ML20066E157

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Rev 1 to Analysis to Support Removal of Vermont Yankee Toxic Gas Monitoring Sys
ML20066E157
Person / Time
Site: Vermont Yankee Entergy icon.png
Issue date: 12/31/1990
From: Burns E, Burns K, Snooks J
ERIN ENGINEERING & RESEARCH, INC., YANKEE ATOMIC ELECTRIC CO.
To:
Shared Package
ML20066E147 List:
References
YAEC-1759, YAEC-1759-R01, YAEC-1759-R1, NUDOCS 9101180110
Download: ML20066E157 (36)
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Analysis to Support Removal of the

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, Vermotit Yankee Toxic Cas Monitoring System ls lI I

.I December 1990 Major Contributorst J. II. Snooks Yankee Atomic Electric Company Yankee Atomic Electric Company E. Burns ERIN Engineering and Research, Inc.

Yankee Atomic Electric Company Nuclear Services Division 580 Main Street Bolton, Massachucetts 01740 WPP44/77

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TABLE OF CONTENTS Page I LIST OF TABLES.................................................... 111 l

1 LIST OF FIGURES................................................... iv

1.0 INTRODUCTION

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2.0 BACKGROUND

................,....................................... 2 3.0 APPLICABLE REGUI ATORY CRITLA I A. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 4.0 ANALYSIS METl10DOLOGY.................. ........................... 8 5.0. II I GIIWAY ANALY S I S R ES U LTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 5.1 llazardous Chemical Identification........................... 11 5.2 Control Room Concentrations................................. 11 1 6.0 RAILROAD ANALYSIS RESULTS.......................................... 15 I 6.1 6.2 6.3 llazardous Chemical Identification. .........................

Control Room Concentrations.................................

Probability of Control Room Uninhabitability................

15 15 16 6.4 Frequency of Significant Radiological Release............... 20 6.5 Treatment of Uncertainties..................................

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7.0 CONCLUSION

S....................................................... 30 6

8.0 REFERENCES

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, w-1.IST OF TAP.I.ES l Number Title Pay 5.1 Control Room Concentrations of llarardous chemicals from 13 liighway Accidents 6.1 List of flazardous Chemicals Transported by Railroad 26 6.2 Control Room Concentrations of lineardous Chemicals f rom 27 8 Railroad Accidents 6.3 Summary of Lost Control Room liabitability 28

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5- 5-1 General Site Area Within Five Miles 14 l

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) 6-1 Discrete Segments Along Railroad Track 29 i  :

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l.0 INTRODUCTION n

This report documents an analysis which supports removal of the Toxic Cas Monitoring System (TCMS) at the Vermont Yankee Nuclear Power Station (VYNPS). The applicable regulatory requirements and the basis for the current TCMS are reviewed. The analysis results demonstrate that the regulatory requirements can be met for VYNPS without the need for a TCMS.

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2.0 BACKGROUND

The TChs is described in VYNPS FSAR Section 7.19. Technical Specification requirements appear in Sections 3,2 and 4.2 of the VYNPS Technical Specifications. The TGMS samples Control Room IIVAC intake air f or I five toxic gases. TGMS trip occurs when mo..itored gas concentrations reach predetermined setpoints. TCMS trip causes Control Room isolation dampers to close and the Bottled Gas Pressurization System to initiate. These automatic actions are designed to provide Control Room operators with at least two minutes to don breathing apparatus before the Control Room air reaches toxic limits.

I The VYNPS TGMS was installed to meet NUREG-0737, item III.D.3.4 I (Reference 1), requirements for Control Room habitability following a postulated off-site chemical release. NUREG-0737 Item III.D.3.4, requires Control Room habitability evaluations for three different hazardst

1. Radiological Release
2. On-Site Chemical Release ,
3. Off-Site Chemical Release Vermont Yankee's original evaluation (Reference 2) showed thatt
1. For radiological releases, "... no further safeguards are necessary to meet Control Room habitability requirements."
2. ... no potential hazard was defined for Control Room personnel from toxic chemical materials stored on-site."
3. "... the following toxic chemicals shipped via the rail line have been identified as potential hazards to the Control Room personnel"!
  • Ammonia

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  • Vinyl Chloride
  • Methanol Thus, off-site chemical releases were the only basis for the TGMS at VYNPS.

The NBC Safety Evaluation Report (Reference 3) concluded that upon I completion of the modifications to provide a detection system for the five gases listed above, the criteria of NUREG-0737, Item III.D.3.4, would be met.

VYNPS installed the TGMS in 1983 and submitted appropriate Technical Specification changes to fulfill the NUREG-0737, Item III.D.3.4 requirements.

8 Since its installation, the VYNPS TCMS has been an operational burden.

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W actual evetits involving toxic gas concentrations have occurred. These spurious alarms require operator response, hence, are a distraction to Control Room operators. Operators tend to lose confidence in the validity of such alarms, which reduces the effectiveness of the system in a real emergency. As it is, each TGL unit is unavailabic about 10 percent of the time due to preventive maintenance and about 10 percent of the time due to corrective action.

g These operational difficulties stem from the fact that it is difficult 4 to maintain instrument calibration within the narrow range required to detect minute quantities of toxic chemicals (e.g., 5 ppm for chlorine). The extent I of these dif ficulties was not anticipated at the time the TGMS was installed.

