Bulletin 96-01: Control Rod Insertion Problems
OMB No. 3150-0012
NRCB 95-02
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, DC 20555-0001
March 8, 1996
NRC BULLETIN 96-01: CONTROL ROD INSERTION PROBLEMS
Addressees
This bulletin is being sent to all holders of pressurized-water reactor (PWR)
operating licenses (except those licenses amended to possession only status).
It is expected that recipients will review the information for applicability
to their facilities and consider actions, as appropriate, to avoid similar
problems. However, action is only requested from PWR licensees of
Westinghouse-designed plants.
Purpose
The U.S. NRC is issuing this bulletin to accomplish the following:
(1) Alert addressees to problems encountered during recent events in which
control rods failed to completely insert upon the scram signal.
(2) Assess the operability of control rods, particularly in high burnup fuel
assemblies.
Background
South Texas Project
On December 18, 1995, with South Texas Unit 1 at 100 percent power, a pilot
wire monitoring relay actuation caused a main transformer lockout, which
resulted in a turbine trip and a reactor trip. While verifying that control
rods had inserted fully after the trip, operators noted that the rod bottom
lights of three control rod assemblies were not lit; the digital rod position
indication for each rod indicated six steps withdrawn. A step is equivalent
to 1.59 cm [5/8 inch], and the top of the dashpot begins at 38 steps.
Boration of the reactor coolant system was occurring, with the charging pump
suction aligned to the refueling water storage tank. One rod did drift into
the fully inserted rod bottom position within 1 hour, and the other two rods
were manually inserted later. During subsequent testing of all control rods
in the affected banks, the rod position indication for the same three
locations, as well as a new location, indicated six steps withdrawn. As
compared to prior rod drop testing, no significant differences in rod drop
times were noted before reaching the upper dashpot area for any of the control
rods. Within 1 hour after the rod drop tests, two of the rods drifted to the
rod bottom position and the other two were manually inserted. All four
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March 8, 1996
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control rods were located in XLR fuel assemblies that were in their third
cycle, with burnup greater than 42,880 megawatt days per metric ton uranium
(MWD/MTU).
Wolf Creek Plant
On January 30, 1996, after a manual scram from 80 percent power, five control
rod assemblies at the Wolf Creek plant failed to insert fully. Two rods
remained at 6 steps withdrawn, two at 12 steps, and one at 18 steps. At
Wolf Creek, a step is equivalent to 1.59 cm [5/8 inch] and the top of the
dashpot begins at approximately 30 steps. Three of the affected rods drifted
to fully inserted within 20 minutes, one within 60 minutes, and the last one
within 78 minutes. The results also indicate that there was some slowing down
of affected rods before they reached the dashpot. After the scram, the
licensee initiated emergency boration because all rods did not insert fully.
During subsequent cold rod drop tests, the same five rods, plus an additional
three rods, failed to fully insert. All of the affected rods were in
17x17 VANTAGE 5H fuel assemblies, with burnup greater than 47,600 MWD/MTU.
North Anna Plant
On February 21, 1996, during the insert shuffle in preparation for loading
North Anna 1, Cycle 12, two new control rod assemblies could not be removed
with normal operation of the handling tool from the fuel assemblies in the
spent fuel pool in which they were temporarily stored. The control rod
assemblies were removed using the rod assembly handling tool in conjunction
with the bridge crane hoist. The two affected fuel assemblies were VANTAGE
5H assemblies, which had achieved 47,782 MWD/MTU and 49,613 MWD/MTU burnup
during two cycles of irradiation.
Discussion
At both South Texas units, a 14-foot active fuel length core design is used.
Several differences between the standard 12-foot active fuel design and the
14-foot design are as follows: the 14-foot fuel design is approximately
76.2 cm [30 inches] longer than the standard fuel assembly design, it has
10 mid grids compared to 8, and the dashpot region is 25.4 cm [10 inches]
longer and comprises a double dashpot. The control rod radial clearances
above and in the dashpot region of the 14-foot fuel assembly are similar to
those of the standard design. The South Texas core contains three different
17x17 fuel types--Standard XL, Standard XLR, and VANTAGE 5H--all of which are
designed and fabricated by Westinghouse. This was the first operating cycle
with VANTAGE 5H fuel. The core also contains 57 silver-indium-cadmium rods.
