ML072150109

Generic Letter 2004-02 - 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

ML072150109
Person / Time
Site:	Watts Bar
Issue date:	08/01/2007
From:	Smith J D Tennessee Valley Authority
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
GL-04-002, TAC MC4730
Download: ML072150109 (10)

Text

August 1, 2007 10 CFR 50.54(f)

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)

References:

1) 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"

2) TVA Letter to NRC dated September 1, 2005, "Watts Bar Nuclear Plant (WBN) Unit 1- NRC

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)"

3) TVA Letter to NRC dated April 11, 2006, "Watts Bar Nuclear Plant (WBN) Unit 1 - 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)"

U.S. Nuclear Regulatory Commission Page 2 August 1, 2007

4) TVA Letter to NRC dated July 3, 2006, "Watts Bar Nuclear Plant (WBN) Unit 1 - Generic Letter 2004 Request for Additional

Information Regarding the Nuclear Regulatory

Commission Staff Audit on the Containment Sump

Modifications (TAC No. MC4730)"

5) TVA Letter to NRC dated August 8, 2003, "Watts Bar Nuclear Plant (WBN) Unit 1 - Response to

Bulletin 2003 Potential Impact of Debris

Blockage on Emergency Sump Recirculation at

Pressurized-Water Reactors" 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 2, 3, and 4.

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.

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

following the fall 2007 outage, TVA has determined that in U.S. Nuclear Regulatory Commission Page 3 August 1, 2007 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 early

February 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 1st day of August 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 August 1, 2007 Enclosures cc (Enclosures): NRC Resident Inspector Watts Bar Nuclear Plant 1260 Nuclear Plant Road Spring City, Tennessee 37381 Mr. Brendan T. Moroney, 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

ENCLOSURE 1 E1-1 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 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.

E1-2 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 115% larger than the strainer opening size

to eliminate any potential blockage in the ECCS injection path.

The changes to the fuel bottom nozzles meet the licensing basis

requirement that the strainer openings be the smallest opening in

the ECCS flow path.

The debris generation calculation and successor downstream impact

evaluations are currently being revised to reconcile the fiber

quantities considered due to Min-K insulation and 3M fire wrap

relative to that tested in the WBN strainer test.

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.

E1-3 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 cannot adversely impact sump operation.

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

potential loss of net positive suction head (NPSH) 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. The probability

of a large break loss of coolant accident (LOCA) remains

extremely low, as is the probability of a small break LOCA that

may require recirculation. The Watts Bar Unit 1 containment is

compartmentalized making total debris transport to the sump

strainers less likely. The time to switchover to recirculation (approximately 10 minutes after initiation of an event) allows

for debris settling.

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.

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, some of the debris could travel around to the other side of

containment to collect on the strainer modules on the opposite

side. There are only small quantities of fibrous material in

containment. Stainless steel reflective metallic insulation is

the predominant insulation type. There are no break locations or

break sizes in the reactor coolant system that will result in

min-K or 3M fire wrap being debris in the sump without the

presence of substantial amounts of reflective metal insulation (RMI) debris. 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 low

sump pH throughout the accident period. This limits corrosion of

light metals and limits chemical effects. All 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 through the strainer openings showed that

the lengths of such fibers were too short to be of concern.

E1-4 Analyses by the vendors and NRC have shown that very little open

area at the bottom of the core is needed to maintain long term

core cooling and acceptable fuel clad temperatures. The number

of new bottom nozzles already installed on fuel currently in WBN

substantially exceeds the flow area needed to assure core cooling

post-LOCA. Based on the visual examination of the bypass debris

from the tests of the WBN strainer, no fuel blockage would occur

with the old bottom nozzles. Using very conservative bypass

fractions it is theoretically, but not physically, possible to

block the old bottom nozzles. With the removal or banding of the

fibrous insulation in the next outage, these theoretical bypass

fractions will be reduced to acceptable values with the old

bottom nozzles. Approximately 2/3 of the core will have the new

nozzles in place after the spring 2008 outage. The old nozzles

will only be present on twice burned fuel assemblies, so there

are no concerns with long term core cooling and clad

temperatures.

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 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.

E1-5 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.