05000397/LER-1989-001, :on 890112,discovered Four New Unanalyzed Failure Modes in Containment Nitrogen Inerting Sys.Caused by Inadequate Design Procedures.Design Change Initiated to Install low-temp Cutoff Device

:on 890112,discovered Four New Unanalyzed Failure Modes in Containment Nitrogen Inerting Sys.Caused by Inadequate Design Procedures.Design Change Initiated to Install low-temp Cutoff Device

ML20086A789
Person / Time
Site:	Columbia
Issue date:	11/14/1991
From:	Arbuckle J, John Baker WASHINGTON PUBLIC POWER SUPPLY SYSTEM
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
GO2-91-207, LER-89-001, NUDOCS 9111190361
Download: ML20086A789 (8)

LER-1989-001, on 890112,discovered Four New Unanalyzed Failure Modes in Containment Nitrogen Inerting Sys.Caused by Inadequate Design Procedures.Design Change Initiated to Install low-temp Cutoff Device

Event date:
Report date:
Reporting criterion:	10 CFR 50.73(a)(2)
3971989001R00 - NRC Website

text

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WASHINGTON PUllLIC POWER SUPPLY SYSTEM l' O. flas 968

• 3n00 George \\Yashington \\Yuv

• Hi<hland. \\Yashington 99352 Docket No.

50-397 November 14, 1991 G00-91-207 Document Control Desk U.S. Nuclear Regulatory Commission Washington, D.C.

20555

Subject:

NUCLEAR PLANT NO. 2 LICENSEE EVENT REPORT NO. 89-001-02

Dear Sir:

Transmitted herewith is Licensee Event Report No. 89-001-02 for the HNP-2 Plant.

Inis report is submitted in response to the report requirements of 10CFR50.73 and discusses the items of reportability, corrective action taken, and action taken to preclude recurrencc.

Very truly yours,

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HNP'2 Plan Man ger CMP:Ig

Enclosure:

Licensee Event Report No. 89-001-02 cc:

Mr. John B. Martin, NRC - Region V Mr. C. Sorensen, NRC Resid(nt Inspector (M/D 901A)

INPO Records Center - Atlanta, GA Ms. Dottle Sherman, ANI Mr. D.L. Hilliams, BPA (H/D 399)

NRC Resident Inspector - Halk Over Copy 1~li ?- 2.

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On January 12, 1989, during an engineering evaluation of the Containment Nitrogen inerting (CN) System, four new unanalyzed failure modes were discovered.

These fdilure modes all have the potential to impact safety reldted equipment required to attain safe shutdown of the reactor.

A summary of the failure modes is os follows:

1)

Failure Mode 1 - Loss of auxilidry steam or pressure control to the "high flow'* nitrogen line, 2)

Failure Mode 2 - A break in the " low flow" nitrogen line or loss of electric heater on the " low flow" line 3)

Failure Mode 3 - A tornado missile causes failure of liquid nitrogen storage tank and/or associated piping 4)

Failure Mode 4 - Non-mechdnistiC rupture of liquid nitrogen stordge tank or liquid lines beneath the tank.

failure Modes 1 and 2 involve potential damage to safety related compenents due to contoct with liquid nitrogen and/or low temperatures.

Failure Modes 3 and 4 involve the potential for oxygen starvation of all three divisions of emergency diesel ;enerators under certain low probability conditions, immediate corrective action was to modify procedures to require additional operator coverage and provide specific guidance to ensure correct response to failure mode conditions.

The root cause of the event was determined to be inadequate design procedures in effect at the time that this system was originally designed.

As a result of an ingineering evaluation, a design change was initiated to instdll low temperature isolation valves in both the low and high flow sides of the CN system.

This event posed no threat to the health and safety of either the public or Plant personnel.

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Event Description

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On January 12, 1989 as a result of an engineering evaluation of the Containment Nitrogen Inerting-(CN) system, four new failure modes of this non-safety related system were' identified.

