cases with other control room characteristics and low wind speed, the maximum operator exposure occurs two minutes after isolation and is primarily due to in- filtration. The six control room types listed in the table span the expected range of protections required for most plants. Other combinations of the signifi- cant parameters are possible, but those listed in the table should provide sufficient guidance for most cases.

This guide does not address the protection of in- dividuals either outside the control room or within the control room but not directly involved in reactor operations. Breathing apparatus should be provided and be readily accessible throughout the plant in order to eliminate the need for personnel to seek shelter in the control room during a chlorine release.

The features and procedures described in this guide apply to plants having conventional ventilation systems. Any different methods of protection proposed will be evaluated on a case-by-case basis.

C. REGULATORY POSITION

Control room operators should be protected against the effects of an accidental chlorine release as described below.

1. Liquified chlorine should not be stored within

100 meters of a control room2 or its fresh air inlets.

(Small quantities for laboratory use, 20 lbs or less, are exempt.)

2. If a chlorine container having an inventory of

150 lbs or less is stored more than 100 meters from the control room or its fresh air inlets, the capability for manual isolation of the control room should be provided.

3. For single container quantities exceeding 150 lb, the maximum allowable chlorine inventory in a single container stored at specified distances from the con- trol room or its fresh air inlet is given in Table 1 for control room Types I through VI (described below).

For each control room type, the maximum allowable chlorine inventory in a single container is given as a function of distance from the control room. If there are several chlorine containers, only the failure of the largest container is normally considered unless the

2 Control room is defined to include all zones serviced by the emergency ventilation system.

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,lo w

w TABLE 1 MAXIMUM ALLOWABLE CHLORINE INVENTORY IN A SINGLE CONTAINER

Control Room Type Control Room Characteristics Isolation Air Exchange Local Remote Time Rate - Normal Detectors Detectors (sec)

(hrI

Air Exchange Rate -

Isolated (hr 4 )

0.06 Maximum Weight (1000 Ib) of Chlorine as a Function of Distance from Control Rooma

100 m

200 m

300 m

500 m

2000 m

(330 ft)

(660 ft)

(980 ft)

(1640 ft)

(6560 ft )

10

1200

II

• D

III

IV

V

VI

Yes Yes Yes Yes Yes Yes No No No No Yesb Yesb

10

4

10

4

1

0.3

0.3

0.06

0.06

0.015

0.06

0.015

1

2

6

8

70

1

0.5

5

12

40

3400

6

14

36

2700

20

60

230

32000

20

50

120

5000

2

4

10c

10c

1

180

380

1300

60000

aTo determine allowable chlorine inventories for distances between those given in this table, log-log interpolation is acceptable.

bsee Regulatory Position 3.e for an alternative to remote detectors.

cThe isolation time of 10 seconds refers to detection by local detectors.

If detection is by remote detectors, isolation time is effectively zero since detection and isolation will be accomplished before the chlorine reaches the control room isolation dampers.

containers are interconnected in such a manner that failure of a single container could cause a chlorine release from several containers.

a. Type I control rooms should include the follow- ing protective features:

(1) Quick-response chlorine detectors located in the fresh air inlets. Within 10 seconds3 after arrival of the chlorine, detection should initiate complete closure of isolation dampers to the control room.

(2) A normal fresh air makeup rate of less than one air change per hour. The fresh air inlet should be at least 15 meters above grade.

(3) Low-leakage construction with an equivalent air exchange rate of less than 0.06 hr-I

when all penetrations are exposed to a 1/8-inch water gage pressure differential. Construction details should be provided to show that this limit is met.

(4) Low-leakage dampers or valves installed on the upstream side of recirculation fans or other loca- tions where negative systems pressure exists and where inleakage from chlorine-contaminated outside air is possible.

b. Type II control rooms should include the protective features of paragraph a except that the isolation time should be 4 seconds or less rather than

10 seconds or less.

c. Type III control rooms should include the protective features of paragraph a except that the normal fresh air makeup rate should be limited to 0.3 air change per hour or less.

d. Type IV control rooms should include the protective features of paragraph a except that the isolation time and the normal air exchange rate should be equal to or less than 4 seconds and 0.3 air change per hour, respectively. In addition, the con- trol room isolated air exchange rate should be reduced to 0.015 air change per hour or less (see description of required leak rate verification test in Regulatory Position 5).

e. Type V control rooms should include theprotec- tive features of paragraph a with the addition of remote chlorine -detectors located at the chlorine storage and unloading location. These additional detectors should be placed and the detector trip points adjusted so as to ensure detection of either a leak or a container rupture. A detector trip signal should accomplish automatic isolation of the control This is the time interval between the time the chlorine concentration exceeds 5 ppm at the isolation dampers and the time the dampers are completely closed. Note that if the chlorine detectors are upstream from the isolation dampers, credit will be allowed for the travel time between the detectors and the dampers.

room before chlorine arrives at the isolation dampers. The detector trip signal should also set off an alarm and provide a readout in the control room.

An alternative to the installation of remote detectors would be to provide an isolation system using local detectors but having an isolation time of effectively zero. This can be accomplished by ensuring that the time required for chlorine to travel from the chlorine detector to the isolation damper, within the inlet ducting, is equal to or greater than the time required to detect the chlorine and close the isolation damper.

f. Type VI control rooms should include the protective features in paragraph e except that the control room isolated air exchange rate should be reduced to 0.015 air change per hour or less. For isolated exchange rates between 0.0 15 hr- ' and 0.06 hr-' , linear interpolation of the weights given for control room Types V and VI in Table 1 can be made.

Verification testing is required within this range of exchange rates (see Regulatory Position 5).

