ML20133A532

Forwards Task Control: Mods to GDC 4 Requirements for Protection Against Postulated Pipe Ruptures, Preliminary Draft Regulatory Analysis & Draft NRC Regulatory Agenda Entry,For Approval

ML20133A532
Person / Time
Issue date:	10/15/1984
From:	Speis T Office of Nuclear Reactor Regulation
To:	Harold Denton Office of Nuclear Reactor Regulation
References
FRN-50FR27006, FRN-51FR12502, RULE-PR-50 AB76-1, AB76-2-10, NUDOCS 8508050497
Download: ML20133A532 (10)

Text

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D October 15, 1984 NOTE T0: Harold R. Denton, Director Office of Nuclear Reactor Regulation FROM: Themis P. Speis, Director Division of Safety Technology, NRR

SUBJECT:

LEAK-BEFORE-BREAK RULE i

In response to your memorandurr. of June 29, 1984, requesting RES initiation of rulemaking to codify the use of advanced fracture mechanics technology in the regulatory process by modifying GDC 4 requirements for protection against postulated pipe ruptures, RES has prepared the enclosed task control I

form, preliminary draft regulatory analysis, draft NRC Regulatory Agenda l

! I recomend that you concur entry, and memorandum seeking EDO approval.

I I

with initiation of this task by signing the task control form. The enclosed material has been reviewed by MTEB, and has the concurrence of DE. RSCB will follow the program to ensure that the schedule is expedited to the extent possible, in order to minimize staff efforts in processing exemptions.

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W kW Themis P. Speis, Director Division of Safety Technology, NRR

Enclosures:

1. Memorandum for W. Dircks fm R. Minogue

2. Task Control Fona

3. Draft NRC Regulatory Agenda Entry

4. Preliminary Draft Regulatory Analysis 85030'50497 841015 PDR PR 50 50FR27006 PDR

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'"~'"' This action Will allow the use of fracture mechanics analysis for excluding the dynamic effects of pipe rupture (pipe whip and jet impingement) from the design basis.

Pipe break requirements for ECCS performance, containment sizing and environmental quali-fication are not affected. All fluid system piping is covered. First use of the rule is expected for eliminating the double-ended pipe rupture in primary loops of PWRs, leading to removal of some pipe whip restraints and jet shields.

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f Draft NRC Regulatory Agenda Entry TITLE: Modifications to GDC 4 Requirements for Protection Against Postulated Pipe Ruptures CFR CITATION: 10 CFR 50, Appendix A e

ABSTRACT:

The proposed rule would allow the exclusion of some pipe ruptures presently postulated in the design basis. Only dynamic effects such as pipe whip and jet impingement are affected. Other design requirements, including those relating to environmental qualification of electrical and mechanical equipment, containment design and ECCS performance are not influenced by this rule. The proposed rule would permit licensees to use newly developed analytical methods involving widely accepted advanced fracture mechanics theories for determining that certain pipe ruptures need

, not be treated in the design basis for dynamic effects.

i Implementation of the rule would facilitate the removal of unnecessary pipe whip restraints and jet shields from existing nuclear power plants. This would reduce inservice inspection costs j and, in addition, would reduce inspector radiation exposure. A benefit derived from the rule would be the avoidance of extensive exemptions to General Design Criteria 4, which would be the only acceptable alternate to the proposed rule. This rule may only

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require minimum addition and/or modification of the existing text of GDC 4. The need and urgency for addressing the issue stems wholely from the widespread acceptance of the analysis results and the research findings pertat.ning to pipe rupture coupled with increasing confidence in its applicability. Prior to the last few years, there was no sound technical basis for excluding certain pipe ruptures from the design basis. Now it is clear that it is possible to defend the exclusion of PWR primary loop double-ended guillotine pipe ruptures, and that the scope may be extended to other piping.

Two staff years is estimated to undertake this rulemaking. The notice of proposed rulemaking should appear in the Federal Register in July 1985.