The four affected rods were found in twice-burned Standard XLR fuel
assemblies.
During subsequent testing, the rod drop traces revealed no significant change
in dashpot entry time; however, the affected rods did not show recoil on the
rod drop trace. Recoil is a dampening affect that is normally seen in the
traces as a result of contact of the control rod assembly spider hub spring
against the fuel assembly. When similar rods in Unit 2 were tested, the
results revealed no adverse indications. One rod did show the "no recoil"
effect but inserted fully into the core.
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At Wolf Creek, subsequent cold, full-flow testing of all of the control rod
assemblies indicated that eight control rods, including the five control rods
that did not fully insert following the reactor trip on January 30, 1996, did
not fully insert when tripped. One control rod in core location H2 paused at
96 steps, stopped at 90 steps, and slowly inserted to 30 steps over the next
2 hours. The control rod was then manually inserted. The seven other
affected rods stopped at various heights in the dashpot region, five of which
fully inserted within 22 minutes. One of the other two drifted to the bottom
within 1.5 hours, while the remaining rod needed to be manually inserted. The
remaining 45 rods fully inserted when dropped, although a number of the rods
did not exhibit the expected number of recoils. Of the total 53 control rod
assemblies, the assembly at core location H2 (the only rod slowing outside the
dashpot region) is a hafnium control rod, while the remaining are silver-
indium-cadmium control rod assemblies. However, subsequent inspection of the
hafnium rod did not indicate any adverse dimensional change. The licensee
retested all rods that stuck, as well as those rods that failed to recoil more
than twice, and the results were similar to the previous testing.
At North Anna, the two affected control rods were removed and were inserted
into a series of other fuel assemblies. No additional binding problems were
observed. However, difficulty was experienced when another control rod was
inserted into the two affected fuel assemblies. On the basis of this result,
the licensee determined the cause of the binding problem to be related to the
fuel assemblies and not the control rods. Subsequent control rod drag testing
data indicated a correlation of control rod drag force to assembly burnup and
a significant increase in drag force at assembly burnups greater than
45,000 MWD/MTU.
These three events, as well as several similar events at foreign reactors,
raise concerns about the operability of control rods in high burnup fuel
assemblies. Although most of the testing to date has demonstrated that the
control rods have reached the dashpot region of the guide tube and that
adequate shutdown margin has been maintained, there have been indications of
degraded rod drop times and a stuck rod well above the dashpot region. Thus,
there is concern that these events may be precursors of more significant
control rod binding problems in which required shutdown margins and rod drop
times may be violated.
Requested Actions
To ensure that the required shutdown margin is maintained during a reactor
trip, all licensees of Westinghouse-designed plants are requested to take the
following actions:
(1) Promptly inform operators of recent events (reactor trips and testing)
in which control rods did not fully insert and subsequently provide
necessary training, including simulator drills, utilizing the required
procedures for responding to an event in which the control rods do not
fully insert upon reactor trip (e.g., boration of a pre-specified
amount).
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(2) Promptly determine the continued operability of control rods based on
current information. As new information becomes available from plant
rod drop tests and trips, licensees should consider this new information
together with data already available from Wolf Creek, South Texas,
North Anna, and other industry experience, and make a prompt
determination of control rod operability.
(3) Measure and evaluate at each outage of sufficient duration during
calendar year 1996 (end of cycle, maintenance, etc.), the control rod
drop times and rod recoil data for all control rods. If appropriate
plant conditions exist where the vessel head is removed, measure and
evaluate drag forces for all rodded fuel assemblies.
a. Rods failing to meet the rod drop time in the technical
specifications shall be deemed inoperable.
b. Rods failing to bottom or exhibiting high drag forces shall require
prompt corrective action in accordance with Appendix B to Part 50 of
Title 10 of the Code of Federal Regulations (10 CFR Part 50).
(4) For each reactor trip during calendar year 1996, verify that all control
rods have promptly fully inserted (bottomed) and obtain other available
information to assess the operability and any performance trend of the
rods. In the event that all rods do not fully insert promptly, conduct
tests to measure and evaluate rod drop times and rod recoil.
