ML19260C478
| ML19260C478 | |
| Person / Time | |
|---|---|
| Issue date: | 08/31/1979 |
| From: | Harold Denton Office of Nuclear Reactor Regulation |
| To: | Gilinsky V, Hendrie J, Kennedy R NRC COMMISSION (OCM) |
| References | |
| REF-GTECI-A-07, REF-GTECI-CO, TASK-A-07, TASK-A-7, TASK-OR NUDOCS 8001030653 | |
| Download: ML19260C478 (4) | |
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'SHanauer MEMOPANDUM FOR:
Chairman Hendrie Comissioner Gilinsky Commissioner Kennedy Commissioner Bradford j C)% $
Comissioner Ahearne FROM-Harold R. Denton, Director h Office of Nuclear Reactor Regulation
[ Lee V. Gossick, Executise Director for;0perations THRU:
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SUBJECT:
MARK I CONTAIN"ENT POTENTIAL PROALE" On Tuesday, August 28, 1977 the staff identified a potential significant concern relating to the condensation loads on the downcomers in Mark I containments. The Mark I design is used in 22 operating plants.
This concern was discussed in the Daily Highlights for August 10 and August 31 The phenomena under consideration is but one of a nu-6er of various loads that have been evaluated under the Mark I Containment Long Tem Program.
The staff is currently developing acceptance criteria to begin implementation of the Mark I long Term Program. This particular loariing relates to the possible :)verstressing of the downcomer - vent header system which could cause steam to bypass the suppression pool water.
Details of this phenomenon m presented in the enclosure.
Over the past few days, we have had several discussions with representatives of BWR licensees, the Mark I Owners Group, and the General Electric Conpar.y.
We have confirmed that all of the Mark I plants have downcomers that are tied together, which, when costined with the Owners Group's preliminary evaluation, leads us to conclude thct the potential concern is not as severe I
as originally believed.
We are continuing to evaluate this concern and will meet with representatives of the Mark I Owners on Wednesday, September 5,1979, to review their further assessment and determine what action, if any, will be necessary. We will continue to keep you informed of our progress.
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ENCLOSijRE POTENTIAL CONCERN RELATING TO CONDENSATION LOADS ON DOWNCOMERS IN MARK I CONTAINMENTS
Background:
Following a postulated loss-of-coolant accident (LOCA) in a Mark I containment, steam will mix with the air.a the drywell and flow through the vent system into the suppression pool where the steam will condense.
The vent system consists of a main vent, a vent header and downcomers extending into the suppression pool, as snown in Figure 1.
As the flow through the vent system continues the relative concentration of air in the vent flow will decrease as the inventory of air in the drywell is depleated. When the air concentration is low enough, " condensation oscillation" loads will occur on the suppression chamber walls and the vent system downcomers due to the rapid formation and collapse of the steam bubbles in the suppression pool.
In order to quantify the condensation loads for the Mark I Containment Long Term Program, the Mark I Owners Group performed full-scale, prototypical tests.
The Full-Scale Test Facility (FSTF) is a 221/2 sector of a Mark I suppression chamber, containing four pairs of downcomers.
The facility was instrumented to provide thermal-hydraulic and structural response data.
Ten tests were conducted to cover small, intermediate and large break accidents with both steam and liquid blowdowns.
High amplitude condensation oscillations occur for large-break LOCAs and are characterized by a random motion of the steam-water interface outside the end of the downcomer. The rapid collapse of the irregular steam bubble in the pool causes lateral loading on the downcomers. The magnitudes and directions of the lateral loads have been inferred from strain measurements in the downcomers and vent header in FSTF. These measurements were taken for both
" tied" and " untied" downcomers.
" Tied" downcomers have a strap which connects the ends of a pair of downcomers together.
Staff Concern:
The driving frequency observed in the test data for condensation oscillations was in the range of 4 - 8 hz, primarily at approximately 5.5 bz. The equivalent stresses (from measured strains) for " untied" downtomers during condensation oscillations were above the ASME allowable yield stress but below the material load bearing capacity.
The staff had estimated that the natural frequency of the FSTF downcomer vent header system could be about 6.9 hz for
" untied" downcomers. Our concern was that in the event that the condensation oscillation driving force was to occur at the natural frequency of the downcomer - vent header system, a dynamic amplication of the load would result.
The staff has estimated that, for the " untied" downcomers, a dynamic amplification of as much as a factor of six greater than the loads measured in FSTF could
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,2 occur at resonant conditions. Such an increase in the load would result in fatigue failures of the downcomers. Since our previous discussions, the Mark I Owners Group has identified data showing that the natural frequency of the downtomer system of one operating plant was about 19 - 20 bz, hence well away from the driving force frequency.
In addition, the Mark I Owners Group has reviewed the downcomer designs in all plants and they have found that all plants have downcomer " ties" and, by comparisons of the relative downcomer-vent header stiffnesses, they conclude that the natural frequency should remain well above the driving frequency.
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FIGURE 1 Corposite Section Through Suppression Chareer I
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