Information Notice 1996-16, BWR Operation with Indicated Flow Less than Natural Circulation

BWR Operation with Indicated Flow Less than Natural Circulation

ML031060210
Person / Time
Site:	Beaver Valley, Millstone, Hatch, Monticello, Calvert Cliffs, Dresden, Davis Besse, Peach Bottom, Browns Ferry, Salem, Oconee, Mcguire, Nine Mile Point, Palisades, Palo Verde, Perry, Indian Point, Fermi, Kewaunee, Catawba, Harris, Wolf Creek, Saint Lucie, Point Beach, Oyster Creek, Watts Bar, Hope Creek, Grand Gulf, Cooper, Sequoyah, Byron, Pilgrim, Arkansas Nuclear, Three Mile Island, Braidwood, Susquehanna, Summer, Prairie Island, Columbia, Seabrook, Brunswick, Surry, Limerick, North Anna, Turkey Point, River Bend, Vermont Yankee, Crystal River, Haddam Neck, Ginna, Diablo Canyon, Callaway, Vogtle, Waterford, Duane Arnold, Farley, Robinson, Clinton, South Texas, San Onofre, Cook, Comanche Peak, Yankee Rowe, Maine Yankee, Quad Cities, Humboldt Bay, La Crosse, Big Rock Point, Rancho Seco, Zion, Midland, Bellefonte, Fort Calhoun, FitzPatrick, McGuire, LaSalle, Fort Saint Vrain, Shoreham, Satsop, Trojan, Atlantic Nuclear Power Plant
Issue date:	03/14/1996
From:	Crutchfield D Office of Nuclear Reactor Regulation
To:
References
IN-96-016, NUDOCS 9603110159
Download: ML031060210 (9)

UNITED STATES

NUCLEAR REGULATORY COMMISSION

OFFICE OF NUCLEAR REACTOR REGULATION

WASHINGTON, D.C. 20555-0001 March 14, 1996 NRC INFORMATION NOTICE 96-16: BWR OPERATION WITH INDICATED FLOW LESS THAN

NATURAL CIRCULATION

Addressees

All holders of operating licenses or construction permits for boiling-water

reactors (BWRs).

Purpose

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information

notice to alert addressees to two instances of power operation in which the

core flow appeared to be less than that normally attributed to operation on or

near the natural circulation line. It is expected that recipients will review

the information for applicability to their facilities and consider actions, as

appropriate, to avoid similar problems. However, suggestions contained in

this information notice are not NRC requirements; therefore, no specific

action or written response is required.

Description of Circumstances

Nine Mile Point Nuclear Station Unit 1

On February 1, 1995, Nine Mile Point Unit 1 (NMP1) had a recirculation pump

runback event as a result of pump maintenance activities. The reactor was

being operated at 77 percent of rated power and on the 78-percent rod line.

When the runback occurred, one of the five pumps was out of service for

maintenance. Speed of the four operating pumps was reduced to about

10 percent of rated speed. The resulting minimum indicated flow was about

13 percent and the corresponding power level was about 30 percent. (The

facility operating procedures require that flow be greater than 20 percent to

be in the RUN mode.) With four pumps operating at 10-percent speed, the

resulting flow would be expected to be a little higher than at natural

circulation. Natural circulation flow for NMP1 is generally shown as being

about 21 to 26 percent for a power of about 30 percent. Thus, the indicated

flow rate was lower than the rate that would normally be considered for the

natural circulation line on the power/flow map.

There was a turbine trip from a high water level signal as a result of a steam

and feedwater flow mismatch, but no reactor scram occurred since steam flow

was within turbine bypass capacity. Operators reduced power by inserting high

worth control rods and increased the indicated flow to above the natural

circulation line by increasing the speed of the recirculation pumps.

Investigation by the utility, General Electric, and an NRC inspection team

attributed the indicated low flows to inaccurate instrumentation, with lack of

recent low-flow calibration as a suspected cause. The indicated low flow was

9603110159 0

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- <-2-IN 96-16 March 14, 1996 not compatible with previous analysis and measurements, which indicated the

flow rate would be at least 24 percent following a trip of all 5 pumps from

similar initial conditions.

Peach Bottom Atomic Power Station Unit 2

On June 3, 1995, while the reactor was being operated at about 32 percent of

rated power and in single-loop operation prior to planned maintenance, a full

recirculation pump runback to minimum speed occurred. This was the result of

maintenance activities while attempting to place a reactor feed pump in

standby. Reactor power was reduced to 26 percent, and the indicated core flow

was about 32 percent of rated flow. The operator knew the reason for the

runback and, as directed by the control room supervisor, restored core flow to

conditions that existed before the event. After the event, the reactor

engineer reviewed core performance during the reduced-flow condition and

determined that the power/flow had apparently reached a flow that was 0.5 to

2.0 percent (depending on the instrumentation used) lower than the flow

corresponding to the natural circulation line-flow (as displayed, for example, in Figure 1.1-1 of the Peach Bottom Technical Specifications).

