Information Notice No. 86-40: Degraded Ability to Isolate the Reactor Coolant System from Low-Pressure Coolant Systems in BWRS
SSins No: 6835
IN 86-40
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF INSPECTION AND ENFORCEMENT
WASHINGTON, DC 20555
June 5, 1986
Information Notice No. NO 86-40: DEGRADED ABILITY TO ISOLATE THE REACTOR
COOLANT SYSTEM FROM LOW-PRESSURE COOLANT
SYSTEMS IN BWRS
Addressees:
All nuclear power reactor facilities holding an operating license (OL) or a
construction permit (CP)
Purpose:
This notice is provided as a supplement to Information Notice (IN) 84-74 on
interfacing systems loss-of-coolant accidents (LOCA) in boiling water
reactors (BWRs) which would bypass primary containment Two recent events
are described where the high-pressure reactor coolant system could not be
fully isolated from low-pressure piping systems outside of primary
containment
It is expected that recipients will review the information for applicability
to their facilities and consider actions, if appropriate, to preclude
similar problems from occurring at their facilities However, suggestions
contained in this information notice do not constitute NRC requirements;
therefore, no specific action or written response is required
Description of Circumstances:
Pilgrim
On February 13, 1986, Pilgrim experienced a "design pressure" alarm on the B
loop of the residual heat removal (RHR) system because of leakage through a
check valve (1001-68B) and a motor-operated isolation valve (1001-28B, the
outboard, normally closed, isolation valve) in the line (see Figure 1) The
RHR system has a design pressure of 450 psig compared with a 1250 psig
design pressure of the reactor coolant system In addition to design
pressure alarms, the piping up to the 28B valve had become warm Several
design pressure alarms had occurred during the preceding several weeks The
licensee's prior corrective action had been to vent the lines On February
13, the licensee closed the normally open 1001-29B valve and opened the 28B
valve to isolate the system The licensee planned to operate in this
configuration until a scheduled refueling and maintenance outage in
September 1986
8606030014
IN 86-40
June 5, 1986
Page 2 of 3
On April 11 and 12, 1986, the closed 29B valve began leaking Several high
pressure alarms were reported The licensee bled off the line to reduce
pressure and began an orderly shutdown, but within a short period of time,
the high pressure alarm was again received The 28B valve was closed, but
the leakage problem continued The shutdown rate was then increased until
the unit scrammed because of other problems An NRC augmented inspection
team was dispatched to the site to investigate these RHR valve problems and
two problems in other systems
Duane Arnold
On March 15, 1986, while reducing power for an outage, Duane Arnold reported
closing the outboard LPCI (low-pressure coolant injection mode of RHR)
isolation valve MO-2004 due to leakage through the inboard isolation valve
MO-2003 (see Figure 2)
Because the leakage flow was relatively small, the p across the check
valve inside of containment (CV-2002) was not high enough to seat the check
valve The problem was discovered when it was noted that the pressure
controllers on the RHR heat exchangers were indicating a pressure of 450
psig rather than the normal 70 psig RHR system relief valves had lifted to
keep pressure at or below 450 psig Closing the MO-2004 valve stopped the
leakage, but because the plant has loop selection logic, both loops of LPCI
were rendered inoperable The plant continued its planned shutdown and
reached cold shutdown on March 16 The leaking valves were repaired during
the outage
Discussion:
The underlying cause of this problem is leaking valves, one of which is
inaccessible with the plant at power Possible solutions include increased
surveillance, preventive maintenance, and reliability-based replacement
Other BWR systems that can be subject to similar problems include but are
not limited to core spray, high pressure coolant injection system (see IE
Information Notice 84-74), and reactor core isolation cooling
It is possible for leakage to exist from the reactor coolant system to a low
pressure system without causing a high pressure alarm or lifting of safety
valves in the low pressure system For example, for the preceding events, if
the check valve at the discharge of one of the RHR pumps is leaking, the
reactor coolant will flow to the suppression pool Hence, slowly increasing
level in the suppression pool is one indication that there is leakage from
the reactor coolant system to the low pressure RHR system, and degradation
of the reactor coolant pressure boundary If leakage through the degraded
valves were to increase suddenly, a severe accident could result, as
described in the following paragraph Further, it should be noted that such
leakage does not meet the intent of general design criteria 14, 30, and 54
of Appendix A to 10 CFR Part 50
IN 86-40
June 5, 1986
Page 3 of 3
The leaking of primary coolant into RHR lines that were never meant to
contain fluid at that temperature and pressure can cause a number of
incidents: over-pressurization with possible faulting of the low pressure
line and a LOCA, steam binding of one or more of the RHR pumps, and
waterhammer The safety significance of these events is the increased
probability of core melt and releases in excess of 10 CFR 100 limits The
Office of Nuclear Reactor Regulations has designated this topic Generic
Issue number 105, "Interfacing Systems LOCA at Boiling Water Reactors," and
has given it a "high" priority A generic letter concerning staff actions
relating to this topic is being considered
No specific action or written response is required by this information
notice If you have any questions about this matter, please-contact the
Regional Administrator of the appropriate regional office or this office
Edward L Jordan, Director
Division of Emergency Preparedness
and Engineering Response
Office of Inspection and Enforcement
Technical Contact: Mary S Wegner
(301) 492-4511
Attachments:
1 Simplified RHR Diagrams
2 List of Recently Issued IE Information Notices
Page Last Reviewed/Updated Tuesday, March 09, 2021