ML061020313
ML061020313 | |
Person / Time | |
---|---|
Site: | Sequoyah |
Issue date: | 04/11/2006 |
From: | Pace P L Tennessee Valley Authority |
To: | Document Control Desk, Office of Nuclear Reactor Regulation |
References | |
GL-04-002, TAC MC4717, TAC MC4718 | |
Download: ML061020313 (18) | |
Text
April 11, 2006
U. S. Nuclear Regulatory Commission
ATTN: Document Control Desk
Washington, D.C. 20555-0001
Gentlemen: In the Matter of ) Docket Nos. 50-327 Tennessee Valley Authority ) 50-328
SEQUOYAH NUCLEAR PLANT (SQN) UNITS 1 AND 2 - GENERIC LETTER 2004 POTENTIAL IMPACT OF DEBRIS BLOCKAGE ON EMERGENCY
RECIRCULATION DURING DESIGN-BASIS ACCIDENTS AT PRESSURIZED
WATER REACTORS - RESPONSE TO REQUEST FOR ADDITIONAL
INFORMATION (TAC NOS. MC4717 AND MC4718)
The purpose of this letter is to provide TVA responses to NRC's request for additional information (RAI) for SQN dated
February 10, 2006. The responses to this RAI supplements
TVA letters dated March 7, July 21, and September 1, 2005, concerning NRC's generic letter.
The enclosure provides the TVA responses for SQN. Please
direct questions concerning this issue to J. D. Smith at
(423) 843-6672.
I declare under penalty of perjury that the foregoing is
true and correct. Executed on this 11th day of April 2006.
Sincerely, Original signed by James D. Smith for:
P. L. Pace
Manager, Site Licensing and
Industry Affairs
Enclosure
cc: See Page 2 U.S. Nuclear Regulatory Commission Page 2 April 11, 2006
cc (Enclosure):
Mr. Edgar D. Hux
94 Ridgetree Lane
Marietta, Georgia 30068
Mr. Douglas V. Pickett, Senior Project Manager
U.S. Nuclear Regulatory Commission
Mail Stop 08G-9a
One White Flint North
11555 Rockville Pike
Rockville, Maryland 20852-2739
Mr. William T. Russell
400 Plantation Lane
Stevensville, Maryland 21666
E-1 ENCLOSURE SEQUOYAH NUCLEAR PLANT (SQN)
UNITS 1 AND 2 GENERIC LETTER 2004-02 RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION
The following provides TVA's response to NRC's request for
additional information letter dated February 10, 2006 concerning
Plant Materials
- 1. (Not applicable).
- 2. Identify the amounts (i.e., surface area) of the following materials that are:
(a) submerged in the containment pool following a loss-of-coolant accident (LOCA),
(b) in the containment spray zone following a LOCA:
- aluminum
- zinc (from galvanized steel and from inorganic zinc coatings) - copper
- carbon steel not coated
- uncoated concrete Compare the amounts of these materials in the submerged and
spray zones at your plant relative to the scaled amounts of
these materials used in the Nuclear Regulatory Commission (NRC) nuclear industry jointly-sponsored Integrated Chemical
Effects Tests (ICET) (e.g., 5x the amount of uncoated carbon
steel assumed for the ICETs).
TVA Response 2 The following quantities of materials are present in the SQN
containment
Aluminum - 887 ft 2 Submerged - 200 ft 2 Zinc - 957,893 ft 2 Submerged - 71,592 ft 2 Copper - 26,000 ft 2 Submerged - 26,000 ft 2 Carbon Steel - 373,120 ft 2 Submerged - 74,624 ft 2 Uncoated Concrete - 78,586 ft 2 Submerged - 7,859 ft 2
E-2 The ICET totals for aluminum and copper were 45 and 7.4 times the SQN amount respectively. SQN has 1.3 times the
zinc of the ICET tests. The ICET test used an exposed
concrete area of 12,630 ft 2 all submerged. The ICET value is 0.2 times the SQN value. The ICET value for carbon steel
is small compared to SQN. The actual surface area used for
scaling was not provided in the test plan, or the Test 1 or
5 test report. The surface to volume ratio at SQN is 60
times the ICET value although much of that number is above
the sump and not subject to containment spray. Examples of
this are all of the steam generator shells, the pressurizer, and the upper parts of the reactor vessel. These quantities
are representative of the entire containment, not just the
quantities either in the sump or subject to spray. As an
example, the zinc values include ice baskets and inorganic
zinc paint on the containment shell. Neither of these is in
the sump pool or subjected to containment spray.