This is evidenced by the fact that the TGMS requires actual calibration every one to two weeks to remain within Technical Specification setpoint limits, yet i the Technical Specificaticn calibration frcquency is once per operating cycle.

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3.0 APPLICABLE RECUT.ATORY CRITERIA The basis for the VYNPS TGM5 is hTREG-0737 Item III.D.3.4 " Control Room Habitability Requirements" (Reference 1). As noted in Section 2.0 above, the TGMS was installed only to meet the 'of f-site release" requirements of Reference 1. No plant changes, including this proposed change, have occurred that would impact the conclusions of References 2 and 3 regarding the I " radiological release" and "on-site chemical" portions of the Reference 1 requirements. Thus, removal of the TGMS requires only that the off-site chemical hazards be evaluated. l Given that the scope of analysis here involves only off-site chemical l

release, Reference 1 provides guidance by identifying the appropriate Standard Review Plan (SRP) sections and Regulatory Guides (RG). These are as follows:

RP 2.2.1-2.2.2 " Identification of Potential Hazaids in Site W Vicinity" SRP 2.2.3 " Evaluation of Potential Accidents" SRP 6.4 " Habitability Systems" RG 1.95 " Protection of Nuclear Power Plant Control Room 1 Operators Against an Accidental Chlorine Release" I RG 1.78 " Assumptions for Evaluating the Habitability of a Nucicar Power Plant Control Room During a Postulated Har.ardous Chemical Release" The guidance provided by these documents is summarized below in the context of this proposed change:

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I' SRP 2.2.1 and 2.2.2, " identification of Potential Hazards in Site Vicinity" g This document is used to identify prJ.c-ntial external hazards. Hazards h which are identified as resulting f rom the presence of hazardous materials or activities in the vicinity of the site are to be reviewed further under I SRP 2.2.3. As discussed above, the hazard applicabic to this proposed change is the transportation of toxic chemicals near the site.

SRP 2.2.3, " Evaluation of Potential Accidents" i Given a potential hazard identified under SRP 2.2.1 and 2.2.2, SRP 2.2.3 is used to determine which events must be considered further as y design basis events. The SRP 2.2.3 acceptance criteria states:

"The identification of design basis events resulting from the presence of hazardous materials or activities in the vicinity of the plant is acceptable if the design basis events include each postulated type of accident for which the expected rate of occurrence of potential exposures in excess of the 10CFR, I Part 100, guidelines is estimated to exceed the NRC staff objective of approximately 10-7 per, year. Because of the difficulty of assigning accurate numerical values to the expected rate of unprecedented potential hazards generally considered in this review plan, judgement must be I

I used as to the acceptability of the overall risk presented.

"The probability of occurrence of the initiating I events leading to potential consequences in excess of 10CFR, Part 100, exposure guidelines should be estimated using assumptions that are as I representative of the specific site as is practicable. In addition, because of the low probabilities of the events under consideration, I data are often not available to permit accurate calculation of probabilities. Accordingly, the expected rate of occurrence of potential exposures in excess of the 10CFR, Part 100, guidelines of approximately 10-6 per year is acceptable if, when combined with reasonable qualitative arguments, the realistic probability can be shown to be lower."

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1 SRP 2.2.3 goes on to say that if accidents involving the release of chemicals do not meet the above acceptance criteria, an evaluation of the effects of these analyses on Control Room habitability should be made using SRP 6.4, "liabitability Syste.as."

No SRP 2.2.3 evaluation of the probability of such events was performed  ;

in Vermont Yankee's original response to NUREG-0737. Item III.D.3.4. Instead, the deterministic criteria of SRP 6.4 and its supporting regulatory guides (Regulatory Guides 1.78 and 1.95) were used.

SRP 6.4. "flabitability Systems" The toxic gas portion of SRP 6.4 makes reference to Regulatory Guide 1.78 as the method to be used to determine whether the quantity or I location of toxic material is such that additional analysis is necessary. The referenced methods to be used in any additional analyses are Regulatory I Guide 1.78 and Regulatory Guide 1.95.

Regulatory Guide 1.95. " Protection of Nuclear Power Plant Control Room Opera' tors Against an Accidental Chlorine Release" l Regulatory Guide 1.95 was developed to provide Control Room operator protection from an accidental on-site chlorine release.

I It uses the methodology of Regulatory Guide 1.78 to calculate the allowable weight of a single chlorine container as a function of distance from the Control Room.

Since Vermont Yankee has no on-site storage of chlorine, the Regulatory Guide 1.95 analysis is not applicable.

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% 1 Regulatory Guide 1.78._" Assumptions for Evaluating the Habitability of a Nuclear Power Plant Control Room Durin La Postulated linrardous Chemical Release" The purpose of Regulatory Guide 1.78 is to identify those chemicals which could result in Control Room uninhabitability. Screening criteria are given in terms of the proximity (within a flue-mile radius) and f requency (10 per year for truck and 30 per year f or rall) of shipment. A representative list of hacardous chemicals and their toxicity limits is also provided.

