ML20207P634

Safety Evaluation Accepting Util 851120 & 860324 Proposals to Eliminate Arbitrary Intermediate Pipe Breaks in Select List of High Energy Piping Sys

ML20207P634
Person / Time
Site:	Catawba
Issue date:	08/31/1986
From:	Office of Nuclear Reactor Regulation
To:
Shared Package
ML20207P625	List: ML20207P624 ML20207P634
References
NUDOCS 8701160346
Download: ML20207P634 (5)

Text

.

.o

.\

ENCLOSURE CATAWBA NUCLEAR STATION UNITS 1 & 2 SAFETY EVALUATION FOR THE ELIMINATION OF ARBITRARY INTERMEDIATE PIPE BREAKS Docket Nos. 50-413/414 August, 1986 U.S. Nuclear Regulatory Commission Office of Nuclear Reactor Regulation Division of PWR Licensing-A Engineering Branch e

4,4 P

.o.

4- In letters from H. B. Tucker to H. R. Denton dated November 20, 1985 and March 24, 1986, Duke Power Company requested staff approval to eliminate arbitrary intermediate pipe breaks in a select list of high energy piping systems in Catawba, Unit I and in the Pressurizer Surge Line and the Main Feedwater Line in Unit 2. In a letter from T. M. Novak to H. B. Tucker dated April 2,1984, the staff previously approved the elimination of these types of breaks in a number of other high energy piping systems in Unit 2.

The guidance in Standard Review Plan 3.6.2, Branch Technical Position (BTP)

MEB 3-1 for postulating pipe breaks in ASME Class 1, 2 and 3 high energy piping can be sumarized as follows.

Breaks should be postulated at the following locations:

1. At teminal ends.

2. At intermediate locations where the cumulative usage factor exceeds a threshold value (fatigue criteria applicable to Class 1 piping only).

3. At intemediate locations where the maximum stresses exceed a threshold value.

4. If two intermediate locations cannot be determined by 2 and/or 3 above, breaks shall be postulated at the two highest stress locations between the terminal ends.

Since the inception of these criteria during the early 1970's, criterion 4 above has come to be known as " arbitrary intermediate breaks" (AIBs). Criteria 1, 2 and 3 above provided protection at the most probable locations of possible failure in a piping system. However, it was also believed that equipment in close proximity to the piping throughout its run might not be adequately designed for the environmental consequences of a postulated pipe break if the break postulation proceeded on a purely mechanistic basis using only high stress and teminal end breaks. Therefore, for added protection, criterion 4

s .

was included. As the pipe break criteria were implemented by the industry,- .

the impact of these criteria became apparent on plant reliability and costs as well as on plant safety. Although the.overall criteria in RTP MER 3-1 have

! resulted in a viable method which assures that adequate protection has been provided to satisfy the requirements of GDC 4, it became apparent that the

~

particular criterion requiring the postulation of arbitrary intermediate pipe breaks could be overly restrictive and may result in an excessive number of pipe rupture protection devices which do not provide a compensating level of safety. Consequently, in 1983, the staff began to re-evaluate the AIB criteria and arrived at the following general conclusions:

1. Designing for AIBs is a difficult process because the location of the ,

J highest stress points tends to change several times due to the iterative process involved in the seismic design of piping systems.

i

2. Inservice Inspection is made more difficult and time-consuming because of

these devices blocking access to welds.

3. Restricted access may also increase occupational radiation exposure during repair, maintenance and decontamination operations.

1

4. There is increased heat loss to the surrounding environment because of cutting back piping insulation near these devices.

5. Unanticipated stresses may be introduced if the pipe rupture protection devices inadvertently come in contact with the pipe.

6. Considerable cost is involved in designing, constructing and maintaining l an excessive number of pipe rupture protection devices. Utilities have l- estimated dollar savings in the range of $2 to 5 million per unit.

l

_3 t 7. Arbitrary intemediate breaks are postulated at locations in the piping system where pipe stresses and/or cumulative usage factors are well below ASME Code allowables. Such postulation necessitates the installation and maintenance of complicated mitigating devices to afford protection from -

dynamic effects such as pipe whip and/or jet impingen:ent. When these selected break locations have stress levels only slightly greater than the rest of the system, installation of mitigating devices lends little to enhance overall plant safety.

8. The combined operating history of domestic cormercial nuclear plants (extensive operatirg experience of over 900 reactor years) has not shown the need to provide protection from the dynamic effects of arbitrary internediate breaks.

The staff's evaluation cercluded that elimination of AIEs wculd be beneficial. As a result, since April,1984, the staff has approved requests from several utilities to eliminate AIBs. For these specific plants, the staff considered that the elimination of AIBs was not to apply to piping systems in which stress corrosion cracking, larger unanticipated dynamic loads such as steam or water hanmer, or themal fatigue in fluid miyirg situations could be expected to occur. However, NUREG-1061, " Report of the USNRC Piping Review Comittee", volume 3, af ter reaching the same general conclusiens discussed above states: "After additional review, it is realized that ir certain systems and for certain materials, thermal fatigue and stress corrosion cracking cannot be absolutely excluded from piping cperation, nor can steem er water hamer. It may also never be possible to specify precise

' acceptable levels' of thermal fatigue and stress corrcsion cracking, nor to I

assure analytically that these levels would not be exceeded. However, if these unanticipated severe conditions were to occur, the break would most likely be located at the terminels ends, at the connections to corrponents, and at other locations which introduce higher stress concentration or that exceed the stated threshold limits in SRP 3.6.2. These locations are not affected by relaxing this requircrent." Accordingly, Volume 5 of NUREG-1061 recorrrrended l

l l

. 4-

~

g that Section 3.6.2 (MEB 3-1) of the SRP be revised to incorporate proposed changes elimirating the requirements for mechanical pipe rupture protection against AIBs in all systems, including those subject to stress corrosion cracking, fatigue, and dynamic loads. The staf f is currently implener. ting '

this recerrendation as a part of Generic Issue 119, " Piping Review Committee Recomendations".

Based on the above discussion, the staff concludes that the applicant's request to elin.inate AIBs as presented in the letters referenced above is acceptable and can be implemented without urdue risk to the health and safety of the public. It should be noted that the staff's approval to eliminate AIBs is applicable only to protection devices required to protect against the dynamic effects of AIBs. For environn. ental qualification of eovirrent in the vicinity of the elirirated AIBs, Duke Power Company has stated that the environments'1 arelyses for Catawba Units 1 and 2 are performed independent of the high erergy pipe whip / jet impingement analyses. Therefore, the environnental analysis methods crd results will not be affected by the elimination of arbitrary interinediate breaks.