- 3. Identify the amount (surface area) and material (e.g., aluminum) for any scaffolding stored in containment.
Indicate the amount, if any, that would be submerged in the
containment pool following a LOCA. Clarify if scaffolding
material was included in the response to Question 2.
TVA Response 3 There is a provision to store some scaffolding in containment that is either submerged in the sump or subject
to containment spray. The total light-metal inventory
associated with the stored scaffolding is approximately
615 ft 2 of zinc. This inventory is included in the total zinc inventory discussed in the response to Question 2.
All other scaffold type structures present in the lower
compartment are considered to be permanent plant features
and are also included in the response to Question 2.
- 4. Provide the type and amount of any metallic paints or non-stainless steel insulation jacketing (not included in the
response to Question 2) that would be either submerged or
subjected to containment spray.
TVA Response 4 There are no paints or non-stainless steel insulation
jackets not included in the response to Question 2.
Containment Pool Chemistry
- 5. Provide the expected containment pool pH during the emergency core cooling system (ECCS) recirculation mission
time following a LOCA at the beginning of the fuel cycle and
at the end of the fuel cycle. Identify any key assumptions.
E-3 TVA Response 5 The expected sump pH is between 8.0 and 8.4 for a LOCA at any time during the fuel cycle. The sump pH range includes
conditions for the beginning and end of core life, the
minimum and maximum quantities of boron and buffering agent
in the reactor coolant system (RCS), the accumulators, the
refueling water storage tank, and the ice condenser. The
range also includes the maximum and minimum water and ice
volumes. The temperature variation of the RWST and
accumulators was included in developing this range.
- 6. For the ICET environment that is the most similar to your plant conditions, compare the expected containment pool
conditions to the ICET conditions for the following items:
boron concentration, buffering agent concentration, and pH.
Identify any other significant differences between the ICET
environment and the expected plant-specific environment.
TVA Response 6 ICET 5 is the test most representative of the SQN
environment. The boron concentration in the test is 2800
parts per million (ppm) versus a plant concentration of 2500 to 2700 ppm.
The buffer is sodium tetraborate with a weight range of 11,070 to 13,284 pounds. The weight range is based on the amount of ice assumed to be in the ice condenser
(2,225,880 to 2,671,056 pounds). The test pH ranged from 8.0 to 8.5 and the plant pH (8.0 to 8.40) is essentially identical. The amount of aluminum is much higher in ICET 5
than is present in the plant. Since this is the predominant
precipitant, this is significant. The other significant
difference is that the ICET temperature is much higher than
the SQN post-LOCA temperature. ICET 5 showed concentrations
of dissolved aluminum of 55 milligrams per liter (mg/l) and
calcium of 35 mg/l. A correlation developed by Westinghouse
from separate effects precipitation test data (WCAP-16530)
showed a total of 9.6 mg/l for the precipitants at SQN based on the total weight of the precipitants. The precipitants
predicted by the Westinghouse correlations were composed
mainly of NaAlSi 3 O 8 with a small amount of AlOOH.
- 7. For a large-break LOCA (LBLOCA), provide the time until ECCS external recirculation initiation and the associated pool
temperature and pool volume. Provide estimated pool
temperature and pool volume 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> after a LBLOCA.
Identify the assumptions used for these estimates.
E-4 TVA Response 7 The minimum time for the start of residual heat removal (RHR) recirculation from the sump is approximately
485 seconds or 8.1 minutes. This is the time assuming both
emergency core cooling system (ECCS) and containment spray
trains are running. The pool volume is 59,487 ft 3 with a temperature of 177.5 o F. The pool volume at 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> is 72,425 ft 3 with a pool temperature of 132.3 o F. The sump temperature is based on single train operation with the
maximum ultimate heat sink temperature and highest refueling
water storage tank temperature. The minimum refueling
water storage tank (RWST) injected volume was used.
Plant-Specific Chemical Effects
- 8. Discuss your overall strategy to evaluate potential chemical effects including demonstrating that, with chemical effects
considered, there is sufficient net positive suction head
margin available during the ECCS mission time. Provide an
estimated date with milestones for the completion of all
chemical effects evaluations.
