ML072760449
| ML072760449 | |
| Person / Time | |
|---|---|
| Site: | Watts Bar  |
| Issue date: | 10/02/2007 |
| From: | Smith J D Tennessee Valley Authority |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| GL-04-002, TAC MC4730 | |
| Download: ML072760449 (11) | |
Text
October 2, 2007
U. S. Nuclear Regulatory Commission ATTN: Document Control Desk
Washington, D.C. 20555-0001
Gentlemen: In the Matter of ) Docket No. 50-390 Tennessee Valley Authority )
WATTS BAR NUCLEAR PLANT (WBN) UNIT 1 - GENERIC LETTER 2004 POTENTIAL IMPACT OF DEBRIS BLOCKAGE ON EMERGENCY
RECIRCULATION DURING DESIGN-BASIS ACCIDENTS AT PRESSURIZED
WATER REACTORS - REQUEST FOR EXTENSION OF COMPLETION DATE FOR
CORRECTIVE ACTIONS (TAC NO. MC4730) - REVISED
References:
Generic Letter 2004 Potential Impact of
Debris Blockage on Emergency Recirculation
During Design Basis Accidents at Pressurized
Water Reactors - Request for Extension of
Completion Date for Corrective Actions (TAC
No. MC4730)
- 2) NRC Generic Letter 2004-02 dated September 13, 2004, "Potential Impact of
Debris Blockage on Emergency Recirculation
During Design Basis Accidents at Pressurized-
Water Reactor"
Generic Letter (GL) 2004-02: Potential Impact
of Debris blockage on Emergency Recirculation
During Design Basis Accidents at Pressurized
Water Reactors (PWR) - Second Response (TAC
No. MC4730)"
U.S. Nuclear Regulatory Commission Page 2 October 2, 2007
Generic Letter 2004-02 Potential Impact of
Debris blockage on Emergency Recirculation During Design Basis Accidents at Pressurized
Water Reactors (PWR) - Response to Request
for Additional Information (TAC No. MC4730)"
Letter 2004 Request for Additional Information Regarding the Nuclear Regulatory Commission Staff Audit on the Containment
Sump Modifications (TAC No. MC4730)"
Response to Bulletin 2003 Potential
Impact of Debris Blockage on Emergency Sump
Recirculation at Pressurized-Water Reactors"
This letter revises TVA's Letter to NRC dated August 1, 2007 (Reference 1) based upon discussions with the NRC Staff after
that letter was submitted. The changes are denoted by
revision bar. NRC Generic Letter (GL) 2004-02 (Reference 1)
requested that licensees provide information regarding the
potential impact of debris blockage on emergency recirculation during design basis events. TVA provided the
requested information in References 3, 4, and 5.
Item 2(b) of GL 2004-02 states that all actions should be
completed by December 31, 2007; provide justification for not
implementing the identified actions during the first
refueling outage starting after April 1, 2006, and describe
how the regulatory requirements discussed in the Applicable
Regulatory Requirements section will be met until the
corrective actions are completed.
During the fall 2007 outage for WBN Unit 1, corrective
actions associated with GL 2004-01 were partially
implemented. New sump strainers were installed with
increased surface area, the orifice in the high head
injection flow path was resized to allow the throttle valves
in this flow path to be opened further, and the old Steam
Generators were replaced with uncoated Steam Generators to
reduce the amount of coating debris transported to the sump.
U.S. Nuclear Regulatory Commission Page 3 October 2, 2007
In response to question 1 of Reference 5, TVA stated that as
a result of the revised debris generation analysis, some of the fiber quantities due to Min-K insulation and 3M fire wrap
have increased with respect to that tested in the WBN
strainer test and that WBN was looking at several options to
reduce these quantities to within the tested configuration.
These options include: credit for additional jet shielding
due to robust barriers and large structures, material testing
under jet impingement loading to reduce the zone of influence (ZOI) for encapsulated fiber, removal of material, and/or sump strainer re-testing. Based on additional jet
impingement testing following the fall 2006 outage, TVA has
determined that in order to meet the fibrous debris loading
for the tested sump configuration that some Min-K insulation
needs to be replaced with reflective metal insulation and
some will require installation of additional restraint bands
to prevent damage.
