Information Notice 2013-14, Potential Design Deficiency in Motor-Operated Valve Control Circuitry

Potential Design Deficiency in Motor-Operated Valve Control Circuitry

ML13144A834
Person / Time
Issue date:	08/23/2013
From:	Laura Dudes, Kokajko L Division of Construction Inspection and Operational Programs, Division of Policy and Rulemaking
To:
Farnan M
References
IN-13-014
Download: ML13144A834 (5)

UNITED STATES

NUCLEAR REGULATORY COMMISSION

OFFICE OF NUCLEAR REACTOR REGULATION

OFFICE OF NEW REACTORS

WASHINGTON, DC 20555-0001 August 23, 2013 NRC INFORMATION NOTICE 2013-14: POTENTIAL DESIGN DEFICIENCY IN

MOTOR-OPERATED VALVE CONTROL

CIRCUITRY

ADDRESSEES

All holders of and applicants for an operating license or construction permit for a nuclear power

reactor under Title 10 of the Code of Federal Regulations (10 CFR) Part 50, Domestic

Licensing of Production and Utilization Facilities, except those that have permanently ceased

operations and have certified that fuel has been permanently removed from the reactor vessel.

All holders of and applicants for a nuclear power reactor early site permit, combined license, standard design certification, standard design approval, or manufacturing license under

10 CFR Part 52, Licenses, Certifications, and Approvals for Nuclear Power Plants.

PURPOSE

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice (IN) to alert

addressees to a potential control circuit design deficiency in motor-operated valves (MOVs) that

could result in incorrect valve position indication with the valve in an improper position during a

loss-of-coolant accident (LOCA). The NRC expects that recipients will review the information

for applicability to their facilities and consider actions, as appropriate, to avoid similar problems.

Suggestions contained in this IN are not NRC requirements; therefore, no specific action or

written response is required.

DESCRIPTION OF CIRCUMSTANCES

In an event report dated September 21, 2012, Exelon Generation Company notified the NRC

that several MOVs at Limerick Generating Station, Units 1 and 2 could remain partially open

following the initiation of an automatic isolation signal in response to a design-basis LOCA.

Specifically, when power is interrupted to the actuator of certain MOVs during the shedding of

loads associated with the plants as-designed LOCA response, the MOVs may not automatically

resume operation once power was restored. Additionally, the valve position indicating lights

would incorrectly indicate that the valves were fully closed when the actual valve position could

be as much as 15 percent open. Multiple primary containment isolation valves (PCIVs) in

different systems at Limerick, Units 1 and 2, were susceptible to this condition.

BACKGROUND

With IN 1985-20, Motor-Operated Valve Failures due to Hammering Effect, the NRC staff

summarized a design deficiency in which certain MOVs repeatedly cycled at the end of their

operating travel. The IN described a phenomenonknown as hammeringduring which

ML13144A834 relaxation of the gearing within the actuator of a closed MOV could lead to repeated attempts to

further close the valve as long as the MOV continued to receive a valve-close demand signal.

Such a continuing signal might occur during a sealed-in accident signal (e.g., containment

isolation signal) or if a plant operator held the control switch in the closed position. MOV

hammering can lead to burn-out of the MOV motor or damage to the valve and actuator. MOV

hammering can also force the valve disc into the seat such that re-opening of the valve is

difficult.

Supplement 1 to IN 1985-20 describes certain types of MOVs that are susceptible to

hammering. In particular, MOVs with low gear ratios that are commonly used in applications

requiring high-speed valve actuation are susceptible to hammering. After these MOVs are

closed and their torque switch contacts open, internal forces exerted through the torque switch

pinion and Belleville washer assembly within the MOV actuator can cause the torque switch to

reclose and resupply power to the MOV motor because the actuator gearing does not provide

sufficient resistance to motion. Consequently, MOVs that use these gear sets are characterized

as nonlocking. Conversely, MOVs with high gear ratios can resist the internal forces on the

actuator gearing; therefore, they do not allow the torque switch to re-close and re-supply power

to the motor. MOVs with high gear ratios are not susceptible to hammering and are

characterized as locking.

IN 93-98, Motor Brakes on Valve Actuator Motors, the NRC staff summarized an MOV issue

related to motor brakes installed in MOVs to minimize the inertial loads during valve closure

after control switch trip. Motor brakes can be used to help avoid hammering in MOVs with

nonlocking gear mechanisms. However, improper sizing and operation of motor brakes might

not prevent hammering, and can result in problems with the performance of the MOV as

discussed in IN 93-98.

DISCUSSION

Certain system designs may require high-speed valve actuation and, thus, necessitate the

application of MOVs with actuator gearing that is nonlocking. In these cases, licensees or

vendors may have designed features within the MOV circuitry to avoid the hammering issue.

