Information Notice 1993-32, Nonconservative Inputs for Boron Dilution Events Analysis

Nonconservative Inputs for Boron Dilution Events Analysis
	ML031070505
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:	04/21/1993
From:	Grimes B; Office of Nuclear Reactor Regulation
To:	;
References
	IN-93-032, NUDOCS 9304150024
	Download: ML031070505 (9)
	v • d • e

UNITED STATES

NUCLEAR REGULATORY COMMISSION

OFFICE OF NUCLEAR REACTOR REGULATION

WASHINGTON, D.C. 20555 April 21, 1993 NRC INFORMATION NOTICE 93-32: NONCONSERVATIVE INPUTS FOR

BORON DILUTION EVENT ANALYSIS

Addressees

All holders of operating licenses or construction permits for pressurized

water reactors (PWRs).

purpose

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

notice to inform addressees that nonconservative assumptions have been

identified in the analysis of licensing basis boron dilution events. 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.

Background

The licensees for several Westinghouse-designed plants have used a boron

dilution mitigation system (mitigation system) to detect and mitigate a boron

dilution event in Modes 3 (hot standby), 4 (hot shutdown), and 5 (cold

shutdown). The mitigation system is designed to terminate automatically an

inadvertent boron dilution event occurring with the plant in these operating

modes before the reactor coolant system is diluted sufficiently to result in a

total loss of shutdown margin. A boron dilution event is detected by this

system by monitoring the signals of the source range monitors to determine if

the neutron flux has increased by a specified multiplication factor over a

prescribed time period. When a dilution event is detected, the mitigation

system isolates known dilution paths to the reactor coolant system and

realigns the reactor makeup water system to the refueling water storage tank

to initiate a re-boration.

NRC Standard Review Plan Section 15.4.6, 'Chemical and Volume Control System

Malfunction That Results in a Decrease in Boron Concentration in the Reactor

Coolant (PWR),6 specifies acceptance criteria for an unplanned boron

(moderator) dilution. These acceptance criteria specify that a dilution event

be terminated before total shutdown margin is lost. If operator action

is required during Modes 3, 4, and 5, these criteria specify at least

15 minutes notice be provided to operators from the time at which an alarm

announces an unplanned dilution to the time at which shutdown margin would be

eliminated (criticality). - &t#

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IN 93-32 April 21, 1993 In NRC Generic Letter 85-05, NInadvertent Boron Dilution Events," the staff

concluded that the consequences of boron dilution events would probably not

jeopardize the health and safety of the public and would not warrant

backfitting requirements. However, the generic letter urged each licensee to

verify that its plants have adequate protection against boron dilution events.

Description of Circumstances

In late 1991, while performing a quality assurance surveillance of

Westinghouse supporting calculations for the Comanche Peak Steam Electric

Station, the Texas Utilities Electric Company (the licensee) found that it

could not justify using a generic curve for the relationship between the

inverse count rate ratio and the reactor coolant system boron concentration, as was used in the Westinghouse boron dilution event analysis. This curve and

a source range flux multiplication setpoint of 2.0 (flux-doubling) were the

bases for detecting and mitigating an Inadvertent boron dilution event.- After

further reviewing this concern, in February 1992 the licensee determined that

the generic curve might not be bounding and that the analysis did not include

instrument uncertainties. These deficiencies may cause Comanche Peak to be

outside the licensing basis in the boron dilution event analysis which shows

that the boron dilution mitigation system response will prevent a return to

criticality. The licensee declared the mitigation system inoperable. While

the licensee is working on the long-term solution to correct the problem, it

put in place-the following interim actions: . -

1) Within 4 hours4.62963e-5 days 0.00111 hours 6.613757e-6 weeks 1.522e-6 months of entry into Modes 3, 4, or 5 from Modes 1, 2, or 6 (and