As discussed with the WBN NRC Project Manager, the actions to
replace the Min-K insulation and add additional restraint
bands will be completed during the upcoming refueling outage (Unit 1 Cycle 8 Outage) that is scheduled to start by
February 10, 2008. Enclosure 1 provides the basis supporting
TVA's conclusion that it is acceptable to extend the WBN Unit
1 completion date.
A list of regulatory commitments is provided in Enclosure 2.
If you have any questions concerning this matter, please call
me at (423) 365-1824.
I declare under penalty of perjury that the foregoing is true
and correct. Executed on this 2nd day of October 2007.
Sincerely, Original signed by
J. D. Smith
Manager, Site Licensing
and Industry Affairs (Acting)
Enclosures
cc: See Page 4
U.S. Nuclear Regulatory Commission Page 4 October 2, 2007
JDS:RAS Enclosures cc (Enclosures): NRC Resident Inspector Watts Bar Nuclear Plant 1260 Nuclear Plant Road Spring City, Tennessee 37381
Mr. Brendan T. Moroney, Senior Project Manager U.S. Nuclear Regulatory Commission MS 08G9a One White Flint North 11555 Rockville Pike Rockville, Maryland 20852-2738
U.S. Nuclear Regulatory Commission Region II Sam Nunn Atlanta Federal Center 61 Forsyth St., SW, Suite 23T85 Atlanta, Georgia 30303
E1-1 ENCLOSURE 1 WATTS BAR NUCLEAR PLANT (WBN) UNIT 1 GENERIC LETTER 2004 POTENTIAL IMPACT OF DEBRIS BLOCKAGE ON EMERGENCY RECIRCULATION DURING DESIGN-BASIS ACCIDENTS AT PRESSURIZED WATER REACTORS REQUEST FOR EXTENSION OF COMPLETION DATE FOR CORRECTIVE ACTIONS In Generic Letter (GL) 2004-02, dated September 13, 2004, the NRC staff summarized their bases for concluding that existing
pressurized-water reactors (PWRs) could continue to operate
through December 31, 2007, while implementing the required
corrective actions for NRC Generic Safety Issue 191 (GSI-191),
"Assessment of Debris Accumulation on PWR Sump Performance." In
the following discussion TVA has addressed the "Criteria for
Evaluating Delay of Hardware Changes," as described in SECY 0078, dated March 31, 2006. This discussion supports TVA's
request for extension of the completion date for the corrective
actions at WBN Unit 1 from December 31, 2007, to the completion
of the spring 2008 refueling outage. The proposed extension of
the GSI-191 implementation schedule by approximately 2-months for
WBN Unit 1 does not alter the original conclusions summarized in
GL 2004-02 in which the staff determined that it is acceptable for PWR licensees to operate until the corrective actions are
completed.
SECY-06-0078 Criterion:
The licensee has a plant-specific technical/experimental plan with milestones and schedule to address outstanding technical
issues with enough margin to account for uncertainties.
The licensee identifies mitigative measures to be put in place
prior to December 31, 2007, and adequately describes how these mitigative measures will minimize the risk of degraded ECCS (emergency core cooling system) and CSS (containment spray
system) functions during the extension period.
=
Reason for Request===
In response to question 1 of Reference 4, TVA stated that as a result of the revised debris generation analysis, some of the fiber quantities due to Min-K insulation and 3M fire wrap have
increased with respect to that tested in the WBN strainer test
and that WBN was looking at several options to reduce these
quantities to within the tested configuration. These options include: credit for additional jet shielding due to robust
barriers and large structures, material testing under jet
impingement loading to reduce the zone of influence (ZOI) for
encapsulated fiber, removal of material, and/or sump strainer re-
testing. Based on additional jet impingement testing, TVA has
determined that in order to meet the fibrous debris loading for E1-2 the tested sump configuration that some Min-K insulation needs to be replaced with reflective metal insulation and some requires
installation of additional restraint bands to prevent damage.
