ML20024B481

Discusses C Michelson,Of Tva,Critique of Small Breaks for B&W RCS in May 1978

ML20024B481
Person / Time
Site:	Three Mile Island
Issue date:	04/14/1979
From:	Dunn B BABCOCK & WILCOX CO.
To:	BABCOCK & WILCOX CO.
References
TASK-06, TASK-07, TASK-6, TASK-7, TASK-GB GPU-2706, NUDOCS 8307090006
Download: ML20024B481 (9)

Text

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MICHELSON STORY COMMENTS '

i C. Michelson, of T.V.A. provided  !

B&W with a critique of small breaks for the B&W R.C.S. in

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May of 1978. Also a follow up letter was provided in February of 1979.

In these critiques Mr. Michelson makes the following major points.

1. The HPI may bypass the reactor core and exit the R . C . S ~. directly via the break thus not providing core cooling.

2. The steam generator must remove significant portions of the decay heat for certain sizes of small breaks.

3. Pressurizer level is not a measure of R.C.S. liquid inventory.

4. In later parts of the analysis

the heat source effects of the steam generators must be included in the analysis.

5. Natural circulation may be interupted i- by formation of voids in primary system and if natural circulation 8307090006 790414 PDRADOCK05000g89 P

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is terminated repressurization will occur before loop draining sufficient to establish steam condensation.

In addressing these points it is important to note that the situations and concerns raised by Mr. Michelson are based on the assumption that the R.C. pumps are tripped or not pumping. The discussion here relater to pumps off. B&W has performed analyses with pumps on and off.

in order to determine the worst case.

Although significantly different results are obtain between pumps on and off and system performance is very different the pumps off case is the worst condition.

All of Mr. Michelson's concerns are included in B&W evaluation model results.

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By the points we include these effects as follows: .

1) Because HPI may by pass

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the core B&W chooses break location to maximize that possibility. All breaks are modeled at the R.C. pump discharge between the HPI injection point and J

the eactor vessel. Water injected ,

in the broken cold leg runs directly out the break. Water injected in the intact legs can bypass by intering the downcomer and flowing out the break via the ,

broken cold leg prior to flowing through the core. The point is well made and has been allowed for in our models e

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2) For smaller breaks the steam generator must remove a significant portion of decay heat. The limit becomes a no break case in which all energy must be removed by the steam generators. This consideration is effectively model in our analyses.

3) Pressurizer level is not a good indicator of primary liquid ,

inventory. No operator action should be based on that signal alone.

It is quite possible to have a smaller break causing a slow loss of R.C. inventory and eventual dry out of the core while maintain-ing a full pressurizer if HPI

- is terminated prematurely. The  ;

only positive indication of reactor i

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5 liquid inventory is a subcooled indication on all RCS pressure and temperature indicators excepting those in the pressurizer.

4) The heat contained in the steam generators severly alters' the course of events for those breaks large enough to depressurize below the control pressure for the steam generators. 'This source must and is considered in our analyses.

5) Natural circulation will be interupted during the small break, a repressurization is probable for smaller breaks, and it would be shown in our evaluations. Our only disagreement with Mr. Michelson is his statement i

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- 6 that natural circulation ends when the vessel head starts putting steam into the hot leg. The escape ratio of steam and water is such that natural circulation will continue for a considerably longer period based on froth flow in the hot legs.

B&W provides bounding small break evaluations in its SAR material. The spectrum of evaluation is based on the following: -

1) The CFT line accident,

2) one break for which LPI CFT and HPI help mitigate consequences,

3) one break for which only CFT and HPI help mitigate consequences,

4) one break which is solely dependent on HPI, and

5) additional breaks to confirm that the above have provide bounding evaluations.

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, l For the very small breaks no evaluation is made for the following reasons.

1) Because of thb vent valves and the once through nature of the steam generator steam condensation in the generator must occur before the reactor core is uncovered.

2) If steam condensation is happening the RCS pressure will be at or arround 1200 psia.

, 3) The existing larger break (HPI only mitigating system) arrived at lower system inventory and 1200 psia earlier and at higher decay heat levels. This evaluation showed that at that time I.HPI prevented core uncovery.

, 4) At the later time I HPI must i

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. 8 also be able to prevent core uncovery because decay heat levels are lower.

5) System repressurization to code safeties is expected and of no known consequence. ,

This condition and possible system oscillations durring transition to code safeties pose no safety concerns. The condition can be avoided if desired by

1) increasing steam generator levels.and depressurizing the generators.

2) Bumping or starting an R.C. pump to mix primary steam with cold water, and

3) transition to decay heat removal system.

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4 However, the operator has at least 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br /> of BWST and can recirculate from the sump even after that so there is no concern about operating on the code safeties.

Mr. Michelson addresses some 29 other issues in his report. Discussion of each issue would require extensive interchange and be best accomplished in an open and limited scope meeting.

We believe allowance has been made for each of these issues within our design and design verification. The report is well thought out and although confrontation may exist on some minor points, it describes fairly well the course of events durring very small breaks.

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