TVA Response 8 TVA included concentrations of chemical precipitants in head
loss testing performed for the advanced design containment
sump strainers that significantly bound the concentrations present in the SQN post-LOCA sump inventory. For the
purpose of the test, TVA used quantities of aluminum oxides
and calcium carbonate corresponding to 90 mg/l. Both of
these materials are insoluble in water at the temperature of
the test loop. The advanced sump strainer design included a
10 percent (%) increase in required strainer area to accommodate anticipated chemical effects. The testing confirmed that the strainer area is sufficiently large that
a fiber bed cannot form. Chemical precipitants are not a
head loss contributor when there is not a fiber bed.
Further, the quantity of chemical precipitants is very small
compared to the total quantity of particulate debris
evaluated and tested based on the debris generation
calculations. Thus the head loss is not sensitive to the
quantity of chemical precipitants present. Based on these
considerations, the 10% increase in advanced design strainer
flow area to accommodate the chemical effects of the post-
LOCA sump recirculation inventory is considered to be
adequate. TVA has completed all actions associated with chemical effects evaluations accordingly.
E-5 9. Identify, if applicable, any plans to remove certain materials from the containment building and/or to make a
change from the existing chemicals that buffer containment
pool pH following a LOCA.
TVA Response 9 TVA already uses a buffer (sodium tetraborate) that is one of the alternative buffering materials being considered by
the industry. There is no need to remove any materials from
the containment to deal with chemical effects at SQN.
- 10. If bench-top testing is being used to inform plant specific head loss testing, indicate how the bench-top test
parameters (e.g., buffering agent concentrations, pH, materials, etc.) compare to your plant conditions. Describe
your plans for addressing uncertainties related to head loss
from chemical effects including, but not limited to, use of
chemical surrogates, scaling of sample size and test
durations. Discuss how it will be determined that
allowances made for chemical effects are conservative.
TVA Response 10 See the response to Question 8.
Plant Environment Specific
- 11. Provide a detailed description of any testing that has been or will be performed as part of a plant-specific chemical
effects assessment. Identify the vendor, if applicable, that will be performing the testing. Identify the
environment (e.g., borated water at pH 9, deionized water, tap water) and test temperature for any plant-specific head
loss or transport tests. Discuss how any differences
between these test environments and your plant containment
pool conditions could affect the behavior of chemical
surrogates. Discuss the criteria that will be used to
demonstrate that chemical surrogates produced for testing (e.g., head loss, flume) behave in a similar manner
physically and chemically as in the ICET environment and
plant containment pool environment.
TVA Response 11 TVA has not performed and does not anticipate performing
plant specific chemical effects tests. TVA has performed
plant specific head loss tests for the advanced design
containment sump strainer design. These tests were
performed by AREVA and did include consideration of chemical
precipitants in the recirculation inventory as discussed in E-6 the response to Question 8
. The test environment was a flume test using tap water at cold temperatures. The chemicals included in the test inventory were chemically
inert at the test temperatures and behaved similar to other particulate debris in the inventory
. Because no fiber bed can form on the SQN sump screen as the result of the amount of screen area available compared with the available fiber, chemical precipitants do not impact head loss in a manner
any different than other particulate debris.
As indicated in the response to Question 8, testing confirmed that the
advanced design strainer head loss is not sensitive to the
quantity of chemical precipitants present for the SQN post-
LOCA sump recirculation inventory.
- 12. For your plant-specific environment, provide the maximum projected head loss resulting from chemical effects (a)
within the first day following a LOCA, and (b) during the
entire ECCS recirculation mission time. If the response to
this question will be based on testing that is either
planned or in progress, provide an estimated date for
providing this information to the NRC.
TVA Response 12 The testing done for SQN did not provide a separate effect
evaluation of the head loss due to chemical effects. The
total head loss was minimal for all debris. Given the low
quantities of chemical precipitants and that no fiber bed is
present to act as a filter it is judged that the maximum
head loss due to chemical effects at the end of the first
day is zero. Similarly, the maximum head loss due to
chemical effects at any time during the 30 days following a
LOCA is judged to be zero.
ICET 1 and ICET 5 Plants
- 13. Results from the ICET #1 environment and the ICET #5 environment showed chemical products appeared to form as the
test solution cooled from the constant 140 o F test temperature. Discuss how these results are being considered
in your evaluation of chemical effects and downstream
effects. TVA Response 13 The quantities of materials used in the SQN tests were based
on the amounts of dissolved material present in ICET 5.
