UNITED STATES

NUCLEAR REGULATORY COMMISSION

OFFICE OF NUCLEAR REACTOR REGULATION

WASHINGTON, D.C. 20555

August 3, 2005

NRC INFORMATION NOTICE 2005-24: NONCONSERVATISM IN LEAKAGE DETECTION

SENSITIVITY 

ADDRESSEES

All holders of operating license or construction permits for nuclear power reactors, except those

that have permanently ceased operations and have certified that fuel has been permanently

removed from the reactor.

PURPOSE

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

addressees that the reactor coolant activity assumptions for containment radiation gas channel

monitors may be nonconservative. As a result, the containment gas channel may not be able to

detect a 1 gallon-per-minute (1-gpm) leak within 1 hour. It is expected that the 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

Several nuclear power plant licensees have reported problems with the detection capabilities of

containment radiation gas channel monitors. The following gives several examples of these

reports.

On May 2, 2005, the McGuire nuclear power plant licensee reported that the containment

atmosphere radioactivity monitors were not sensitive enough for their intended function of

detecting a 1-gpm reactor coolant system (RCS) leak within 1 hour (Licensee Event Report

(LER) 50-369/2005-01, ADAMS Accession No. ML051310167). This resulted in a Severity

Level IV noncited violation. 

The McGuire licensee declared the atmosphere monitors inoperable and performed

compensatory actions in accordance with plant technical specifications. The compensating

actions were to (1) establish temporary alarm setpoints to provide earlier notification should a

significant RCS leak occur, (2) instruct operators on other methods of RCS leak detection, (3)

establish sensitivities as low as practical based on actual RCS radioactivity levels, (4)

periodically review the sensitivities for revision as needed, (5) provide additional training as

needed, and (6) consider submitting a license amendment request to clarify the capabilities of

the leak detection instrumentation.

ML051780073
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In February 2005, NRC inspectors at the Catawba nuclear power plant identified a noncited

violation of Technical Specification 5.4.1.a, “Written Procedures,” because the licensee failed to

establish and maintain an adequate procedure for the required containment atmosphere

radioactivity monitor surveillance in that the associated alarm function was not set or tested to

alarm at a value equivalent to 1 gpm in 1 hour for a realistic current reactor coolant activity level

(NRC Integrated Inspection Report 50-413/2005-02 and 50-414/2005-02, ADAMS Accession

No. ML051160367). 

The Catawba licensee also declared these channels to be inoperable and is performing

compensatory actions in accordance with plant technical specifications.

In June 2003, an NRC inspection made a similar finding at Callaway (NRC Inspection

Report 50-483/2003-04, ADAMS Accession No. ML032020562) that resulted in a noncited

violation. The gas channel monitor was not capable of performing its design basis function of

detecting a 1 gpm RCS leak within 1 hour. The calculation for the gas channel monitor

response used an RCS source term corresponding to an assumed 0.1 percent failed fuel but,

because of improved fuel performance and RCS chemistry control, the plant operated with an

RCS source term several orders of magnitude smaller.

The Callaway licensee responded to this situation similarly by (1) declaring the gas channel out

of service to prevent its being credited for leakage detection and (2) considering a license

amendment request to revise the final safety analysis report and technical specification bases

to reflect actual leakage detection capabilities.

DISCUSSION

The NRC requires licensees to use a means of detecting and, to the extent practical, identifying

the location of any sources of RCS leakage (Title 10 of the Code of Federal Regulations,

Part 50, Appendix A, “General Design Criteria [GDC] for Nuclear Power Plants,” Criterion 30,

“Quality of Reactor Coolant Pressure Boundary”). The NRC provided guidance on meeting

GDC 30 in Regulatory Guide (RG) 1.45, “Reactor Coolant Pressure Boundary Leakage

Detection Systems.” Some licensees committed to using RG 1.45 as the basis for meeting

GDC 30.

RG 1.45 states that an acceptable means would provide for adequate sensitivity and response

time of all leakage detection systems to detect a leakage rate of 1 gpm in less than 1 hour. 

Further, the acceptable means would employ at least three separate detection methods. Two

of these methods are monitoring sump level and sump flow and monitoring airborne particulate

radioactivity. The third method is either monitoring the condensate flow rate from air coolers or

monitoring airborne gaseous radioactivity. The guide also states that a “realistic” primary

radioactivity concentration should be assumed when analyzing the sensitivity of leak detection

systems.

During original plant licensing, the typical calculation for the technical specification for gas

channel monitor response used an RCS source term corresponding to an assumed 0.1 percent

failed fuel. Nowadays, because of improvements in fuel performance and RCS chemistry

control, the actual RCS source term can be orders of magnitude smaller. Though desirable, a
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small source term can result in reduced leakage monitoring capabilities. Using a realistic RCS

source term, a 1 gpm RCS leak would likely not be detected by a gas channel monitor for a

much greater time than within 1 hour. The 0.1-percent failed fuel assumption introduces a

nonconservatism into the technical specifications. Guidance on resolving such a

nonconservatism is given in NRC Administrative Letter 98-10, “Dispositioning of Technical

Specifications That Are Insufficient to Assure Plant Safety.”

The consistency of leakage detection systems with RG 1.45 has been questioned at several

nuclear power plants. See NUREG/CR-6861, “Barrier Integrity Research Program,” December

2004 (ADAMS Accession No. ML043580207) for a good discussion of detector sensitivities.

This information notice requires no specific action or written response. Please direct any

questions about this matter to the technical contact(s) listed below or the appropriate Office of

Nuclear Reactor Regulation (NRR) project manager.

/RA/ By David C. Trimble Acting For/

Patrick L. Hiland, Chief

Reactor Operations Branch

Division of Inspection Program Management

Office of Nuclear Reactor Regulation

Technical Contacts: Michael Peck, RIV Vernon Hodge, NRR

573-676-3181 301-415-1861

E-mail: msp@nrc.gov E-mail: cvh@nrc.gov

Note: NRC generic communications may be found on the NRC public Website,

http://www.nrc.gov, under Electronic Reading Room/Document Collections.