This condition was discovered during performance of corrective action for-the event reported in LER 88-034-00 " Pipe failure Caused by-

[

Introduction of liquid Nitrogen into the Primary Containment Supply Purge Piping Due to Personnel / Design / Maintenance Problems"._ An engineering review of the CN System

- design, being performed to investigate potential f ailure modes, found four new f ailure modes which could result in the degradation of safety related equiptrent as summarized below (see attached diagram figure 1):

a)-

Failure Mode 1 - Loss of auxiliary steam or nitrogen pressure control (failure of high N2 flow pressure control valve CN-PCV-6) resulting'in liquid nitrogen i-being introduced into the "high flow" Inerting header.

This header is not

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capabic of withstanding cryogenic temperatures.

i-b)

Failure Mode 2 - A break in the " low flow" line or loss of electric heater on i.

the " low flow" line when this line is supplied from the bottom of the liquid Nitrogen Tank CN-lK-1.

Again, liquid nitrogen is introduced into piping not designed for cryogenic temperatures, i

c)

Failure Mode 3 - A tornado missile that causes failure of the exposed CN-TK-1

'and associated piping outside the buildings that results in release of-large 4

quantities of liquid nitrogen which might starve the emergency diesel 4

generators of oxygen.

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Failure Mode 4 - Non-mechanistic rupture of the Liquid Nitrogen Storage Tank

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CN-TK-1-_ or the liquid lines beneath the tank that might result in oxygen starvation of the emergency diesel generators.

At 1500 hours0.0174 days <br />0.417 hours <br />0.00248 weeks <br />5.7075e-4 months <br />, these new failure modes were identified as conditions which alone could have-prevented the fulfillment of the safety func tiot, of the Emergency AC

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Electrical Power Distribution System..At 1701 hours0.0197 days <br />0.473 hours <br />0.00281 weeks <br />6.472305e-4 months <br />, the NRC Operations Center was notified via the Emergency Notification System that this condition was reportable 7

under the four hour report requirement of 10CFR50.72(b)(2)(lii).

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- Immediate Corrective Action

~As'a-result of actions taken during the previous CN System event reported in-LER 88-034-00, the system operating procedure had been modified to require an operator to be stationed at the CN System skid whenever high volume containment inerting was being performed.

Guidance was glweis to the operators via night orders that the only

- duty-of this_ additional operator was to isolate the nitrogen supply line if nitrogen effluent temperrture dropped to 30'f.

lhis action was to limit entry of liquid nitrogen into piping-not-designed for extreme low temperature use in response to failure Modes l'and 2.

Additionally, a night order was issued which requires an alarming oxygen monitor to be used whenever taking-suction with the Control Room Ventilation system from the remote air intake nearest the CN skid without use of the normal system intakes.

This measure was taken to mitigate the effects of possible reduction in oxygen content of control room air due to induction of nitrogen which potentially could be r: dased'in large quantity as a result of CN System failure in response to failure Hode 4.

Further Evaluation and Corrective Ac!lon

(

Further Evaluation 1.

This event is being reported as an event or condition which alone could have-prevented the fulfillment of the safety function of structures or systems that are needed to :

(A)

Shut down the reactor an maintain it in a safe shutdown condition per the requirements of 10CfR50.73(a)(2)(V).

E.

1he root cause of'this event was determined to be inadequate design procedures which were in existence at the time that the Containment Nitrogen-Inerting System was originally designed.

During intial plant construction and-startup, the design procedures then in ef fect did not cause single f ailure analyses or-break pipe analyses to be performed on these types of non-safety related systems.

The CN System was not reviewed as a system which supported safety related equipment.

Also, dt that time, no piping outside the " Power Block" was considered-for analysis.

Based on this root cause, no additional corrective action is necessary.-

In light of the improvements to-the design procedures which have been made since.

that time, the probability for recurrence has already been signif icantly minimized, lhe self-initiated Safety System functional Inspection being i

performed by the Supply System Engineering Assurance Group will continue to-j

. attempt to discover additional system design deficiencies of a similar nature, i

Therefore, no additional corrective action is needed to examine other systems.

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3.

The potential for the four unanalyzed failure modes and the compensdtory I

measures presenting an unreviewed safety question was recognized.

An ar'olysis per 10CFR50.59 was completed which concluded that an unreviewed saf ety quest ion s'

did not exist.

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There were no structures, components or systems that were inoperable at the start of the event that contributed to the event.

5.

A comprehensive engineering calculation was perfornied pertaining to a non-mechanistic rupture of the Liquid Nitrogen Storage lank (f ailure Mode 4).