TIMETABLE:

Notice of Proposed Rulemaking 7/00/85 l

i LEGAL AUTHORITY:

i 42 USC 2201; 42 USC 5846

. EFFECTS ON SMALL BUSINESS AND OTHER ENTITIES:

i Not applicable.

l AGENCY CONTACT: "

John A. O'Brien Office of Nuclear Regulatory Research Washington,-D.C. 20555 301 443-7860 4

Preliminary Draft Regulatory Analysis Task

Title:

Modifications to GDC 4 Requirements for Protection Against Postulated Pipe Ruptures

1. Statement of Issue.

Recent investigations using both deterministic and probabilistic fracture mechanics methodologies have demonstrated that for the specific case of the primary circuits of pressurized water reactors, a double-ended guillotine rupture does not occur. Efforts are currently underway to learn if these findings can be extended to other high energy piping systems.

In the context of the definition of " Loss of Coolant Accident", the requirements of General Design Criterion 4 have led to the placement of massive pipe whip restraints and jet shields near piping to

- mitigate the consequences of postulated pipe rupture accidents. Many experts believe that these accident mitigation featurcs actually degrade overall reliability of piping because they reduce the effectiveness of inservice inspection, and because of difficulties and potential errors in installation, or reinstallation, which could actually increase the likelihood of pipe rupture. When the General Design Crtieria were promulgated, there were no conclusive methods to demonstrate that certain pipes could not rupture. The past several years have witnessed the development of advanced fracture mechanics 4

technology. These advanced fracture mechanics' techniques deal with i relatively small flaws in piping components (either postulated or real) and examine their behavior under various pipe loads. The objective of efforts which utilize ~ these methods has been to r demonstrate that the detection of small flaws by either inservice

inspection or leakage monitoring systems is assured long before the flaws can grow to critical or unstable sizes which could lead to 1arge break areas such as the double ended guillotine break (DEGB) or

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its equivalent. The concept underlying such analyses is referred to

!- as " leak-before-break" (LBB).

In sumary, the issue is that requirements of GDC 4 as applied in the context of the definition of LOCA led to a situation where unnecessary. design features have been added to nuclear power plants to forestall accidents which do not occur, and now indications are

' that these design features reduce reliability and increase inspector radiation exposure. A need exists to allow exclusion of certain pipe ruptures when supported by acceptable analyses. It is emphasized that not all pipe whip restraints and jet shields will be eliminated as a result of this rulemaking; the scope of affected piping has not been determined. However, at the very least, primary system piping of PWRs will be affected.

2. The Need and Urgency for Addressing the Issue The need and urgency for addressing the issue stems wholely from the widespread acceptance of the analysis results and the research findings pertaining to pipe rupture coupled with increasing confidence in its applicability. Prior to the last few years, there was no sound technical basis for excluding certain pipe ruptures from the design basis. Now it is clear that it is possible to defend the exclusion of PWR primary loop double-ended guillotine pipe ruptures, and that the scope may be extended to other piping. These analytical results have been applied to the resolution of Unresolved Safety Issue A-2 already. Acceptance criteria for applying these results pertaining to leak-before-break generally are being published by the NRC staff in NUREG-1061, Volume 3, and by the American Nuclear Society in ANS-58.2. The new knowledge that certain piping does not rupture mandates the removal of pipe whip restraints and jet shields both from the point of safety and economics. Rulemaking action would, in addition, promote investigations to learn which other situations would also permit the removal of pipe whip restraints and jet shields.

3. Alternatives to Rulemaking Studies performed by Lawrence Livermore National Laboratory under contract to the Office of Nuclear Regulatory Research have shown that the presence of pipe whip restraints near piping can increase pipe rupture probabilities by a factor of ten and pipe leak probabilities by a factor of one hundred.

Given that the NRC staff, the ACRS, the CRGR and the nuclear industry have determined that the postulation of certain pipe rutpures may have a detrimental effect on public health and safety as well as having adversely affected construction and maintenance economics, it is imperative that steps be taken to allow removal of design features from both plants in operation and under construction which produce these negative effects. General Design Criterion 4, however, would require these design features. Thus only two potertial courses of action are acceptable: exemptions to GDC 4 and rulemaking. The ELD has expressed the view that extensive use of exemptions to authorize the elimination of pipe whip restraints is inapproprlate.

Nonetheless, now that the analytical methods are available for application and acceptance criteria for implementing the analytical methods are being developed by the NRC and industry groups, there will be significant motivation to seek exemptions. It therefore appears most rational to undertake rulemaking at this time.