In summary, the first two actions requested by the bulletin ensure that all
affected plants respond in a proactive manner to recent industry experience.
The second two requested actions support data collection that will permit the
staff to more effectively assess this issue and determine if further
regulatory action is needed.
Required Response
Pursuant to Section 182a, the Atomic Energy Act of 1954, as amended, and
10 CFR 50.54(f), all licensees of Westinghouse-designed plants must submit the
following written information:
(1) Within 30 days of the date of this bulletin, a report certifying that
control rods are determined to be operable; actions taken for Requested
Actions (1) and (2) above; and the plans for implementing Requested
Action (3) and (4).
(2) Within 30 days of the date of this bulletin, a core map of rodded fuel
assemblies indicating fuel type (materials, grids, spacers, guide tube
inner diameter) and current and projected end of cycle burnup of each
rodded assembly for the current cycle; when available, provide the same
information for the next cycle.
(3) Within 30 days after completing Requested Action (3) for each outage, a
report that summarizes the data and that documents the results obtained;
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this is also applicable to Requested Action (4) when any abnormal rod
behavior is observed.
Address the required written information to the U.S. Nuclear Regulatory
Commission, ATTN: Document Control Desk, Washington, DC 20555-0001, under
oath or affirmation. In addition, submit a copy of the report to the
appropriate regional administrator.
Related Generic Communications
. NRC Information Notice 96-12: "Control Rod Insertion Problems"
. Letter from R.A. Newton (WOG) to Document Control Desk (NRC), "Response to
NRC Questions Concerning Incomplete RCCA Insertion," February 23, 1996.
Backfit Discussion
This bulletin is an information request under the provision of 10 CFR
50.54(f). The objective of the actions requested in this bulletin is to
verify that licensees are complying with the current licensing basis for the
facility with respect to shutdown margin and control rod drop times. The
issuance of the bulletin is justified on the basis of the need to verify
compliance with the current licensing basis with respect to shutdown margin,
control rod drop times, and proper operator action when control rods are not
promptly inserted into the reactor.
Paperwork Reduction Act Statement
This bulletin contains information collections that are subject to the
Paperwork Reduction Act of 1995 (44 U.S.C. 3501 et seq.). These information
collections were approved by the Office of Management and Budget, approval
number 3150-0012, which expires June 30, 1997. The public reporting burden
for this collection of information is estimated to average 240 hours per
response, including the time for reviewing instructions, searching existing
data sources, gathering and maintaining the data needed, and completing and
reviewing the collection of information. The U.S. Nuclear Regulatory
Commission is seeking public comment on the potential impact of the collection
of information contained in the bulletin and on the following issues:
1. Is the proposed collection of information necessary for the proper
performance of the functions of the NRC, including whether the
information will have practical utility?
2. Is the estimate of burden accurate?
3. Is there a way to enhance the quality, utility, and clarity of the
information to be collected?
4. How can the burden of the collection of information be minimized,
including the use of automated collection techniques?
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Send comments on any aspect of this collection of information, including
suggestions for reducing this burden, to the Information and Records
Management Branch, T-6 F33, U.S. Nuclear Regulatory Commission, Washington, DC
20555-0001, and to the Desk Officer, Office of Information and Regulatory
Affairs, NEOB-10202 (3150-0012), Office of Management and Budget,
Washington, DC 20503.
The NRC may not conduct or sponsor, and a person is not required to respond
to, a collection of information unless it displays a currently valid OMB
control number.
If you have any questions about this matter, please contact one of the
technical contacts listed below or the appropriate Office of Nuclear Reactor
Regulation (NRR) project manager.
signed by
Dennis M. Crutchfield, Director
Division of Reactor Program Management
Office of Nuclear Reactor Regulation
Technical contacts: Laurence Kopp, NRR
(301) 415-2879
Internet:lik@nrc.gov
Margaret Chatterton, NRR
(301) 415-2889
Internet:msc1@nrc.gov
Lead Project Manager: Kristine Thomas, NRR
(301) 415-1362
Internet:kmt@nrc.gov
Attachment: List of Recently Issued NRC Bulletins
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