ML19260C449
| ML19260C449 | |
| Person / Time | |
|---|---|
| Issue date: | 11/02/1979 |
| From: | Burger C NRC OFFICE OF NUCLEAR REGULATORY RESEARCH (RES) |
| To: | Aycock M, Hanauer S Office of Nuclear Reactor Regulation |
| References | |
| REF-GTECI-A-01, REF-GTECI-PI, TASK-A-01, TASK-A-1, TASK-OR NUDOCS 8001030436 | |
| Download: ML19260C449 (4) | |
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c WASHlf:GTON, D. C. 20t n5 N0y 0 21979 MEMORANDUM FOR:
S. H. Hanauer, Director Unresolved Safety Issues Prcgram M. B. Aycock, Deputy Director Unresolved Safety Issues Prg ram FRL!!:
C. 11. Burger, Task I:anager Task Action Plan A-1,!!ater Hammer
SUBJECT:
RESPONSE TO THE ACRS REQUESTS Rett.ED TO UNRESOLVED SAFETY ISSUES TASK ACTION PLAN (TAP) A-1, WATER HAMMER ACRS Request:
Thennal-Hydraulics During the 230th ACRS Meeting (June 1979),
"i;r. Plesset requested that the NRC staff provide information during the June 19-20 ECCS Subcommittee Meeting regarding the potential mechanical forces that can be developed by injecting cold water into steam and steam / water mixtures, and hot pipes."
The ACRS also recommends that each licensee and construction permit helder should examine a wide range of anomalous transients and degraded accident conditions which might lead to water hammer.
Tiethods of controlling or preventing such conditions should be evaluated, as should research to provide a better basis for such evaluations.
The Committee expects it would be appropriate to have such studies done generically first, for classes of reactor designs and system types.
With regard to the above reouest made by Dr. Plesset concerning the potential mechanical forces that can be developed by injecting cold water into steam and stea n/ water mixtures, and hot pipes, it is noted here that the physical conditions of the steam / water contact are critical in determining the magnitude of the potential forces.
If cold water is sprayed as droplets into a steam atmosphere in a vessel, such as a tank fcr example, condensatior, will occur and the forces will correspond to the change in the saturation conditions.
These are generally mild forces and the mechanical effects would be expected to be relatively mild.
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S. H. Hanauer M. B. Aycock 2
For the injection of cold water masses, instead of droplets into a vessel containing a steam atmosphere, the steam / water interface may remain relatively calm so that condensation occurs at the water boundary in a quiet way.
Under these circumstances, the forces would again be likely to be relatively mild.
If, on the other hand, the steam / water interface is turbulent, it is conceivable that some steam can be trapped by the surface water to form a steam bubble within a subcooled water system.
Under this circumstance, the steam condensation coula be extremely rapid and a water hammer situation arises.
However, the determination of the magnitude of the forces, due to the bubble collapse, is not straightforward for many reasons.
For example, steam entrapment must necessarily occur relatively near the water surface because the steam bubble cculd not penetrate too deeply into the water without condensing.
In such a case, depending upon the bubble size, the water velocity into the void space would determine the magnitude of the pressure according to the Joukowski relationship P = fCV.
But on the other hand, the transient time, which affects the impulse load on the systems, is determined by the proximity of the surfaces to the region of collapse. That is to say, if the water surface is closer than the structural surface, then the local pressure by the water hammer can be relieved through the steam / water interface before the pulse reaches any structural surfaces.
In this case, the effects are very local and probably of little damage consequence.
In the event that the structural surfaces are closer to the initial region of collapse than the water surface, the steam / water interface still controls the magnitude of the impulse, but some structural loading will occur.
For each foot of submergence, for excmple, the duration of the impulse would be 2_L_ _ at _ 2
_.0004 seconds or about one-half of C
4500 a millisecond.
This is the maximum value of the pul e width if the bubble collapse occurs at the structural surface; this value would be reduced in proportien to the distance of the structural surface from the initial bubble collapse region.