For those chemicals that are not eliminated by the proximity / frequency screening criterio. Regulatory Guide 1.78 provides a methodology to calculate 9

Control Room concentration versus time after an accidental release. The acceptance criterion is that the time from detection to the time when the toxicity limit is reached must be at least two minutes to allow operators to don self-contained breathing apparatus.

The analysis of Reference 2 showed that this two-minute criterion was satisfied for all chemicals except five. The '1GMS was designed to provide automatie detection and Control Room isolation such that this two-minute criterion could be met for these five chemicals.

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4.0 ANALYSIS _ METHODOLOGY This report addresses chemical releases from off-site transportation I accidents. Both railroad and highway transportation routes are considered.

The general *nethodology applied to each transportation route consists of the I following steps. Note that Steps 1 and 2 involve deterministic analyses, and Steps 3 and 4 involve probabilistic analyses. The probabilistic analyses are only performed if the deterministic screening criteria of Steps 1 and 2 are exceeded.

5 Step 1 - Hazardous Chemical Identification I for the purposes of this analysis, a chemical is identified as hazardous if it meets all of the following criterlat I

a. Transportation of a chemical occurs within a five-mlie radius of g

g the plant.

b. Shipment frequency is greater than ten per year for truck and 30 per year for rail.

I c. Chemical appears on either the Regulatory Guide 1.78 list of potentially hazardous chemicals or the Environmental Protection I Agency's (EPA's) list of Extremely Hazardous Substances (Reference 6).

Step _2 - Control hoom concentrations Foc each chemical identifled at hazardous in Step 1, a calculation of Control Room concentration versus time .s performed. The accident is assumed to occur at the transportation route's closest proximity to the plant. No credit is taken for the TGMS. These calculations were performed using the W HAZARD computer code (References 7 and 8), following the guidance of Regulatory Guide 1.78. The HAZARD code accounts for such parameters as chemical volatility, atmospheric dispersion, and Control Room intake / exhaust flow.

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The toxicity limits used in these calculations are based on the "Inmediately Dangerous to Lif e and licalth" (IDLil) concentrations published by the National Institute of Occupational Safety and Health (NIOSH)

(Reference 9). IDLil values have been developed with acute human toxicity as the priocipal consideration. The IDLil provides the basis from which the EPA I developed levels of concern when emergency planning for Ells releases (Reference 6).

Chemicals are judged not to require an automatic detection system if

a. Control Room concentrations never reach toxic limits, or
b. There is at least two minutes between the time that the toxic gas is detectable by smell by the operating crew and the time that the toxic limit is reached.

I Step 3 - Probability of Control Room Uninhabitability Chemicals which can reach toxic levels faster than two minutes after detection are identified in Step 2. These chemicals require an automatic detection system to satisfy the deterministic criteria of Regulatory Guid? 1.78. Ilowever, the deterministic criteria of Regulatory Guide 1.78 are I only required for potential hazards whose frequency of causing a radiological rGease in excess of 10CFR100 limits exceeds the SRP 2.2.3 guideline of IE-7 I to IE-6 per year. For these chemicals, the annual probability of Control Room uninhabitchility is calculated using the following informationt R = Annual accident rate per mile.

I Rc = Conditional probability of a significant chemical release, given an accident.

N = Annual number of shipments within a five-mile radius of the plant.

I L = Length, in miles, of transportation route within a five-mile I radius of the plant.

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I F = Annual frequency of wind speeds and stability classes that could disperse plume to plant.

E A potential accident involving release of a chemical is judged to meet the SRP 2.2.3 criteria if the frequency of Control Room uninhabitability is less than the SRP 2.2.3 lower bound of 1E-7 per year, blep 4 - Frequency of Signifleant Radiological Release l

The SRP 2.2.3 guideline of IE-7 to 1E-6 actually refers to the frequency of a 100FR100 fission product release involving significant core damage. In order to cause such a release, the Control Room uninhabitability

nust lead to a plant trip with operating crew and equipment f ailure leading to failure of a critical safety function such as core cooling or decay heat removal. Step 4 calculates the frequency of a 100FR100 fis,ston product release resulting f rom an of f-site chemical release.

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5.0 HIGHWAY AWALYSIS RESU1]E The h'ghway analysis involves Steps 1 and 2 from Section 4.0, " Analysis Methodology," Sections 5.1 and 5.2 below demonstrate that Regulatory Guide 1.78 requirements are met. Accordingly, Steps 3 and 4 were not I necessary.

5.1 Harardous Chemical Identification Contacts were made with both state government agencies and chemical distributors (Reference 12) to identify tNe types and quantities of chemicals transported past Vermont Yankee on U.S. Route 91, the most frequently traveled major highway (see Figure 5-1).

E The chemicals identified during the survey were compared to Regulatory

0. tide 1.78 and EPA's list of Extremely liarardous Substances (Ells)

(Reference 6). EllSr are those chemicals that, because of, extreme toxicity, are most likely to cause severe toxic effects in humans who are exposed to them due to an accidental release. Chemicals appearing on either list wero considered for further evaluation.

