IEB 80-17 Supplement 4, Failure of Control Rods to Insert During a Scram at a BWR (Generic Letter 80-111)
SSINS No.: 6820
UNITED STATES IEB 80-17 Sup. 4
NUCLEAR REGULATORY COMMISSION
OFFICE OF INSPECTION AND ENFORCEMENT
WASHINGTON, D.C. 20555
December 18, 1980
IE Supplement 4 to Bulletin N. 80017: FAILURE OF CONTROL RODS TO INSERT
DURING A SCRAM AT A BWR
NRC staff evaluation of failures of the continuous monitoring system (CMS)
for the scram discharge volume (SDV) at an operating DWR has identified the
need for licensee actions in addition to those requested by IEB 80-17 and
Supplements 1-3. The purpose of these actions is to provide assurance that
the CMS has been tested to demonstrate operability as installed, remains
operable during plant operation, and is periodically surveillance tested to
demonstrate continued operability.
The occurrence of CMS failures at Dresden Nuclear Power Station was
discussed in IE Information Notice 80-43, which was issued on December 5,
1980 to those operating BWR's with CMS recently installed. Subsequently,
investigation into the cause of the failure to receive the alarm with the
SDV essentially full revealed several items which required correction,
1. Excess portions of transducer cable were placed in physical positions
which would increase external noise sensitivity.
2. The UT transducers were not placed in a physical position to optimize
3. A certain amount of "cross-talk" was occurring between redundant
transducers located a few feet apart on the same run of 4" pipe.
Station and vendor personnel shortened and rerouted transducer cables to
improve noise rejection. Vendor specialists optimized transducer placement
and synchronized both transducers to the same ultrasonic instrument internal
clock to minimize cross-talk and improve signal to noise characteristics.
Following these actions the CMS appeared to function properly.
Further difficulties were encountered when apparently minor quantities of
water leaked into the SDV as a result of control rod drive scram valve
maintenance activities and minor scram outlet valve leakage. It appears
that the transducers are located on a section of SDV piping which forms a
local low point. Accordingly, small amounts of water can accumulate to a
depth which triggers the high level alarm (at 1-1/4") before the water
drains to the instrument volume. The licensee in conjunction with the NSSS
vendor, performed a unit specific analysis for a conservative high alarm
setpoint and reset the alarm point to 2-1/2". The system now appears to
function properly. A five second alarm time delay was also installed to aid
in rejecting spurious alarms.
IEB 80-17 Sup. 4
December 18, 1980
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Potential malfunction modes which are still of concern are:
1. The capability of the CMS to adequately determine level of water for the
entire range of depths which may occur during slow and rapid fill
conditions, that is, beam penetration capability.
2. The potential for loss of transducer sensitivity during periods of rapid
flow, or when the water being detected is turbulent or mixed with
entrained air or steam bubbles.
The ability of the CMS installed in your facility to operate in respect to
these concerns should be considered in the preparation of your response to
this bulletin. The following actions are requested in addition to those
specified in IE Bulletin 80-17 and Supplements 1 through 3.
Actions to be Taken by Licensees of Operating BWR's Using CMS
1. Bench Test of CMS
Make available the following information which describes the CMS design
and the bench tests which have been performed to demonstrate system
operability and sensitivity:
(a) System description including a schematic of the apparatus and
(b) Type of sensing device and characteristics (include response
characteristics versus temperature).
(c) Calibration criteria, including transmission losses.
(d) Training and testing of personnel performing the calibration test.
Items a through c above may be referenced by the licensee if the
information has been submitted to the NRC by the equipment manufacturer.
2. Operability Test of CMS
Prior to conducting the operability test, verify that the CMS on the SDV
is installed and calibrated in accordance with the vendor
In order to provide assurance of operability of the CMS, if not already
performed conduct an operability test within 14 days of the date of this
bulletin. In this test, inject a sufficient amount of water into each
SDV header to determine that the ultrasonic transducers are adequately
coupled to the SDV piping and that the trip alarm function of the CMS
will perform satisfactorily. The test may be performed by single
(multiple) rod scram tests while operating. No water may be introduced
into the SDV header while the reactor is operating except using the
scram function. Independent level measurement must be used to verify
CMS operation and proper calibration.
