ML16340D024

Provides Suppl to 820907 Memo Re Flow Blockage by Steam During Natural Circulation in Pwrs.Accident Analyses Upon Which Operating Procedures Are Based Give No Credit for Condensation by Repressurization

ML16340D024
Person / Time
Site:	Diablo Canyon
Issue date:	12/06/1982
From:	Etherington E Advisory Committee on Reactor Safeguards
To:	Shewmon P Advisory Committee on Reactor Safeguards
Shared Package
ML16340D023	List: ML16340D022 ML16340D024
References
NUDOCS 8212220438
Download: ML16340D024 (8)

Text

CA0 ate),

P O~

~O

~

g

~O a*+

UNITEDSTATES NUCLEAR REGULATORYCOMMISSION ADVISORYCOMMITTEEON REACTOR SAFMUARDS WASHINGTON,D. C. 20555 December 6, 1982

SUBJECT:

~

~

~

~

REMORA)DUN FOR:

P.

G. Shewmon, Chafrman ACRS Hembers e

FROM:

H.

E

rington, em

~0 SUPPLEMENT TO HENORANDUN "FLOM BLOCKAGE BY STEAN DURING NATURAL CIRCULATION IN PWRs,"

DATED $EPTEHBER 7, 1982 I

I'.

It has been found desirable.'to release the referenced internal memo as a

public document, aqd there is some indication that the memo mav have been interpreted as implying that a serious problem exists.

This was not the intention, and fs'hard1y consistent with the conclusion (Item )7) that "The Committee may want to review the final disposition of this problem, and to be assured that the various possibilities are reflected fn sufficiently flexible and understandable operating procedures."

It was thought unnecessary to devel. on Item )4, and the misunderstanding probably arose from the wording of thfs "item:

"The main concern fs a 88W System with a SBLOCA that fs too small t.

dp I

KP y

I off\\ tlrf ly p

tt ftl I

p sure safety systems, yet too large to permit the inventory ta be maintained by the high pressure pumps.

Loss of inventory leads to formation of steam voids at hfgh spots, and the possibility of "degraded" modes of heat transfer as discussed fn Sections 9 and 10."

This supplement fs a discussion of the implications of Item )4.

A steam void-blocking a candycane can be elfmfnated either by fn situ con-densatfon, or by transport of steam to the condensing surfaces of the steam generator.

The previous m~a indicates that condensation by heat transfer processes associated with +pressurization may be too slow to be effective, and fn situ condensatfon by other pr6cesses fi somewhat con)ectural, It therefore appears nece'ssary to review conditions that will induce bulk trans-port of steam from the candycane.

r BSW and NRC have long recognized that some LOCAs are characterized'y a stage

.fn whfch natural circulation fs interrupted by steam accumulation at the top

'f the candycanes, but analyses of these LOCAs indicate that the condition fs transient, and that the flow of coolant. (eater or steam) fs re-established without assuming fn situ condensatfon at the top of the loop. It fs useful to distinguish between twa cases of a vafd that completely blocks flow, f.e.,

that does not extend far enough down fnto the steam generator tubes to permit cooling fn the bailer-condenser made:

1.

system at saturation, 2..

system subcoaled, 8212220438 8212i4 PDR ADQCK 05000275

ra

V P.

G.

Shewmon ACRS Members In the first case, continuing loss of iriventory and generation of steam will cause the void to grow until it extends far enough into the steam generator tubes to sustain stable cooling in the boiler~<<denser mode.'he second case could develop from a loss of inventory followed by repres-surizat$ on with the reactor coolant pumps unavailable.

This condition was discussed in the attachment to Hr. Denton's letter of Hay 7, 1982, to Dr. +ers.

The temperature of the water will increase and, in the absence of automatic or manual corrective measures,. the volume of the water inventory and the pressure will,increase.

The result could be:

I.

Sufficient contraction of the void by compression to permit spillover from the hot leg would re-establish cooling

$ n,the single-phase natural circulation mode (once coolant flow is re-established by spillover it would cool,and condense the butible quickly) or 2.

Saturation will 'be reached, in which case boiling would occur in the system; if boiling extends to the hot legs, or if steam enters the hot Iegs, the void will become enlarged until it extends far enough into the steam generator tubes for cooling in the boiler-condenser mode to be established, or

'< h.

3.

The pressure will reach the PORV or SV setting, without conditions (I) or (2) being established; loss of inventory and boiling will eventually lead to condition 2.

(Activation of high-pressure in-fection pumps would modify this course of events).

0 In all three cases natural circulation (single-phase or boiler-condenser)

$ s established without assuming any in situ condensation.

Conclusions 1..Accident analyses on which operating procedures are based give no credit

. for condensation by repressur ization.

To the degree that any condensation may occur, it is.likely to be an effect that will change the time at which a transition from one mode of cooling to another will occUr but not the basic mechanism by which ft occurs.

The term "boiler-condenser mode" is used here to include the condition in which'steam, rising in the hot leg, causes slug flow and massive carryover of water into the steam-generator tubes.

I have referred to the boiler-condenser mode as a "degraded" mode of cooling.

I believe the NRC Staff does not dis-tinguish between the effectiveness of liquid-phase natural circulation and the boiler-"~-denser mode.

I agree that the boiler-condenser mode is an effective mode of heat transfer~

~ J

)g g~

V ll

P.

G.

Shewmon ACRS Members 3

3.

Although no safety issue has,been identified, operator action would be beneficial to expedite elimination of a steam block and curtail the transient.

However, different conditions require different actions, and it would be necessary that the operator recognize the particular condition in order to take appropriate action.

4.

suggestion in Item 17 of the previous memo (quoted in the first para-graph of this memo) was probably unnecessary, because ACRS Subcommittees have been reviewing. Technical Guidelines for Emergency Operating Procedures (EOps), including those-by BSM, and the. matter All be receiving additional Committee attention in its continuing review.

f 1

I

yl u

BOARD NOTIFICATION BN '25/125A

• w/enclosure

~Document COntrol-(50 275/323/361/362')*'RC POR*

L PDR*

, TERA*

NSIC*

LBP3 Reading*

G. Knighton*

Project Manager* BBuckley, HSchierling, HRood J.

Lee T.

M. Novak M. Stine D. 'Eisenhut/R.

Purple M. Williams H. Denton/E.

Case*'PAS J.

Youngblood A. Schwencer E.

Adensam C.

Thomas R. Vollmer H. Thompson R. Mattson S.

Hanauer Ati.orney; OELD*

E. L. Jordon J.

M. Taylor Regional Adminstrator Re's-dcrrt Inspector W. J. Dircks, EDO (3)

E. Christenbury, OELD'.

Scinto, OELD A. Bennette (1)*

S. Black cc:

Board Service List

~ y