ML20151X974

Status Rept Evaluation of Lasalle Instability Event

ML20151X974
Person / Time
Site:	LaSalle
Issue date:	04/08/1988
From:	Office of Nuclear Reactor Regulation
To:
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ML20151X958	List: IA-88-362, Status Rept Evaluation of Lasalle Instability Event ML20151X955 ML20151X962 ML20151X974
References
FOIA-88-362 NUDOCS 8808260172
Download: ML20151X974 (20)

Text

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a STATUS REPORT EVALUATION OF LASALLE INSTABILITY EVENT APRIL 8, 1988 i

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BACKGROUND BWR thermal hydraulic stability was the subject of Generic Issue B-15f. Early BWRs were designed to maintain decay ratios less than 0.5 (a decay ratio = 1.0 corresponds to an undamped limit cycle oscillation). Later core designs tended towards decay ratios of 1.0 due to higher powe- density cores and changing fuel design characteristics.

In the resolution of Generic Issue B-19, core designs which were potentially unstable (DR = 1.0) under natural circulation operating conditions were approved with the provision that operating procedures and technical specifications would assure that neutron flux oscillations indicative of core instability would be readily detected and suppressed as required by GDC 12.

Licensees were informed by Generic Letter 86-02 that such procedures and s technical specifications must be implemented for new reload cores unless it I could be demonstrated by approved calculational methods that the core was stable throughout permissible operating regions of the power / flow map.

Calculated core decay ratios of less than 0.80 by General Electric methods were approved as acceptable evidence of core stability.

Enclosure 1 is a sumary of the B-19 resolution package, including key slides,

' presented to CRGR in late 1985.

LASALLE LaSalle Unit 2 did not have fully implemented procedures and technical specifications in accordance with the B-19 resolution because the calculated decay ratio for the current operating cycle is 0.60. They did havp technical specifications and procedures for stability surveillance under conditions of single loop operation (SLO), but had declined suggestions by the staff that these should also apply to two loop operation because higher decay ratios were certain for future reloads.

LaSalle Unit 1 is currently under review for reload. The licensee has been i informed that stability technical specifications will be required for that reactor even though the calculated decay ratio 's 0.75 (indicative that it is lessstablethanUnit2).

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LASALLE EVENT PROBLEM AREAS

1. INSTRUl1ENTATION (See description in Enclosure 2)

(1) Detection Concerns Slow Pen response time and chart speed of APRM strip chart recorders reduce indicated amplitude of oscillations.

Time delay relays for the LPRM Hi and APRM Hi alanns delay recognition.

6 Running Average of APRM signal in Power / Flow circuit delays or prevents reactor trip at lower power levels with reactor oscillations.

• If oscillations are regional in nature and LPRM signals are out of phasa.

LPRM inputs to APRMs will tend to cancel each other so that oscillations are not evident on the APRMs.

(2) Event Evaluation Concerns Hi Speed Recording of APRM signals used for event evaluation was triggered by Low Water Level signal - not always a condition of the instability event. This data is needed to assess the nature and magnitude of neutron flux oscillations and the safety of restjirt after an instability event.

• LaSalle and some other BWRs do not have high speed data recording instrumentation which can be comitted for availability during plant operation.

(3) Potential Resolution

• Oscillations were re' cognized on APRM strip charts for LaSalle - Ask licensees to review their capability - Identify monitoring deficiencies and evaluate the need for design modifications and/or more reliance on LPRM meters. Analyses indicate that sufficient margin exists to safety limits so that significant error in the indicated magnitude of neutron flux oscillations could be tolerated.

High Speed Recording Instrumentation with high availability during power s operation and triggered by selected LPRM signals in event of signal alarms should be required. This instrumentation should be required operable when deliberately operating in regions of potential instability (mostly during Startup, controlled Shutdown, and Single Loop Operation).

• On Line Surveillance techniques using noise analysis permit measurement of decay ratio based on minimal signal inputs. ORNL, under NRC contract, has developed such an algorithm and benchmark cases to enable evaluation of similar techniques now comercially available.

• For regional oscillations where half of the core is 180' out of phase with the other half, the APRM does show oscillations (of reduced magnitude) since all LPRMs comprising the APRM signal are not 180' out of phase and the local oscillations are non-linear in shape; i.e,., the peak magnitude is further from the average than the minimum of the oscillation.

This will result in an oscillation when the two signals from 180' out of phase oscillations are added. The sumation of the signals will appear to be twice the frequency of the individual signals. This phenomenon was observed on a foreign BWR/6 plant which experienced out of phase regional oscillations.

