Information Notice 2017-01, Reactor Coolant System Leakage from a Control Rod Drive Threaded Connection
| ML16344A371 | |
| Person / Time | |
|---|---|
| Issue date: | 05/03/2017 |
| From: | Louise Lund, Mcginty T Division of Construction Inspection and Operational Programs, Division of Policy and Rulemaking |
| To: | |
| Mensah T, NRR/DPR/PGCB, 301-415-3610 | |
| References | |
| TAC MF8698 IN-17-001 | |
| Download: ML16344A371 (4) | |
ML16344A371 UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
OFFICE OF NEW REACTORS
WASHINGTON, DC 20555-0001
May 3, 2017
NRC INFORMATION NOTICE 2017-01:
REACTOR COOLANT SYSTEM LEAKAGE
FROM A CONTROL ROD DRIVE
THREADED CONNECTION
ADDRESSEES
All holders of 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 combined 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 inform
addressees of operating experience regarding reactor coolant system (RCS) leakage through a
control rod drive threaded connection. 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 IN are not NRC requirements; therefore, no specific
action or written response is required.
DESCRIPTION OF CIRCUMSTANCES
On October 21, 2015, Watts Bar Unit 1 started up from a refueling outage. On
October 23, 2015, operators noted higher than expected unidentified RCS leakage1
(0.19 gallons per minute (gpm) total, 0.07 gpm identified, and 0.12 gpm unidentified) but
chemistry analysis concluded the high unidentified leakage was not from the RCS. The
containment radiation monitors confirmed rising activity for particulates; however, Tennessee
Valley Authority, the licensee, attributed the rise to be typical for post refueling outage operation.
On October 29, 2015, chemistry samples confirmed short-lived isotopes, indicative of an RCS
leak. From October 30 through November 5, 2015, unidentified leakage ranged from 0.19 gpm
to 0.24 gpm.
1 The definition of identified leakage in Watts Bar technical specifications (TS) is leakage into the containment
atmosphere from sources that are both specifically located and known either not to interfere with the operation of
leakage detection systems or not to be pressure boundary leakage, while the definition for unidentified leakage is all
leakage (except RCP seal water injection or leakoff) that is not identified leakage. The definition for pressure
boundary leakage in TS is leakage (except primary to secondary leakage) through a nonisolable fault in an RCS
component body, pipe wall, or vessel wall. On November 5, 2015, Watts Bar Unit 1 was operating at 100 percent power when a shutdown
bank control rod lost electrical power and, as designed, dropped into the core. Reactor power
was reduced to 74 percent per procedure to meet TS limits with one shutdown bank control rod
inserted fully into the core. Operators then performed a reactor shutdown on
November 7, 2015, entering a maintenance outage to search for the previously identified RCS
leak and to determine the cause of the dropped rod. Upon entry into containment with the RCS
depressurized and temperature less than 200 degrees Fahrenheit, the leak was not apparent
until the control rod drive mechanism (CRDM) missile shields were removed. Once the CRDMs
were exposed, plant personnel discovered reactor coolant leaking from the threads of the
CRDM housing eyebolt. As shown in Figure 1, beneath the eyebolt is a vent plug that was
found to be hand-tight rather than tightened to its recommended torque value. The vent plug
itself forms a mechanical joint which is part of the RCS pressure boundary. The threaded
eyebolt is not a part of the RCS pressure boundary.
The shutdown bank control rod that dropped into the core on November 5, 2015, was located
adjacent to the rod with the leaking CRDM vent plug. Borated water made its way into the cable
connection housing of the adjacent shutdown bank control rod. The resulting corrosion caused
an electrical ground and failed connector, which led to the dropped rod. Borated water spraying
past the CRDM housing eyebolt also caused boric acid deposits to accumulate on the reactor
vessel head and adjacent cabling and components.
CRDM vent plugs had not been previously identified by the licensee as a potential source
location for RCS leakage. The licensee had required a periodic visual inspection of the CRDM
eyebolts. Following discovery of the leakage, the licensee removed the eyebolt and found the
vent plug hand-tight. This was never checked during the periodic inspections, as the vent plug
was not identified as a component that should be checked. The licensee implemented
corrective actions that inspected the vent plug, reinstalled and torqued it, then installed and seal
welded a new vent cap in place of the eyebolt, with the vent plug remaining as the pressure
boundary component. Additional corrective actions included extensive cleaning of boric acid, inspection of surrounding CRDM electrical connectors, identification of all threaded/flanged
pressure boundary connections, and revisions of procedures to provide additional guidance
Figure 1: Simplified drawing of reactor head with rod travel housing, vent plug, and eyebolt pertaining to potential leak paths on the reactor vessel head. The licensee also inspected
seven other CRDM vent plugs for torque as a part of their extent of condition review.
DISCUSSION
Although the limit for unidentified leakage per Watts Bar Unit 1 TS is 1 gpm for Modes 1-4, a
site-specific administrative limit exists to shut down for leakage well below the limit specified in
TS. In this instance, there were indications of RCS leakage prior to the rod drop including
repetitive containment radiation monitor alarms, containment air samples with positive results
for RCS leakage, and RCS operational leakage calculations returning higher-than-expected
values. The boric acid buildup, caused by the leak, resulted in a dropped rod and subsequent
reactor shutdown, during which the source of the leakage was identified. This event
demonstrates that leakage, at rates well below the TS limit, can cause adverse consequences
such as boric acid deposits and electrical faults.
CONTACT
This IN requires no specific action or written response. Please direct any questions about this
matter to the technical contacts listed below or to the appropriate Office of Nuclear Reactor
Regulation project manager.
/ra/
/ra/ (Paul G. Krohn for)
Louise Lund, Director
Timothy J. McGinty, Director
Division of Policy and Rulemaking
Division of Construction Inspection
Office of Nuclear Reactor Regulation
and Operational Programs
Office of New Reactors
Technical Contacts: Catherine Nolan, NRR/DIRS
301-415-1535
E-mail: Catherine.Nolan@nrc.gov
Note: NRC generic communications may be found on the NRC public Web site, http://www.nrc.gov, under NRC Library, Document Collections.
ML16344A371; *concurred via email
TAC No. MF8698 OFFICE
TECH EDITOR
NRO/DCIP/CIPB/BC
NRO/DSRA/SPSB/BC
NRR/DE/EPNB/BC
R-II/DRP/RPB6/BC
NAME
JDougherty*
VHall*
ADias*
DAlley*
ABlamey*
DATE
12/7/16
3/13/16
3/16/17
3/17/17
4/03/17 OFFICE
NRR/IOEB/DIRS/TL
NRR/IOEB/DIRS/BC
NRR/DIRS/D
R-II/DRP/D
NRR/DPR/PGCB/PM
NAME
CNolan*
HChernoff
CMiller*
JMunday*
TMensah
DATE
4/04/17
4/04/17
4/10/17
4/04/17
4/11/17 OFFICE
NRR/DPR/PGCB/LA
NRR/DPR/PGCB/BC
NRO/DCIP/D
NRR/DPR/D
NAME
ELee*
SStuchell*
TMcGinty (PKrohn for)
LLund
DATE
4/12/17
4/11/17
4/17/17
5/3/17