The ICET 5 test report did not provide quantities of
precipitants formed. Using data from the Westinghouse
Owner's Group (WOG) Chemical Effects Tests, a quantification
of the precipitants generated when the sump cooled to E-7 approximately 70 o F was made. The quantities of chemical precipitant surrogates used in the flume tests were 9 times
greater than would be seen in the plant. These quantities
then contributed to the head loss measured and in the
quantities of material that was carried through the strainer
and would impact a downstream effects evaluation.
Trisodium Phosphate Plants
- 14. (Not applicable).
- 15. (Not applicable).
Additional Chemical Effects Questions
- 16. (Not applicable).
- 17. (Not applicable).
- 18. (Not applicable).
- 19. (Not applicable).
- 20. (Not applicable).
- 21. (Not applicable).
- 22. (Not applicable).
- 23. (Not applicable).
- 24. (Not applicable).
Coatings Generic - All Plants
- 25. Describe how your coatings assessment was used to identify degraded qualified/acceptable coatings and determine the
amount of debris that will result from these coatings. This
should include how the assessment technique(s) demonstrates
that qualified/acceptable coatings remain in compliance with
plant licensing requirements for design basis accident (DBA)
performance. If current examination techniques cannot
demonstrate the coatings' ability to meet plant licensing
requirements for DBA performance, licensees should describe
an augmented testing and inspection program that provides
assurance that the qualified/acceptable coatings continue to
meet DBA performance requirements. Alternately, assume all
containment coatings fail and describe the potential for
this debris to transport to the sump.
E-8 TVA Response 25 TVA performed head loss testing assuming all coatings failed
whether qualified or not. The head loss from the strainer
for this test condition was similar to the head loss based
on a 10 diameter (D) ZOI for qualified coatings and is a
small fraction of the net positive suction head (NSPH)
available. Thus, no further assessment of the condition of
coatings in the plant is needed.
Plant Specific
- 26. (Not applicable).
- 27. (Not applicable).
- 28. (Not applicable).
- 29. (Not applicable).
- 30. The NRC staff's safety evaluation (SE) addresses two distinct scenarios for formation of a fiber bed on the sump
screen surface. For a thin bed case, the SE states that all
coatings debris should be treated as particulate and assumes
100% transport to the sump screen. For the case in which no
thin bed is formed, the staff's SE states that the coatings
debris should be sized based on plant-specific analyses for
debris generated from within the zone of influence (ZOI) and
from outside the ZOI, or that a default chip size equivalent
to the area of the sump screen openings should be used (Section 3.4.3.6). Describe how your coatings debris
characteristics are modeled to account for your plant-
specific fiber bed (i.e. thin bed or no thin bed). If your
analysis considers both a thin bed and a non-thin bed case, discuss the coatings debris characteristics assumed for each
case. If your analysis deviates from the coatings debris
characteristics described in the staff-approved methodology
above, provide justification to support your assumptions.
TVA Response 30 The SQN advanced design containment sump strainer has been
designed to preclude the formation of a fiber bed (thin or
thick) for post accident sump recirculation operation. To
confirm this design objective, a series of flow
transport/blockage tests were performed. The design basis
test case was performed with all failed coatings simulated
as 10 micron particles. This test was intended to maximize
small particulate transport to the sump screen and serve as E-9 a limiting case for thin bed blockage effects. Upon confirmation that the strainer design will preclude thin bed
formation, additional tests were performed to evaluate other
sump blockage mechanisms. These tests included 1) the
limiting failed coating size for maximum strainer blockage (i.e., the size of the failed coatings in this case were
approximately 1/8" square and 5 mils thick and were
considered small enough to maximize transport and large enough to maximize strainer blockage); 2) the maximum coating inventory (i.e., the coating quantities for phenolic
and inorganic zinc coatings were increased to reflect the
total amount of qualified and unqualified coatings inside
containment); and 3) the maximum latent debris inventory (i.e., the quantity of assumed latent dust and dirt was
increased by an order of magnitude to bound latent debris
effects). There was very little change in measured head
loss in all cases. The head loss difference between the
particulate cases and the chip cases was less than 0.05 feet. 31. Your submittal indicated that you had taken samples for latent debris in your containment, but did not provide any
details regarding the number, type, and location of samples.
Please provide these details.