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l II. EVALVATION OF LASALLE EVENT VERSUS GI B-19 CONCLUSIONS l

1 (1) Predicted Core Stability Concerns

• GE predictive methods were approved with 20% uncertainty. Predicted decay ratio for LaSalle conditions was 0.60 (40% uncertainty). The only obvious deviation from prediction assumptions was in core water level and associated natural circulation flow rate. Have calculations been discredited as acceptable evidence of core stability?

- GE is working on LaSalle decay ratio calculations using refined inputs of conditions at the time of the event. These are scheduled for submittal on May 15 and should identify deficiencies in the origina? calculation.

- Preliminary review indicates that 22 of 24 operating BWR 4/5/6 reactors have already implemented technical specifications which require innediate (within 15 minutes) action to reduce power after a two pump trip, and also incorporate neutron oscillation (APRM/LPRM) i surveillance provisions consistent with the Generic Issue B-19 technical resolution for both Single Loop Operation (SLO) and Two l Loop Operation (TLO). It is beltewed that operators of these~ plants would have responded to the two pump trip by rod insertion prior to scram. Corrective measures have been initiated for LaSa,11e Units 1 and 2. Corrective measures are needed for Browns Ferry Units 1, 2, I

and 3 prior to restart. Predictive calculations of Decay Ratio are I not needed for plants with appropriate technical specifications.

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- 6 of 7 BWR3 reactors have SLO tech specs and probably have adequate i

specs for two pumps tripped conditions, even though they are less l susceptible to neutron flux oscillations because of lower power density. The adequacy of stability related technical specifications l .. . . . - _ _ . .- - - _ _

for these reactors and for the 3 BWR 1/2 reactors will be further evaluated.

(2) Applicability of Existing B-19 Analyses for Fuel Integrity Limit Cycle oscillations to just below the 120% scram setpoint were analyzed by GE in response to NRC questions. Transient events were analyzed using the REDY system; they were initiated from the bottom of the neutron flux oscillation. Bundle peaking factors were assumed constant based on Vermont Yankee stability tests. The following events

, were analyzed and no fuel safety limits were violated.

Moderate Frequency Events

- Generator Load Rejection

- Turbine Trip

- Loss of 100* F Feedwater Heating

- Inadvertent HPCI Startup FeedwaterControllerFailure(MaximumDemand)

- Control Rod Withdrawal Error Infrequent Events r

- Generator Load Rejection with Bypass Failure

- Turbine Trip with Bypass Failure

• These analyses appear to bound the LaSalle event except for the assumption of constant bundle peaking factors. Based on the initiation of LPRM Hi alarms at APRM level of 87% the staff estimates that the Peaking Factor shifted fran 2.112 prior to the event to a value of 2.91 at the time of the LPRM alarm. This corresponds to a peak neutron flux

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1evel of 343% (relative to rated core average) at the time of scram. At l steady state, this would be equivalent to 19.1 kw/f t LHGR. Actual maximum LHGR with the oscillations is estimated to be 11.6 kw/ft.

Preliminary conclusions of the staff are that the GE analyses could probably account for the increased power peaking without violation of Safety Limits for global oscillations of the type experienced at LaSalle, even if the oscillations were to continue undetected until an anticipated transient occurred. However, we believe that regional oscillations in combination with increased power peaking would challenge 6 Safety Limits in advance of a high neutron flux scram. Therefore, manual detection and suppression must be reliable.

The most severe consequence expected from any instability event is fuel damage with some loss of fuel cladding integrity. Even then, significant radiological consequences are not likely.

III. APPROACH FOR RESOLUTION OF ISSUES Although the review of licensee /GE response to questions may impact the actions and schedule for resolution of sta'aility issues, an outline of present plans follows.

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(1) AIT Report ,.

l Based on discussions with the AIT team leader, the following items will be addressed in the AIT report.

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Technical Specifications acceptable to both the licensee and the staff are being developed. The major revisions to existing LaSalle technical specifications are:

- No Pumps Operating Immediately reduce thermal power by rod insertion and observe APRM/LPRM noise. Trip reactor on indication of high noise levels.

- Two Loop Operation APRM/LPRM surveillance and noise limits when above 80% Rod Control 6 Line at less than 45% power.

• Operating Procedures acceptable to the staff to replace interim standing orders are being developed. Key improvements to existing procedures for LaSalle and other BWRs are:

- Designate high worth rods for sequential insertion comencing within two minutes after two pump trip.