Mitigative Measures
- 1. Measures Completed at WBN Unit 1
During the Unit 1 Cycle 7 (U1C7) refueling outage that concluded in November 2006, TVA implemented changes to the plant that included the installation of the new containment sump strainer
design, installation of a resized orifice in the high head
injection flow path and replacement of the old coated steam
generators with non-coated steam generators.
The new strainer is a significant improvement over the original
design and increases the available flow area from approximately
200 ft 2 to approximately 4600 ft
- 2. The openings in the new strainer are round holes with a diameter of 0.085 inch. This is substantially smaller than the 1/4 inch rectangular mesh of the original sump screens. Testing of the new sump strainer has been conducted that showed very low head loss with extremely
conservative debris loadings.
A review of the Emergency Core Cooling System (ECCS) injection pathways was performed and determined that a change to the
throttle position of the high head injection flow path throttle
valves was necessary to ensure debris would not be trapped in the
throttle valves. To allow for the further opening of the
throttle valves the orifice in the flow path was replaced with
one having a smaller bore size. The new orifice and the throttle
valve position changes were accomplished during the U1C7 outage.
These changes ensured that the orifice and the throttle valve openings were at least 50 percent larger than the strainer opening size to eliminate any potential blockage in the ECCS injection path.
- 2. Containment Cleanliness As discussed in Reference 5, WBN has a formal program for inspecting and cleaning areas inside containment. Technical Instruction TI-12.07, "Containment Access" and procedure SPP-
10.7, "Housekeeping/Temporary Equipment Control," provide
guidelines for entering/exiting containment, acceptance criteria
for housekeeping/cleanliness to ensure no loose debris is left in containment, and for storage of materials inside containment
during MODE 4 and higher.
TI-61.003, "Ice Condenser Loose Debris Log," records, tracks, and
evaluates debris that is found in the ice condenser system to
ensure that this debris does not impact sump operation.
E1-3 3. Procedural Guidance Emergency operating procedure, ES-1.3, "Transfer to Containment Sump," contains guidance to the operators for monitoring the containment sump for blockage. This procedure provides for
monitoring the ECCS pumps and Containment Spray pumps for sump
blockage and ability to deliver necessary flow. Monitoring of
containment sump level to determine the necessity to refill the refueling water storage tank (RWST). Also with the assistance of
the Technical Support Center (TSC) personnel, the procedure
provides for guidance on reducing containment spray and ECCS flow
to single train operation to reduce the rate of debris
accumulation.
- 4. Risk Evaluation Included in Generic Letter 2004-02 were the following observations regarding risk significance that remain valid through the proposed extended implementation period to the
completion of the Spring 2008 refueling outage. WBN performed a
risk evaluation and concluded the probability of a Large Break
LOCA resulting in core damage during the two month extension is less than 1E-06. The probability of a Large Break LOCA in the
WBN PRA model is 2.67E-06 per year. It was conservatively
assumed that the amount of fibrous material released from the
existing Min-K insulation inside containment results in a debris
layer over the containment sump screen that causes the loss of
NPSH to ECCS pumps. It is also assumed that failure of the ECCS
pumps in recirculation mode eventually results in core damage.
The probability of core damage is then equal to the initiating event probability of a Large Break LOCA or 2.67E-06 per year.
The probability of Large Break LOCA resulting in core damage over
the estimated 2 months needed for the extension request is 4.39E-
07 or less than 1E-06. As stated in the analysis the assumption
that the Large Break LOCA results in sump blockage and core
damage is conservative because 1) leak before break was not
credited; 2) some of the fibrous insulation may be mixed with
reflective metallic insulation and there may not be enough flow
to dislodge this debris; and 3) mitigative measures to deal with
sump blockage were not credited.