TVA Response 31 A quantitative latent debris walkdown was performed at SQN.
This walkdown was an as found at the start of the refueling
outage. There had been no special containment cleaning.
The walkdown involved the collection of debris samples from
31 locations inside the reactor containment building
selected to provide a representative sample of the latent
debris preset in the containment building. The sample
collection area for each location varied in size from 1 ft 2 to 70 ft 2. The samples collected were analyzed for both quantity and type of debris. The latent debris from the
sampled areas was then projected for the entire containment
building based on the total amount of surfaces similar to
those surveyed.
The walkdown found small quantities of particulate debris such as rust, paint, and dust. The
quantity found would scale to a total containment quantity
of 24.5 pounds. Only a few latent fibers were found. The
latent particulate quantities are insignificant compared to
the paint debris. TVA used the NEI latent debris recommendation of 200 pounds total and 12.5 ft 3 of fiber to design and test the SQN advanced containment sump strainers.
This assumption is extremely conservative compared to the results of the SQN walkdown.
E-10 32. How will your containment cleanliness and foreign material exclusion (FME) programs assure that latent debris in
containment will be controlled and monitored to be
maintained below the amounts and characterization assumed in
the ECCS strainer design? In particular, what is planned
for areas/components that are normally inaccessible or not
normally cleaned (containment crane rails, cable trays, main
steam/feedwater piping, tops of steam generators, etc.)?
TVA Response 32 Procedures are in place to inspect and clean the
containment. These were the procedures in place when the
latent debris walkdown was performed. Given that the
quantities of material found were either insignificant (fibers) or overwhelmed by break generated debris (particulates), no special inspections are planned or can be
warranted given the extra dose to personnel and that there
is no safety benefit.
- 33. Will latent debris sampling become an ongoing program?
TVA Response 33 Latent debris sampling will not be an ongoing program. See
the response to Question 31 for more details.
- 34. Based on the low amount of fibrous debris from other sources, has the potential for the "thin bed effect" from
Latent fiber only been evaluated? If so, what were the
results? TVA Response 34 Yes. The strainer is sized such that a thin bed cannot
form. A latent fiber quantity of 12.5 ft 3 is too small to form a thin bed on a flat screen the size of the SQN
strainer much less for an advanced screen design.
- 35. You indicated that you would be evaluating downstream effects in accordance with WCAP-16406-P. The NRC is currently
involved in discussions with the WOG to address
questions/concerns regarding this WCAP on a generic basis, and some of these discussions may resolve issues related to
your particular station. The following issues have the
potential for generic resolution; however, if a generic
resolution cannot be obtained, plant-specific resolution
will be required. As such, formal RAIs will not be issued
on these topics at this time, but may be needed in the E-11 future. It is expected that your final evaluation response will specifically address those portions of the WCAP used, their applicability, and exceptions taken to the WCAP. For
your information, topics under ongoing discussion include:
- a. Wear rates of pump-wetted materials and the effect of wear on component operation
- b. Settling of debris in low flow areas downstream of the strainer or credit for filtering leading to a change in
fluid composition
- c. Volume of debris injected into the reactor vessel and core region
- d. Debris types and properties
- e. Contribution of in-vessel velocity profile to the formation of a debris bed or clog
- f. Fluid and metal component temperature impact
- g. Gravitational and temperature gradients
- h. Debris and boron precipitation effects
- i. ECCS injection paths
- j. Core bypass design features
- k. Radiation and chemical considerations
- l. Debris adhesion to solid surfaces
- m. Thermodynamic properties of coolant TVA Response 35 No response is required at this time.
- 36. Your response to GL 2004-02 question (d) (viii) indicated that an active strainer design will not be used, but does
not mention any consideration of any other active approaches (i.e., backflushing). Was an active approach considered as
a potential strategy or backup for addressing any issues?
TVA Response 36 The SQN strainer showed a head loss of approximately 0.03
feet for a case where all coatings were assumed to fail and
all debris fell directly on the strainer. This compares to
an available NPSH of about 17 feet. No additional active
features were needed.
- 37. The NRC staff's SE discusses a "systematic approach" to the break selection process where an initial break location is
selected at a convenient location (such as the terminal end
of the piping) and break locations would be evaluated at 5-
foot intervals in order to evaluate all break locations.
For each break location, all phases of the accident scenario
are evaluated. It is not clear that you have applied such
an approach. Please discuss the limiting break locations
evaluated and how they were selected.