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- Concurrently moritor APRM/LPRM signals and trip reactor if limits are exceeded.

Open issues for follow up study will be identified. These will include:

l r l - Instrumentation issues

- Generic impact on technical specifications and operating procedures

- ATWS

- Reliance on predictive calculations of decay ratio

- Other outstanding questions and status of response I

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Licensee Performance

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- Response to event

- Reporting of event

- Evaluation of event Lessons Learned Characteristics of the Event.

- Recirculation Flow coastdown normally results in a drop in power to below the 80% flow control line (FCL), with subsequent rise in power s along the natural circulation line as the effects of lower feedwater temperature are introduced into the core. A lag of 5 to 15 minutes is expected before a power level high enough to cause instability is reached (5 minutes were available in the Unit 2 event). The staff believes this time interval is sufficient to reach a stable FCL by rod insertion. Therefore, inmediate SCRAM (or equivalent) is prudent if APRM/LPRM surveillance indicates instability in spite of rod insertion following a two pump trip. This approach will be reflected in both the procedures and the technical specifications for LaSalle Units 1 and 2.

- The magnitude of the APRM neutron flux oscillations (100% peak to peak) with escalation to the 118% trip setpoint was much larger than expected by General Electric Company and an order of magpitude larger than experienced in the Vermont Yankee stability tests.

However, analyses submitted on the NEDE-24011 docket (see paragraph II(2))byGEletterdatedSeptember 30, 1983, in response to a staff request, are applicable to the LaSalle event and indicate that no fuel damage is expected. i l

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- Preliminary study of the LaSalle event indicates that instability events with associated loss of feedwater heating result in a dow'nward shif t and increase in power peaking which will significantly increase the magnitude of local fuel channel oscillations.

Recomendations for LaSalle (2) Information Letter Prepared Lessons Learned Characteristics of Event Tech Spec / Procedure Concerns

• New Model Standard Tech Spec (LaSalle)

• Generic Letter may follow.

(3) Generic Letter (Modify GL 86-02)

Required Procedure Revisions (Next Reload)

• Required TS Revisions (Probably none for plants conforming with GL 86-02 tech spec recomendations.) ,.

• Required Review of Instrumentation for detection / alarm / trip in response to neutron flux oscillations

• Criteria for detection / response instrumentation

• New Criteria for Stability Tech Specs Relative to Decay Ratio Calculations

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ENCLOSilPE 1

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SELECTED VIEW GRAPHS s

FROM CP.GR PRESENTATION OF GI B-19 RESOLUTION

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MEASURE OF STABILITY IS DECAY RATIO m

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FOR BWRs, DECAY RATIO IS A FUNCTION OF FUEL CCSIGN CORE OPERATING CONDITIONS FUEL BURNUP

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511. No. 380 -

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Aegion 2 - Surveillance of APRMs and LPRMs required.

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DECAY RATIO (RELGAD NO.) . .

PigI ' EARLIER CYCif LATFR CYCLE ,

1. DRESDEN 2 *

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2. DRESDEN 3 .42(3) .48(5) -

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DECAY RATIO INCREASES AS Grd" dN FCCEEDS INTO LAIER CYCLE

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2. FOR MOST CASES, DECAY RATIO IS ,. LARGER THAN .5.

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• B-19 TECHNICAL RESOLUTION INVOLVES A. APPROVAL OF PREVIOUSLY UNAPPROVED CALCULATION METHODS AND A DECAY RATI0 CRITERION OF 0.75/0.80 B. APPROVAL OF A LICENSING OPTION TO UPGRADE THE TECHNICAL SPECIFICATIONS FOR AVOIDANCE OF LESS STABLE OPERATING CONDITIONS AS A MEANS TO:

(1) LICENSE CORES WHICH DO NOT MEET THE DECAY RATIO h

CRITERION (2) REMOVE EXISTING RESTRICTIONS ON !!NGLE LOOP OPERATION (3) REMOVE LICENSE CONDITIONS REQUIRING NEW STABILITY ANALYSES FOR EACH RELOAD CYCLE C. ENHANCEMENT OF THE CAPABILITY TO PREDICT THE THERKAL HYDRAULIC

• STABILITY CHARACTERISTICS OF A CORE DESIGN.

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BWR STABILITY TECHNICAL SPECIFICATIONS

• BASES -

1. GDC 10 - CORE SHALL BE DESIGNED TO ASSURE SAFDL IS NOT EXCEEDED FOR ANY CONDITION OF NORMAL OPERATION INCLUDING A00's.