- 5. Safety Features and Margins in Current Configuration/Design Basis The WBN containment sump incorporates many design features that help to minimize the possibility of strainer blockage. The containment sump is located in the containment floor below the
refueling canal to provide protection from high energy pipe
failures. The lower containment is an open, one-level area. The
only drains which are used to route water to the sump are the two large refueling cavity drains and the twenty ice condenser floor
drains. These drains route water away from the sump strainers.
E1-4 There are two entry paths to the sump area, separated by approximately 320 degrees around lower containment. The water
fills the floor areas and covers the sump entrance. This
provides two entry points into the sump area on opposite sides.
Thus, if a break were to occur near one of the sump entry points, water would travel around to the other side of containment to the
strainer modules on the opposite side. Stainless steel
reflective metallic insulation (RMI) is the predominant
insulation type used in the lower containment. There are no
break locations or break sizes in the reactor coolant system that
will result in Min-K being debris in the sump without the
presence of substantial amounts of RMI debris. The Min-K panels
are generally interspersed within the quadrants opposing the
containment sump inlet area. The volume of insulation contained
in a given replacement Min-K panel is small compared to the
volume of RMI that exists on the host piping segment. The Min-K
panels having the largest volume are located on two elbows on the
main steam lines from steam generators 1 and 4. These Min-K
panels will be replaced with RMI during the U1C8 refueling
outage. With the exception of the Min-K panels installed on the
two main steam line elbows, shielding by the steam generators, reactor coolant pumps, and the reactor cavity wall would allow
any single break to release fibers into the sump water that would
only marginally exceed the scaled volume under which the prototype strainer module was tested. The blowdown from a high
energy line break that would destroy the Min-K panels on the main
steam line elbows would also generate RMI debris from the host
piping segment and also from the RMI installed on the nearby
steam generators. The subsequent washdown by containment spray
and the break flow would result in the accumulation of a pile of
RMI and Min-K debris on the containment floor. Lower containment
would fill with water during the post-LOCA injection phase and
immediate release of significant amounts of Min-K captured in the
debris pile is not expected. The mitigative measures described
in item 3 above provide procedural guidance to respond in the
event of a gradual release of Min-K fibers from the debris pile
and subsequent collection on the strainer. The containment sump
has a high water level compared to most containment designs which
provides large margins in available net positive suction head (NPSH). This coupled with the low head losses established during
the testing shows that the sump screens have large safety
margins. WBN as an ice condenser used sodium tetraborate as the
buffering agent, has low temperature in the sump water, and
maintains a relatively neutral sump pH throughout the accident
period. This limits corrosion of light metals and limits
chemical effects. Testing performed to date supports the conclusion that chemical effects are not an issue at WBN. The
small hole size selected for the new strainer prevents any large
material with a potential to block fuel, ECCS injection pathways, or the containment spray nozzles from bypassing the strainer.
While it could be assumed that long fibers could pass through the
strainer openings with a potential to block fuel a visual microscopic examination of the type of material that passed E1-5 through the strainer openings showed that the lengths of such fibers were too short to be of concern.
The Westinghouse fuel installed in WBN, depending on vintage, has
two different protective features for foreign material exclusion.
WBN core design incorporates Westinghouse RFA-2 fuel with debris
filtering bottom nozzles. The size of the holes used in the
debris filtering nozzle are larger than the sump screen hole
size. Currently, approximately 2/3 of the core has a protective
grid (p-grid) design with a leading edge offset slightly from the
debris filtering bottom nozzle. This feature provides further
protection for the fuel from debris entering the bottom of the
fuel assembly during normal operation that might cause wear or
fretting damage to the fuel. This grid is situated immediately
above the bottom nozzle such that for some flow openings, the
grid cruciform bisects the inlet flow openings, whereas for other
holes, a single strap crosses the opening. This causes the
characteristic flow dimension to be smaller than the debris
filtering nozzle opening size. For the cruciform bisected holes, the opening is slightly smaller than the new sump screen hole
size, and for the remainder it is slightly larger than the screen
hole size. WBN has contracted with Westinghouse to provide an
alternate p-grid design on future fuel furnished to the plant.