One approach to avoid MOV hammering, such as implemented at Limerick, relies on a limit

switch contact to serve as an isolation permissive function. When the isolation permissive limit

switch contact is closed, a sealed-in signal to close the valve (e.g., containment isolation) would

be allowed to energize the valve close circuit. Once the valve reaches a certain position in its

travel, the isolation permissive limit switch contact opens. During the continued MOV operation, the MOV circuitry is designed to allow current to flow around the open limit switch contact. After

the torque switch opens, power is interrupted to the MOV motor and the valve travel stops.

If the torque switch re-closes inadvertently (such as by relaxation of the actuator gearing), the

MOV circuitry does not allow the current to flow around the open limit switch contact, and power

is not restored to the MOV motor. The MOV circuitry can be designed to allow power to be

restored to the MOV motor when valve operation is needed, such as by use of the MOV hand

switch in the control room.

In a recent 10 CFR 50.72 Immediate Notification Requirements for Operating Nuclear Power

Reactors, event notification (EN No. 48334), Exelon reported that several MOVs at Limerick, Units 1 and 2, that used an isolation permissive limit switch could potentially remain partially

open during the plants designed response to a LOCA. The followup 10 CFR 50.73 License

Event Report System, licensee event report (LER No. 05000352-2012007, Agencywide Documents Access and Management System (ADAMS) Accession No. ML12293A100)

provides further details on the MOV vulnerability that was discovered during a licensee-led

evaluation of electrical system voltage that would be expected to occur during a LOCA. The

evaluation identified that the MOVs with an isolation permissive limit switch setting were set to a

value of 5 percent to 15 percent of open travel. This represents a dead zone in the valve close

circuitry. The licensee determined that if power is interrupted to the affected valve actuators

after the isolation permissive limit switch contact opened, but before the valve reached its

closed position, the affected PCIVs could potentially remain as much as 15 percent open. This

condition could occur if, during the plants designed response to a LOCA, the load shed

sequence occurred when a valve was within the dead zone. In this case, once power was

sequenced back to the MOV, it would not resume motion because the isolation permissive limit

switch contact would be open, thus, preventing the actuation signal from reaching the MOV

motor. Furthermore, the valve indicating lights would indicate fully closed because the close

position indicating light contacts share the same limit switch rotor as the isolation permissive

limit switch contacts, and the valve would indicate closed when entering the set dead zone. The

conditions just described could leave a valve open by as much as 15 percent, although the

valve position indication would indicate closed.

Upon discovery of this condition, the licensee declared the PCIVs inoperable and implemented

design changes to remove this vulnerability in MOVs at Limerick, Units 1 and 2. The licensees

corrective actions are described in the referenced LER 05000352-2012007. This LER provides

further details on this issue and the licensees response. The licensee is considering long-term

corrective action to modify the MOV gearing for the affected MOVs to install locking gear sets to

prevent potential torque switch hammering and to allow removal of the limit switch permissive

circuitry. This would allow the MOVs to restart during a LOCA power restoration sequence to

fully close the valves and to provide accurate valve position indication. This design change

would also obviate the need for the affected limit switch contacts for these valves, thereby

eliminating the dead zone and the root cause of the issue.

This issue and LER were reviewed by NRC inspectors and dispositioned as a licensee- identified, non-cited violation, of very low safety significance. This LER was closed in the

Limerick Generating Station NRC Integrated Inspection Report 05000352/2012005 and

05000353/2012005, dated February 5, 2013; see page 35, section 4OA3.2 and page 40,

section 4OA7 of the inspection report for more details (ADAMS Accession No. ML13036A364).

In this licensees particular situation, the issue was determined to be of very low safety

significance. However, similar problems at other licensees might have greater safety

significance.

The NRC expects that recipients will review the information, links, and references provided in

this IN for applicability and consider actions, as appropriate for their facilities to avoid similar

problems. However, no specific action or written response to the NRC is required for this IN.

CONTACT

This information notice does not require any 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 NRC project manager.

/RA/ /RA/

Laura A. Dudes, Director Lawrence E. Kokajko, Director

Division of Construction Inspection Division of Policy and Rulemaking

and Operational Programs Office of Nuclear Reactor Regulation

Michael Farnan, NRR

301-415-1486 E-mail: Michael.Farnan@nrc.gov

Note: NRC generic communications may be found on the NRCs public Web

site, http://www.nrc.gov, under NRC Library/Document Collections.

ML13144A834 *via email TAC 1740

Region-1 OFFICE NRR/DE/EPTB Tech Editor NRR/DE/EPNB NRR/DE NRR/DORL

Limerick SRI

NAME MFarnan CHsu* EDiPaolo* TLupold PHiland MEvans

DATE 7/ 16 /13 7/ 19 /13 7/ 25 /13 7/ 16 /13 7/ 17 /13 7/ 23 /13 NRR/DPR/PGCB NRR/DPR/PGCB NRO/DCIP/ D NRR/DPR/DD NRR/DPR/D

OFFICE NRR/DPR/PGCB

NAME CHawes MKing D Pelton LDudes SBahadur LKokajko

DATE 7/ 25 /13 7/ 25 /13 8/ 15 /13 8/ 19 /13 8/ 22 /13 8/ 23 /13