ML20031F128
| ML20031F128 | |
| Person / Time | |
|---|---|
| Site: | Oconee  |
| Issue date: | 04/19/1977 |
| From: | Liaw B Office of Nuclear Reactor Regulation |
| To: | Shao L Office of Nuclear Reactor Regulation |
| Shared Package | |
| ML13319A640 | List:
|
| References | |
| FOIA-81-313 NUDOCS 8110190221 | |
| Download: ML20031F128 (5) | |
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UMTED STATES NUCLEAR REGULATCRY c0MMisSION
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$ '%$'4 APR 191977 MEMORANDUM FOR:
L. C. Shao, Chief. Engineering Branch, Division of Operating Reactors THRU:
R. J. Stuart, Section B Leader. Engineering Branch, Division of Operating Reactors FROM:
B. D. Liaw, Engineering Branch, Division of Operating Reactors
SUBJECT:
TRIP REPORT - B&W FACILITIES AT MT. VERNON, INDIANA On April 4,1977, D. G. Eisenhut, J. C. Guibert and B. D. Liaw met with the manufacture support engineering staff of B&W Mt. Vernon, Indiana,.
facilities to discuss the differences in steam generator hardware designs between the Oconee units and other B&W operating units (Arkansas Unit 1, Three Mile Island, and Rancho Seco). Following this discussion, a tour of B&W manufacturing facilities was conducted with a special emphasis on the assembly process of the once-through steam generators (OTSG's).
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A list of participants in the discussion is attached.
i The following is a summary of the highlights of the discussion and i
significant findings during the plant tour:
1.
B&W has formulated a tentative program for dealing with the Oconee steam generator tube leak problem, and has indicated to us that they will discuss tne program with Duke Power sometime in the week of April 11, 1977.
Following the discussion between B&W and Duke Power, a meeting with NRC will be scheduled.
2.
Preliminary indications are that the proposed program will consist of two parts; the short tenn corrective actions and the long term efforts to find the cause(s) of the crack initiation and possible mechanical modifications to prevent further tube degradation and leakage. The detailed content of the program will be presented to NRC by Duke Power after they approve B&W's proposal.
3.
Regarding the possible initiation mechanism. B&W pointed out that Oconee units have undergone daily tests'of the turbine stop valve operability. Since th.is increased frequency of control valve testing has only occurred at Oconee, it was speculated that the associated additional thennal and flow transients n...
ave caused
Contact:
B. D. Liaw O
28060 g1slosa7 81-313 PDR
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higher than expected stresses in tubes in certain locations of the tube bundle.
4.
In reviewing the drawings, B&W also pointed out that the auxiliary feedwater spray arrangement which is unique to Oconee generators constantly injects cold water (about 40*F) directly to peripherial tubes near the upper tube sheet. No details on the possible consequences of this operation were indicated.
5.
In their fatigue calculations, the pretension in tubes was not included in their evaluations.
6.
During the plant tour, they pointed out a steam generator which was being assembled for installation in a Gennan plant. This design was different in that it utilizes several extra sets of support rods. They indicated that the Germans postulate different main steam line break conditions which require the extra support rods.
7.
Regarding the justification for the safe continued operation, they still rely upon the preliminary responses to our initial round of questions that were transmitted to Duke Power previously. The justification was based upon the low probability of a pipe break and the consequences of such an event. A summary of this argument is attached.
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s-B. D. Liaw Engineering Branch Division of Operating Reactors
Enclosures:
As stated -
cc:
V. Stello R. Beineman D, Eisenhut J. Knight R. Bosnak S. Pawlickt J. Guibert R. Stuart W. Kazelton J. Neighbors F. Almeter B. D. Liaw
ATTACHMENT A ATTENDEES - 4/4/77 4
B&W NRC i
R. Douglass - Mt. Vernon D. G. Eisenhut C. Bruny - Mt. Vernon J. C. Guibert D. McCabe - Mt. Vernon B. D. Liaw W. Keywort - Lynchburg, VA 4
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i ATTACHMENT B i
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(1).
REASON FOR CONTINUED SAFE OPERATION:
All of the defects which have been visually observed at tne Oconee site have been circumferential cracks of varying length. One of these failed tubes, tube 77/23 of generator 28, was removed for detailed examination.
Evidence obtained f em the fracture surface of this tube indicates that from an initiation site of unknown origin the crack propagated as a thru-wall defect due to the application of a high cycle fatigue loading.
It 5
5 was deduced that approximately 1 x 10 to 3 x 10 cycles were required for the crack to travel its total observed length of roughly 240'.
The only known source of loading which would involve this number of cycles is fl'ow induced vibration. This would occur with most prominence in the fundamental mode of the tube, or at a frequency of about 40 Hz.
From this infonna' tion, it is apparent that an initial defect would propagate to a detectable leak in approximately 1 to 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br />.
Since the propagating mechanism is flow induced vibration, the defect is not " stable".
It will rapidly progress around the circumference of the tube as long as there is flow of sufficient enargy to drive it. However, the crack formed will produce an identifiable leak and the unit will be shut down promptly. Therefore, the probability of the occurrence of a major accident during the time between leak and shutdown is low.
As for the consideration of tube integrity under accident conditions, a samil defect induced in the tube rapidly propagates by flow induced vibration to a crack with a detectabic leak rate. This " fast break" phenomenon and the resultant plant shutdown procedure constitutes sufficient assurances that the chances of a " degraded" tube, or a tube at a state of incipient failure, being subjected to accident loading conditions is low. Small defects which are in the process of forming but which have not yet propagated to a detectable leak do not substan-tially affect the gross structural integrity of the tube. Consequently, j
loads induced during an accident condition should not cause a tute which i
is in this particular state to fail.
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(2). CONSEQUENCE ANALYSIS OF A MSLB IN TERMS OF 0FF SITE DOSAGE:
A study has been made of the environmental consequences of a steam line break accident followed by the rupture of a steam generator tube such that a large primary-to-secondary leak rate (640 gpm) exists in the affected steam generator. The 640 gpm leak rate is at reactor operating temperature and is approximately equivalent to the Oconee FSAR leak rate of 435 gpm at the density for cold conditions. For this analysis it is assumed as in the Oconee FSAR Section 14.1.2.9. that the reactor coolant leakage continued unabated for three hours before the reactor coolant system can be cooled down and the leakage tenninated.
In evaluating the environ-mental consequences, the Oconee FSAR Section 2.3 meterology was used, that is for a ground level release from 0 to 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br />, the atmospheric dispersion factor (X/Q) at the exclusion area boundary is 1.16 x 10-4 sec/m { The 3
reactor coolant system iodine inventory is based on 1.0% defective fuel and the source tenns from the Oconee FSAR Section 11.3.
The reactor coolant iodine cor. centration is 4.6pCi/cc of dose-equivalent I-131. The thyroid dose at the exclusion area boundary was calculated as follows:
The steam generator tube rupture causes all of the iodine activity in the entire reactor coolant to be released through the steam line break directly to the atmosphere.
It was assumed that there was no further release of iodine from the fuel into the reactor coolant as a result of'the steam I
line break transient.
It was also conservatively assumed that the entire iodine release occurred over a two-hour time period. The resulting thyroid dose at the c clusion area boundary is 91 rem. Therefore, the j
entire reactor coolant volume with an iodine inventory corresponding to 1.0% defective fuel can be released directly to the atmosphere via ruptures in one or more steam generator tubes following a steam line break since the re.sulting thyroid dose is well below the 10 CFR 100 guideline of 300 rem.
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