ML20087Q088

Safety Balance for Elimination of RCS Main Loop Pipe Break Protective Devices

ML20087Q088
Person / Time
Site:	Vogtle
Issue date:	03/23/1984
From:	BECHTEL GROUP, INC.
To:
Shared Package
ML20087Q084	List: ML20087Q083 ML20087Q088
References
GL-84-04, GL-84-4, NUDOCS 8404100295
Download: ML20087Q088 (19)

Text

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Safety Balance for the

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Elimination of Reactor Coolant System Main Loop Pipe Break Protective Devices 4

Vogtle Electric-Generating Plant Units 1 and 2 i

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Prepared for Georgia Power Com.pany.

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.by Bechtel Power Corporation, Los Angeles Division

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March 23, 1984

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Safety Balance for the Elimination of Reactor Coolant System Main Loop Pipe Break Protective Devices Page I.

Introduction I-1 II. ' Safety Balance Assessment Summary and Conclusions II-1 III. Development of' Safety Balance III-l A.

Risk Avoidance Attributable to Protection from Dynamic III-l Effects Associated with Pipe Breaks 1.

Public Health III-l 2.

Occupational Exposure.- Accidental III-4

'B.-

Reduction in~0ccupational Radiation Exposure (ORE)

III-6 Resulting from a Decision Not to Use Protection Against Dynamic Effects Associated with Pipe Breaks 1.

Occupational Exposure - Operational III-6 a.

Inservice Inspection III-6 b.

Maintenance III-9 IV.

References IV-1 C

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Introduction

.This report presents a safety balance evaluation of the consequences of eliminating the protective' devices currently employed in the design of the Vogtle Electric Generating Plant, Units 1 and 2 (VEGP) to mitf. gate dynamic effects associated with postulated breaks in the reactor coolant system (RCS) main loop piping. This assessment uses methods suggested in

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._the " Leak Before Break Value-Impact Analysis" attached to the Nuclear i

Regulatory Cummission's (NRC) Generic Letter 84-04 (Reference 1).

Plant

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specific data and the generic data developed in Reference 1, as well as

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-various public' documents, are used to perform the safety balance evalua-tion for VEGP. The evaluation is performed in terms of public health and

-occupational accident risk avoidance attributable to the protection

-provided f'or dynamic. effects associated with postulated breaks in the RCS primary loop versus the reduction in Occupational Radiation Exposure

-(ORE)lresulting from a decision not to use such protecti;n.

The man-rem savings is presented in tabular form and listed as lower,

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upper, and nominal values. These. represent the range of values' expected

,at VEGP; however,.there are conservatisms included in the analysis of the ORE which tend to lower the estimated man-rem: savings over the entire i,

-These are explained as follows:

range of values.

_ The man-rem savings: associated with not installing jet. impingement A..

. barriers are not included in'this analysis. -The elimination of jet

impingementLbarriers'and associated supporting structures will result in increased work efficiency'due to' improved access for

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maia*enance.. These factors are not considered in this analysis.

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'Only man-rem savings associated with'not installing _ pipe whip

. restraints are analyzed.

.B.

Conservatively. low estimates. of man-rem exposures a're used when calculating the. total exposure due to the removal and reinstallation of pipe. whip restraints 'for. access to perform inservice inspection (ISI).

It is assumed that it takes two persons, two shifts to remove each pipe whip restraint and another two shifts.for reinstal-lation. The expected exposure rates in the vicinity of the reactor coolant piping are in the range of 0.02 to 0.2 rem /hr. This corre-sponds.to an expected dose of 1.3 to 12.8 rem per restraint per ISI.

40 man-rem per restraint per ISI is used as a maximum based on industry experience.

The'1.0, 10 and 40 man-rem per restraint per ISI values used in this analysis are conservative when compared to the expected values' based on' expected dose rates near reactor coolant piping.

C.

Conservatively low estimatesJof the increased work efficiency due to

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improved _ access for ISI or maintenance'(based on fewer interferences

-with the pipe whip restraints and supporting stractural members) are used. _An increased work' efficiency of 1% is'used for ISI and reactor.

