Information Notice 1992-39, Unplanned Return to Criticality During Reactor Shutdown

Unplanned Return to Criticality During Reactor Shutdown
	ML031200314
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:	05/13/1992
From:	Rossi C; Office of Nuclear Reactor Regulation
To:	;
References
	IN-92-039, NUDOCS 9205060036
	Download: ML031200314 (12)
	v • d • e

UNITED STATES

NUCLEAR REGULATORY COMMISSION

OFFICE OF NUCLEAR REACTOR REGULATION

WASHINGTON, D.C. 20555 May 13, 1992 NRC INFORMATION NOTICE 92-39: UNPLANNED RETURN TO CRITICALITY DURING

REACTOR SHUTDOWN

Addressees

All holders of operating licenses or construction permits for nuclear power

reactors.

Purpose

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

notice to alert addressees to recent events involving unplanned returns to

criticality caused by the cooldown of the reactor coolant system during reactor

shutdowns. 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

The licensees for the following plants recently experienced unplanned

criticalities during reactor shutdowns: the Monticello Nuclear Generating

Plant, the Big Rock Point Nuclear Plant, and the Grand Gulf Nuclear Station.

Monticello

On June 6, 1991, an unplanned criticality occurred at the Monticello Nuclear

Generating Plant during a reactor shutdown when an unanticipated cooldown

occurred in conjunction with rod insertion. The licensee initiated this

shutdown to repair a leaking safety-relief valve shortly after the plant was

returning to power following a refueling outage.

Since this shutdown occurred shortly after the cycle startup, the reactor did

not generate sufficient decay heat to produce the steam needed to supply the

normal steam system loads and still maintain pressure in the reactor. As

control rods were being inserted using notch insertion, the reactor coolant

system pressure and temperature began to decrease. As long as the operator

continued to insert control rods, the reactor remained subcritical.

However, when the operator stopped inserting control rods to review and

evaluate plant conditions, the cooldown continued, adding sufficient

reactivity to overcome the negative reactivity from the insertion of control

rods and causing the reactor power to increase. The reactor power continued to

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IN 92-39 May 13, 1992 increase until an intermediate-range monitor (IRM) tripped on a high-high flux

setpoint, causing a reactor scram.

Further details of the event may be found in Licensee Event Report

50-263/91-15 or NRC Inspection Report 50-263/91-13.

Big Rock Point

On November 30, 1991, an unplanned criticality occurred at the Big Rock Point

Nuclear Plant during a reactor shutdown when a cooldown occurred in conjunction

with a rod insertion. The event occurred during a scheduled shutdown for a

refueling outage.

After taking the main turbine off line and bringing the reactor to a

subcritical condition, the licensee delayed actions to continue inserting

control rods while changing shifts. However, the reactor coolant system

continued to cool down because the normal steam system loads were removing

heat. The cooldown added sufficient reactivity to eventually overcome the

effects of the control rods, and the reactor returned critical. The operators

noticed the condition in approximately 2 minutes and took the appropriate

actions to return the reactor subcritical.

Further details of the event may be found in Licensee Event Report

50-155/91-OO9-orJNRC-Inspection-Report 50-155/91-25. _ _ _

Grand Gulf

On December 30, 1991, an unplanned criticality occurred at the Grand Gulf

Nuclear Station during a reactor shutdown when a cooldown occurred in

conjunction with a rod insertion. The event occurred during a scheduled

shutdown for a maintenance outage to replace a recirculation pump shaft. To

minimize the release of plated-out radioactive material from the fuel cladding

and channels to the reactor coolant system, a slow power reduction and a slow

cooldown were prescribed for this shutdown.

The slow power reduction and the effects of an earlier outage minimized reactor

decay heat and xenon peaking. The operators were driving rods into the core

individually because gang drive for the control rods was unavailable. This

made the reactivity insertion slower than normal. The operators inserted

rods to reduce flux to range 3 on the IRMs and stopped rod insertion in order

to perform a source range monitor surveillance. Indicated power reached IRM

range 1. However, the reactor coolant system continued to cool down because

the normal steam system loads were removing heat. In order not to affect the

source range monitor surveillance, the shift supervisor elected to not insert

control rods and alerted the operators to the possibility of a return to

criticality as the cooldown continued. The reactivity added by the cooldown

eventually overcame the effects of the control rods. The reactor returned to a

critical condition, and reactor power increased on a reactor period between 300

to 800 seconds. The operators, having been trained on a similar event at

another BWR, were monitoring the condition and expected the power to increase

to the point of adding heat, where the fuel and moderator temperature coeffi- cients would halt the reactor power increase. As the power increased, the

IN 92-39 May 13, 1992 reactor operator ranged the IRMs to prevent a reactor scram. Power level

remained on scale. The power increase was terminated with the IRMs reading on

range 7 and 8. The operators completed the source range monitor surveillance, then resumed inserting rods and successfully completed the shutdown.

Further details of the event may be found in NRC Inspection Report

50-416/92-04 and Licensee Event Report 50-416/91-16.

Discussion

The underlying principle of reactivity management is to maintain the reactor

in the desired condition by properly anticipating, controlling, and responding

to the plant's changing parameters. The experience at Monticello, Big Rock

Point, and Grand Gulf indicates that shutdowns in situations with low decay

heat present a unique challenge to reactivity control if inadvertent recritica- lity is to be prevented. These events emphasize the importance of the operator

giving continuous attention to plant parameters during a shutdown. Further, accepting recriticality without prior management approval and procedures, i.e.,

an ad-hoc approach, raises concerns due to the lack of opportunity for contin- gency planning.

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.

Va&res'E.Rossi, Director

Division of Operational Events Assessment

Office of Nuclear Reactor Regulation

Technical contacts: Melvyn Leach, RIII

(708) 790-5559 Geoffrey Wright, RIII

(708) 790-5695 Paul O'Connor, NRR

(301) 504-1307 Attachment: List of Recently Issued NRC Information Notices

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IN 92-39

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LIST OF RECENTLY ISSUED

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0 92-38 Implementation Date for 05/12/92 All holders of OLs or CPs

the Revision to the EPA for nuclear power reactors, Manual of Protective non-power reactors and

Action Guides and Pro- materials licensees auth- tective Actions for orized to possess large

Nuclear Incidents quantities of radioactive

material.

1 92-37 Implementation of the 05/08/92 All Nuclear Regulatory

Deliberate Nisconduct Rule Commission Licensees.

92-16, Loss of Flow from the Re- 05/07/92 All holders of OLs or CPs

Supp. 1 sidual Heat Removal Pump for nuclear power reactors.

during Refueling Cavity

Draindown

I 92-36 Intersystem LOCA Outside 05/07/92 All holders of OLs or CPs

Containment for nuclear power reactors.

' 92-35 Higher Than Predicted Ero- 05/06/92 All holders of OLs or CPs

sion/Corrosion in Unisol- for nuclear power reactors.

able Reactor Coolant Pres- sure Boundary Piping Inside

Containment at A Boiling

Water Reactor

92-34 New Exposure Limits for

Airborne Uranium and

Thorium

05/06/92 All licensees whose opera- tions can cause airborne

concentrations of uranium

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and thorium.

92-33 Increased Instrument 04/30/92 All holders of OLs or CPs

-u Response Time When for nuclear power reactors.

0 Pressure Dampening

rn> Devices are Installed

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D2 zzz> Operating License

u) D- I CP - 'Construction Permit

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