ML20041A260
| ML20041A260 | |
| Person / Time | |
|---|---|
| Issue date: | 01/21/1982 |
| From: | Hanauer S Office of Nuclear Reactor Regulation |
| To: | Jason White AFFILIATION NOT ASSIGNED |
| Shared Package | |
| ML20041A261 | List: |
| References | |
| REF-GTECI-A-49, REF-GTECI-RV, TASK-A-49, TASK-OR NUDOCS 8202190340 | |
| Download: ML20041A260 (3) | |
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January 21, 1982 RECE!VED 1.
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f4r. Joseph 11. White III U m ar a n n a ssai g
11 South Merion Avenue e
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Mulyua Dryn flawr, Pennsylvania 19010
Dear Mr. White:
6 Your Decenher 29, 1981 letter sent to the NRC Connissioners with freedon of infornation requests concerning pressure vessel enbrittle ent has been referred to ne for a technical response.
It is the consensus of our working group nenbers who are workinq on the pressurized then-al shock (PTS or "enbrittlenent") issue that the problem is not as significant at BWRs as.it is at NRs. However, I an unaware of any particular, single individual who is responsible for this position.
It is sinply the opinion held by knowledgeable staff nenbers and their nanagenent. Peasons for their opinions are well founded in the basic differences between a NR and a BUR.
BURS are quite different frm PURs both in basic design and in operating condition.
Both types of differences contribute to the staff's consensus, as discussed below. First, as Mr.~ Basdekas wrote, the fast neutron flux at the RWR vessel wall is substantially lower than for a PUR. This causes the vessel to be noch less enbrittled for a given period of operation than a PWR. This lower flux is due to the basic design of a BUR which requires nore space between the ruclear core and the pressure
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vessel for the racirculation flew path. That extra space is largely full of water, and the water reduces the neutron flux which causes the-enbri tti m"en t.
There are additional reasons for the staff's consensus that Mr. Basdekas did not nention. F f rst. BUR pressure vessels are about 6" thick at the 3
nos* 1rradiated (enbrittled) location, whereas PUR vessels are about 8" thick due to the higher PUR operating pressure (about 2250 psig for a j-PUR cmpared to 1050 ?sig for a BWR, i.e., another basic design difference).
With respect to pressurized themal shock (the event which could cause an enbrittled vessel to fail), this difference in thickness is inportant because it results in lower themal stress intensity at the tip of a crack.
The 5asic conditions under which a BUR operates makes it nuch less likely that the sinultaneous rapid cooling and high pressure conditions necessary to create a significant PTS event will occur.' That is, BWRs q
operate by desian with saturated water, i.e., the water exists as an
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'quilibrium nixture of steam and liquid water at the boiling tmperature P
for the existing pressure (about 550T at abcut 1050 psig, just as 212T is the boiling tencerature at etnnspheric pressure O psig). The result
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f1r. Joseph H. White I'. is that any rapid ecolir.g of the water in the vessel will inherently result in a cor '
,nding pressure decrease, thereby precluding the necessary condu..ons for PTS of sinultaneous rapid cooling and high pressu re.
PWRs operate by design with subcooled water inside the pressure vessel considerably below the saturation temperature, and under those conditions it is easier to cool the water and maintain or re-establish high pressure to create PTS conditions.
The above argument that PTS events are much less probable for BWRs is borne out by operating experience. There have been no severe PTS events at domestic or foreign BHP.s.
In summary, 24Rs have less embrittlement due to the lower neutron flux at the vessel wall (so the material is better able to stand stresses);
there would be less themal stress intensity at the crack tip during PTS events due to the thinner wall, and less pressure stress due to the lower pressure; and finally it is very much less probable that a PTS event will occur since it is very difficult to sinultaneously produce the necessary low temperature with high pressure conditions.
I hope the above addresses your basic concern, which I interpret to be that NRC may not be properly considering the PTS issue for BWRs.
I am aware that this letter does not answer your specific requests for data. The reason is that we do not have extensive, femal data of the type you requested for B'lRs (nor do we need it for the above reasons).
He do have a considerable amount of infomal knowledge of flux and fluence measurements at BURS which reinforce the staff's above stated consensus, but it does not exist in a fomal report femat that we could meaningfully send to you. He are not responsible for and do not have the non-NRC, non-government, and industry studies you requested.
We do consider the PTS concern to be important for PWRs and we have a full time task nanager, Dr. Roy Woods, whose assignment is resolution of PTS concerns.
If you would like more detail than has been included in this brief letter, he will be pleased to talk to you if you call him at (301)492-4714 Sincerely, S. H. Hanauer, Director Division of Safety Technology Office of Nuclear Reactor Regulation
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DISTRIBUTION Central Files (A-49)-
GIB Reading H. Denton J. Felton E. Case E. Goodwin PPAS D. Eisenhut R. Mattson R. Vollmer H. Thompson P. Check B. Snyder S. Hanauer F. Schroeder K.' Kniel N. Anderson R. Woods F. Karas, ADM (F0IA-82-5)
S. Cavanaugh (NRR-82-016)
C. Twigg J. Mullin J. Butts A-49 Task File N
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