2. GDC 12 POWER OSCILLATIONS WHICH CAN RESULT IN CONDITIONS EXCEEDING SAFDL ARE NOT POSSIBLE OR CAN BE RELIAEl.Y A'O '

READILY DETECTED AND SUPRESSED 6

• PREVIOUS TECHNICAL SPECIFICATIONS BELIEVED OK BECAUSE -

1. BWR CORE DESIGNS WERE BELIEVED TO BE STABLE UNDER PERMISSIBLE OPERATING CONDITIONS BASED ON OPERATING EXPERIENCE AND PREDICTIVE BERAVIOR USING UNAPPROVED ANALYTICAL METHODS (DECAY RAT 10 :50.5)

2. IT WAS EXPECTED THAT POWER OSCILLATIONS WOULD BE DETECTED AND SUPPRESSED BY APRM OVERPOWER SCRAM CHANNELS BEFORE EXCEEDING SAFDL

3. OPERATION IN REGION OF LOW STABILITY MARGIN (SINGLE LOOP OR ~

NATURAL CIRCULATION) WAS LIMITED (e.g.,12 hrs.) BY TECH. SPECS.

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• NEW INFORMATION BECAME AVAILABLE -

1. VERMONT YANKEE STADILITY TESTS SHOWED THE OCCURRENCE OF POWER OSCILLATIONS UNDER PERMISSIBLE NATURAL CIRCULATION CONDITIONS AND INDICATED THAT STABILITY PREDICTIONS (PREDICTED DR = 0.85 VS. ACTUAL DR2:1.0) HERE NON-CONSERVATIVE.

2. STABILITY TE3TS AT TWO FOREIGN REACTORS SHOWED THE OCCURRENCE OF LOCAL OSCILLATIONS WHICH WERE OUT OF PHASE AND OF MUCH GREATER MAGNITUDE THAN THE CORE AVERAGE - APRM OVERPOWER SCRAM CHANNELS DO NOT PROVIDE THE DETECTION / SUPPRESSION CAPABILITY CLEARLY REQUIRED BY GDC 12

• RESOLUTION - UPGRADE TECHNICAL SPECIFICATIONS

1. REQUIRE IMMEDIATE CORRECTIVE ACTIONS AFTER INCURSIONS (e.g. LOSS OF PUMP (s)) INTO LOW FLOW OPERATING REGIONS WHERE CORZ DESIGN STABILITY 15 NOT ASSURED BY PREDICTIVE CALCULATIONS (INCLUDING UNCERTAINTIES)

2. PROVIDE FOR SURVE!LLANCE OF LPRMs AS NEEDED TO ASSURE THAT POWER OSCILLATIONS WOULD BE Rt.IABLY DETECTED AND SUPPRESSED ,

PER GDC 12 l

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• IMPLEMENTATION OF B-19 TECHNICAL RESOLUTION

1. ALL LICENSING ACTIONS (e.g., NEW OLs) MUST CONSIDER THE NEW INFORMATION AND ITS IMPACT ON TECHNICAL SPECIFICATIONS ASSURING COMPLIANCE WITH GDC 10 AND GDC 12

2. LICENSEE INITIATED TECHNICAL SPECIFICATION CHANGE REQUESTS RELATING TO MODIFICATIONS IN FUEL DESIGN OR CORC OPERATING

. CONDITIONS WHICH REDUCE THE STABILITY MARGIN CANNOT BE

APPROVED WITHOUT CONCURRENT CONSIDERATION OF THE NEW IHFOR.'4ATION.

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3. BWR LICENSEES SNetMbEE INFORMED BY GENERIC LETTER'OF THE TECHNICAL RESOLUTION OF GENERIC ISSUE B-19 SO THAT LICENSEES CAN:

A. AVOID ERR 0NEOUS 50.59 DETERMINATIONS FOR RELOAD COREl WHICH DO NOT MEET THE APPROVED STABILITY DECAY RATIO .

CRITERION INCLUDING UNCERTAINTIES B. PROPERLY ASSESS THE IMPACT OF THE RESOLUi'ON ON FUTURE LICENSING SUBMITTALS C. PROCEED WITH SUBMITTAL OF REVISED TECHNICAL SPECIFICATIONS IF DESIRED TO REMOVE EXISTING RESTRICTIONS ON SLO OR TO REMOVE LICENSE CONDITIONS REQUIRING NEW STABILITY ANALYSES FOR EACH RELOAD CYCLE.

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