The alternate p-grid design has a shortened grid height which effectively raises the grid further above the bottom nozzle and
provides a larger flow dimension than the sump screen opening
size for all the nozzle openings. The normal operation
protection for the fuel is not significantly impacted by the
alternate p-grid while the post accident performance is improved
by reducing the likelihood that particulate material passing
through the sump screen will be blocked by the protective grid.
Approximately 1/3 of the core currently has this design which was
first incorporated into the fuel during the last refueling.
Westinghouse has evaluated WBN operation with mixed p-grid/
alternate p-grid design. The evaluation was based on an existing
parametric core blockage study conducted for Sequoyah Nuclear
Plant with applicability to Watts Bar Nuclear Plant. The study
examined arbitrary core blockage percentages from 20 percent to
80 percent using natural circulation flow at the time of sump
switchover. Cladding temperatures were determined to be
acceptable for all blockages studied and it was concluded that
sufficient flow area is therefore available to protect the core
for design basis events.
WBN has NRC approval to invoke the leak-before-break methodology to eliminate the dynamic effects (pipe whip and jet impingement)
of postulated reactor coolant piping (hot leg, cross-over leg, and cold leg) ruptures from the design basis of the plant. The
approval was based on the conclusion that the probability or
likelihood of large pipe breaks occurring in the primary coolant
loops is sufficiently low. The leak would be detected and the unit brought to a safe shutdown condition prior to the occurrence E1-6 of a large pipe break. While the leak-before-break methodology was not used in determining the debris loading on the sump
strainer, it does provide additional margin in the overall sump
strainer design.
- 6. Unblocked Sump Strainers and Drainage Paths Surveillance Instructions, 1-SI-304-2 "18 Month ECCS Containment Sump Inspection" is performed during each refueling outage to ensure that the containment sump suction pit is free of debris
and that the sump components, including strainers, show no
evidence of degradation.
Inspections to assure that the refueling canal drains are not
blocked that direct the flow of containment spray water from the
upper compartment to the lower compartment are performed by
procedure 1-SI-72-3 "Containment Refueling Canal Drains." These
drains are accessible during plant operation and are inspected
every 92 days.
There are 20 ice condenser floor drains which drain to the floor in lower containment on the outside of the sump strainers. 1-SI-
61-9, "18 Months Ice Condenser Floor Drains Visual Inspection",
ensures that these floor drains, associated pipe, and valves are
free of ice, frost, or debris and each valve seat is free of any
corrosion, pitting or cracking.
There are two additional drainage paths available to supply water
to the area inside the polar crane wall. These two paths are
floor drains that are located in accumulator rooms 3 and 4.
These drains are verified clear and free flowing every other
outage in accordance with procedure 1-TRI-40-901, "Accumulator
Rooms 3 and 4 Open Ended Crane Wall Drains ASME Section XI
Unimpaired Flow Test". In addition, 1-SI-304-2 requires
inspection of the drains for no blockage each refueling outage, in conjunction with the sump screen inspection.
Conclusion Based on the above discussion TVA has determined that overall plant safety will be maintained until the corrective actions are completed during the spring 2008 refueling outage.
ENCLOSURE 2 E2-1 WATTS BAR NUCLEAR PLANT (WBN) UNIT 1 GENERIC LETTER 2004 POTENTIAL IMPACT OF DEBRIS BLOCKAGE ON EMERGENCY RECIRCULATION DURING DESIGN-BASIS ACCIDENTS AT PRESSURIZED WATER REACTORS REQUEST FOR EXTENSION OF COMPLETION DATE FOR CORRECTIVE ACTIONS COMMITMENT LIST The following provides a list of commitments in this submittal which are being tracked by TVA's commitment process.
- 1. The actions to replace the Min-K insulation and add additional restraint bands will be completed during the upcoming refueling outage (Unit 1 Cycle 8 Outage) that is scheduled to start in early February 2008.