once per every 14 days thereafter while in Modes 3, 4, or 5), the

licensee will verify (unless startup is in progress) that those valves

that could contribute to the boron dilution event are closed and secured

in position; or

2) Within 4 hours4.62963e-5 days 0.00111 hours 6.613757e-6 weeks 1.522e-6 months of entering Mode 5, the licensee will ensure that only one

reactor makeup water pump (dilution source) is aligned to the supply

header. Following entry into Mode 3, 4, or 5 from Modes 1, 2, or 6, each

crew of the control room staff will receive a briefing to discuss the

type of reactivity changes that could occur during a boron dilution

event, the indications of a boron dilution event, and the actions

required to stop a dilution; commence immediate boration and establish

the required shutdown margin. For extended shutdowns, this briefing will

be repeated for each crew prior to resumption of control room duties

following an off duty period which exceeds 7 days. During time periods

when this option is used, the source range will be monitored for

Indication of unexplained increasing counts and inadvertent boron

dilution every 15 minutes.

Discussion

The inverse count rate ratio used in the Comanche Peak analysis of the boron

dilution event is based on the generic curve supplied by Westinghouse. The

IN 93-32 April 21, 1993 curve is used to predict the time at which a source range flux-doubling signal

would occur as criticality is approached during a boron dilution event. Using

actual Comanche Peak Unit 1 inverse count rate ratio data, the licensee

determined that the flux-doubling signal and alarm detected by the mitigation

system may not meet the acceptance criteria for the boron dilution event since

an appreciable amount of dilution would already have occurred before the

signal was generated.

The licensee also found that the flux multiplication setpoint of 2.0 does not

include the instrument uncertainties that should be applied to determine the

flux multiplication setpoint. These uncertainties include the statistical

variation of neutron flux, the variations in the linearity of the log level

amplifier as a function of counts per second, and uncertainties in numerous

amplifiers and the multiplexer. Thus, an actual flux signal could exceed the

flux-doubling setpoint used to analyze the boron dilution event. This

condition could result in a loss of shutdown margin before the mitigation

system terminates an inadvertent boron dilution event. (This problem may be

applicable not only to plants that rely on an automatic mitigation system, but

also to those that rely on a flux-doubling signal to generate an alarm for

operator intervention.) Alternatively, setting the setpoint too

conservatively could unnecessarily allow the mitigation system to be activated

and the reactor coolant system to be borated. This sequence of events would

have detrimental effects on operation.

On March 23, 1992, the staff met at NRC Headquarters'with'the'licensee and

Westinghouse to discuss issues discussed above. The licensee informed the

staff that it had investigated various alternatives and found none readily

available to restore the operability of the mitigation system but would

continue to evaluate this problem with Westinghouse. Later, on

December 15, 1992, the staff met with Westinghouse and licensees of

Westinghouse-designed plants which use the mitigation system. It was

concluded that the mitigation system was unreliable because of the large

uncertainties associated with the indication of a true flux doubling by the

nuclear instruments. Therefore, this problem may be common to all PWRs that

rely on a flux-doubling signal to mitigate a boron dilution event before

criticality can occur.

This information notice requires no specific action or written response. If

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

the technical contact listed below or the appropriate Office of Nuclear

Reactor Regulation (NRR) project manager.

Brian K. Grimes, Director

Division of Operating Reactor Support

Office of Nuclear Reactor Regulation

Technical contact: Larry Kopp, NRR

(301) 504-2879 Attachment: List of Recently Issued NRC Information Notices

Attachment

April

Pag 29te1 1992 Im

LIST OFRECEINLY ISSIJED

IN INFOUSTION NTICES

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UNITED STATES

NUCLEAR REGULATORY COMMISSION FIRST CLASS MAIL

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WASHINGTON, D.C. 20555-0001 USNRC

PERMIT NO. G-87 OFFICIAL BUSINESS

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TN 93-32

rl 21, 1993 K> *e 3 of 3 curve is used to predict the time at which a source range flux-doubling signal

would occur as criticality is approached during a boron dilution event. Using

actual Comanche Peak, Unit 1, inverse count rate ratio data, the licensee