[Federal Register Volume 62, Number 97 (Tuesday, May 20, 1997)]
[Notices]
[Pages 27629-27632]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 97-13189]
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NUCLEAR REGULATORY COMMISSION
Proposed Generic Communication; Control Rod Insertion Problems
AGENCY: Nuclear Regulatory Commission.
ACTION: Notice of opportunity for public comment.
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SUMMARY: The Nuclear Regulatory Commission (NRC) is proposing to issue
a bulletin supplement that will request addressees to take actions to
ensure the continued operability of the control rods. These actions
will ensure that adequate shutdown margin is maintained and that the
control rods will satisfactorily perform their intended function of
effectively terminating the fission process during all operating
conditions in accordance with the current licensing basis for each
facility. The NRC is seeking comment from interested parties regarding
both the technical and regulatory aspects of the proposed bulletin
supplement presented under the Supplementary Information heading.
The proposed bulletin supplement has been endorsed by the Committee
to Review Generic Requirements (CRGR). The relevant information that
was sent to the CRGR will be placed in the NRC Public Document Room.
The NRC will consider comments received from interested parties in the
final evaluation of the proposed bulletin supplement. The NRC's final
evaluation will include a review of the technical position and, as
appropriate, an analysis of the value/impact on licensees. Should this
bulletin supplement be issued by the NRC, it will become available for
public inspection in the NRC Public Document Room.
DATES: Comment period expires June 19, 1997. Comments submitted after
this date will be considered if it is practical to do so, but assurance
of consideration cannot be given except for comments received on or
before this date.
ADDRESSES: Submit written comments to Chief, Rules Review and
Directives Branch, U.S. Nuclear Regulatory Commission, Mail Stop T-6D-
69, Washington, DC 20555-0001. Written comments may also be delivered
to 11545 Rockville Pike, Rockville, Maryland, from 7:30 am to 4:15 pm,
Federal workdays. Copies of written comments received may be examined
at the NRC Public Document Room, 2120 L Street, N.W. (Lower Level),
Washington, D.C.
FOR FURTHER INFORMATION CONTACT: Margaret S. Chatterton, (301) 415-
2889.
SUPPLEMENTARY INFORMATION:
NRC Bulletin 96-01 Supplement 1: Control Rod Insertion Problems
Addressees
This bulletin supplement is being sent to all holders of
pressurized-water reactor (PWR) operating licenses (except those that
have certified that they are permanently shutdown). 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 and Babcock and Wilcox designed plants.
Purpose
The U.S. Nuclear Regulatory Commission (NRC) is issuing this
supplement to Bulletin 96-01 to: (1) Alert addressees to the issues
concerning incomplete control rod insertion as a result of distortion
of the thimble tubes, (2) request all licensees of Westinghouse and
Babcock and Wilcox designed plants take actions to ensure the continued
operability of the control rods, and (3) require that all licensees of
Westinghouse and Babcox and Wilcox designed plants send to the NRC a
written response to this bulletin supplement relating to the actions
and information requested in this supplement.
Background
Incomplete control rod insertion has been previously addressed by
the NRC in Information Notice (IN) 96-12, ``Control Rod Insertion
Problems,'' dated February 15, 1996, and Bulletin 96-01, ``Control Rod
Insertion Problems,'' dated March 8, 1996. Bulletin 96-01 requested
actions to ensure that all affected plants respond in a proactive
manner to recent industry experience and support data collection that
permitted the staff to more effectively assess this issue and determine
whether further regulatory action was needed. Since Bulletin 96-01 was
issued, there has been extensive investigation of the issue, including
evaluation of plant data (trip, rod drop time, recoil and drag data),
spent fuel pool testing, Zircaloy material property review, and review
of worldwide experience.
[[Page 27630]]
Description of Circumstances
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. One rod
drifted 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 control rods were
located in XLR fuel assemblies, which 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 the fully inserted position 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 Unit 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 burnups of 47,782 MWD/MTU and 49,613 MWD/MTU during two cycles
of irradiation.
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 contained three different 17x17 fuel types-
Standard XL, Standard XLR, and VANTAGE 5H-all of which are designed and
fabricated by Westinghouse. The core contained 57 control rods, all of
which are 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 effect
that is normally seen in the traces as a result of contact of the
control rod assembly spider hub spring with the fuel assembly. The
testing of similar rods in Unit 2 revealed no adverse indications. One
rod showed no recoil but inserted fully into the core.
When rod drop tests were performed at South Texas Unit 1 on March
4, 1996, seven rods failed to fully insert. The stuck rods were in fuel
assemblies with burnups from 43,500 to 47,500 MWD/MTU. All seven
stopped at 6 steps from the bottom. Again there was no significant
degradation in the rod drop times.
During end-of-cycle (Unit 1 Cycle 6) rod drop tests on May 18,
1996, 11 rods did not fully insert; 9 stuck at six steps and 2 stuck at
twelve steps. Two of the rods were in fuel assemblies with lower
burnups--32,200 and 35,400 MWD/MTU.