Impulses of this short a duration are not exoected to impose significant loads on the structures.
The actual magnitudes of the forces and irrpulse loads are difficult to calculate in general.
The water collapse velocity, in particular, is an unknown quantity.
However, some insight into the possible magnitudes of these events may be inferred from the Mark I and II containment tests if the subcooling conditions are appropriate. These tests were reverse in nature in that, steam was injected into water (instead of water into steam), in which case the bubble penetration would likely be deeper than in the case under consideration here.
For this reason, the Mark I and II impulse loads may be upper bounds for water injection into steam, provided the subcooling is the same in both cases and air is not present.
For steam / water mixtures, the situation is such that the fluid is near or at saturation.
In this case, the injection of the cold water can lead to encapsulation of a steam bubble by the cold water.
Here again, 1700 017
S. H. Hanauer M. B. Aycock 3
the bubble condensation can be quite rapid, if the water is cold enough, leading to rapid acceleration of the water into the void space. The magnitude of the pressure pulse is again somewhat indeterminate because of the difficulty in defining the Joukowski pressure peak. Also, the geometry and configuration affect the pressure wave transit time, as noted above, so that the detennination of the impulse lcad also requires that the configuration be specified.
For injection of cold water into steam / water mixtures, it is also to be noted that the subcooling that occurs locally to the steam bubble, is a critical parameter in determining the nature of the bubble collapse and the subsequent loads; that is, whether it is inertially dominated, thermally dominated, or is in a transition region, sometimes referred to as an "anomolous" region because of multiple pressure peaks.
The local subcooling is difficult to predict because of the difficulty in tracking the injected cold water.
If it mixes with the hot water first, then encapsulates a steam pocket, the results will differ, perhaps markedly, than if the cold water immediately encapsulates a steam pocket.
The latter possi' lity leads to an inertially dominated pulse, the former case tends toard a thermally dominated collapse, with the development of relatively mild pressure peaks.
The injection of cold water into hot pipes can cause, in addition to the thermal shock, steam entrapment and rapid bubble collapse if the pipe is initially steam filled.
As indicated above, the Joukowski pressure magnitudes or the impulse cannot be calculated at present with any degree of confidence or reliability.
The staff is presently attempting to establish a program in which a calculational tool will be developed to be used in estimating the magnitude of the water hammer forces and structural loading due to steam bubble collapse.
This is discussed below.
Another aspect of cold water injection into hot pipes containing steam is that local depressurization can occur as a result of steam condensation.
This local depressurization would then induce a steam flow transient within the system that could cause transient structural loads because of slug flow. The magnitudes of such effects are unknown at present.
At the present time, the NRC staff has completed several phases of the generic effort on water hammer.
This has included the issuance of reports by EG&G covering a systematic review of the Licensing Event Reports ar.d incidents involving water hammer, the formulation of calculational methods, and the issuance of NUREG-0582, " Water Hammer in Nuclear Power Plants." The calculational methods are an effort to evaluate the concequences of water hammer under degraded conditions such as, check valve closure associated with an upstream pipe break and the discharge into a voided line in order to evaluate the consequences of reactor spray initiation.
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S. H. Hanauer M.'B. Aycock 4
In addition, the staff i<, planning a meeting with utilities to discuss with them the safety significance of the many reported water hammer events in their plants and to identify the licensee's efforts to deal with this problem. An effort is also underway t; the staff to establish a program to provide a calculational model with which to evaluate the consecuences of water hammer induced by steam bubble collapse, combined with a long-term program which will involve testing and analysis, and be directed toward determining the limits of subcooling required to cause water hammer, the resulting forcing function and the conditions required to initiate the mechanism.
The results of this program can establish the means of preventing or controlling the water hammer events in order to finally resolve the water hammer problem.
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Charles W. Burger, T x Manager Task Action Plan A-1, Water Hammer cc:
D. Fischer, NRR F. Cherny, NRR C. Tan, NRR J. Zwolinski, NRR 1700 019