4. Addressing the Issue Through Rulemaking The issue will be addressed by a modification to General Design Criterion 4. The candidate modification is given below, where the underlined portion indicates the added text:

1 (added text underlined, none of the old text is deleted)

Criterio 4 - Environmental and missile design bases. Structures, i systems and components important to safety shall be designed to accomodate the effects of and to be compatible with the environmental conditions associated with normal operation, maintenance, testing and postulated accidents, including loss-of-coolant accidents. These structures, systems and components I shall be appropriately protected against dynamic effects, including the effects of missiles, pipe whipping and discharging fluids, that may result from equipment failures and from events and conditions outside the nuclear power unit. Dynamic effects associated with i

loss-of-coolant accidents may be excluded from the design basis when fracture mechanics analyses demonstrate that postulated leakage size flaws in fluid system piping will not >ropagate unstably. Postulated fluid system piping flaw sizes, locautons, orientations, and their

propagation shall be established consistent with the design basis for

the p' ping.

5. Benefits and Costs i The staff has stated in NUREG-1061, Volume 3 that:

l "The LPipe Break] Task Group is aware that the NRC is initiating i rulemaking to preclude the need to issue exemptions by permitting the use of advanced fracture mechanics as an alternate approach to requiring the postulation of pipe

< ruptures. The Task Group supports this initiative and strongly recomends that rulemaking be pursued expeditiously. The basis recommendation, as supported elsewhere in the i for this value-impact and other sections of this report, is that a net safety as well as economic gain accrue from the elimination of massive protection devices in nuclear power facilities.

> particularly those intended to prevent whipping of ruptured

pipes."

Elsewh.:re in the same document the staff writes:

"Any detailed value-impact assessment of changes in pipe break criteria requires knowledge beforehand of the specific changes

themselves. Nevertheless, a review of assessments already completed for specific pipe break issues implies the following I

general conclusion: for eliminatien of the

• double-ended-guillotine break (DEGB) as a design basis for PWR reactor coolant loop piping:

i (a) Elimination of pipe whip restraints would only negligibly 1 increase public and occupational radiation exposure (ORE) resulting from pipe break accidents.

(b) Elimination of pipe whip restraints would improve access to pipe welds for inservice inspections, and thereby i significantly reduce ORE during inspection. Improved access would also reduce ORE during normal plant maintenance, although to a lesse.r degree. This benefit would apply both to operating plants and to plants under construction.

(c) For operating plants not already having pipe whip restraints, eliminating the DEGB -- and thus the need to install restraints -- would reduce ORE during installation.

For the sixteen A-2 plants, installation and maintenance of these restraints would avoid the occurrence of ORE by about 11,000 man-rem compared to a small increase in public risk and accident avoided occupational exposure of less than 5 man-rem."

i Regarding item (a) and (c) above, recent research results from Lawrence Livermore National Laboratory not available to the Pipe

Break Task Group have indicated that elimination of pipe whip restraints may actually produce a small decrease in public and occupational radiation when the prospect of improper installation of pipe whip restraints is considered. Improper installation of pipe j whip restraints may increase pipe leak and rupture probabilities.

The industry perspective on value-impacts regarding the use of the leak-before-break hypothesis is reflected by the Atomic Industrial Forum and the Bechtel Power Corporation comments in NUREG-1061, Volume 3. According to the Atomic Industrial Forum, costs associated

with pipe whip restraints are as follows:

l o Design, procurement and construction costs related to pipe rupture hardware is from $20M to $40M per unit.

o Number of pipe whip restraints in a " typical" light water reactor is from 250 to 400 per plant.

o Installation times, including design, etc., is from 150,000 to 250,000 man-hours.

The preceding figures will vary from plant to plant, depending on its status, whether being designed, in early sta approaching a near-term operating license , or(NTOL)ges operating. of construct ,

Bechtel Power Corporation figures are similar as indicated below:

o Design analyses, materials and construction is $100,000 per I

restraint.

o Number of restraints on a typical plant is 300.

I o Overall costs per nuclear unit is from $30M to $50M.

o Manpower per typical plant for overall installation of restraints is 250,000 man-hours per 500-600 MWe LWR.

i Relevant excerpts from the Atomic Industrial Forum Discussion of Value Impacts regarding application of the leak-before-break concept follow:

3 "Value-impacts reported by industry to date identify significant i

cost savings and operational radiation exposure (ORE) reductions that can be realized based on NRC approval of pipe ruptures i

._. ~_. . _ . _ _ _ _ . . _ _ - _ _ _ _ _ _ _ , _ . . _ _ - _ . _ . _ _ _ _ . _ _ _ . . - _ . _ _ _ _ , . -

postulated at specific locations and subsequent elimination of the associated pipe ruptures mitigation hardware, i.e. , pipe whip restraints and jet barriers.