. coolant pump.(RCP) maintenance. Actual increases in. work efficiency

_are. expected to. exceed 10%.when considering the, instruments and

. equipment that must ba manipulated during these activities and the

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. genera 1' difficulty of working in protective clothing..An increased work efficiency of 5% (with a low of 1%'and a high of 10%) is used I-2 m

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.for special maintenance of the steam generator. This value is higher than the ISI and RCP maintenance value of 1% due to the close proximity of two of the pipe whip restraints to the area below the steam generator. However, it too is anticipated to be greater than

'10%, and may be as high as 20%, when considering the tightness of this area and the need-to manipulate the type of equipment necessary for performing tube plugging, tube welding, and sludge lancing while wearing protective clothing.

- As these conservatisms affect values which are the major contributors to the man-rem savings, the overall analysis is similarly conservative in its expression of man-rem savings.

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I-3 b

II.

Safety Balance Assessment Summary and Conclusions A summary of the results of the safety balance is shown below. The nominal dose estimates support the request to not require consideration of the dynamic effects of pipe breaks in the RCS main loop in the VEGP

. design basis.

Nominal Lower Upper Value (man-rem)

Estimate Estimate Estimate Public Health (*)

-1.0

-8 0

OccupationalEyposure

-0.3

-5 0

(Accidental)

Occupational Exposure (Operational) a)

' Inservice 649 68 2576

-Inspection b)

Maintenance 79 14 221 Total Quantified 727-69' 2797 Value-

'(a) 'The. estimates shown here are given in negative terms to reflect the

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decrease in man-rem savings. The_ upper and lower estimates are transposed from the values presented in section III.A to reflect a more conservative estimate of the total quantified upper and lower values by presenting larger upper. estimates and smaller-lower estimates.

II-1.

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III : Development of Safety Balance A.

' Risk' Avoidance Attributable to Protection from Dynamic Effects Associated with Pipe Breaks i.

.1.

Public Health i

Dose estimates derived in Reference 1 are found to be

. conservative and bound the results calculated for VEGP for

'the following reasons:

a.

Reference 1' assumed a uniform population density of

-340 people per square-mile around the reactor site and 4

.a 50-mile release radius model. The expected average

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population density at the VEGP site is 117 people per square mile in the year 2028. A total of 99.7 percent of that population is expected to live between 10.to

.50' miles away from the plant (Reference 3, Section 2.1).

b.

As calculated in Reference 1, the off-site population doses 6-6-

6 are 5.4 x 10 man-rem, 4.8 x 10 man-rem, 5.4"x 10 man-rem, 6

5 2.7 x.10 man-rem, 1.0 x 10 man-rem, 1.5 x 10 man-rem, and 2300 man-remifor the seven WASH-1400 release categories.

.The calculated'offsite population dose ~from a large' LOCA'at 3

'the VEGP site is 3.26 x.10 man-rem. thyroid dose and 13.6 man-rem whole body dose'(Reference 3, Section 7.1).

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The~ nominal estimate of added risk to public health for plants-that use a two-loop configuration was estimated to be 0.006 man-rem / plant year:(py) in Reference 1.

For VEGP this number is adjusted to account for the four-loop design. This results in a nominal risk of:

4 Risk'= 7 x 0.006 = 0.012 man-rem /py.

Upper estimate risk calculations are'made using procedures similar to those of the nominal estimates.

No corrections are necessary for the number calculated in reference 1 because this frequency 'is per plant year and not based on the number of loops. The upper estimate risk is:

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Risk = 0.1 man-res/py.

Theflower estimate is assumed to be 0.

Multiplying each of.the risk calculations by the number of-

_ years of expected plant life (2 plants x 40 yr = 80 py) results in the VEGP public. risk increase of:

. Total Added Riski (man-rem)

~ Nominal Estimate 1.0

-Upper Estimate o8. 0 Lower Estimate-0 eg

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The nomina'l estimate from Reference 1 of the total increase in' core melt frequency for'not providing protection against s

dynamic effects associated with pipe breaks is used and adjusted for the larger number of loops in the VEGP design.