I From the list of chemicals identified, the following were considered I potential " toxic" hacardst

  • Anhydrous Ammonia
  • Sulfur Dioxide
  • Sulfuric Acid
  • Propane (LPG) l 8 5.2 Control Room Concentrations Loss of Control Room habitability occurs when the Control Room i

concentration of a hazardous chemical exceeds its " toxic limits" in less than two minutes after detectio.. TW toxic limit in this analysis is considered l

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I' to be the National Institute of Occupational Safety and llealth (N10Sil)

"Inunediately Dangerous to Life and llealth (IDLil)" concentration I (Reference 9). This value is the maximum concentration from which an operator could escape within 30 minutes without experiencing any escape-itnpairing or irreversible health effects.

To calculate Control Room concentration, the computer program IIAZARD

  • I was used (References 7 and 8). Control Room habitability is maintained if Control Room concentrations never equal or exceed the IDLil value, or if there  ;

is at least two minutes between the time the toxic gas is detectable by smell by the operator and the time that the IDLil value is reached. Two minutes is considered sufficient time for a trained operator to put a self-contained breathing apparatus into operation (Reference 11).

I The results (from Reference 12) are shown in Table 5.1. Also listed is the quantity of the largest container shipped and the IDLil value.

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As shown, the maximum Control Room concentration for each chemical is less than its corresponding IDLil value. The " toxic limit," therefore, is not l reached, and Control Room habitability will be maintained in the event of a postulated highway accident.

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I' TA!5LF 5.1 Control Room Concentrations of flazardous Chemicals f from }!ighway Accidents

!' Maximum Control P,oom

Chemical Quantity _ Concentration IDLil Chlorino 1 ton 23 ppm 30 ppm Annonia 16 tons 258 ppm 500 ppm i 1 i

Sulfur Dioxide 1 ton 15 ppm 100 ppm l Sulfuric Acid 6,500 gal <1 ppm 20 ppm

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Propane 10,000 gal 1,491 ppm 20,000 ppm lI iI 4

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6.0 RAILROAD ANALYSIS RESULTS The railroad analysis involves Steps 1 through 4 fram Section 4.0,

" Analysis MetMdology." Sections 6.1 and 6.2 below show that, of the hazardous chemicale identifled, only chlorine exceeds Regulatory Guide 1.78 criteria. Therefore, probabilistic analyses (Sections 6.3 and 6.4 below) were performed for chlorine.

I 6.1 Harardous Chemical Identification Two railroads operate along the tracks adjacent to Vermont Yankeet Central Vermont Railroad and Sprin6 field Terminal Railway Company (formerly Boston and Maine Railroad). Each was contacted and asked to provide a list of hazardous chemicals transported (Reference 13). The railroad tracks opposite Vermont Yankee across the river are no longer in service (Reference 14).

As in Section 5.1, the chemicals listed were compared to negulatory g Guide 1.78 ani EPA's list of EHS (Reference 6). Chemicals appearing on either W list were considered for further evaluation.

The chemicals considered toxic are listed in Table 6.1. However, only chlorine, propane, nitrogen, and carbon dioxide were evaluated because they exceed 30 shipments per year.

6.2 Control Room Concentrations As in Section 5.2, the computer code HAZARD (Rt.ferences 7 and 8) was I used to calculate Control Room concentrations. Control Room habitability maintained if Control Room concentrations never equal or exceed the IDLH is value, or if there i, at least two minutes between the time the toxic gas is detectable by smell by the operators and the time that the IDLH value is reached.

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For chemicals that act only as asphyxiants (e.g., nitrogen), Control Room habitability is lost if the oxygen content drops below 18 percent (Reference 15). It is assumed the chemicals will not be detected prior to reaching the asphyxiation level, thus the two-minute warning criterion is not used. Percent oxygen content is calculated as 0 % = 20.9 ( , dare X 2 00 is the percent by volume of the hazardous chemical.

I The results (from Reference 13) are listed in Table 6.2. The IDLil value, detection threshold, and the time f rom detoction to IDLII are also l

listed where applicabic.

I As shown, the maximum Control Room carbon dioxide concentration is below the IDL11 value. The nitrogen concentration (31,127 ppm) it equivalent to 3.1 percent by volume. This value would reduce the Control Hoorn oxygen content to 20.25 percent, which is well above the reconnended minimum of 18 percent (Reference 15).

The maximum Control Room concentrations of both propane and chlorine are above their respective IDLil values. 110 wever, propane meets the regulatory guide of more than two minutes between the time the gas is detected and the P time the IDLil value is reached. The time dif f erence for chlorine, howev'er, is only about one minute.

Carbon dioxide, nitrogen, and propane meet Regulatory Guide 1.78 criteria to maintain Control Room habitability. This is not the case for chlorine. Accordingly, chlorine was evaluated further to determine the probability of Control Room uninhabitability.

