ML18025B029
| ML18025B029 | |
| Person / Time | |
|---|---|
| Site: | Browns Ferry  |
| Issue date: | 08/05/1980 |
| From: | Michelson C NRC OFFICE FOR ANALYSIS & EVALUATION OF OPERATIONAL DATA (AEOD) |
| To: | Ahearne J, Gilinsky V, Hendrie J NRC COMMISSION (OCM) |
| Shared Package | |
| ML18025B030 | List: |
| References | |
| NUDOCS 8008140572 | |
| Download: ML18025B029 (12) | |
Text
ply~I pgi,g>>f rIPff(g Rp I.;Il0Y AUG 5 19Ã Distribution:
AEOD Reading File (w/o enc.)
AEOD Chron. File (w/o enc.)
CJHeltemes, AEOD (w/o enc.)
CMichel'son, AEOD (w/o enc.).
.WJDircks, EDO
- KCornell, EDO
Chairman Ahearne Coamissioner Gilinsky Coomissioner Hendrie Commissioner Bradford FROrl:
THRU:
SUBJECT:
Car lyle ffichelson, Director Office for Analysis and Eva'luation of Operational Data W l 'l 'l l am J Dircks (SiEned) )Villiamj. DirckS Acting Executive Directorr Operation~
AEOD REPORT ON THE BRO'HNS FERRY 3 PARTIAL FAILURE TO, SCRAM Of) JUNE 28, 1980 As requested during the AEOD presentation on the FY82 budget (Ref: Chilk memo, dated July 28, 1980), enclosed for'our information is a copy of'he.AEOD report covering the June 28,
'f980 Browns Ferry 3 partial failure to scram event, This report has been forwarded to NRR for appropriate action.
I would be pleased to provide any clarification or f'urther infoPmation that you may desire.
priginel Signed by Qariyle Michelson Carlyle Michelson, Director Office for Ana'lysis and Evaluation of Operational Data
Enclosure:
AEOD Repot t on the Bro,4II57'EOD CMichelson 8/,I /80 EDO 8/S'/80
'NRC FORM 318 (9.76) NRCM 0240
- AU.S, GOVERNMENT PRINTING OFFICE<-1979 289 369
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EXECUTIVE
SUMMARY
OF THE BROWNS FERRY 3 PARTIAL FAILURE TO SCRAM ON JUNE 28 1980 Browns Ferry 'Unit No.
3 experienced a partial failure of its scram system on June 28, 1980, while shutting down for, a scheduled feedwater system main-tenance.
The failure occurred when the control room operating personnel initiated a manual scram at low power which was the next step in the normal shutdown evolution.
Upon scram actuation, all of the control rods on the West side of the core inserted properly.
However, most of the rods on the East side of the core failed to fully insert; stopping at positions ranging from 00 to 46 (fully.withdrawn) with an average insertion of about 20 positions.
In all, a total of four reactor scrams over a period of 14 minutes was required to complete full-in insertion of the East side control rods before normal shutdown operations could be resumed.
Shortly after the Browns Ferry 3 event, the Office for Analysis and Evaluation of Operational Data (AEOD) initiated an independent study of the occurrence, including the Browns Ferry 3 scram system design and operation and the special scram system tests and inspections which were performed at the plant site during the days imnediately following the event.
The principal purpose of this study was to,provide an independent assessment of the event cause and to determine th lessons learned and recommend corrective actions to prevent recurrence.
The
.AEOD review focused, for the most part, on the scram system design and the adequacy of the design features which protect against the loss of scram capabi lity and provide containment isolation.
The AEOD study has identified possible fundamental deficiencies in the present scram discharge volume/scram instrument volume vent and drain arrangements.
These deficiencies cast doubt upon the ability of the scram discharge volume protection and isolation features to adequately perform their intended functions.
In view of these deficiencies, AEOD has,recormended changes to provide corrective
4t
action.
These changes go beyond the imnediate corrective actions being taken at BF-3 and other BWRs in the short term as a result 'of IE Bulletin 80-17.
However, they are not intended to address the ATWS'oncerns and related modifications which are being. considered for the longer term.
