ML20058G968
| ML20058G968 | |
| Person / Time | |
|---|---|
| Site: | Mcguire, Catawba, McGuire  |
| Issue date: | 11/07/1990 |
| From: | Tuckman M DUKE POWER CO. |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| TAC-75138, TAC-75141, NUDOCS 9011140140 | |
| Download: ML20058G968 (11) | |
Text
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o Duke hwrr Company (TN)373 40ll butlear l'roduction ikpartment I'0 tha llWi Charlotte, hC!! Col ILWI DUKEPOWER November 7, 1990 i
U. S. Nuclear Regulatory Commiscion ATTN Document Control Desk Washington, D.C.
20555
Subject:
McCulte Nuclear Station Docket Numbers 50-369 and -370 Catawba Nuclear Station Docket Numbers 50-413 and -414 Topical Report BAW-10174,
" Hark-BW Reload LOCA Analysis for Catawba and McGuire" i
Revised Responses to Questions (TACS 75138-141)
By letter dated March 27, 1990, the NRC staff requested additional information and the subject Topical Report.
This information was provided by letters f rom 11. B. Tucker dated June 7. July 25, and August 8,1990.
Attached are revised responses to Questions 15 and 28, which were originally transmitted in the August 8, 1990 letter.
Portions of the responses which have been revised are indicated by vertical lines in the right-hand margin.
t If there are any questions, please call Scott Gewehr at (704) 373-7581.
Very truly yours, i
f.1'[wh l
M. S. Tuckman, Vice President Nuclear Operations SAG /240/1cs Attachment l
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9011140140 901107 f.'DR ADOCK 05000369 PDC
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i U. S. Nuclocr Regulstery Connicsitn-November 7,1990 l
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xc Mr. S. D. Ebneter Regional Administrator U. S. Nuclear Regulatory Commission Region II 101 Marietta Street, NW, Suite 2900 Atlanta, Georgia 30323 Mr. Tim Reed. Project Manager Office of Nuclear Reactor Regulation U. S. Nuclear Regulatory Commission One. White Flint North, Mall Stop 9H3 Washington, D.C.
20555 Mr. R. E. Martin, Projects Manager Office of Nuclear Reactor Regulation U. S. Nuclear Regulatory Commission One White Flint North, Hall Stop 9H3 o
Washington, D.C.
20555 Mr. L. L. Losh B & W Nuclear Services 3315 Old Forest Road P. O. Pax 10935 Lynchburg, Virginia 24506-0935 Mr. W. T. Orders NRC Resident Inspector Catawba Nuclear Station.
Mr. P. K. VanDoorn NRC Resident Inspector McGuire Nuclear Station
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15.
Due to the slope of the axial shape on either side of the peak power location, the power at the PCT location was less than at the peak power location (see Figure 8-3).
Clarify if a worst case axial shape could be defined within the Catawba and McGuire technical specification limits so that the axial shape was flatter in the vicinity of the exial peah and the power was higher at the PCT location.
For example, in the " 0 ft axial peak case, the axial peak was in node 14 and the PCT occurred in node 12, at 6.9 ft.
Would a higher PCT have been calculated if the axial shape was flatter around the peak power location so that additional power was applied at the 6.9 ft elevation?
A similar question applies to all other cases, t
Response
The limit on power shapes imposed within plant technical specifications because of 14CA is restricted to specification of the peak power as a function of elevation in the core.
No provision is made to specify a shape or a distribution of the total peak between the axial and radial factors.
To determine power shapes with which the allowable peak power versus core position values will be
- set, conservative, pragmatic, and phenomenological factore are balanced.
In so doing, the RSG LOCA evaluation model requires that five separate power shapes, each peaked at a different elevation in the core, be evaluated.
The basic philosophy for determining the power shapes is to use realistic power shapes with normal but high peaking and increase both the axial and radial peaking in a reasonable l
fashion until the absolute or the desired power limit is reached.
Although operation is allowed up to and beyond the limits (the limiting is by administrative control such that being above a limit only requires a plant to take control measures to return below the limit), it is seldom if ever achieved, and there is no most probable way by which peaking would increase to the limits.
Therefore, by starting with real shapes and pushing them to the limit in reasonable ways, the power shapes that result can be described as possible shapes if operation at the limiting local power levels were to 7
occur.
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While the power shapes used are representative of shapes possible for operation at the limits of local power, they are bounding for the operations of the plant because they are at the limits of allowable operation.