The results of that calculation, which was perf ornied by a contrac tor, are presented in the Safety Significance section of this LtR.

Further Corrective Actlon An Engineering evaluation was perf ormed to determine the need for design changes to the CN System. As a result of the evaluation, a de5ign change was initiated to install low temperature cut-off devices (isolat.lon valves) in both the low and high flow sides of the system.

The low temperature isolation valves have been installed in the low flow side of the system.

Installatiori of the isolation valves in the high flow side of the system is also complete; however, procedural restrictions have been established to operate the valves manually pending resolution of problems associated with the automatic actuation switches.

The isolation valves will provide adequate protection against Failure Mode 1 and 2 events by isolating the nitrogen in the storage tank any time the flowing nitrogen temperature f alls below -20*f on the high flow side and -30*F on the low flow side.

Safety Significance

Any potential for adverse safety consequences as a result of this event occurred during the time between initial startup of the CN System and the time compensatory measures were implemented.

If the low probability condition posed by f ailure Modes 1 or 2 had occurred during this period in conjuction with a Design Basis Loss of Coolant Accident (itself a low probability event), it is possible that the ability l

to shut down the reactor and maintain it in a safe shutdown condition could have t c compromised.

Neither f ailure Mode 3 nor Failure Mode 4 represents a credible

(/ent which could have occurred during this period.

Since the postulated CN System f ailure modes did not occur, no adverse safety significant consequences actually resulted.

The following is a sumri)ary of the safety significance for each postulated f ailure mode:

o failure Modes I and 2 - Both failure modes potentially could have caused piping breaks within the various rooms in the Diesel Generator and Reactor Buildings through which these lines are routed.

Consequently, the safety related equipment in these rooms (primarily emergency diesel generators and related l

components) could have been exposed to liquid nitrogen resulting in conditions beyond the design capabilities of this equipment.

Without compensatory measures applied to avert these failure modes, the resultant impact on safety related equipment could have compromised the ability of the equipment to achieve its design f unc tion.

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f ailure Mode 3 - A tornado which produces a missile suf ficient to topple arid /cr ruuture the CN Liquid Nitrogen Storage lank and associated piping would include enough wind to provide mixing and dilution to the point that the emergency diesel generators would not be starved for oxygen.

lhe ability of the reactor to achieve safe shutdown is not compromised in this failure mode, o

failure Mode 4 - In accordance with the guidelines of Standard Review Plans 3.6.1 and 3.6.2, a moderate encrgy crack was assumed to occur in the piping associated with the nitrogen tank.

The analysis completed for this event indicates that the only Plant equipment directly af f ected by the crack and subsequent nitrogen release were the three emergency diesel generators.

Dispersion analysis for the most limiting crack size could result in a loss of the three diesel generators for up to eight minutes. However, from a safe shutdown analysis perspective, this is acceptable because a loss of offsite power need not be assumed concurrent with an assumed moderate energy pipe crack if the event does not lead to a reactor scram or turbi je trip.

Because the postulated crack and resultant conserences do not result in a reactor scram or i

turbine trip, offsite power remains o.allable and reliance on the diesel generators is rat required.

The potential for the loss of nitrogen supply to indirectly cause a turbine trip or reactor scram due to loss of the nitrogen supply to the Main Steam Isolation Valves (MSIVs) was also evaluated, lhe design of the MSIV system is such that the check valves and accumulators have been shown by testing to maintain adequate nitrogen supply pressure for at least 60 minutes to hold the MSIVs open.

A loss of nitrogen pressure would also alarrii in the Control Room and operator response to such an alarm would be to line up alternate pressurized air / nitrogen supplies, thereby, preventing MSIV isolation.

As a result, the safety significance associated with this failure mode is minimal due to 1) the short time (eight minutes) the emergency diesel generators would be inoperable, 2) the design characteristics of the MSIV nitrogen supply, and 3) operator actions to prevent MSIV closure.

Accordingly, a non-mechanistic failure of the liquid nitrogen storage tank would not compromise the ability to shutdown the reactor and maintain it in a safe shutdown condition.

This event posed no threat to the health and safety of either the public or Plant personnel.

Similar Events

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