6.3 Probability of Control Room l'ninhabitability A hazard model was used to calculate the annual probability (P) of Control Room uninhabitability in the general form WPP44/77

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l P = (R) x (Re ) x (N) x (L) x (F) l wheret g R = Annual train accident rate per mile.  :

k Re = Conditional probability that a significant chemical release I will occur, given an accident.

N = Annual number of shipments within five-mile radius of plant.

L = Length, in miles, of railroad track within five-mile radius of plant.

I F = Annual f requency of wind speeds and stability classes that could disperse plume to plant.

I The probability of loss of Centrol Room habitability, given a release, varies as a function of distance f rom the Control Room air intake, wind direction, speed, and atmospheric stability. To account for these variables, the analysis was performed over 17 discrete segments along the railroad track and the probability summed for each segment. (See Figure 6-1)

I The probability of loss of Control Room habitability was calculated by assuming that the midpoint of each segment represents the average distance for I the segment of track being analyzed. The total probability, P, therefore, becomes the sum of the probabilities associated with each segmenti n

P = 1 ((Ri ) x (Ni ) x (L1 ) x (R eg) x (Fi)], where n = 17 i=1 I

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The parameters Rg and Ng are constants and do not vary between segments. The parameters R ei, Lg , and 1F , on W o h hand, m y b segment to segment. The general form of the hazard model, hence, becomest i P = R x N x [ [(Lg) x (Re {) x (Fi H I i=1

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(k) Egiltgr Accident - There is no published accident rate for I chlorine. Instead, the average accident rate for railroads, in general, during the five-year period from 1984 to 1988 was used (Reference 16).

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l This annual average is 5.40 x 10-6 accident / mile.

I (N) Annual Numhar_of Shipmenia - Trom Table 6.1, the annual number of I

i chlorine care transited is 60.

(Li) SegmenLLengtha_nLEsilroad_Irath._in_tiiles_IhaLChintint_Would Irang_it Within_Ilyg_tillen of Plant - From Figure 6-1, which shows the railroad and the general site area, the total length of track was measured to be 10.7 miles. Segment lengths (L i

) are listed in Table 6.3.

(Re{) C.Qndi11onni Probabilily_gl S.ignificant Rhlorine R CitaE.C - Given an accident, there must be a release to affect the plant. In addition, given a release, it must be significant to be of concern. Therefore, R is W eg product of the probability of a release given an accident and the probability of a significant release given a release; R =R ri x R,i.

ei The probability of a release given an accident, Rrg, was estinated from the accident data involving all hazardous materials, chlorine being a hazardous material (Reference 16).

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The average rate for the five-year period, from 1984 to 1988, is 0.124 releases / accident, which is assumed constant for each segment.

The probability of a significant release given a release, R, , was determined from actual chlorine incident data (Reference 17). During the 1

f I years 1971 through 1989, there were 64 chlorine railcar incidents that l

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involved a release. Of these, the largest was 17,440 gallons, and the 1 smallest was one gallon, flowever, to be significant, the release must be of l suifleient volume to produce toxic limits inside the Control Room.

To determine the minimum volume needed to produce toxic levels within

the Control Room for each of the 17 segments, the program flAZARD (keferences 7

! and 8) was used. The probability of a significant release given a release, R, , f or each segment is simply the number of releases in the dath base that j were greater than or equal to the minimum volume for each segmeni: divided by k 64, the total number of releases in the data base. The results are shown in f Table 6.3.

) (Fi ) frohnhili1Lni_.h'iruLantitahili11Alasaes_lhaL_Cattidliipirie ,

Plume to_P.lant - Adverse meteorological conditions include both the likelihood of the stability class occurring and the likeliheod of the wind blowing toward J the Control '?oom intake.

I To account for the many wind sectors along the railroad track, the I midpoint at each segment was analyzed (using ilAZARD) to determine what stability classes and wind speeds produced toxic limits inside the Control Room given a worst-case accident. In other words, an analysia was performed for each segment over the reven stability classes and the five wind speed categorics to determine which conditions resulted in the loss of Control Room habitability.

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The toxic limit cot...itions were then compared to a site meteor (logical joint frequency anelysis (Reference 20) to calculate the annual frequency (Fi) that toxic limits can be expected in the Control Room for each segment.

Combining the segment analysis input l'arameters (see Table 6.3), the annual probability of chlorine reaching toxic levels in the Control Room is as follows:

I 17 F=$.40x10-6x60x0.124x((R, xLixF}

i i=1 i P = 4.39 x 10-7 per year 6.4 Frequency of Significant Radiological Release Sections 6.1 and 6.2 show that an off-site chlorine release is the only I, event which does not satisfy the deterministic criteria of Regulatory Guide 1.78. SRP 2.2.3 requires that such an accident be considered as a design basis event only if the frequency of exceeding 10CFR100 guidelines for fission prnduct release exceeds IE-7 per year (a value of IE-6 per year is acceptable when combined with reasonable qualitative arst_ments that the realistic probability is lower).

I A 10CFR100 fission product release is a result of significant core damage. Thus, this section estimates the core damage frequency due to an I off-site chlorine release and compares this to the SRP 2.2.3 actuptance guidelines.