Mid-cycle (Unit 1 Cycle 7) testing was performed on January 25,
1997, when the burnup reached approximately 32,000 MWD/MTU on the most
burned rodded assembly in the new cycle. During this test two rods
stuck at six steps. Both control rods were located in V5H fuel
assemblies, which were in their second cycle with burnups of 26,100 and
27,400 MWD/MTU.
On February 8, 1997 when South Texas Unit 2 shutdown for refueling,
four rods stuck at six steps and one rod stuck at twelve steps. The
associated fuel assembly burnups were 39,800 to 52,700 MWD/MTU. Four of
these five rods had shown zero or one recoil during rod drop testing in
January 1996. Although all rod drop times were within technical
specification limits, increases in rod drop times were observed for
some rods. Examination of the rod drop traces showed marked differences
from previous normal traces. Thus indicating resistance above the
dashpot area.
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;
the remaining rod needed to be manually inserted. The remaining 45 rods
fully inserted when dropped, although a number of them did not exhibit
the expected number of recoils. Of the total 53 control rod assemblies,
the assembly at core location H2 (the only rod stopping outside the
dashpot region) was a hafnium control rod; the remaining were 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
results of 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
was 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 that the cause of the binding was
related to the fuel assemblies and not the control rods. Subsequent
control rod drag testing data indicated a correlation of control rod
[[Page 27631]]
drag force to assembly burnup and a significant increase in drag force
at assembly burnups greater than 45,000 MWD/MTU.
Regulatory Requirements and Guidance
10 CFR part 50, Appendix B, Section XI, ``Test Control'' requires
that ``a test program shall be established to assure that * * *
structures, systems, and components will perform satisfactorily * * *''
The requested actions described below will assure that adequate
shutdown margin is maintained and that the control rods will
satisfactorily perform their intended function of effectively
terminating the fission process during all operating conditions in
accordance with the current licensing basis for each facility.
Regulatory guidance for the control rods is stated in General
Design Criterion (GDC) 26, of Appendix A to 10 CFR part 50,
``Reactivity Control System Redundancy and Capability,'' of Appendix A
to 10 CFR Part 50 which specifies ``Two independent reactivity control
systems of different design principles shall be provided. One of the
systems shall use control rods, preferably including a positive means
for inserting the rods, and shall be capable of reliably controlling
reactivity changes to assure that under conditions of normal operation,
including anticipated operational occurrences, and with appropriate
margin for malfunctions such as stuck rods, specified acceptable fuel
design limits are not exceeded.''
In addition, GDC 29 ``Protection against anticipated operational
occurrences,'' states that the protection and reactivity control
systems shall be designed to assure an extremely high probability of
accomplishing their safety functions in the event of anticipated
operational occurrences.
Worldwide experience of incomplete control rod insertion problems
(other than those caused by debris, foreign material, or control rod
drive mechanism problems) has shown that the primary cause was thimble
tube distortion caused by excessive compressive loads. This problem has
been limited to fuel designs that incorporate small-diameter
(approximately 0.5 inch) thimble tubes. Current data show that
distortion significant enough to cause incomplete insertion has not
occurred below certain burnup levels. Thus small-diameter thimble tube
fuel designs are considered acceptable up to those burnup levels. In
order to meet the current licensing basis for each facility, the
ability to insert the control rods needs to be demonstrated for burnups
that exceed these burnup levels. This ability can be demonstrated
through testing at intervals or by a rigorous engineering analysis.
Discussion
The root cause explanation for the Wolf Creek event was that the
increased compressive load was caused by greater than expected fuel
assembly growth. The phenomenon appears to be dependent on a number of
factors, including burnup, temperature, and power history, the
interaction of which is not clearly understood. Nothing in this root
cause explanation would preclude other fuel designs from exhibiting
similar behavior at different combinations of burnup, power history,
and core exit temperature. In addition, unknown factors may also
contribute to the observed behavior.
The root cause of the incomplete control rod insertions at South
Texas Project has been identified as excessive fuel assembly guide tube
distortion in the dashpot. The reason for the distortion is inadequate
resistance to buckling in the fuel assembly design under required loads
and burnup.
The NRC staff has evaluated the data obtained as a result of
Bulletin 96-01 and determined that while most of the high drag data has
been in high-temperature plants, there have been a number of cases of
high drag in lower temperature plants. High drag has been correlated
with thimble tube distortion. Thus, it is not clear that plants with
lower temperatures are not susceptible to thimble tube distortion,
which can lead to incomplete control rod insertion.
Although fuel with intermediate flow mixing grids (IFMs) would
appear to be stiffer and thus less susceptible to distortion, it has
not been shown that this fuel is not susceptible to thimble tube bowing
from compressive loads. Furthermore, since the mid-spans would be
strengthened, the top and bottom spans might be the most susceptible
portions of the fuel assembly and distortion of the top span could lead
to control rods sticking very high in the core. Thus, the staff still
considers this fuel susceptible to thimble tube distortion which can
lead to incomplete control rod insertion.
Although incomplete control rod insertion has only been experienced
in a small number of fuel assembly designs to date, the NRC staff
believes that all designs that incorporate small-diameter thimble tubes
need to be examined, since these small-diameter thimble tubes appear to
be susceptible to distortion and thus susceptible to control rod
binding problems at high burnup levels.