. Experience has shown that it is very difficult to assess an exact value impact. No standard value-impact methodology has been used due to the many variables involved in computing cost savings and ORE reductions. However, it is industry's belief that the magnitude of cost savings and ORE reductions are so 4 compelling and beneficial that there is no need to rigorously quantify it."

"Where utilities have made specific requests, cost savings on the order of several million dollars and ORE reductions of several hundred man-rem are reported possible. The differenca between the AIF and uSlity estimates 1s due to the fact that 4

the Alt estimate is basc on the total cost of all required pipe rupture hardware while the utility estimates are based on cost savings on eliminating hardware associated with a limited scope NRC request."

"Value impact estimates can vary considerably from alant to plant due to fundamental differences in design. For nnstance.

value impact estimates for a Combustion Engineering plant and a Westinghouse plant with similar megawatt ratings can be expected to be different due to design variances. Likewise, value impact differences can be expected among plants with the same NSSS In 1

vendor, e.g., 2, 3, and 4-loop Westinghouse plants.

addition, depending upon which lead A/E is contracted for the plant, design differences lead to different value impacts.

For A/Es, the major difference is layout. Further differences can originate as a result of design oecisions made by the utility."

" Plant status variables are important when considering value impact estimates. For example, a value impact estimate for an NT0L would include proportionately large contributions due to engineering and construction cost savings. A value impact estimate for an operating plant would ' include proportionately large contributions for removals costs and penalties for radiation exposure incurred during the removal."

1 More specific value-impact information is attached as Enclosures A, B, C and D to this preliminary draft regulatory analysis. A sumary and comentary of the contents of these enclosures follows.

Enclosure A This value-impact analysis dated April 23, 1984 and submitted by Texas Utilities Generating Company deals with the elimination of large primary loop pipe rutpures at Comanche Peak, Units 1 & 2. It concludes that occupational' radiation exposure would be reduced by 155 man-rem and cost savings of $2.7 million would,be realized if only jet shields were not i

-. _._ - . _,._ ._. ,~ , _ _.. . _ _,__._ _ _ ,_ _ _ _ ._,.. _ ._ . _ _ _ _ _ . _ . . , _ . - _ . _ . _ _ _ _

! l installed. No consideration is given to pipe whip testraints, except for

! restraint shim costs. Additionaly, twenty-five weeks of replacement power i costs due to fuel load delay would be saved. These figures are the total for both units.

Enclosure B i Entitled " Safety Balance for the Elimination of Reactor Coolant System i Main Loop Pipe Break Protective Devices" and dated March 23, 1984, this document deals with Units 1 & 2 of the Vogtle Electric Generating Plant i owned by Georgia Power Company. Unlike Enclosure A, only man-rem savings associated with not installing pipe whip restraints are analyzed. No cost savings are investigated, 'ud the nominal estimates of occupational

. radiation exposure is 727 ' man-rem for both units with a range of 69 to i 2797 man-rem.

Enclosure C Dated Septmeber 14, 1984, and prepared for Catawba Unit 2 owned by Duke Power Company, this enclosure contains a summary of estimated cost savings and operational benefits for elimination of primary loop pipe breaks.

Results indicate a reduction of 600 man-rem and a cost savings of $1.3 million.

Enclosure D Relating to South Texas Project. Units 1 and 2, owned by Houston Lighting and Power Company and dated April 5,1984, this document, like Enclosure B, treats only man-rem savings associated with not installing pipe whip i restraints. No cost savings are investigated, but the nominal estimate of

[ occupational radiation exposure is 171 man-rem for both units with a range of 20 to 656 man-rem.

It is anticipated that similar results will be applicable to the reactor coolant loops of all PWRs and some BWRs indicating total reductions in occupational radiation exposure of the order of 10,000 ' man-rem and cost savings of $100 million. Moreover, since these estimates may be found to be applicable to piping other than the reactor coolant loop, substantial increments in these value impacts may result.

6. NRC Resources and Schedu' ling Two staff years is estimated to undertake this rulemaking. The notice of proposed rulemaking should appear in the Federal Register _

in July 1985. (

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