E-12 TVA Response 37 The inside diameters of the primary RCS pipes are 29 inches
for the hot legs, 27.5 inches for the cold legs, and 31
inches for the crossover legs. A break in one of the 31-
inch crossover legs would create the largest ZOI. However, depending on the exact location of various types of
insulation, a break in the smaller hot or cold legs could
result in the generation of a larger quantity of debris.
Therefore, to analyze this scenario, the worst case break
location and corresponding debris generation was considered
for all 4 loops. Iterations were performed which showed the
limiting break location to be the 31-inch crossover leg
pipe. Then, a 28.6D ZOI was used for all materials except
qualified coatings. A 26D zone of influence was used for
all materials except paint. A 10D zone of influence was
used for paint in the base case. Subsequently, TVA assumed
all coatings failed, thus the minimum zone of influence is
26D. The volume of the lower compartment is 280,000 ft
- 3. The volume of the sphere for a 26D zone is 1,690,000 ft
- 3. The limiting break was a crossover leg double ended rupture.
This pipe has an inside diameter of 31 inches. This
results in a sphere diameter of almost 68 feet. The
distance from the outside of the biological shield wall
around the reactor vessel to the crane wall is approximately
31 feet. Moving a break at 5-foot intervals along the pipe
does not result in a different ZOI. Thus, where the break
is located on the pipe has absolutely no impact on how much
debris is generated.
- 38. Were secondary side breaks (e.g., main steam, feedwater) considered in the break selection analyses? Would these
breaks rely on ECCS sump recirculation?
TVA Response 38 No secondary side breaks require sump recirculation for
mitigation. Thus, they did not need to be considered in the
evaluation.
- 39. The staff SE refers to Regulatory Guide 1.82 which lists considerations for determining the limiting break location (staff position 1.3.2.3). Please discuss how these
considerations were evaluated as part of the Sequoyah break
selection analyses.
TVA Response 39 TVA used the following criteria to determine limiting break
locations.
E-13 Break 1: Largest Potential for Debris Generation
The largest quantity of insulation in containment is located
in the RCS loops near each of the steam generators (SG) and
reactor coolant pumps (RCPs). Due to the size of the
primary RCS loop piping and the quantity of insulation in
close proximity to these pipes, a double-ended guillotine
break of one of the primary loop pipes presents the limiting
case for SBLOCAs and LBLOCAs at SQN. The inside diameters
of the primary RCS pipes are 29 inches for the hot legs, 27.5 inches for the cold legs, and 31 inches for the
crossover legs. Clearly, a break in one of the 31-inch
crossover legs would create the largest ZOI. However, depending on the exact location of various types of
insulation, a break in the smaller hot or cold legs could
result in the generation of a larger quantity of debris.
Therefore, to analyze this scenario, the worst case break
location and corresponding debris generation was considered
for all 4 loops.
Break 2: Two or More Types of Debris
All of the breaks discussed above encompass this break
scenario since reflective metallic insulation (RMI) and
coatings are the only debris present in the lower
compartment.
Break 3: Most Direct Path to the Sump
Given the sump location, all breaks in the lower compartment
proper have a direct path to the sump. Since the ECCS
recirculation sump is in close proximity to the RCS piping
in Loop 4, a break in this loop would have the most direct
path to the sump.
Break 4: Largest Particulate to Insulation Ratio
RMI, latent particles, and coatings are the only debris
present inside the crane wall in the lower compartment. RMI
does not transport as easily as particulates and is not a
major factor in developing head loss. The latent particulate
source is independent of break location. The limiting break
is the one that produces the most coatings debris. A
thorough analysis has shown that a break in each of the
crossover legs near the SG nozzle yields the most coating
debris. Small break LOCAs do not produce a large quantity
of debris.
E-14 Break 5: Potential Formation of the Thin-Bed Effect
SQN has no fibrous material in containment that is a
potential debris source for the sump. The sump strainer
area is large enough that there is not enough fiber in
containment to form a thin bed on a flat screen of this
size, much less an advances strainer design. As such, this
criteria does not affect the break selection process.
- 40. The licensee did not provide information on the details of the debris characteristics (debris size distribution)
assumptions other than to state that the Nuclear Energy
Institute (NEI) and SE methodologies were applied. Please
provide a description of the assumptions applied in these
evaluations and include a discussion of the technical
justification for deviations from the SE-approved
methodology.