6.4.1 Initiating Event I The initiating event considered here is an off-site transportation accident which leads to toxic chlorine concentrations in the Control Room within two minutes after operator detection. As calculated in Section 6.3, the frequency of this initiating event is 4.39 x 10-7 per year.

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This initiating event does not lead directly to core damage. For core damage to occur, a plant trip must occur, and there must be failure of a critical safety function, e.g., the core injection or containment heat removal function must fail.

I 6.4.2 Plant Trip Occurs Since chlorine is not hallucinogenic, it is assumed that the operators, even if incapacitated, will not act in such a way as to deliberately place the I

plant in an unsafe state. Rather, they simply will not be able to respond positively if required to do so. This means that they will not be able to respond to events set in motion by a transient. For Vermont Yankee, it is conservatively estimated that there are an average of six transients per year I that result in reactor scram and demand for a safe shutdown. In addition, it is estimated that operators take actioh to prevent such occurrences an additional ten' times per year (Reference 18). Assuming that the operating crew is incapacitated for an average of four hours *, the probability that a shutdovn challenge occurs during this four-hour period is calculated as X = 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br /> x 16 challenges / year = .0073 ^

8,760 hours0.0088 days <br />0.211 hours <br />0.00126 weeks <br />2.8918e-4 months <br /> / year Even if the operators are unable to respond to a transient challenge, j

I core melt and the release of radioactive material to the environment do not follow directly. The possibility that the plant can remain in a safe, stable state given a scram challenge is estimated below, based on an evaluation of the core coolant injection and containment heat removal critical safety functions.

I 6.4.3 Coolant Iniection Fails I The emergency core cooling systems at Vermont Yankee are automatically I initiated without the need for operator action. The emergency core cooling

  • This time was chosen because the accident will occur, on the average, halfway through a typical operating crew shift of eight hours.

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function can be satisfied by either the 511gh Pressure Coolant Injection (HPCI)- 0 System or the Reactor Core Isolation Cooling (RCIC) System. The emergency core cooling function can elso be satisfied by either the Low Pressure Coolant Injection (LPCI) System or the Low Pressure Core Spray (LPCS) Syst'em after successful operatien of the Automatic Depressurir.ation System (ADS).

I Thus, the failure probability of the core injection function can be l

approximated by the following equation:

Core Injection Failure = [HPCI x RCIC] x .(ADS + (LPCI x LPCS)]

Where:

HPCI = Probability of HPCI System Failure I RCIC = Probability of RCIC System Failure ADS = Probability of ADS System Failure LPCI = Probability of LPCI System Failure LPCS = Probability of LPCS System Failure I The probability of High Pressure Injection System failure [HPCI x RCIC) is conservatively estimated to be 1.0 (guaranteed failure).

The Automatic Depressurization System is initiated on either of the

,I following accident signals:

  • Low-low reactor water level sustained for eight minutes.
  • High drywell pressure and low-low reactor water level.

I The 120-second ADS timer is initiated if either of the signals is present. The ADS valves will automatically opun to relieve pressure if the water level has not been restored ag if any of the low pressure ECCS pumps I are running.

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The ADS initiation causes the RPV to depressurite rapidly. The failure probability for automatic depressurization is taken from previous PRAs (Ref erences 18 and 19) with the f ailure probabilities for operator initiation I and external nitrogen supply both set to 1.0. This results in a failure probability over four hours of lE-3 for the failure to depressurize. This system is highly reliable because it is a redundant safety system designed to actuate automatically under these conditions. The TMI modification to ADS which results in depressurization on low level only contributes to the low assessed failure probability. Low pressure systems will reliably inject following deprersurization because of the automatic design of the low pressure system response.

I 'Ine f ailure probability of the multiple Low Pressure Injection Systems (two LPCI and two Core Spray) can be estimated by comparison with other similar plants and their PRAs. The failure probabilities are as follows:

I Combined Conditional Failure Probability Over 24 Hours for LPCI and Core Spray Reference I 7.7E-5 Limerick PRA (Reference 18) 6.25E-4 Shoreham PRA (Reference 19)

The Limerick design has individual reactor pressure vessel nozzle injections for LPCI which is different from Vermont Yankee's configuration.

I The arehsm configuration is similar to Vermont Yankee's and, therefore, it is usad as t.he basis, and a factor of three increase is used to conservatively cover dif f erences in the two systems. Therefore, the failure probability of the low pressure ECCS is conservatively taken to be 1.8E-3 for the purposes of the evaluation of a four-hour mission time. This leads to a total conditional failure probability of adequate low pressure injection (ADS + (LPCI x LPCS)]

of 2.8E-3 over a four-hour mission time without operator intervention.

I Thus, the combined probability of coolant injection failure is estimated to be 2.8E-3.

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6.4.4 Containment Heat Removal Failure I Even if the coolant injection function is successful, core cooling systems can be jeopardized if the Main Steam Isolation Valves (MSIVs) are closed, and if the containment heat removal function fails. We assume (conservatively) that the MSIVs close when the reactor trips. The probability of the containment heat removal function is estimated from past PRA evaluations (References 18 and 19). The estimate for RHR unavailability without operator recovery actions is 4E-4. This is dominated by hardware failures which, for this analysis, are assumed unrecoverable despite having approximately 20 to 30 additional hours for repair.

