Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
SSINS No.: 6835
IN 86-48
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF INSPECTION AND ENFORCEMENT
WASHINGTON, D.C. 20555
June 13, 1986
Information Notice No. 86-48: INADEQUATE TESTING OF BORON SOLUTION
CONCENTRATION IN THE STANDBY LIQUID
CONTROL SYSTEM
Addressees:
All boiling water reactor (BWR) nuclear power facilities holding an
operating license (OL) or a construction permit (CP).
Purpose:
This notice is to alert recipients to a potentially significant problem that
has been observed with the sampling and testing of the sodium pentaborate
solution concentration in the standby liquid control system (SLC) at several
BWR nuclear power plants. It is expected that recipients will review the
information for applicability to their facilities and consider action, if
appropriate, to preclude a similar problem 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:
Review of licensee event reports (LERs) over the past several years
indicates continuing problems in maintaining volume and concentration of the
solution within Technical Specification (TS) limits. Inspections have also
revealed inadequate preoperational testing which failed to prevent
operational problems with concentration. Several of the problems caused
plants to initiate shut downs as required by TS limits. Usually, the proper
level and concentration was recovered in less than 8 hours but in one case
recovery took twice that time. Values both lower and higher than the TS
requirements were reported. Low boron concentration or low tank level
reduces protection against an anticipated-transient-without-scram (ATWS)
event due to reduced negative reactivity worth of the boron solution. High
boron concentration increases the risk of forming crystals of boron that
could render the system inoperable. However, none of the LERs or
inspections reported situations in which the SLC would be rendered
inoperable by boron crystallization or not capable of shutting down the
reactor as required.
Discussion:
Eight LERs reported boron solution concentration too low (ranging from 94
percent to 99 percent of the TS limit) while 6 reported concentration too
high
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IN 86-48
June 13, 1986
Page 2 of 3
(ranging from 100.1 percent to 106 percent of the TS limit). The causes
reported included incorrect tank levels, inadequate accounting for system
leakage, inadequate accounting for the specific gravity of sodium
pentaborate, inferred loss of boron, and increased water evaporation rate
with increased solution temperature. The problems of tank level measurement
and mixing and testing procedures are discussed below in more detail.
Storage Tank Level
At LaSalle 2, NRC inspection revealed that the operations group and the
chemistry group used 3 different methods to measure the tank level and
results were in frequent disagreement. In some instances, the
disagreement was as large as 200 gallons (about 4 percent). The 3
methods involved level meter readings, sight glass readings, and
readings of a measuring tape attached to a plumb bob. As a corrective
measure, the licensee is specifying the use of the last method, because
it is apparently the most reliable method.
At Susquehanna 2, level was reported to be 8 percent less than the TS
requirement (about 17 percent error in volume). Level was measured
using a steel tube from which air bubbles continuously exited near the
bottom of the tank and tank level was inferred from the air pressure.
This approach was rendered inaccurate by tube blockage built up by the
alternate wetting and drying of the tip of the tube. The licensee
modified this system for periodically cleaning the tube with a brush
and is establishing an additional alternative method of level
measurement using an ultrasonic device.
Mixing
Over the course of 3 inspections of preoperational testing of the SLC
at LaSalle, Perry, and Clinton, the NRC has identified apparent
noncompliance related to whether the SLC can generate a uniform
solution. The involved licensees did not adequately meet their
commitment to implement Regulatory Guide 1.68, "Initial Test Programs
for Water-Cooled Nuclear Power Plants." In particular, the air sparger
subsystem was not tested properly. Also, the sampling methodology (dip
samples were or might be taken from near the liquid surface at these
plants) necessary to satisfy surveillance requirements of TS was not
demonstrated to yield valid results.
Concerning preoperational testing of the SLC, Appendix A of Regulatory
Guide 1.68 gives in part:
(3) Standby Liquid Control System Tests. Demonstrate proper operation
of the system with demineralized water. Verify proper mixing of
solution and adequacy of sampling system. . . .
Verify operability of. . .air spargers. . . .
The testing observed consisted of filling the solution tank with
demineralized water, commencing air flow through the sparger, and
verifying that
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IN 86-48
June 13, 1986
Page 3 of 3
the distribution of air bubbles was even. This procedure does not
demonstrate uniform mixing of the solution and it does not determine
what sparging time is required to ensure that dip samples represent the
tank's contents.
The sodium pentaborate solution stratifies over time. Because TS
surveillance tests are performed on a monthly basis, the solution may
not be well mixed at the time of the test. If the solution is not
adequately mixed, a sample will be unrepresentative of the tank's
contents. This may lead to accepting the inferred concentration
erroneously or diluting or strengthening the solution erroneously and
thus allowing the concentration to be outside the TS requirement.
The licensees at LaSalle, Perry, and Clinton have found that an
adequate way to verify proper mixing is to initially fill the tank to
the concentration required by the facility's TS, allow a settling
period equal to the maximum allowed time between surveillances, and
apply air sparging of the solution at a given rate with periodic axial
sampling until the difference in concentration between the axial
samples is within a predetermined variance (e.g., twice the sampling
accuracy). The cumulative sparging time that yields this variance then
is the minimum sparging time to be used for all future surveillances
for the specified sparging rate. Any decrease from the sparging rate
used in the above test would necessitate retesting to determine an
acceptable sparging time.
For those plants past the preoperational test phase, if the original
testing performed on the SLC air sparger subsystem was inadequate, the
TS surveillance tests may not be, yielding valid results.
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 Contacts: Vernon Hodge, IE
(301) 492-7275
Roger D. Lanksbury, RIII
(312) 790-5579
Attachment: List of Recently Issued IE Information Notices