Discussion

NRC staff investigation of these two events indicates that some plant

operators may not have a complete understanding of at least four aspects of

such events.

(1) Natural circulation may not occur, or may not appear to occur, precisely

where it is depicted on the power/flow maps normally available to

operators, for example, in technical specifications, the Bases or

procedures. Usually, these maps are only intended to be illustrative

and approximate. The correct values change with cycle, time in cycle, and other core and coolant system characteristics. In addition, the

observed flow also depends on the accuracy of the relevant instruments

and may not be correct because of, for example, calibration problems.

(2) Flow instruments are usually calibrated at high flow rates. Calibration

information obtained at low flows can reduce uncertainties about the

accuracy of the instruments involving low flow situations.

(3) Operation (on a given rod line) at flow lower than that on the natural

circulation line, if real, can be more unstable than operation on the

natural circulation line. For the NMPI event, the NRC inspection team

calculated that the reactor would have been unstable at the indicated

minimum flow (13 percent) if the event had been initiated from

conditions on a somewhat higher rod line. The stability regions for

NMP1 have now been conservatively extended below the natural circulation

line, and operators are aware of the potential for increased instability

at lower flows.

IN 96-16 March 14, 1996 (4) It was pointed out in NRC inspection report Reference No. 50-220/95-80

[9504070141] for the NMP1 event that the technical specification for the

NMP1 safety limit with low flow, did not have a logical or sufficient

Basis (B 2.1.1.b) for the safety limit. The specification, which says, in part, that the power should be less than 25 percent of rated thermal

if the flow is less than 10 percent of rated, does not follow from the

information in the Basis.

The flow related-safety limit addresses flow below 10 percent and flow

above 10 percent. Above 10-percent flow, the limit is stated in terms

of the minimum critical power ratio (MCPR) and the MCPR safety limit.

The Basis states that there are sufficient data to validate the MCPR

correlation down to about 10-percent flow. Thus, with power/flow

information for any given state of the reactor, the relationship to the

MCPR safety limit can be determined. However, no relevant information

for below 10-percent flow is provided as a basis for the stated

25-percent power safety limit. The basis (which, like the specifica- tion, is essentially the same for most BWRs, and the new standard

specification NUREG-1434) indicates that the critical power ratio

correlations are valid down to about 10-percent flow, but does not

address the below-10-percent-flow critical power correlations or any

other relevant data or analysis for that region. Instead, it states

that minimum flow will be no less than about 20 percent, the lowest flow

that was anticipated under low-flow conditions as a result of the BWR

flow geometry. At this flow, a power of 50 percent was found to be

within minimum critical power limits and thus, it is stated, the

specification of a 25-percent limit is conservative. However, no

information is provided to validate the translation to 10 percent or

lower flow conditions, the flow range of the low-flow specification.

This aspect of the safety limit will be further examined by the NRC, and

action may be taken to correct NUREG-1434.

IN 96-16 March 14, 1996 This information notice requires no specific action or written response. If

you have any questions about the information in this notice, please contact

one of the technical contacts listed below or the appropriate Office of

Nuclear Reactor Regulation (NRR) project manager.

Dennis M. Crutchie ,Wrector

Division of Reactor Program Management

Office of Nuclear Reactor Regulation

Technical contacts: Howard J. Richings, NRR

(301) 415-2888 Internet:hjr@nrc.gov

T. Jerrell Carter, NRR

(301) 415-1153 Internet:tjc@nrc.gov

Attachment: List of R tly Issued NRC Information Notices

Aid ' -9"11 r kC,

Attachment

IN 96-16 March 14, 1996 LIST OF RECENTLY ISSUED

NRC INFORMATION NOTICES

Information Date of

Notice No. Subject Issuance Issued to

96-15 Unexpected Plant Perform- 03/08/96 All holders of OLs or CPs

ance During Performance for nuclear power reactors

of New Surveillance Tests

96-14 Degradation of Radwaste 03/01/96 All holders of OLs or CPs

Facility Equipment at for nuclear power reactors

Millstone Nuclear Power

Station, Unit 1

96-13 Potential Containment 02/26/96 All holders of OLs or CPs

Leak Paths Through for nuclear power reactors

Hydrogen Analyzers

96-12 Control Rod Insertion 02/15/96 All holders of OLs or CPs

Problems for nuclear power reactors

96-11 Ingress of Demineralizer 02/14/96 All holders of OLs or CPs

Resins Increases Potential for pressurized water

Stress Corrosion Cracking nuclear power reactors

of Control Rod Drive

Mechanism Penetrations

96-10 Potential Blockage by 02/13/96 All holders of OLs or CPs

Debris of Safety System for nuclear power reactors

Piping Which is Not Used

During Normal Operation

or Tested During Surveil- lances

96-09 Damage in Foreign Steam 02/12/96 All holders of OLs or CPs

Generator Internals for pressurized water

reactors

96-08 Thermally Induced Pres- 02/05/96 All holders of OLs or CPs

sure Locking of a High for nuclear power reactors

Pressure Coolant Injec- tion Gate Valve

OL = Operating License

CP = Construction Permit

IN 96-16 March 14, 1996 This information notice requires no specific action or written response. If

you have any questions about the information in this notice, please contact

one of the technical contacts listed below or the appropriate Office of

Nuclear Reactor Regulation (NRR) project manager.