This, in combination with I
the use of five separate axial distributions, provides broad l
coverage of the possible power shapes that can be achieved and f
makes the LOCA limits power distributions, as a
- set, essentially bounding for operation of the plant.
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1 Because the LOCA limits shapes are derived from abstractions of actual shapes, they are curved over the entire length of the core.
Within the vicinity of the peak, however, the j
curvature is not so severe that complete flattening would seriously alter the results achieved. To provide a measure of the effect of total local flattening of the power shapes a study was made that, using the LOCA limits cases as a base, adjusted the cladding-to-vapor temperature difference at each axial position by the ratio of the peak local power to the l
local power at the axial position. The results are considered I
credible for the grid spans above and below the span that contains the peak power in the base.
Within this range the peak cladding temperatures were increased by 10 to 50 F.
The highest cladding temperature within the LOCA limits set would increase by 49 F.
Therefore, further flattening of the power l
shapes will not substantially alter the results obtained by current practice.
In summary, a set of five power shapes are utilized for the LOCA limits studies under the BWFC LOCA evaluation model.
These shapes provide, as a set, a realistic bound to the possible power shapes that can occur at the limits of plant op3 ration.
- Moreover, the effects of an increase in the flatness of the power shapes has been shown to be limited to a
60 F possible increase in peak cladding temperature.
Therefore, the methodology of the BWFC RSG LOCA evaluation
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i 28.
On page A.3 the statement is made that the only effect of the i
mixed core that needed to be considered for the reflood phase of the LBLOCA is the whole core pressure drop.
The following i
questions are related to this statement.
Justify why the pressure drop difference for the two fuel a.
assemblies does not cause a flow diversion effect during the reflood phase similar to the blowdown phase. Provide i
appropriate data or analy es to support your conclusions.
l Response: The large break LOCA reflood phase is modelled
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with a one-dimensional simulation that conservatively
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ignores radial effects within the core.
Some flow diversion toward the Mark-BW is likely and could be represented in more detailed modelling in a larger calculation.
Those techniques would also substantially reduce reflood cladding temperatures by incorporating 9
two-or three-dimensional effects that cause preferential flow to the hot assemblies.
Because it would be inconsistent to evaluate a negative multi-dimensional effect without consideration of the positive effects, only the overall resistance impact of the OFA versus Mark-BP assemblies was included in the reflooding comparison.
The combined ef fects of added assembly resistance and i
two-or three-dimensional reflooding can be discussed considering two possible core arrangements:
(1) a Mark-BW assembly as the hot assembly surrounded by OFA assemblies or (2) an OFA assembly as a hot assembly surrounded by Mark-BW assemblies.
In either situation the flow diversion, if any, will be toward the Mark-BW.
Therefore, with diversion allowed, the cooling of the Mark-BW assembly under the first arrangement would be enhanced over present evaluations and that condition need not be considered further.
For the second core arrangement, with an OFA assembly as a hot assembly, any flow diversion toward the Mark-BW
j would tend to reduce the flow of water entering the bottom of the OFA assembly. The pressure drop difference j
between the OFA and the Mark-BW is small (less than 5 percent), and any flow diversion toward the Mark-BW would be limited to the square root of that difference (about 2 percent).
Such a diversion is not large and would be compensated for by a buildup of elevation head in' the Mark-BW assemblies. The pressure drop across the nominal core during reflooding is less than one psi.
Converting 5 percent of the core pressure drop into elevation head shows that the water level in the Mark-BW need be only I
0.12 feet higher than that in the OFA to eliminate the diversion of flow.
At 2 percent flow diversion that difference in elevation head will be established within the first 30 to 40 seconds of the reflooding transient.
Thus, any net flow diversion is short-lived.
As demonstrated by the SCTF and CCTF experiments, the conservatism of not considering multi-dimensional effects on core reflooding f ar out weighs the effects of any possi.le flow diversion caused by the difference in the design of the two assemblies.
Within tne hotter fuel assemblies, substantially more water is boiled and l
entrained by the reflooding process than within the average and colder assemblies.
As the core fills, hydrostatic head imbalances are set up that lead to l
greater flow of water to the hottest assemblies and correspondingly less to the coldest assemblies.
As a reasonable estimate, a measure of the flow imbalance is the radial power peak, whereby inlet velocities for the hot assembly 30 to 50 percent higher than those of the average channel can be expected.
The result is a flattening of the cladding temperatures across the core radially and a reduction of the hot spot cladding l
temperatures of 300 to 500 F.