Bulletin 96-01 requested actions through calendar year 1996 only.
However, the staff believes that continued actions, as stated in this
supplement, are necessary in order to resolve the concerns about small-
diameter thimble tube distortion leading to incomplete control rod
insertion.
While the tests performed in response to Bulletin 96-01 did not
reveal any additional incomplete control rod insertions and all rod
drop times measured met the Technical Specification limits for drop
times to top of the dashpot, there were other disturbing results. The
drag measurements resulted in dashpot drag above the criteria in three
plants and higher than normal drag in an additional six plants. Thimble
tube measurements were above the criteria in six plants and high in
three other plants. In addition, during measurements in the spent fuel
pool control rods could not be fully inserted under their own weight in
several plants.
Safety Assessment
The staff considers the potential for thimble tube distortion
caused by high burnup and excessive compressive loads, leading to
incomplete control rod insertion, a safety issue. In the absence of
corrective actions that clearly eliminate the problem, the staff
remains concerned about the ability to fully insert the control rods.
The safety significance depends on the amount of shutdown margin lost
because of incomplete control rod insertion. Were the control rods to
stick high in the core, the reactor could not be shut down by the
control rods, and other means, such as emergency boration, would be
required.
At this time, the NRC staff considers all fuel designs that
incorporate a small-diameter thimble tube to be potentially susceptible
to thimble tube distortion caused by excessive compressive loads.
Although the problem has only been observed in Zircaloy thimble tubes,
the possibility of thimble tube distortion needs to be addressed for
fuel assemblies incorporating other materials.
Requested Actions
In order to ensure the continued operability of the control rods,
all licensees of Westinghouse and Babcock and Wilcox designed plants
are requested to verify the full insertability and rod drop times by
testing control rods in fuel assemblies with burnups greater than
35,000 MWD/MTU for assemblies without IFMs for 12 foot cores
[[Page 27632]]
40,000 MWD/MTU for assemblies with IFMs for 12 foot cores
25,000 MWD/MTU for assemblies in 14 foot cores
upon first reaching the limit(s) and approximately every 2,500 MWD/MTU
until the end of cycle. In addition, end-of-cycle rod drop time tests
and drag testing of all rodded fuel assemblies should be performed.
Licensees are requested to submit their anticipated schedule for
testing within 30 days of the date of this bulletin supplement. Within
30 days after completion of each set of testing, licensees are
requested to submit a report that summarizes the data and documents the
results obtained.
In order to meet the current licensing basis for each facility, the
ability to insert the control rods needs to be demonstrated for burnups
greater than those presented in the bulletin supplement. This ability
can be demonstrated through testing at intervals specified above, or by
a rigorous engineering analysis.
Required Response
Pursuant to Section 182a, the Atomic Energy Act of 1954, as
amended, and 10 CFR 50.54(f), all licensees of Westinghouse and Babcock
and Wilcox designed plants must submit the following written
information under oath and affirmation:
Within 30 days of the date of this bulletin supplement, a response
indicating whether the requested actions will be taken and a schedule
indicating when the actions will be performed. Licensees who choose not
to take the requested actions must describe in their response any
alternative course of action that they propose to take, including the
basis for the acceptability of the proposed alternative course of
action, and the schedule for completion of the alternative.
If, in the course of responding to this bulletin, a licensee
determines that it is not in compliance with the Commission's rules and
regulations, the licensee is expected to take corrective actions in
accordance with the requirements of Section XVI of 10 CFR part 50,
Appendix B.
Address the required written responses to the U.S. Nuclear
Regulatory Commission, ATTN: Document Control Desk, Washington, DC
20555-0001. In addition, submit a copy of the response to the
appropriate regional administrator.
Related Generic Communications
NRC Information Notice 96-12, ``Control Rod Insertion Problems''
NRC Bulletin 96-01, ``Control Rod Insertion Problems.''
Backfit Discussion
This bulletin supplement transmits an information request pursuant
to the provisions of Section 182a of the Atomic Energy Act of 1954, as
amended, and 10 CFR 50.54(f) to determine whether addressees are taking
appropriate action to ensure continued operability of the control rods.
To the extent that the actions requested herein by addressees are
considered backfits, the backfits are justified under the compliance
exception of the backfit rule, that is, 10 CFR 50.109(A)(4)(i).
10 CFR Part 50, Appendix B, Section XI, ``Test Control'' requires
that ``a test program shall be established to assure that * * *
structures, systems, and components will perform satisfactorily * * *''
The requested actions previously described will assure that adequate
shutdown margin is maintained and that the control rods will
satisfactorily perform their intended function of effectively
terminating the fission process during all operating conditions in
accordance with the current licensing basis for each facility.
The objective of the actions requested in this bulletin supplement
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 and control rod drop times.
Dated at Rockville, Maryland, this 13th day of May, 1997.
Seymour H. Weiss,
Acting Deputy Director, Division of Reactor Program Management, Office
of Nuclear Reactor Regulation.
[FR Doc. 97-13189 Filed 5-19-97; 8:45 am]
BILLING CODE 7590-01-P