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78

Information Notice No. 93­78: Inoperable Safety Systems at a NonPower Reactor

UNITED STATES

NUCLEAR REGULATORY COMMISSION

OFFICE OF NUCLEAR REACTOR REGULATION

WASHINGTON, D.C. 20555

October 4, 1993

NRC INFORMATION NOTICE 93‐78: INOPERABLE SAFETY SYSTEMS AT A NON‐POWER

REACTOR

Addressees

All holders of operating licenses or construction permits for test and

research reactors.

Purpose

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

notice to alert addressees to a problem which resulted in inoperable safety

systems at a non‐power reactor. 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

On April 28, 1993, after a number of spurious scrams had occurred, the

licensee for the University of Virginia Research Reactor shut down the reactor

and began troubleshooting activities to determine the cause of the problem.

The reactor had been experiencing spurious scrams for some time and the

licensee had concluded that the scrams were not caused by electrical

transients or line noise. The licensee suspected the reactor scram logic to

be the cause of the scrams. The reactor scram logic consists of two trains

that cause power to different magnets to be cut off on receipt of a scram

signal. This in turn releases control rods and shuts down the reactor. While

investigating the problem, the senior reactor operator (SRO) interchanged

components of the scram logic system between the two trains. Among these

components were solid‐state relays and mixer‐drivers (M/Ds) which act as a

28‐channel "OR" gate in the scram logic. After consulting the facility safety

analysis report and visually inspecting the exterior of the components, the

SRO assumed that the interchanged components were identical. However, the

M/Ds had been internally modified in the 1970s to tie unused inputs together

and were no longer identical.

After the M/Ds had been exchanged and no spurious scram signals were received

for 30 minutes, the reactor administrator and the SRO started the reactor.

Neither the reactor administrator nor the SRO recognized that the
troubleshooting activities were actually a maintenance or modification

function and that testing to verify the operability of the reactor scram

system was required before the reactor was restarted.

9309290007.

IN 93‐78

October 4, 1993

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After operating for approximately 5� hours, the reactor was shut down by

driving the rods into the core. With three of the four control rods seated,

licensee personnel then introduced a scram signal into the reactor

electronics using the intermediate range channel switch. However, the

electronics did not generate a scram signal as expected. The licensee

investigated and found that the M/Ds were not internally identical as

described above. Further investigation showed that the following scram

signals were inoperable: the two power level scrams, intermediate range

period scram, primary coolant low flow scram, loss of power to the primary

pump scram, intermediate channel range switch scram, and key switch scram.

The licensee returned the M/Ds to their original positions and tested the

reactor to ensure that the reactor electronics had not been damaged.

To prevent a recurrence of this problem, the licensee made the following

changes: (1) revised the facility standard operating procedures (SOPs) to

clearly define maintenance and troubleshooting activities, (2) added a

checklist to the SOPs to specifically control maintenance activities, and

(3) added a checklist to the SOPs to verify the operability of the reactor

safety systems after an unplanned reactor scram. Other changes were also made

to the SOPs to ensure management control over maintenance.

The licensee checked the reactor electronics against the schematics and found

that changes had been made to console modules that were not reflected in the

schematics. The licensee found two instances where externally identical

components were internally different and therefore not interchangeable. After

completing this check, the licensee labeled the modules that had been modified

to clearly indicate that they were unique and not interchangeable.

Discussion

The circumstances described above demonstrate the importance of recognizing

and controlling maintenance and modification actions. Clearly defining

maintenance activities in facility procedures and providing training to

personnel can be effective methods for controlling such activities. Also

important to reactor safety is updating schematics of reactor electronics to

reflect modifications to safety systems. Performing testing of the affected

safety systems after activities of the type described above may prevent the

occurrence of similar events.

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IN 93‐78

October 4, 1993

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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.

/s/'d by AEChaffee/for
Brian K. Grimes, Director

Division of Operating Reactor Support

Office of Nuclear Reactor Regulation

Technical contacts: A. Adams, Jr., NRR

(301) 504‐1127

C. Bassett, RII

(404) 331‐5570

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