ML19323C478
| ML19323C478 | |
| Person / Time | |
|---|---|
| Issue date: | 04/25/1980 |
| From: | Catton I Advisory Committee on Reactor Safeguards |
| To: | Okrent D Advisory Committee on Reactor Safeguards |
| Shared Package | |
| ML19323C477 | List: |
| References | |
| ACRS-R-0878, ACRS-R-878, NUDOCS 8005150697 | |
| Download: ML19323C478 (3) | |
Text
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April 25,1980' O
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T0:
D. Okrent gn05150 FROM:
I. Catton
SUBJECT:
Breac'h of Containment by a Core Melt
REFERENCE:
1.etter from Ivan Catton to David Okrent dated 6 March 1980
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The question posed is whether or not'it is feasible and practical to design a containment that can withstand a core melt.
It is my opinion that to do so is both feasible and practical.
Of course there will be a number or 3 hurdles to overcome in arriving at a design.
I will attempt to substan-tiate my opinion in the following paragraphs by first addressing existing plants ano then give my ideas about new plants.
Before discussing LWRs, however, I would like to call your attentioh i
to brevious work in this area for LMFBR's.
Designs for core catchers were proposed for FFTF and CRBR. A number of crucible materials were evaluated and both passive (time delay) and active systems were considered. The German reactor SNR 300 will have an actively cooTed crucible using depleted U02 as a sacrificial material.
Several ideas for core retention have come out of efforts of the GE advanced reactor group. A firm in Germany was found that for what I remember to would make standard size bricks out of depleted U0 2 be a reasonable cost. The depleted UO2 was needed to absorb the thermal shock from the melt and protect the active cool.ing system.
The designs were not fully evaluated but had potential for being successful. When one considers th3t the fuel melt from an LMFBR has an energy density that is an order of magnitude greater than an LWR one sees t' hat the design of a core catcher for a LWR will be less difficult.
A number of aspects of a core melt accident were discussed in the above referenced letter, which dealt with Indian Point and Zion.
They : are repeated here in part.
1.
Steam explosions will probably not occur in-vessel if the pressure is above 7-10 bars.
Even if a steam explosion were to occur in-vessel, recent SANDIA work shows that there is little chance of a missle that could penetrate the containment. The only missile that might be of concern was the control rod drive.
Some plants have missile shields for. this already and plants without could install one. An ex-vessel steam explosion will only occur if water is in the reactor cavity before the vessel is penetrated or enters shortly thereafter (before the molten po.ol solidifies and while gas is still being generated by concrete decomposition). The ex-vessel steam explosion will probably not do much damage and it appears that acceleration of missiles that will penetrate the containment is unlikely.
Further con-firmation of this opinion is needed to assure that damaging the shield wall, moving the vessel or some other aspect will not lead to containment penetra-High steam generation rate will occur if water precedes the melt and tion.
the resultant high steam generation rate needs to be a factor considered in seeking mitigation measures.
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2.
In-vassel cara coolability is presently not well (nough understood to fully d; scribe thn core meltdown process. Programs presently und:rway in Germany and the US may yield sufficient information at some time in the future to describe the process. At this time one can only bound the problem and must It should assume that penetration of the vessel occurs early in the worst way.
b2 mentioned that it is not really clear what the worst way is.
For example.
a jet of fuel resulting from. a hole in the bottom of the vessel might erode a hole in the base mat with subsequent erosion of the hole being greater than if the entire vessel lower dome failed dumping all the molten fuel at one time.
3.
Ex-vessel core debris coolability will depend strongly on whether or not water is in the cavity.
If water is in the cavity in sufficient quan-tify before the vessel is penetrated, the core debris will be quenched as it tnters. A sufficient quantity of water is a pool deep enough to prevent erosion
/of the base mat.
It is not clear how deep this is. SANDIA programs under-way, however, could help answer this question.
If a reflux path is available the gare debris will probably not dry out and re-melt. This opinion is based on past work at TREAT, UCLA, ANL and SANDIA.that shows.that c =.0.45 is a[ reasonable void fraction and that an average particle size of 500 pm is to be expected.
For c =.45 and 500 pm particle sizes the entire core and -
a g(eat deal of steel (125 tons of fuel and steel).will remain cool ble.
If vessel penetration occurs when no water is in the reactor cavity, a great deal of penetration of the base mat may occur. The amount of penetra-tion occurring during the period when the core debris is molten is predictable.
Once l't freezes a complicated process occurs and the amount of penetration l
is not predictable. Again, studies are underway in Germany (their strong interest results because they do not allow water into the reactor cavity) that will answer this question within the.next couple of years.
Use of a liner in the cavity could buy time for plant personnel to get water into the cavity.
The debris could enter the dry cavity and become particulates. The gas flow from the decomposing concrete might block water added later from entering the bed.
It is not known whether the cooling by the gases from the decomposing gases will be sufficient to preclude re-melting.
This sequence needs further study if it cannot be shown that water will always pr ede the melt.
To sunnarize, in existing plants'where water precedes the melt in suf 3 '
ficient quantities and can be resupplied, penetration of the base mat will most likely not occur.
Under these conditions an ex-vessel steam explosion will probably take place with the possibility of a great deal of steam gene-ration that must be accommodated. The possibility of damage of the biological shield or shifting of NSSS components leading to containment damage needs to be further assessed.
When water is 'not available, the ch'ances of base mat penetration are much greater. The conclusion is that a water supply needs to be assured. A cavity liner of depleted U0,.A10, Mg0 or some similar 2
23 refractory or sacrificial material should be considered.
A containment building could be designed based on present information to preclude molten core penetration. A conceptual design that has redundant cooling capability as well could include the following features:
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1, Concrete that minimizes gas genaration on decomposition and has the best possible refractory characteristics.
2.
Several courses of depleted UO2 bricks actively cooled at the concrete-brick interface similar to the SNR 300 core catcher.
The heat sink U02 could be an existing plant system.
3.
A steel liner to protect the U0 bricks. _
2 4.
A cavity flooding capability and a method of refluxing to insure that the cavity stays flooded.
Such a system requires very little new technology and depends on no new research.
It should also be relatively inexpensive.
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