ML19221A389
| ML19221A389 | |
| Person / Time | |
|---|---|
| Site: | Crane  |
| Issue date: | 04/13/1979 |
| From: | Cunningham M NRC OFFICE OF NUCLEAR REGULATORY RESEARCH (RES) |
| To: | NRC OFFICE OF NUCLEAR REGULATORY RESEARCH (RES) |
| References | |
| NUDOCS 7905220432 | |
| Download: ML19221A389 (5) | |
Text
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X'. I I Degradation to Core iteit 4
Consideration has been given to the unlikely possibility that all cooling to the core might still be lost and a meltdown of the fuel occurring.
Detailed computer analyses have been performed to examine the progression of such a meltdown in the presence of varying degrees of ccntainment engineered safety features (ESFs), :.
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The ESFs will not significantly affect the progression of a postulated molten core itself but are important in mitigation of radioactive releases.
The major assumptions in these analyses are:
(l) all ficw and cooling to the core is assumed to stop at 14 days (t ) after reactor trip; g
' (2) the reactor coolant system (RCS) is water-solid; and (3) decay heat levels and core temperatures are consistent with the 14 days o f decay. The progression of the accident is as follows:
-t
+ 28.4 hrs.
g s
Water level in the RCS has dropped to the top of the core due to boiloff out the pressurizer safety /
relief salves.
-t
+ 31.4 h rs.
g Water level continues to d.op and fuel temperatures reach the melting point; core melt begins.
-t
- 36.1 h rs.
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g A large fraction o f the core (75-80%) has become molten and falls into the vessel lower head; the reactor vessel begins mel ting.
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- t + 36.2 hrs.
g Vesesi head fails due to the combined effect of
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temperature and pressure stresses (the RCS is, at s2500 psia).
The molten core drops into the reactor cavity.
Up to this point the containment pressure has been slo af increasing due to steam release from the RCS.
In the pessimistic case where the containment spray system and the reactor building cooling system (RBCS) (fan coolers) are assumed not to be operating, containment
.ressure is about 46 psia at t + 36.2 hrs. When the vessel fails, g
a pressure pulse is generated because of the release of the RCS pressure and rapid steam generation when the molten core falls into the water in the reactor cavity.
Containment pressure peaks at about 70 psia.
If hydrogen burning occurs at this time, roughly an additional 15 psi would ba added to the pressure peak.
The ;ombined pressure loadings at the time of vessel failure indicate a critical time in the meltdown progression.
If the combined loads were sufficient to fail the containment (an uniikely circumstance), then a significant radioactive release to the envi ron, ment could occur. However, on a more realistic basis, contain-ment would be expected to sustain this pressure transient.
After a temporary que :hing of the molten core by water in the reactor
'avi ty, the core-basemat concrete interaction begins.
The progression of the interaction is not signi ficantly affected by the operability 160 025
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3 o f containmen t ESFs.
From the time to + 38. 5 h rs, to t the core penetrates roughly 40 cm. i
+ 58.5 hrs.,
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nto the basem,at.
During this 1
progression and with containment ESFs operati f
ng, the containment piessure rises slowly, but is calculatr d not t I
pressure of about 130 psia.
o reach the failure As such, radioactive releases are ver" small.
In the pessimistic case where containment ESF i b d
s are not operating, the core-concrete interaction causes a steady increas cb 14
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'res s ure. At time t e in containment g + s12 days, pressure has risen to the failure G
point of the containment (sl30 psia) due to c without containment heat removal (i.e ontinued steam genera tion h
cooling system).
., without the reactor building Because of the long time periods involved i accidut sequence, the amount of radioactive matn this environment is calculated to be minor.
erial released to the Consideration was given to the possibility th t cccur at various times throughout the course hydrogen burning would a
of the accident.
magnitudes of the resultant pressure spikes The to the existing pressure within the cont iwere calculated and added a nment.
as it is released from the primary system If the hydrogen barns the containment atmosphere are relativel, the rates o f energy containment pressure is minor.
y low and the ef fect on the If, on the other hand, the hydrogen accumula t'.s above flammable 1imits before igniti en and then burns rapidly, significant pressure increases can result.
time of potential hydrogen b'urning occu The mos t critica ?
rs at the t m of sci failure
4 (t + 36.2 hrs.), as discussed above At this time, the composition of g
the containment atmosphere is near er barely in the flammable range.
3urning of the hydrogen down to the flammability limit,would produce a small pressure increment as previously noted. While it appears
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highly unlikely, the worst.ccmbination of hydrogen deflagration F
e (complete reaction) together with steam production from the quenchingM.%
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of core debris would lead M the prediction of containment failure $
For the cases in which containment cooling functions, analyses indicate that the deflagrction of hydrogen does not appear to threaten containment shortly following melt down. However, as time proceeds, the concentration of hydrogen in containment will continue to rise.
If the oxygen content of the containmen; has not been decreased by prior berning, eventually a hydrogen concentration would be achieved which could fail containment.
At this time, the air-borne incentration of radionuclides would be y
low and the consequences of the additional release would be comparatively minor.
The possibility of steam explosions of sufficient energy to rupture containment was also considered in these analyses.
Such explosions might occur-at two times : when the mciten core falls into the water
. in the lower vessel head, and when the molten core penetrates the vessel head and falls into water in the reactor cavity.
Steam explosion experimental and analytical research in the recent past suggests that such explosions are not likely in a high pressure environment.
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5 Thus a steam explosion 1s considered highly unlikely to occur when i
the molten core falls into the lower head (when surrounding pressures are approximately 2500 psia).
The effect of a steam explosion cccurring when the vessel head is L'enetrated was next considered; in this case, such an explosion might occur, but that it was judged that ccnt2inment would not.be grossly violated by the limited energetics of such an event.
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