TVA Response 40 TVA did not deviate from the SE-approved methodology.
Stainless steel RMI was assumed to fail as 75% small pieces
and 25% large pieces. Particulate debris was assumed to be
10 micron particles. No large particulate debris was
postulated. Coatings which were assumed to fail as "chips" were assumed to be 1/8 inch in diameter and 5 mils thick. The latent fiber debris was assumed to all be individual fibers.
- 41. Has debris settling upstream of the sump strainer (i.e., the near-field effect) been credited or will it be credited in
testing used to support the sizing or analytical design
basis of the proposed replacement strainers? In the case
that settling was credited for either of these purposes, estimate the fraction of debris that settled and describe
the analyses that were performed to correlate the scaled
flow conditions and any surrogate debris in the test flume
with the actual flow conditions and debris types in the
plant's containment pool.
TVA Response 41 No.
- 42. Are there any vents or other penetrations through the strainer control surfaces which connect the volume internal
to the strainer to the containment atmosphere above the
containment minimum water level? In this case, dependent
upon the containment pool height and strainer and sump
geometries, the presence of the vent line or penetration E-15 could prevent a water seal over the entire strainer surface from ever forming; or else this seal could be lost once the
head loss across the debris bed exceeds a certain criterion, such as the submergence depth of the vent line or
penetration. According to Appendix A to Regulatory Guide
1.82, Revision 3, without a water seal across the entire
strainer surface, the strainer should not be considered to
be "fully submerged." Therefore, if applicable, explain
what sump strainer failure criteria are being applied for
the "vented sump" scenario described above.
TVA Response 42 There are no vents between the sump and the containment
atmosphere when recirculation from the sump is initiated
after a design basis accident.
- 43. What is the minimum strainer submergence during the postulated LOCA? At the time that the re-circulation
starts, most of the strainer surface is expected to be
clean, and the strainer surface close to the pump suction
line may experience higher fluid flow than the rest of the
strainer. Has any analysis been done to evaluate the
possibility of vortex formation close to the pump suction
line and possible air ingestion into the ECCS pumps? In
addition, has any analysis or test been performed to
evaluate the possible accumulation of buoyant debris on top
of the strainer, which may cause the formation of an air
flow path directly through the strainer surface and reduce
the effectiveness of the strainer?
TVA Response 43 The minimum strainer submergence for the limiting LOCA is
9.06 feet at initiation of ECCS switchover to sump recirculation. This rapidly increases to 13.22 feet minimum pool height at containment spray (CS) switchover and remains
at this height for long-term operation in the recirculation mode. The potential for vortex formation at the SQN sump was extensively reviewed by the NRC as part of initial plant licensing. TVA performed a number of tests to support
initial plant licensing that conclusively demonstrated that
no vortex would form. Specifically, the sump design at SQN
showed no air drawing vortex with a pool level of 2.5 feet above the floor for ECCS operation and 5 feet above the
floor for CS operation.
No intermittent surface swirls from supports were demonstrated to exist with a pool level
greater than 8 feet. The water level in the lower
compartment was raised to 13 feet to provide a quiescent
pool surface even with flow rates several times the maximum
ECCS rate. Notwithstanding that, the center line of the E-16 cross over leg where it is close to the sump strainer is over 8 feet below the pool surface. There is no possible way for a break at this location to entrain air into the
sump.
The only potentially buoyant debris present in the sump pool
post-LOCA is the latent fiber. The top of the sump strainer
is over 5 feet below the pool surface and there is very limited fiber. It is physically impossible for these fibers
to form an air flow path to the sump.
- 44. The September 2005 GL response noted that the licensee analyzed the debris transport based on the methodology
described in the NEI guidance report "Pressurized Water
Reactor Sump Performance Evaluation Methodology," NEI 04-07, for refined analyses as supplemented by the NRC's safety
evaluation, as well as the refined methodologies suggested
by the SE in Appendices III, IV, and VI. Please identify
and justify if any exception to either the NEI 04-07 or SE
method was taken, or confirm that no exception was taken.
TVA Response 44 TVA did not calculate a thin bed head loss as the screens
were designed with sufficient surface area to preclude the
formation of a thin bed given the amount of fiber assumed in
the analysis. Walkdown data showed that there is not enough
fiber present to form a thin bed on the current screen.