ML20027A872
| ML20027A872 | |
| Person / Time | |
|---|---|
| Site: | Arkansas Nuclear  |
| Issue date: | 07/30/1982 |
| From: | Eisenhut D Office of Nuclear Reactor Regulation |
| To: | Michelson C NRC OFFICE FOR ANALYSIS & EVALUATION OF OPERATIONAL DATA (AEOD) |
| Shared Package | |
| ML20027A873 | List: |
| References | |
| NUDOCS 8208180173 | |
| Download: ML20027A872 (5) | |
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I JUL 3 0 B32 MDORANDUll FOR: Carlyle Michelson, Director Office of Analysis & Evaluation of Operational Data l
FROM:
Darrell G. Eisenhut, Director Division of Licensing
SUBJECT:
REVIEW OF AEOD ANALYSIS OF ABNORMAL TRANSIENT OPERATING GUIDELINES (ATOG) AS APPLIED TO THE APRIL 9,1981 OVERFILL EVENT AT ARXANSAS NUCLEAR ONE - UNIT 1 In response to your April 9,1982 request for a peer review.of the AEOD case study on the above subject, we have developed the attached comments.
These comments indicate the need for some re-analysis prior to the final l
publication of your case study.
I We also wish to bring to your attention the fact that although the TMI j
Action Plan does not require overfill protection, several B&W plant licensee have taken this opportunity to propose T.ergericy feedwater systen modifications which include overfill protection. Table 1 presents the s*atus of overfill protection on B&W plant *,.
It appears that ongoing actions related to steam generator overfill isrotection will be sufficient to resolve your concerns in this area. We 'will keep you informed of the progress of our review of the systens modifications being proposed in this area.
C"],,,, 3, s a'M e
e Darrell G. Eisenhut Director Division of Licensing Attachments:
As stated
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9i TABLE 1 STATUS OF BW OT5G OVERFILL PROTECTION Plant ANO-1 The licensee has proposed main feedwater isolation on high level plus a safety grade emergemey feedwater level control system j
Crystal River 3 The licensee has proposed. main feedwater isolation on l
high level plus a safety grade emergency feedwater level control system l
Rancho Seco The licensee has proposed main feedwater isolation on high level plus a safety grade emergemcy feedwater level control system Midland The licensee has proposed main feedwater isolation on high level plus a safety grade energe=cy feedwater level control system j
TMI-l The licensee has cor:mited, at the rest:ert hearings.
to provide overfill protection Davis-Besse The licensee has informally indicated -that a system like the ANO-1 overfill protection sys. tem is being considered g
Oconee 1/2/3 These units have control grade high le. vel protection for tripping the main feedwater pumps ((no overfill protection is included for the emergency feedwater i
system)
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a COMMENTS ON TE AEOD CASE STUDY.
"AN ANALYSIS OF THE ABNORMAL TRANSIENT OPERAT]NG GUI DELINES (ATOG) AS APPLIED,
TO TE APRIL 9,1981 OVERFILL EVENT AT ARKANMS NUCLEAR ONE - UNIT 1"
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1.
The analyses of the time to OTSG overfill do riot proper 1,y' account 4 Mi.. -
P for the initial water inventory in the OTSG. The initia,1 decrease in OTSG 1evel following a reactor trip is associated with'a redistri-bution of the water in the generator.
This occurs when the reactor
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^ trip and turbine trip cause a reduction in steam generation and g
y steam flow in the tube bundle region and a corresponding decrease c
in differential pressure.
Since the decreased differential pressure
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cannot support the column of water in the downcomer region, the down-comer level begins to drop.
The response of OTSG level to a typical reactor trip is shown in the attached figures (source: AT03 documen-tation, part II).
The Appendix to the case st,udy shows calculations of the time to overfill the OTSG under various conditions, and states that the initial decrease in OTSG 1evel was caused by the 20 seconds of
' steam flow feedwater flow mismatch plus the time required for the steam generation due to decay heat to drop below the post trip feed-water flow rate. In fact..the main cause of the level decrease was' l
the normal response to a reactor trip. The calculation (P.A-2) showing 4 min, and 15 seconds to OTSG overfill with no initial overcooling is therefore incorrect; the 6 min. and 50 seconds calculated for the April 8,1981 event would not have been t
significantly different if there had been no initial undercooling event.
Similarly, the overfill time of 50 seconds for the cases of full feedwater flow after reactor trip should be 1 minute and 30 seconds.
2.
The case study suggests that the overfill event would have been more severe had it occurred on OTSG-A rather than OTSG-B because the OTSG-A level was higher initially.
This does not appear to be the case.
We have been monitoring the level behavior in these two generators for several years.
Both we and the licensee believe (based on the OTSG heat removal performance) that the generators have been operating with approximately the same mass inventory in j
the tube bundle region. The higher water level in the.0TSG-A down.
comer region appears to be thc result of an increased pressure-drop in the tube bundle, due to crud deposits. Therefor.e. following a.
reactor trip both OTSG 1evels would decrease to approximately the same value.
r, r,e 3.
The order of operator actions, as instructed in the' ATOG, is not-correctly presented on page 9.
Whenever the operator is alerted by a reactor trip signal he is told to push the reactor trip and the turbine trip buttons and to verify trip actuation (reactor power decreasing, all rods on bottom, turbine stop valves shut).
Once he has further verified proper closing of the letdown line, he should observe the status of the main feedwater system. If feed-water has not run back, he should trip the FW pumps (ATOG Part I,Section II, Step 5.0).
The hoi start is verified later. The only
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situation where the opera' tor has to initiate HPI before the FW pump verification / trip step'is for ATWS scenarios (reactor trip
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indications could not be verified).
In this situation a delayed FW trip would not be adverse but beneficial.
4.
The last sentence on pg.10 gives the impression that proper operttor diagnosis and multiple mitigative actions are required before the operator can trip the FW pumps. As explained in item 3, only rapid verification is needed before the operator is instructed to terminate FW. The ANO-1 transient response showed that a rapid 1
operator action for this event is not unrealistic. Also, since the ANO-1 operator did not have procedures based on the ATOG, response times recorded during the ANO-1 transient may not be representative for a plant where the ATOG is implemented.
5.
Page 11 of the case study indicates that the ANO-1 plant does not meet General Design Criterion 13 because it is not designed to withstand an overfill event.
GDC-13 requires instrumentation and control systems for important variables. ANO-1 has both OTSG level instrumentation and a level control system and, therefore, meets GDC-13 with respect to OTSG 1e' vel.
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