ML111530443
| ML111530443 | |
| Person / Time | |
|---|---|
| Site: | Pilgrim |
| Issue date: | 06/01/2011 |
| From: | Pilgrim Watch |
| To: | Atomic Safety and Licensing Board Panel |
| SECY RAS | |
| Shared Package | |
| ML111530440 | List: |
| References | |
| RAS 20407, 50-293-LR, ASLBP 06-848-02-LR | |
| Download: ML111530443 (4) | |
Text
UNITED STATES ATOMlC ENERGY COMMlSSlON WASHINGTON. P.C. 20545 September 20, 1972 2, F, O'teary, L F. E, Kzuesi, RO
'L. Rogers, RS Xere is an idea to kick around, PI cc: E, G, c&,
L J. M. Hendrie, L D, F. Knuth, L R. L, Tedesco, L V, Stello, L
.. G. Lainas, L
'F'ressure-Suppression Containments Recent events have highlighted the safety disadvantages of pressure-sup-pression containments-While they a l s o have some saSety advantages, on balance I believe the disadvantages a r e preponderant.
I recommend t h a t the.GC adopt a policy of discouraging further use of pressure-suppression containments, and t h a t such designs not be accepted f o r construction per-mits f i l e d a f t e r a date t o be decided (say two years a f t e r the policy is adopted),
2, Discussion A pressure-suppression containment system has some means of absorbing the heat of vaporization of the steam i n the f l u i d released t o the containment
- volume, In a l l three GE models, the.steam is forced t o bubble through a pool of water and is condensed.
In the Westinghouse design, the steam is condensed by flowing it over i c e cubes.
The objective is t o reduce the pressure i n the containment through "suppressing" the p a r t i a l pressure of the steam by condensing it. To be'effective, pressure suppression must take piace concurrent with the flow of steam i n t o the containment, and its effectiveness is therefore dependent on the rate at which steam is generated o r released, If some unexpected event s6ould result i n steam generation o r flow greater than the suppression capability, then the steam that is not condensed would add an increment of containment pressure, Since the objective of pressure suppression is t o permit use of a smaller con-tainnent, rated a t lower pressure than would be required without suppres-sion, then incomplete suppression would lead t o overpressurizicg a pressure-suppression contaiomenE so desigrtd.
It may be noted t h a t the Stone and Webster "subatmospherict1 desiga has l i t t l e e f f e c t on the i n i t i a l containment pressure rise due t o an accident, and is therefore,not a "pressure-suppression containment" f o r the present discussion.
I n t h i s design, chilled water sprays a r e used t o reduce 'the containment pressure, and therefore the containment leakage, quickly a f t e r ti postulated LOCA, The pressure capability and vo~ume are designed t o take the f u l l accident, without c r e d i t f o r condensation.
Like all containments, the pressure-suppression designs are required to include margins i n capability.
Experiments have been conducted by GE and Westinghouse t o establish the r a t e of steam generarion that can be accommodated, The pressure-suppression pools, ice condenser, etc,, a r e then sized f o r the double-ended break steam flow, with margins f a r un-'
equal distribution of steam t o the many modular u n i r s of which =he c-on-denser is composed, The r a t e and distribution margins a r e probably ade-qua te, -
More d i f f i c u l t t o assess.is the margin needed when applying the experi-cleats1 data t o the reactor design, Recently we have reevaluate& the 10-year-old CE test r e s u l t s, and decided on a more conservative interpre-tation than has been used a l l these years-by GE (and accepted bp us)
W e
now believe that the former interpretation was incorrect, using data fro= t e s t s not applicable t o accident conditions.
We are requiring an independent evaluation of the i c e condenser design and its bases t o make less probable any comparable misinterpretation of t h i s design.
Sizce the pressure-suppression contaiments a r e smaller than conventional "b"
containments, the same amount of hydrogen, formed i n a postulated acclden:,
would constitute a higher voluine o r weight percentage of the contaiment atmosphere.
Therefore, such hydrogen generation tends t o be 8 more serious problem i n pressure-suppression containments.