I Because adverse impacts on operating crew actions to align containment heat removal systems may be anticipated due to residual effects of the toxic gas outside of the Control Room, only RHR alignment from the Control Room is assuned possible. No recovery or repair actions are assumed possible, and the probability of failure of the containment heat removal function is estimated to be 4E-4.

I 6.4.5 fic.quency_cf Core Damagg I Based on the above, the core damage frequency from this initiating event is estimated as follows:

Core Damage Frequency = initiating event frequency x I probability of plant trip x I (probability of core injection failure +

I probability of containment heat removal failure)

= 4.39E-7 x 7.3E -3 x (2.8E-3 + 4E-4)

= 1E-11 per year I

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This result shows that the estimated core damage frequency due to an off-site chlorine release is far below the SRP 2.2.3 guidelines. Thus, this event does not require consideration as a design basis event, and the Regulatory Guide 1.78 provisions for an automatic detection system are not required.

I 6.5 Treatment of Uncertainties The probabilistic analyses of Sections 6.3 and 6.4 are based on point-estimate values. The point-estimates rsed in this analysis are based on the best available data and engineering judgement. These values were chosen to be "as representative of the specific site as is practicable," as specified in the SRP 2.2.3 guidance.

I SRP 2.2.3 recognizes the " difficulty of assigning accurate numerical values to the expected rate of unprecedented potential hazards," and that judgement must be used as to the acceptability of-the overall risk presented.

The analysis results presented in Section 6.4 show that there are large margins to SRP 2.2.3 guidelines. These margins constitute the " reasonable qualitative arguments" referred to in SRP 2.2.3 that the " realistic probability can be shown to be lower" than the SRP 2.2.3 guidelines.

I Thus, the use of best-estimate values and the large margins in the results satisfy the SRP 2.2.3 guidelines for use of site-specific estimates and reasonable jiMgements in evaluating the risk.

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I TABLE 6.1 List of Hazardous Chemicals Transported by Railroa_d_

l (1) Centra 1(2) Springfield (2) Total i Chemical Vermont Track _

Per Year Carbon Dioxide 395 96 491 Nitrogen 248 ---- 248 Propane (LPG) 60 162 222 Chlorine 60 ---- 60 Sulfuric Acid ---- 24 24 Anhydrous Ammonia 1 6 7 Methyl Alcohol ---- 4 4 Xylene ---- 2 2

Notes
(1) Listed in either Regulatory Guide 1.78 or EPA's Extremely Hazardous Substance List.

(2) Railcars per year.

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TABLE 6.2 Control Room Concentrations of flezardous Chemicals from Railroad Accidents l g Carbon Propane Chlorine g Dioxide Nitrogen

1. Quantity 20,000 gal 30,000 gal 33.500 gal 20,000 gal I 2. Detection
  • Threshold N/A N/A 0.5 ppm 0.2 ppm
3. IDLi!** 50,000 ppm N/A 20,000 ppm 30 ppm
4. Control Room 20,631 ppm 31,127 ppm 28.039 ppm 17.376 ppm I Maximum

>2 min <2 min

5. Time N/A N/A (IDLil-Detect)

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I *Ref erences 21 and '12

    • Reference 9 I i l

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M M M 'M M M M M M M M M E M M M M IABLE._6.,3 -

Summary _o f__Lost t_fostrol_Rocm_Habitabilily (i) (2) (3) (4) (5) (6) (7)

A" al C12 Release Annual A 1ent Number Release Probability Segment Wind / Stability Probability (P)

Segment Race (R) Railcars (N) Probability Length (L i) Frequency (Fi ) 1x2x3x4x5x6 (Rsi)

A 5.40E-06 6.00E+01 1.24E-01 1.56E-02 1.02E+00 0.00E+00 0.00E+00 B 5.40E-06 6.00E+01 1. 24E-01 1.56E-02 1.08E+00 1.44E-03 9.75E-10 C 5.40E-06 6.00E+01 1.24E-01 1.56E-02 1.00E+00 6.11E-03 3.83E-09 D 5.40E-06 6.00E+01 1. 24E--01 1.56E-02 9.50E-01 4.70E-02 2.80E-08 E 5.40E-06 6.00E+01 1.24E-01 3,13E-02 3.20E-01 7.13E--02 2.86E-08 F 5.40E-06 6.00E+01 1.24E-01 1.09E-01 3.00E-01 9.30E-02 1.22E-07 G 5.40E-06 6.00E+01 1.24E-01 1.25E-01 1.90E-01 7.25E-02 6.92E-08 .