4. The following should be applied to all control room types (I through VI):

a. Immediately after control room isolation, the emergency recirculating charcoal filter or equivalent equipment designed to remove or otherwise limit the accumulation of chlorine within the control room should be started up and operated.

b. Steps should be taken to ensure that the isolated exchange rate is not inadvertently increased by design or operating error. For instance, the following should be considered:

(1) An administrative procedure should require that all doors leading to the control room be kept closed when not in use.

(2) Ventilation equipment for the control room and for the adjacent zones should be reviewed to en- sure that enhanced air exchange between the isolated control room and the outside will not occur (e.g., if there is a chlorine release, exhaust fans should be stopped and/or isolated from the control room ven- tilation zone by low-leakage dampers or valves).

(3) A control room exit leading directly to the outside of the building should have two low-leakage doors in series.

c. The use of full-face self-contained pressure- demand-type breathing apparatus (or the equivalent)

and the use of protective clothing should be con- sidered in the development of a chlorine release emergency plan. Because calculations indicate that chlorine concentrations may increase rapidly, emergency plan provisions and rehearsal of these provisions are necessary to ensure donning of breathing apparatus on detection of high chlorine I

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concentrations. Storage provisions for breathing ap- paratus and procedures for their use should be such that operators can begin using the apparatus within two minutes after an alarm. Adequate air capacity for the breathing apparatus (at least six hours) should be readily available onsite to ensure that sufficient time is available to transport additional bottled air from offsite locations. This offsite supply should be capable of delivering several hundred hours of bot- tled air to members of the emergency crew. A

minimum emergency crew should consist of those personnel required to maintain the plant in a safe condition, including orderly shutdown or scram of the reactor. As a guideline, a minimum of five units of breathing apparatus should be provided for the emergency crew.

d. The air supply apparatus should meet the single failure criterion and be designated Seismic Category I. (In the case of self-contained breathing apparatus, the single failure criterion may be met by supplying one extra unit for every three units required.)

The isolation system components should be of a quality that ensures high reliability and availability.

One method to meet these goals is to provide a system that meets the requirements of IEEE-279,

"Criteria for Protection Systems for Nuclear Power Generating Stations." In all cases, the isolation system, recirculating filter system, and air condition- ing system should meet IEEE-279 since they are re- quired to maintain a habitable environment in the control room during design basis radiological events.

Specific acceptance criteria for the chlorine detec- tion system and allied actuating electronics are as fol- lows:

(1) Chlorine Concentration Level. Detectors should be able to detect and signal a chlorine con- centration of 5 ppm.

(2) System Response Time. The system response time, which incorporates the detector response time, the valve closure time, and associated instrument delays, should be less than or equal to the required isolation time.

(3) Single Failure Criteria. The chlorine detection system should be redundant and physically separate to accomplish decoupling of the effects of unsafe en- vironmental factors, electric transients, physical acci- dents, and component failure.

Local detectors should consist of two physically separate channels for each fresh air inlet. Each chan- nel should consist of a separate power supply, detec- tor, actuating electronics, and interconnecting ca- bling. Remote detectors should also consist of two separate channels having detectors located at the chlorine unloading facility.

(4) Seismic Qualification. The chlorine detection system should be designated as Seismic Category I

and be qualified as such.

(5) Environmental Qualification. The detection system should be qualified for all expected environ- ments and for severe environments that could clearly lead to or be a result of a chlorine release. The instal- lation of the detectors should ensure that they are protected from adverse temperature effects.

(6) Maintenance, Testing, and Calibration. The manufacturer's maintenance recommendations are acceptable provided they follow sound engineering practice and are compatible with the proposed ap- plication. A routine operational check should be con- ducted at one-week intervals.

Verification testing and calibration of the chlorine detectors and verification testing of the system response time should be conducted at six- month intervals.

5. The gross leakage characteristic of the control room should be determined by pressurizing the con- trol room to 1/8-inch water gage and determining the pressurization flow rate. (The use of a higher pressure differential is acceptable provided the flow rate is conservatively adjusted to correspond to 1/8-inch water gage.) For air exchange rates of less than 0.06 hr -', periodic verification testing should be per- formed. An acceptable method for periodic testing would be the use of a permanently installed calibrated pressurization fan. The system would have a known pressure-versus-flow characteristic so that the leak rate could be determined by measuring the control room pressure differential. Testing should be conducted at least every six months and after any ma- jor alteration that may affect the control room leakage.

6. Emergency procedures to be initiated in the event of a chlorine release should be provided.

Methods of detecting the event by station personnel, both during normal workday operation and during minimum staffing periods (late night and weekend shift staffing), should be discussed. Instrumentation that has been provided for the detection of chlorine should be described including sensitivity; action in- itiated by detecting instrument and level at which this action is initiated; technical specification limitations on instrument availability; and instructions for maintenance, calibration, and testing. Criteria should be defined for the isolation of the control room, for the use of protective breathing apparatus and other protective measures, and for maintenance of the plant in a safe condition including the capability for orderly shutdown or scram of the reactor. Criteria and procedures for evacuating nonessential personnel from the station should also be defined.

1.95-5

D. IMPLEMENTATION

The purpose of this section is to provide informa- tion to applicants regarding the NRC staff's plans for using this regulatory guide.

This guide reflects current NRC staff practice.

Therefore, except in those cases in which the appli- cant proposes an acceptable alternative method for complying with specified portions of the Commis- sion's regulations, the method described herein is be- ing and will continue to be used in the evaluation of submittals for construction permit applications until this guide is revised as a result of suggestions from the public or additional staff review.

I

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