The small G"c designs (both the light-bulb-and-doughnut and the over-under configura-tions) have t o be inerted because the hydrogen assumed (per Safety Guide 7) would immediately forn an exptosive Lixture, The GE Mod 3 and the Westing-house i c e condenser designs (they have equal volumes) require high-flow circulation and mixing systems t o ensure even d i l u t i o n of the hydrogen t o avoid f l a w a b l e mixtures i n one or' more compartments (see following f o r an addirianal serious disadvantage of 'this needed recirculation and i t s valves) a By contrast, the dry containments only require recombination o r purging s t a r t i n g weeks a f t e r the accident.
/,
A 1 1 pressure-suppression containments a r e divided i n t o two (or more) major volumes, the steam flowing from one t o the other through t h e condensing water o r ice, Any steam t h a t flows from one of these volumes t o the other without being condensed is a potential source of unsuppressed pressure.
Neither the strength nor the leakage r a t e of the divider (between the voIumesJ is tested i n the rurrently approved programs f o r i n i t i a l o r period-i c inservice testing.
Some e f f o r t i s now underway t o devise a legkage test, but none has so f a r been accomplished, Because of l i n i 6 d strength againsr collapse, the "receiving" volume tias t o be provided with vacuum r e l i e f,
In a l l designs except GE Mod 111, t h i s function is p~,sfomed by a group of valves, Such a valve stuck open is a large bypass of the condensation scheme; the amount of steam that thus escapes condensation can overpressurize the containment, Valves do not have a very good r e l i a b i l i t y record, Recently, f i v e of the vacuum r e l i e f valves f o r the pressure-suppression containment of Quad C i t i e s 2 were found stuck partly open.
Moreover, these valves had been modified t o include redundant "valve-closedf' position indiczzors a d test-ing devices, because of recent Reg concerns, The redundant posLtion in-dicators were found not t o indicate correctly the p a r t i c u l a r p=cly open s i t u a t i o n that obtained on the f i v e failed valves.
W e have only recehtly begun t o pay serious attention t o these valves, so previous :.~-rveillance programs have not generally included them, The GE Mod li; desLga has an elegant water-leg s e a l t h a t obviates the need f o r vacuum reltef valves.
T5.e high-capacity atmosphere recirculation systems provided for hydrogen mixing involve additional valves which, i f open a t the wrong t i n e, would constitute a serious steam bypass and thus a potential source of containment
over-pressurization.
These valves a r e large, and must open quickly and r e l i a b l y when recirculation is needed.
I n other engineered safety features,
'I no single valve is relied on f o r such service, y e t redundancy has not been provided even for single f a i l u r e s, open and closed, of these valves.
This -
is a serious mission, since opening a t the wrong time leads t o over-pressur-izarfon, while f a i l u r e t o open when needed i n h i b i t s recirculation.
The snaller s i z e of the pressure-suppression containment, plus the require-nect f o t the primary system t o be contained i n one of the two volumes, has l e d to overcrowding and limitation of access t o reactor and primary system conpnents f o r surveillance and in-service testing.
Separate shielding of con?cnents has tended t o subdivide i n t o compartments the volume occupied.
by che primary system.
(Some coinpartmentation of dry containments also occurs.)
A pipe break i n one of these-compartments creates a pressure d i f f e r e n t i a l ; each compartment must Be designed t o withstand t h i s pressure.
A method of testing such designs has not been developed.
Wnat are the safety advantages of pressure suppression, a p a r t from the cost saving.
GE people t a l k about a decontamination f a c t o r of 30,000 from scrubbing of iodine out of the steam by the w a t e r.
This i s hard t o swallow, but som? decontamination undoubtedly occu"rs, One wonders why GE doesn't do an experiment t o measure i t, and g e t c r e d i t f o r it. The i c e condenser decontamination is measurable but n o t significant.
Recirculation of the containment atmosphere' through the i c e has the potential for rapidly reducing the containment pressure by cooling its atmosphere, 2ut i n the present design there' s not ezmzg;Fr l e s for "bt,, =e J
sprays a r e furnished (in both volumes), j u s t a s i n dry c ~ n t a i ~ e n t s.
Re-circulation through the water i n the GE designs seems not t o have been tried, but may be necessary i n Mod 111 f o r hydrogen control.
We have ng analysis w h e t h e s a y significant cooling w i l l result.
It is by no means clear t h a t the pressure-suppression contai3ments are, over-a i l, significantly cheaper than dry contaianents when.all costs are'included, Information on t h i s point would be useful i n evaluating costs a d benefits, and-should be obtained.
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