This results'in a corc melt: frequency increase of:

core.meltfrequencyincrease=f01x10

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o = 2 x 10

-6 The upper estimate of core melt-frequency increase of 2 x 10 jpy

_ (Reference 1) is applicable for the VEGP analysis. No correction

'for the number of loopsf is necessary because this number is per pl' ant' year. A lower estimate of 0 is used for VEGP. The resulting. total core melt frequency' increase est'imates are as follows:

Increase in Core Melt' Frequency (events /py)

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Nominal Es'timate 2 x 10

-6 Upper Estimate 2 x 10 Lowe'r Estimate 01 (Probabilistic analysis of. the potential for increased risk to <

1 the public health due_to-the' increase in core melt frequency

demo ~nstrates that there is'no credible' increase in

the~ risk

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_to_public health. Because of'the uncertainties in'the' core U

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melt frequency. estimates (References 6 and 7), the increase in core melt frequency is not statistically significant enough to establish a credible difference in the core melt frequency and hence the estimated added risk to public health.

12.

Occupational Exposure (Accidental)

The increased occupational exposure from accidents is estimated to be the product of the change in total core melt frequency and the occupational exposure likely to occur in the event of a major accident. The nominal change in core melt frequency was estimatated as 2 x 10' events /py. The occupational exposure in the event of a major accident has two components.

The' first is the "immediate" exposure to the personnel onsite' during the span of the event and the time necessary i

to achieve short term control. The second is the longer term exposure associated with the cleanup cnd recovery from the accident.-

The_ total avoided occupational exposure is calculated as follows:

D I OA 10 + LTO TOA OA e

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= Total' avoided occupational exposure-TOA N = Number of affected facilities = 2 T = Average plant lifetime = 40 yrs.

dA = Avoided occupational. dose per reactor year D

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P = Change in core melt frequency t'

D

= "Immediate" occupational exposure IO D

= L ng-term occupational' exposure-LTO Results of the calculations are shown below. Uncertainties

- are conservatively propagated' by the use of extremes (e.g.,

-upper-bound DTO +. upper bound DLTO)

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Increase in Immediate(*} Long Term (*}

Total Core Melt Occupational Occupational Avoided-Frequency Exposure' Exposure Occupational (events /

(man-rem /

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Exposure plant-yr event) event)

(man-rem)

. Nominal Estimate 2 x 10 1 x 10 2 x 10' O.3 3

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-6 3

4' Upper Estimate 2 x 10 4 x 10 3 x 10 5

Lower Estimate 0

0 1 x 10' 0

(a) Based on cleanup and decommissioning estimates, Reference 2.

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JReduction'in 0ccupational-Radiation Exposure (ORE) Resulting from a Decision Not to Use Protection Against Dynamic Effects Associated with Pipe Breaks.

1.

Occupational. Exposure - Operational u

a.

Inservice Inspection (ISI) 1

Review of the VEGP design' indicates that the RCS pipe whip restraints are located.such that there is. sufficient

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' access to the RCS piping welds for performing ISI, with the

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exception of.the crossover leg pipe whip Eestraints. There are some interferences posed by.the crossover leg pipeLwhip restraints during ISI and it is assumed that they must be

. removed to facilitate crossover leg piping weld ISI which'

'is required four times.over the-life of the plant-(once e,

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'- -s every 10 years).

Industry experience shows that the radiation' exposure associated with removal and reinstal-lation_of the crossover leg pipe whip restraints ranges from 1 man-rem to 40 man-rem per restraint per ISI with

.a nominal value of 10. man-rem per restraint per ISI.

Since in the VEGP design there are eight pipe whip res-traints per unit which require removal, the nominal reduction in ORE for not installing these-pipe whip V

restraints is estimated as follows:

8f y f, rest a nts x10"*['#**

x4 reduction in ORE = 2 units x 8

= 640 man-rem The upper estimate is based on a 40 man-rem dose per ISI per restraint. The lower estimate is based on a value of 1 man-rem per ISI per restraint.

In addition, with all the RCS pipe whip restraints and supporting structural members removed, improved access is'provided for ISI of the-following:

- 1)~

Reactor coolant piping'(including removal and reinstallation of-the insulation from the vessel nozzle and coolant piping)

2)

Steam generator welds (lower shell)

3). Steam-generatorLtube eddy current testing-t 1

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4..t The annual radiation exposure for performing the above ISI is estimated to'be'11.65 man-rem averaged over a 10 year period

.(Reference 4,. Table 12.4.1-11).

It is further estimated that removal of'he pipe whip restraints will provide improved t

access and increase.the inspection efficiency by 1 percent.