The SCTF and CCTF experiments have demonstrated the improvements in hot
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channel reflood cooling for radially peaked conditions.
(The response to Question 2.a of the second round of questions on the RSG LOCA evaluation model, BAW-10168, i
presents a-discussion of the effects observed in the experiments, or reference can be made to the appropriate experimental reports, references 28.1 and 28.2.)
- Thus, the consequence of simulation of all sources of flow i
diversion is a
substantial decrease in cladding temperatures from the present calculational results.
J As discussed, there is no flow diversion from one fuel assembly to another during reflooding with the I
i calculational approach taken by the BWFC RSG LOCA evaluation model. The RSG LOCA model, in not considering l
the reflooding process at a level of modelling detail sufficient to evaluate reflood flow diversion, j
incorporates conservatisms of far larger effect than those possible because of mixed core-induced flow I
diversion.
Therefore, the evaluation of Appendix A, without the consideration of flow diversion, remains appropriate for the licensing of the transition cores.
b.
Clarify how the fuel loading pattern during mixed core operation affects the possibility of flow diversion from one type of bundle to the other.
Response
The act' fuel loading pattern for a reload is determined relatively late in the reload design process and cannot be readily predicted.
It is reasonable to assume that the transition from a mostly OFA core to a mostly Mark-BW core will take place over three cycles with one third of the fuel replaced in each cycle.
Even after the third cycle, it is likely that the core-design will continue to utilize a few OFA assemblies for several years.
Therefore, the LOCA calculations are not performed to an accuracy level that would require precise knowledge of the fuel loading pattern.
f As explained in the~ response to Part (a) of - this question the consideration of flow diversion during.
j reflooding as a mixed core consequence is not appropriate for calculations with current evaluation models.-
Therefore, the only effect of the~ loading pattern.on the evaluation presented in Appendix A is the degree to wh o;h the mixed cores resistance is changed..
The considerations of Appendix A bound the-extremes of most j
resistive,-all OFA,.to-least resistive, all. Mark-BW.= As the loading pattern and the number of; Mark-BW assemblies in the core change, the reflooding ef fect will transition from a 2 percent decrease in flooding' rate to no effect.
During the first cycle, for example, the decrease should only be about 1.3 percent.
Repeating the conclusions from Part-(a), the evaluation l
of ref'.ooding at a-level of accuracy that could properly treat flow diversion would' result in cladding f
temperatures several hundred degrees lower than those l
predicted by present techniques.
At ' that IcVel of detail, however, care would be required to assure that both the fuel loading pattern and-the skew in steady-state core flow assumed did not limit the core-design.
At present, the conservative one-dimensional reflood-treatment is independent of individual ~ fuel assembly.
placement and, within the constraints 'of the results presented in Appendix A, independent-of cycle design..
Clarify how the above two' items effect cooling of the OFA c.
and Mark-BW assemblies during reflood.
Response
As developed.in the responses to-the other parts of this question,.the evaluation of. the coolability of the mixed core configuration as presented in Appendix A of BAW-10174 is conserv=1tive and appropriate for plant
-licensing.
.ealistically, flow. diversion within the I
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reflooding core would occur because of power distribution
.f effects as well as for fuel assembly. resistance differences.
The results of the SCTF and CCTF experiments (See the response to Question 2' a of the-second set of questions on the RSG LOCA evaluation model, BAW-10168, and references 28.1 and. 28.2) show. a substantially larger. benefit from power distribution-3 induced flow diversion than the expected deficit from the fuel = assembly resistance mismatch.
Thus, although some small amount of fuel assembly-induced flow diversion, resulting in a smaAl temperature increase, is expected during reflood, that increase would be imposed on a -
cladding temperature several hundred degreesLeooler than the present licensing. calculations.
.Therefore, the.
cooling considerations for the mixed core configuration as expressed.in Appendix A of BAW-10174 remain applicable l
and appropriated for licensing, i
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References:
l 28.1 T. Iwamura, M. Osakabe, and Y. Sudo, " Effects of Radical I
Core Power Profile on Core Thermo-Hydraulic Behavior j
(
l during Reflood Phase in PWR-LOCAs,"-Journal of Nuclear Science and Technology, 20(9), pp 743
_751, September 1983.
i 28.2 H. Akimoto, T. Iguchi, and Y. Murao, " Core Radial Profile Ef fact on System and Core Cooling Behavior during RSflood Phase of PWR-LOCA. with CCTF Data,". Journal of e au ear Science and: Technology, 22[7), pp 538 - 550,-July-1985.
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