~findin s
The principal findings which resulted from the AEOD study are summarized'below.
~
The BF-3 scram instrument volume (SIV) Hi Level scram function did not and cannot provide protection against the undetected accumulation of water in the East scram discharge volume (SDV) header with attendant loss of the East b'ank scram capability even during unobstructed venting and draining conditions.
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A single 'blockage in the West header SDV vent or drain line can result in an undetected accumulation of water in both the East and West headers'hich could disable the scram capabi.lity of all control rods.
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With the current SDV/SIV design, a blockage in the SDV drain (or vent) path can cause a partial loss of scram capability and disable the pro-tection function installed to assure detection and corrective action.
o There are numerous actual and potential mechanisms for introducing and retaining water in the SDV with, no accumulation in the SIV.
e The current SDY/SIV design arrangement results in the automatic Hi Level scram safety function being adversely influenced by the nonsafety-related reactor building Clean Radioactive Waste drain system.
o The BF-3 partial scram failure occurrence, together with recent events at other
- BWRs, shows that float-type water level monitoring instruments have a significant -degree of unreliability.
4 I
~ If a scram condition exists which cannot be bypassed in SHUTDOWN or REFUEL mode, then failure to close of the SDV vent or SIV drain valve can result in an unisolatable blowdown of reactor coolant outside primary containment.
o The. emergency operating instructions at BF-3 did not include a procedure or guidance for the operator to follow in the event of a partial or complete scram failure.
Recommendations As a result of the findings described
~
AEOD recommends that the SIV Hi Level scram function should be independent of the SDV vent and drain arrangement.
An acceptable configuration would be to place the SIV tank directly.
under the low end of the 6" SDV header and to connect the top of the SIY tank to the bottom of the SDV header by a short, vertical 6" diameter pipe (rather than the long 2" diameter horizontal pipe).
This arrangement should assure water spillage from the SDV directly into the tank containing the level monitoring instruments.
Furthermore, it would not depend on venting or draining phenomena which are sensitive to blockages.
This would also require that all plants provide two separate SIV tanks; one for each SDV header.
Separate instrument
- volumes, in immediate proximity to their respective
- headers, should assure proper water spillage into the SIVs and should provide adequate redundancy for protection against a total loss of scram capability.
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AEOD recommends that diversity be added to SIY water level monitoring instruments for the SIV Hi Level scram function.
Monitoring techniques such as differential pressure cells, ultrasonic detection, or conductivity may be considered along with others for this purpose.
o AEOD recommends that all vent and drain paths from the SDV and SIV be equipped with redundant, automatic isolation valves.
o AEOD recommends that emergency operating procedures and operator
~ training be provided for both part'ial and complete scram failure events.
Conclusions The Browns Ferry Unit 3 partial scram failure event which occurred on June 28, l980, demonstrated that the present BWR scram system is susceptible to loss of scram capability while operating at power.
Furthermore, the event showed that the loss of scram capability can occur in a way which goes undetected by the operator and unprotected by the reactor protection system.
The AEOD assessment of the BF-3 partial scram failure concludes that the cause of the partial loss of scram capability was the presence of water in the East scram discharge header.
Furthermore, the analysis of the SDV/SIV design configura-tion, together with its vent and drain characteristics, lead AEOD to conclude that several actual and postulated mechanisms exist which can cause the SDV to fill undetected and without protection against such filling.
The analyses also show that certain scram events can result in an unisolated reactor coolant blowdown outside of primary containment following a singl'e isolation valve failure in the SDV/SIV arrangement.
ig
<i
In view of these design.deficiencies, AEOD believes it is necessary that modifications be made to the SDV/SIV arrangement and isolation features.
Specific recommendations for changes in the SDV/SIY design are provided in the report.
These recommendations should be considered along with those from others who are also reviewing the BF-3 event.
AEOD believes,
- however, that the changes described ih its recommendations, which result from the findings provided in its report, are necessary to adequately reduce the risks associated with unreliability of.the BHR scram system which can stem. from the undetected accumulation of water in the scram discharge volume.
0 T*
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