ML20212B760

Requests for Technical Review of Encl Draft in 97-076 Re Inadvertent Draindown at Plant,Unit 1

ML20212B760
Person / Time
Site:	Sequoyah
Issue date:	10/21/1997
From:	Denning R NRC (Affiliation Not Assigned)
To:	Kingsley O TENNESSEE VALLEY AUTHORITY
References
IEIN-97-076, IEIN-97-76, NUDOCS 9710280194
Download: ML20212B760 (7)

Text

. . - . . - - . - . . - - _ - - . . . . -. .- ._- - ...-.-.-- -~.-.-

October 21, 1997 Mr, Oliver D. Kingsty, Jr.

J. Pr

sident, TVA Nucle:r cnd Chhf Nucle:r Officer 16A Lookout Place-

.i 1101 Market Street

• Chattanooga, Tennessee 37402 2801 i

SUBJECT:

REQUEST FOR A TECHNICAL REVIEW OF A DRAFT lt3FORMATIOil NOTICE l REGARDING INADVERTENT DRAINDOWN AT SEQUO'/AH UNIT 1

Dear Mr. Kingsley:

-

The U.S. Nuclear Regulatory Commission is planning to issue an information notice (IN) discussing

the inadvertent draindown event which occurred at Sequoyah Unit 1 on March 24,1997, We ask that j you review the enclosed draft of that IN to ensure the technical information regarding the event is accuratec Your cooperation in this matter is appreciated. Please retum any comments you may have by close of business on October 31,1997. A copy of this request and your response will be placed in the Public Document Room for review by the public. Your response should be mailed to

U.S. Nuclear Regulatory Commission ATTN

William Burton '

, MAllSTOP: 0-11E4 Washington, D.C. 20555 0001 4

Please address any questions you may have on this matter to William Burton of my staff. Mr. Burton can be reached at 301415-2853. If no comments are received by the close of business on i October 31,1997, we will assume the technical information in the notice is correct.

- Sinccely, s I. toriginal signed byt l Robert L. Dennig, Chief Events Assessment and Generic Communications Section Events Assessment, Generic Communications, and SpecialInspections Branch

Division of Reac;or Program Management l- Office of Nuclear Reactor Regulation Docket Nos. 50 327 j/

4 and 50 328 Enclosute: Draft Informa! Ion Notice gg

! cc witncl: See next page -

i- DISTRIBUTION: M, PUBLIC PD23 Rdg/ File

! PECB R/F JWRoe SARichards BBoger RLDc nig ' WFBurton RWHeman FHebdon BClayton . _

OGC ACRS JJohnson, Rll

- DOCUMENT NAME: G;\WFB\TVA x

I . To remive a copy of this docurnent, incicate in the box C= Copy w/o attachment / enclosure E= Copy with attachment / enclosure N = No copy

h0FFICE PEC8 PM:PD23 4 _ SC. pdm)) C:PECB b IhNAME WBurton b)d RWHeman b//h RDe g SRichards YDATE- 10/j/97 - =

10@97 10[/07 -10d/97 7+

E OI fiCIAL RECOR'J COPY t

J ,

1. - noo:

-9710280!?4 971021  ?

, hh hk) h f PDR ADOCK 0501,0327 g ri PDR

Mr. Oliver D. Kingsley, Jr. SEQUOYAH NUCLEAR PLANT Tennessee Valley Authority cc:

Mr. O. J. Zeringue, Sr. Vice President Mr. Pedro Salas, Manager Nuclear Operations Licensing and Industry Affairs Tennessee Valley Authority Sequoyah Nuclear Plant 6A Lookout Placa _ Tennessee Valley Authority 1101 Market Street P.O. Box 2000 Chattanooga, TN 37402-2801 Soddy Daisy, TN 37379 Mr. Jack A. Bailey Mr. J. T. Herron, Plant Manager Vice President Sequoyah Nuclear Plant Engineering & Technical Services Tennessee Valley Authority Tennessee Valley Authority P.O. Box 2000 6A Lookout Place ' Soddy Daisy, TN 37379 1101 Market Street

- Chattanooga, TN 37402-2801 Regional Administrator U.S. Nuclear Regulatory Commission Mr. Masoud Bajestani Region 11 Site Vice President- 61 Forsyth Street, SW.

