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UNITED STATESNUCLEAR REGULATORY COMMISSIONOFFICE OF NUCLEAR REACTOR REGULATIONWASHINGTON, D.C. 20555-0001

December 5, 1997

**Addressees**

All holders of operating licenses for pressurized-water reactors (PWRs), except those licensees who have permanentlyceased operations and have certified that fuel has been permanently removed from the reactor vessel.

**Purpose**

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice to alert addressees to two eventsinvolving inadequate control of reactor coolant system (RCS) inventory. It is expected that recipients will review this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**

SEQUOYAH UNIT 1

At approximately 11 p.m. on March 23, 1997, with Sequoyah, Unit 1 in Mode 5 (cold shutdown), operators initiated adraindown of the unit from solid conditions intending to achieve a pressurizer level of approximately 25 percent. Operatorswere using cold-calibrated level instrumentation to monitor pressurizer level during the evolution, as specified in plantprocedures.

The draindown was interrupted at approximately 1:50 a.m. on March 24, 1997, to perform a diesel generator test and wasresumed at approximately 7 a.m. Throughout most of the draindown, the cold-calibrated level instrument provided levelindications later found to be inaccurate, but continued to trend down along with actual pressurizer level. Plant operators didnot recognize that the cold-calibrated instrument was providing erroneous indications and continued the draindown untilthe problem was discovered at approximately 7:45 a.m. when a member of the incoming operations crew recognized thatthe cold-calibrated instrument had stopped trending down and was holding steady at 38 percent level. The draindown wasstopped and charging flow was increased until the pressurizer was refilled to approximately 25 percent at 9:16 a.m.Approximately 3300 gallons of water were required to refill the pressurizer. During this event, shutdown cooling was notaffected and the core remained covered.

9712030278

The excessive draindown occurred as a result of erroneous level indications due to gas accumulating in the reference leg ofthe cold-calibrated instrument. The gas buildup occurred as a result of depressurization from approximately 325 psig toapproximately 30 psig just before initiating the draindown. The rapid depressurization caused gas within the RCS water tocome out of solution and displace approximately 182 inches of water from the instrument reference leg.

Hot-calibrated level instrumentation was also available to monitor pressurizer level during this evolution. Normally, fromsolid conditions to approximately 21 percent pressurizer level, the cold-calibrated level instrument reads lower than the hot-calibrated level instrument and, therefore, is the more conservative measure of pressurizer level. Both plant procedures andoperator training instruct the operator to monitor the cold-calibrated instrument during draindown from solid conditions to25 percent pressurizer level. During this event, however, the cold-calibrated instrument was reading higher than the hot-calibrated instrument due to the voided reference leg. Plant operators did not recognize that the relationship between thehot- and cold-level instruments was not correct and continued to rely on the cold-calibrated instrument as the primary levelindicator.

Another reactor coolant system draindown event occurred on Unit 1 in 1993 when approximately 10,000 gallons of waterwere drained from the RCS as a result of inaccurate level indication caused by drainage of the reference leg of the cold-calibrated level instrument. In this case, level was lost in the reference leg as a result of leakage past an instrument ventvalve. The corrective actions that were developed as a result of this event included modifications to plant proceduresinstructing the operators to utilize various pressurizer level indications during plant evolutions. However, thesemodifications were only implemented in the licensee's procedures for reduced inventory and not in the plant shutdownprocedures, which were in use during the event.

ZION UNIT 2

On March 7, 1997, with Unit 2 in cold shutdown, the licensee identified a gas bubble in the reactor vessel head. It wasdiscovered when an operator questioned the discrepancy between the computer trend plots for the volume control tank(VCT) and pressurizer levels. The head was vented and approximately 6900 gallons of water were required to refill thehead. It was later determined that vessel level had decreased to approximately 2.6 feet below the vessel flange.

The source of the gas was the nitrogen blanket on the VCT. As nitrogen is water soluble, the nitrogen gas present in thefree space of the VCT tends to dissolve in the VCT water volume until an equilibrium is established. Because the VCTtemperature was lower than that of the RCS, the solubility of nitrogen in the RCS was less than that of the VCT. Whenwater was transferred from the VCT to the RCS, the water was heated, causing nitrogen gas to come out of solution andaccumulate in the reactor vessel head.

Given the geometry and configuration of the facility, the plant conditions and the operating practices at that time, thepotential existed for a void to have been created in the reactor vessel that could have impacted decay heat removal.Additionally, the accumulation of gas in the steam generators could have prevented the preferred alternative method ofRCS cooling due to obstruction of primary natural circulation flow in the steam generators.

The Reactor Vessel Level Indicating System (RVLIS) was available but not in service during this event. The licensee'sprocedures did not require RVLIS to be in service and did not direct the operators to monitor RVLIS during this mode ofoperation (cold shutdown) nor during a loss-of-shutdown-cooling condition.

A precursor to this event occurred on Unit 1 in September of 1996 when Unit 1 experienced an unexpected gasaccumulation in the reactor vessel head. The licensee initiated a root cause investigation. However, approval of theinvestigation and implementation of the corrective actions were postponed by plant management pending completion of theongoing Unit 2 refueling outage. If the identified corrective actions had been implemented in a timely manner on bothunits, the March 7 event probably would not have occurred. Further, had the licensee more aggressively reviewed operatingexperience and applied industry lessons learned, both events might have been avoided.

**Discussion**

These events illustrate the importance of timely review of operating experience and prompt implementation of correctiveactions. In each case, the timely review of previous events and implementation of corrective actions for those events,including modifications to operating procedures and operator training (in particular, procedures and training stressing theimportance of monitoring and understanding the relationship between multiple level indicators) could have prevented thesesimilar loss-of-RCS-inventory events. In addition to monitoring level indications, routine inventory balances provideanother independent method to determine RCS inventory.

**Related Generic Communications**

Information Notice (IN) 96-65, "Undetected Accumulation of Gas in Reactor Coolant System and Inaccurate ReactorWater Level Indication During Shutdown," dated December 11, 1996.

IN 96-37, "Inaccurate Reactor Water Level Indication and Inadvertent Draindown During Shutdown," dated June 18, 1996.

IN 96-15, "Unexpected Plant Performance During Performance of New Surveillance Tests," dated March 8, 1996.

IN 95-03, "Loss of Reactor Coolant Inventory and Potential Loss of Emergency Mitigation

Functions While in a Shutdown Condition," dated January 18, 1995.

Bulletin 93-03, "Resolution of Issues Related to Reactor Vessel Water Level Instrumentation in BWRs," dated May 28,1993.

IN 94-36, "Undetected Accumulation of Gas in Reactor Coolant System," dated May 27, 1994.

IN 93-27, "Level Instrumentation Inaccuracies Observed During Normal Plant Depressurization," dated April 3, 1993.

IN 92-54, "Level Instrumentation Inaccuracies Caused by Rapid Depressurization," dated July 24, 1992.

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.