H 5.40E-06 6.00E+01 1.24E-01 1.2SE-01 1.70E-01 5.48E-02 4.68E-03 I 5.40E-06 6.00E+01 1.24E-01 1.2SE-01 2.20E-01 6.57E-02 7.26E-08 J 5. 40E--06 6.00E+01 1.24E-01 3.13E-02 5.00E-01 3.30E-02 2.07E-08 K 5.40E-06 6.00E+01 1.24E-01 3.13E-02 5.50E-01 5.10E-02 3.53E-08 L 5.40E-06 6.00E+01 1.24E-01 1.56E-02 7.50E-01 2.29E-02 1.08E--08 M 5.40E-06 6.00E+01. 1.24E-01 1.56E-02 3.50E-01 5.99E-04 1.32E-10 N 5.40E-06 6.00E+01 1.24E-01 1.56E-02 1.05E+00 4.79E-04 3.16E-10 0 5.40E-06 6.00E+01 1.24E-01 1.56E-02 1.00E+00 1.20E-04 7.52E-11 P 5.40E-06 6.00E+01 1.24E-01 1.56E-02 9.50E-01 1.20E-04 7.15E-11 Q 5.40E-06 6.00E+01 1.24E-01 1.56E-02 3.00E-01 0.00E+00 LDQFdQQ TOTAL 4.39E-07 WPP44/77

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7.0 CONCLUSION

S The analysis considered in this report evaluates Control Room habitability in order to address NUREG-0737. Item III.D.3.4, requirements for off-site toxic chemical releases. The evaluation includes both deterministic and probabilistic analyses, and takes no credit for the existing TGMS. The deterministic analyses show that the Regulatory Guide 1.78 guidelines are inet I for all chemicals except chlorine. The combined probabilistic analyses show that the probability of a 100FR100 release due to an off-site chlorine release is far below the SRP 2.2.3 guideline for consideration as a design basis event.

Together these analyses show that the TGMS is not required to meet the regulatory criteria specified in NUREG-0737, Item III.D.3.4. Since NUREG-0737, Item III.D.3.4, is the basis for the TGMS, this analysis justifies removal of the TGMS at VYNPS.

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8.0 REFERENCES

1. NUREG-0737, "TMI Action Plan," Item III.D.3.4, and " Clarification to TMI Action Plan," Item III.D.3.4.
2. Letter, FVY 81-8, R. L. Smith (VYNPS) to D. G. Eisenhut (NRC), " Submittal of Information on NUREG-0737. Item III.D.3.4: Control Room Habitability " dated January 12, 1981.
3. Letter, NVY 82-22, D. B. Vassallo (NRC) to R. L. Smith (VYNPS), "Saf ety I Evaluation Report for VYNPS, NUREG-0737. Item III.D.3.4," dated February 24, 1982.

4 Vermont Yankee Nuclear Power Station Final Safety Analysis Report.

5. Vermont Yankee Nuclear Power Station Technical Specifications.
6. " Technical Guidance for Hazard Anaiysis, Emergency Planning for Extremely Hazardous Substances," U.S. Environtental Protection Agency, Federal Emergency Management Agency, U.S. Department of Transportation, December 1987.
7. YAEC-1287, " HAZARD - an Interactive Program for Calculating Toxic Vapor Concentrations " 1981.
8. YAEC Engineering Calculation YC-164, " Validation of Computer Code HAZARD," PC Version, 1990.

I 9. " Pocket Guide to Chemical Hazards," National Institute of Occupational Safety and Health, September 1985,

10. NUREG-75/087, " Standard Review Plan," Sections 2.2.1, 2.2.2, 2.2.3, and 6.4.
11. Regulatory Guide 1.78, " Assumptions for Evaluating the Habitability of a Nuclear Power Plant Control Room During a Postulated Hazardous Chemical Release," June 1974.
12. YAEC Engineering Calculation, VYC-912. " Highway Hazardcus Materials Accident Analysis," 1990.  !
13. YAEC Engineering Calculation, VYC-992, " Railroad Hazardous Materials Accident Analysis Update," 1990.
14. Telephone Communications, Guilford Transportation Industries Inc. - Rail Division, dated August 8, 1990.

I 15. " Threshold Limit Values... 1989-1990," American Conference of Government Industrial Hygenists, Cincinnati, Ohio.

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16. " Accident / Incident Bulletin No. 157. Calendar Year 1988," Federal Railroad Administration Of fice of Safety U.S. DOT, June 1989. ,

1 I 17. U.S. DOT, Research and Special Programs Administration, Letter, Enclosure, dated October 11, 1990.

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18. " Limerick Generating Station Probabilistic Risk Assessment," Philadelphia l Electric Company, Dockets 50-352, 50-353, September 1982.
19. "Probabilistic Risk Assessment Shoreham Kuclear Power Station, Unit 1,"

Science Applications, Inc., SAI-372-83-PA-01, June 1983.

20. YAEC Engineering Calculetion, VYC-916, "VY Lower Level Joint Frequency Wind Analysis," 1990.
21. C. J. Thompson, "A New Look at Odorization - Revisited," Proceedings of the Symposium on LP-Gas Odorization Technology, dated April 18-19, 1989, I Dallas, Texas.
22. " Occupational flealth Guidelines for Chemical llazards," NIOSil, U.S.

Depar tment of Imbor, January 1981.

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