Sequoyah Nuclear Plant Suite 23T85 Tennessee Valley Authority Atlanta, GA 30303-3415 P.O. Box 2000 Soddy Daisy, TN 37379 Mr. Melvin C. Shannon Senior Resident inspector General Counsel Sequoyah Nuclear Plant Tennessee Valley Authority U.S. Nuclear Regulatory Commission ET 10H 2600 Igou Ferry Road 400 West Summit Hill Drive Soddy Daisy, TN 37379 Knoxville,TN 37902 Mr. Michael H. Mobley Director Mr, Raul R. Baron, General Manager Division of Radiological Health Nuclear Assurance 3rd Floor, L and C Annex Tennessee Valley Authority 401 Church Street 4J Blue Ridge Nashville, TN 37243-1532 1101 Market Street Chattanooga, TN 37402-2801 County Executive Hamilton County Courthouse Mr. Mark J. Burzynski, Manager Chattanooga, TN 37402-2801 Nuclear Licensing Tennessee Valley Authority 4J Blue Ridge 1101 Market Street Chattanooga, TN 37402-2801

UNITED STATES NUCLEAR REGULATORY COMMISSION OFFICE OF NUCLEAR REACTOR REGULATION y WASHINGTON, D.C. 20555-0001 3, October 22,1997 NRC INFORMATION NOTICE 97-76: RECENT EVENTS INVOLVING REACTOR COOLANT SYSTEM INVENTORY CONTROL DURING i SHUTDOWN 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.

Purpong The U.S. Nuclear Regulatory Commission (NRC) is issuing this informa. tion 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 cimilar problems. However, suggestions contained in this information notice are not NRC requirements; tl erefore, no specific action or written response is required.

Descriotion of Circumstances SEQUOYAH UNIT 1 At approximately 11 p.m. on March 23,1997, with Sequoyah, Un;t 1 in Mode 5 (cold shutdown),

operators initiated 3 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 moni or pressurizer level during the evolution, as specified in plant procedures.

The draindown was interrupted at approximately 2:30 a.m. on March 24,1997, to perfmm s diesel generator test and was resumed at approximately 7 a.m. Throughout most of th.3 draindown, the cold-calibrated level instrument provided level indications later found to be inaccurate, but continued to trend down along with actual pressurizer level. At approximately 38 percent pressurizer level, the level indication on the cold-calibrated instrument stopped trending down and held steady, although the actua! pressurizer level continued to decrease.

Plant operators did not rocognize that the cold-cWrated instrument was providing erroneous indications and continued the draindown until the problem was discovered at approximately

(N 97-76 DRAFT octobe,22, ,997 P:ge 2 of 4 7:45 a.m. when a member of the incoming operations crew recognized the discrepancy -

between the hot- and cold-calibrated instruments and the reactor vessel level indicating system (RVLIS). 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.

The excessive draindown occurred as a result of erroneous levelindications 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 l 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-levelinstruments was not correct and continued to rely on the cold-calibrated instrument as the primary level indicator. Further, plant procedures did not instruct the operators to monitor other level instruments.

A similar event occurred on Unit 1 in 1993 when approximately 10,000 gallons of water were drained from the RCS as a result ofinaccurate levelindication 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 approximateh 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

IN 97 76 g[ October 22,1997 Page 3 of 4

. 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 configurat:en 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 attemative 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 leamed, both events might have been avoided.

Discussion These eveats illustrate the importance of timely review of operating experience and prompt implemantation 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 t 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 Indicatbn 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 Pehrmance During Performance of New Surveillance Tests,"

dated March 8,1996.

EcE%f22,,ee7 i DRAFT Page 4 of 4 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 934)3, " 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.

l IN 92 54, " Level instrumentation inaccuracles Caused by Rapid Depressurization," dated July 24,1992.

This information notice requires no specific action or written response, if you have any I

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.

Jacl; W. Roe, Acting Director Division of Reactor Program Management Office of Nuclear Reactor Regulation Technical contact: William F. Burton, NRR 501-415-2853 E-mail: wfb@nrc. gov

Attachment:

List of Recently Issued NRC Information Notices

• IN 97 76 Oct:ber ,1997 '

LIST OF R*iCENTLY ISSUED NRC INFORMATION NOTICES i Information Date of N tice No. Subject Issuance issued to l

97 77 Exemptions From the 10/10/97 All holders of OLs for Requirements of nuclear power reactors ,

l Section 70.24 of Title 10 of the fada of Federal Reaulations 97-75 Enforcement Sanctions 09/24/97 All U.S. Nuclear Issued as a Result of Regulatory Commission Deliberate Violations lic9nsees of NRC Requirements 97 74 Inadequate Oversight 09/24/97 All holders of OLs for of Contractors During nuclear power reactors Sealant injection except those who have Activities permanently ceased operations and have certified that fuel has been permanently removed from the reactor vessel 97-73 Fire Hazard in the Use 09/23/97 All holders of OLs for of a Leak Sealant nuclear power reactors except those who hava permanently ceased operations and have certified that fuel has been permanently removed from the reactor vessel OL = Operating License CP = Construction Permit

- ____