5Therefore,' the nominal reduction in ORE due to improved access for ISI is:

reduction in. ORE = 2 units x 0.01 x 11.65 man-rem /yr x 40 yr.

= 9 man-rem The upper and. lower estimates are based on engineering judgement

' that the exposures.will be 20 man-rem /yr and 5 man-rem /yr, respcctively.

The total reduction in ORE for operational occupational exposure

due to ISI if the pipe whip restraints are not installed is:

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' Occupational Radiction Exposure (man-rem)~

Nominal Estimate 649 Upper Estimate-2576 4

68

. Lower _ Estimate

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Maintenance

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Routine Maintenance

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a)

During. power operation There is no' identifiable ~ routine maintenance activity which must be performed inside the steam generator compartment or primary shield wall where the RCS pipe whip restraints are located. However, as a good practice, the utility anticipates performing pipe whip

-restraint gap verification once every 10 years.

It is estimated that an average exposure of 0.1 man-rem will result from gap verification of one pipe restraint. There are 24 pipe whip restraints per unit at'VEGP. The nominal-reduction in ORE for-not; installing pipe whip

= restraints,'and thus not performing gap.

verification ~is:

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a restraints 4 verifications

-reduction in' ORE = 2 units x 24 x

unit plant life s

"*"~#*"

x 0.1

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! restraint, verification

= 20 man-rem 1

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. The upper and lower estimates are based on engineering judgement that the exposures will be 0.5 man-rem and

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0.01 man-rem per restraint per verification, respectively.

b)'

~During-Refueling

a..

4 j1 Routine maintenance will be required on the reactor coolant pump (RCP) seals. As outlined in NUREG-0933 (Reference'5) each RCP is expected to require maintenance once every 2 years, resulting in a nominal

. exposure of 7 man-rem per maintenance activity.

Assuming that pipe whip restraint removal will increase efficiency by 1 percent, the nominal reduction in ORE due to improved access is:

maintenance RCPs activity reduction in. ORE = 2 units x 4 x 0.5 Y#

,1 RCP, year x 0.01 x 7 maintenance activity

= 11 man-rem u

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s-The upper and lower estimates are based on engineering judgement that the exposures will be 15 man-rem and I man-rem per.RCP maintenance activity, respectively, 2.

Special Maintenance 2

Substantial l improvement in access may be realized for g

the following special maintenance activities if the pipe whip restraints on the crossover leg and vertical

. piping at the outlet of'the steam generator are eliminated from VEGP:

i a)

Steam Ge'nerator tube plugging

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b)

Steam Generator tube welding c) -Sludge lancing i

i The annual exposure from performing the above special main-tenance activities is estimated to'be 12.1 man-rem averaged over a -l'O. year period (Reference 4, Table 12.4.1-11).

It is assumed that the removal of the pipe whip restraints will

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provide' improved access'and increase work efficiency by i

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-5 percent. Therefore, the' nominal reduction in ORE due to improved access for.special maintenance is:

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reduction in ORE = 2 units x 0.05 x 12.1 "*"~"'"

yr x 40 yr = 48 man-rem

~The upper and lower estimates are calculated based on assumed 1

work efficiency improvements of 10 percent and 1 percent,

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respectively.

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The total reduction in ORE for operational occupational exposure due to maintenance if the pipe whip restraints are not installed

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=is:

i Occupational Radiation Exposure (man-rem) 1 Nominal Estimate 79

-1 Upper Estimate:

221

-Lower Estimate 14

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.IV.

References 1..

U.S. NRC Generic Letter 84-04 " Safety Evaluation of Westinghouse Topical Reports Dealing with Elimination of Postulated Pipe Breaks

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in PWR Primary Main Loops" dated February l',

1984.

2.

'NUREG/CR-2601, " Technology, Safety'and Costs of Decommissioning

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o Reference Light Water Reactors Following Postulated Accidents,"

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November 1982.

5.

Vogtle Environmental Report, sections 2.1 and 7.1 4.

Vogtle FSAR, section 12.4

' 5.

NUREG.0933, "A Prioritization of Generic Safetiy Issues," 3/31/83 6 '..

WASH 1400.(NUREG-75/014) " Reactor Safety Study," October 1975 N _

7.

~ German Risk Study, NRC Translation 729, May 1980 4

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