IEB 80-17 Sup. 4
December 18, 1980
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3. Interim Manual Surveillance
In the interim 14-day period before the operability test is completed,
perform a manual surveillance for the presence of water in the SDV at
least once pe shift and after each reactor scram. In order to provide
assurance that manual surveillance can detect water accumulation in the
SDV, verify that the method and the operator have been qualified by
testing which uses or simulates the SDV piping and has the ability to
detect different levels of water in the SDV.
Surveillance of SDV manual measurement techniques should be done before
completion of the operability test described in Item 2 above.
4. Full Test of CMS to be Conducted During a Planned Outage
During a planned outage within six months, perform a full CMS test using
the SDV headers:
(a) Admit water into the SDV to establish full rates for several (not
less than three) in-leakage flow rates. The in-leakage rates
should range from approximately the minimum which results in water
accumulation in the SDV to a full scram.
(b) Establish and record the response of the CMS indication and alarm
functions from the trip level to a full SDV. Provide criteria for
replacement or adjustment when exceeding design specifications of
(c) Verify by independent measurement that the alarm initiates at the
proper level setpoint.
5. Operability of CMS During Reactor Operation
The CMS shall be operable prior to reactor startup and during reactor
operation. If the CMS becomes less than fully operable, within 8 hours
perform a manual check for water in the SDV and institute procedures
for a manual check of the SDV each shift and following scram until the
CMS is fully operable. When not fully operable, the CMS should be used
to the extent practical in addition to the manual checks.
If the CMS is not operable within 7 days, the frequency of the manual
check should be increased to once every 4 hours. If the CMS is not
operable within 30 days the plant shall be shutdown.
To demonstrate continued operability of the CMS during reactor
operation, perform periodic surveillance tests for operability of the
CMS. For these periodic surveillance tests, test as much of the CMS as
practical during reactor operation without injecting water in the SDV.
Establish criteria for repair or replacement when the system design
criteria or estimated service life limitations are exceeded. The
frequency of these periodic surveillance checks should be determined by
IEB 80-17 Sup. 4
December 18, 1980
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These periodic surveillance tests should include the following:
(a) determination that the response and power output of the transducer
has not degraded;
(b) visual inspection for adequate condition of the transducer to SDV
coupling material; and
(c) a calibration check of the electronics to assure alarm initiation
in the control room.
Water should be periodically injected into the SDV to perform a CMS
operability and calibration check similar to that specified in Item 2
above. This check should be performed semiannually and during startup
after plant outages where maintenance operations may have taken place
near to CMS equipment.
6. Operating Procedures
Develop procedures for operation, periodic testing and calibration of
the CMS and for repair or replacement when system design specifications
are exceeded. Develop procedures for the calibration and use of the
hand held UT device in the event of a malfunctioning CMS. Notify the
NRC before changing the established CMS alarm level setpoints.
Licensees of all operating BWRs with a CMS shall provide the information
requested in Item 1 and shall submit a report summarizing action taken in
response to each of the above items within 45 days of the date of this
Bulletin Supplement. Accordingly, you are requested to provide within 45
days as specified above, written statements of the above information signed
under oath or affirmation under provisions of Section 182a of the Atomic
Energy Act of 1954. Reports shall be submitted to the Director of the
appropriate NRC Regional Office and a copy forwarded to the Director, NRC
Office of Inspection and Enforcement, Washington, D.C. 20555.
This request for information was approved by GAO under a blanket clearance
number R0072 which expires November 30, 1983. Comments on burden and
duplication should be directed to the U.S. General Accounting Office,
Regulatory Reports Review, Room 5106, 441 Eighth Street, N.W., Washington,
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