original signed by

Dennis M. Crutchfield, Director

Division of Reactor Program Management

Office of Nuclear Reactor Regulation

Technical contacts: Howard J. Richings, NRR

(301) 415-2888 Internet:hjr@nrc.gov

T. Jerrell Carter, NRR

(301) 415-1153 Internet:tjc@nrc.gov

Attachment: List of Recently Issued NRC Information Notices

Document reviewed by Tech Ed on 10/25/96

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IN 96-xx

February xx, 1996 This information notice requires no specific action or written response. If

you have any questions about the information in this notice, please contact

one of the technical contacts listed below or the appropriate Office of

Nuclear Reactor Regulation (NRR) project manager.

Dennis M. Crutchfield, Director

Division of Reactor Program Management

Office of Nuclear Reactor Regulation

Technical contacts: Howard J. Richings, NRR

(301) 415-2888 Internet:hjr@nrc.gov

T. Jerrell Carter, NRR

(301) 415-1153 Internet:tjc@nrc.gov

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DATE _02/14/96 01/02/96 02/97/96 02/ /96

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IN 96-xx

February xx, 1996 This information notice requires no specific action or written response. If

you have any questions about the information in this notice, please contact

one of the technical contacts listed below or the appropriate Office of

Nuclear Reactor Regulation (NRR) project manager.

Dennis M. Crutchfield, Director

Division of Reactor Program Management

Office of Nuclear Reactor Regulation

Technical contacts: Howard J. Richings, NRR

(301) 415-2888 Internet: hjr@nrc.gov

T. Jerrell Carter, NRR

(301) 415-1153 Internet: tjc@nrc.gov

Attachment: List of Recently Issued NRC Information Notices

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96-xx

February xx, 1996 less than 25 percent of rated thermal if the flow is less th 10

percent of rated, does not follow from the information in t e Basis.

The flow related-safety limit addresses flow below 10 percent nd flow above

10 percent. Above 10-percent flow, the limit is stated in t ms of the

minimum critical power ratio (MCPR) and the MCPR safety lim . The Basis

states that there are sufficient data to validate the MCP correlation down to

about 10-percent flow. Thus, with power/flow informatf o for any given state

of the reactor, the relationship to the MCPR safety liht can be determined.

However, no relevant information for below 10-percent low is provided as a

basis for the stated 25-percent power safety limit. he basis (which, like

the specification, is essentially the same for mos BWRs, and the new standard

specification NUREG-1434) indicates that the critscal power ratio correlations

are valid down to about 10-percent flow, but doe~l not address the below-10-

percent-flow critical power correlations or an other relevant data or

analysis for that region. Instead, it argues hat minimum flow will be no

less than about 20 percent. It indicates, i brief, that a critical power

test was done at a flow of 3.5 kg/sec [28,0 0 lb/hrj, which is about 22 percent of rated flow for NMP1 (and is a milar range for other BWRs). That

flow was chosen because that is the lowe flow that was anticipated under

low-flow conditions as a result of the R flow geometry. At this flow, a

power of 50 percent was found to be within minimum critical power limits and

thus, it is stated, the specificatio of a 25-percent limit is conservative.

However, the referenced test was at 2-percent flow, and no information is

provided to validate the translati n to the 10-percent or lower flow

conditions, the flow range of the/low-flow specification. Thus, no logical

basis is provided to establish tfat operation is acceptable at 25-percent

power (or any other power) at ?f below 10-percent flow. This aspect of the

safety limit will be further amined by the NRC, and action may be taken to

correct NUREG-1434.

This information notice re uires no specific action or written response. If

you have any questions ab ut the information in this notice, please contact

one of the technical co acts listed below or the appropriate Office of

Nuclear Reactor Regula on (NRR) project manager.

Dennis M. Crutchfield, Director

Division of Reactor Program Management

Office of Nuclear Reactor Regulation

Technical conta s: Howard J. Richings, NRR

(301) 415-2888 T. Jerrell Carter, NRR

(301) 415-1153 Attachme List of Recently Issued NRC Information Notices

DOCUMENT NAME: A:\RICHINGS.IN

To receive a copy hdocument, Indicate inthe box: 'C' = Copy without enclosures 'E' = Copy with enclosures 'N = No copy

OFFICE P RBRP I SPLB/DSSA I AC:PECB/DRPM I D:DRPM I

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