ML20086Q760
| ML20086Q760 | |
| Person / Time | |
|---|---|
| Site: | Vogtle  |
| Issue date: | 02/16/1977 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20085K039 | List: |
| References | |
| FOIA-83-722 NUDOCS 8402280126 | |
| Download: ML20086Q760 (28) | |
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T INTERIM SAFETY EVALUATION REPORT ON EFFECTS OF FUEL R0D BOWING ON THERMAL MARGIN CALCULATIONS FOR LIGHT WATER P,EACTORS (REVISIONI) 4 February 16, 1977 4
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8402280126 840105
-r POR FOIA BELL 83-722 PDP
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i CONTENTS i
1.0 Introduction I.
2.0 DNBR Reduction Due to Rod Bow
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i 3.0 Application To Plants In The Construction Permit And Operating -
License Review Stage 4.0 Application To Operating Reactors 5.0 References e
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Data have recently been presented (Re_forence 1) to the staff which i
show that previously developed' methods for accounting for the effect f
of fuel rod bowing on departure'from nuclecte boiling in a pressurized water reactor (PWR) may.not contain adequate therml margin when l
unheated rods, such as instrument tubes, are present.
Further experimental verification of these data is in progress.
However an interim measure is required pending a final decision on the validity of thes,e new data.
The staff has evaluated ti.e impact of these data on the performance of all operating pressurized water reactors. Models for treating the effects of fuel rod bowing on thermal-hydraulic performance have been derived. These models are based on the propensity of the individual fuel designs to bow and on.the thermal analysis methods used to predict the coolant conditions for both normal operation and anticipated transients.
As a result of these evaluations the staff has concluded that-in some cases sufficient thermal margin does not now' exist.
In these cases, additional thermal margin will be required to' assure, with high confidence, that departure from nucleate boiling (DNB) does not occur during anticipated transients. This report discusses how these conclusions were reached and identifies the amount of additional margin required.
The models and the required DNBR reductions which result from these models are meant to be only an interim measure until inore data are available.
Because the data base is rather sparse, an attempt was made to treat this problem in a conservative way.
The required DNBR reductions will be revised as more ' ata become d
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The staff review of the amount and consequences of fuel rod
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bowing in a boiling water reactor is now underway.
At present no conclusions have been reached.
When this review reaches a stage i
where either an interim or final conclusion can be reached, the results of this review will be published in a separate safety evaluation report.
It should be noted that throughout the remainder of this e
report, all discussion and conclusions apply only to pressurized water reactors.
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2.0 DNBR Reduction.Due To Rod Bow
2.1 Background
In 1973 Westinghouse Electric presented to the staff the results of experiments in which a 4x4 b'undle of electrically heated fuel f
rods was tested to determine the effect of fuel rod bowing to contact on the thermal margin (DNBR reduction) (Reference 2).
The tests were done at conditions representative of PWR coolant conditions, The
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results of these experiments showed that, for the highest power density at the highest coolant pressure expected ir, a Westinghouse reactor,th'e DNBR reduction due to heated rods bowed to contact was approximately 85.
Fuel bundle coolant mixing and heat transfer computer programs such as COBRA IIIC and THINC-IV were able to accurctely predict the results of these experiments.
Becabse the'end point cobld'be predicted, i
i.e., the DNBR reduction at contact,there was confidence that the DNBR reduction due to partial bow, that is, bow to less than contact could also be correctly predicted.
On August 9,1976 Westinghouse met with the. staff to discuss further experiments with the same. configuration of fuel bundle (4x4) using electrically heated rods.
However, for this set of experiments one of the center 4 fuel rods was replaced by an unheated tube' of the same size as a Westinghouse thimble tube.
This new test configuration was tested over the same range of power, flow and pressure as ~ the earlier tests.
However, with the unheated, larger diameter rod the reduction in DNBR was much larger than in the earlier (1973) tests.
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The data consisted of points corresponding to no intentional bowing (that is, a certain amcunt of bowing due to' tolerances cannot be prevented) and to con' tact.
No data were taken at partial clearance reduc.tions between rods.
i The staff attempted to calculate the Westinghouse results with the COBRA IIIC computer code but could not obtain agreenent with the new data.
Westinghouse was also unable to obtain agreement between their experimental results and the THINCIV computer code.
On August 19, 1976 CE presented results of similar experiments to the staff.
These tests were performed using a 21 rod bundle of electrically heated rods and an unheated guide tube.
Results were 1
l presented for not only the case of. full contact, but also the case of partial bowing.
Both sets of data (Westinghouse and CE) showed similar effects due to variations in coolant conditions.
For both cases, the DNBR reduction became greater as the coolant pressure and the rod power I
increased.
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Because both sets of data showed that plant thermal margins might be less than those intended, the staff derived an interim model to conservatively predict the DNBR reduction.
Since the data with unheated rods could not be predicted by existing analytical methods, empirical models were derived.
These models give the reduction in DNBR as-a function of the clearance reduction between adjacent fuel rods. Two such models were derived, one based on the Westinghouse data and one based on the CE data.
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5-2.2 Model Based on Westinghouse Data As stated in Section 2.1, dat'a were presented by Westinghouse for the DNBR reduction at full contact and with no bow.
No data at I
partial gap closure were presented. Westinghouse proposed, and the l-i staff accepted, a straight line interpolation between these two points as shown in Figure 2.1.
l This approach is conservative if the DNBR reduction does not increase more rapidly than the-straight line reduction shown in j
i Figure 2.1.
Although the date for DNBP reduction due to rod bowing
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in the presence of an unheated fuel rod cannot be predicted by existing analytical methods, one would nevertheless expect tha't the actual behavior would more nearly follow the curved line also shown in Figure 2.1.
According to ~ fhis curved line, the DNBR would be reduced gradually for small amounts of bow.
As the fuel rods (or fuel
'l rod and unheated rod) become close enough so that there is an inter-action, the DNBR would decrcase more rapidly.
No physical mechanism j
has been postulated which would lead to sudden large decreases in the j
i DNBR for small or moderate gap closures. Thus, the straignt line approximation is believed to be an overestimate of the expected behavior.
Experience with critical heat flux tests also supports the i
assumption of a small reduction in DNBR for small amounts of fuel rod bowing.
Experimental measurements of critical heat flux done on test assemblies always have some amount of rod bowing. This.may-be due simply to fabrication tolerances or to electromagnetic-attraction forces set up between electrically resistance heated rods which simulate fuel rods.
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l It should be noted that this behavior (little or no reduction in DNBR for small amount of bowing) is shown by Combustion Engineering data which became available to the staff after the Westinghouse model was derived.
The Combustion Engineering data is discussed in Section 2.3 and the model derived from this data is shown in Figure 2.2.
All manufacturers of reactor cores, including Westinghouse, include a factor in their initial core design to account for the reduction in DNBR that may resislt from pitch reduction from fabrication tolerances and initial rod bow.
The amount of 'this pitch reduction factor varies with the fuel design and the analysis methods which are used.
For any particular core this factor is not varied as a function of burnup.
In developing the interim rod bow penalties described in this report, it became apparent that the penalty should be a function of burnup since the magnitude of rod bow is a function of burnup.
However, to maintain existing thermal margins early in core life when only a small amount of fuel rod bow is anticipated, the initial I
pitch reduction factor was included until such time as the rod bow
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DNBR reduction became greater. This is represented as-the straight L
i horizontal line on Figure 2.1.
4 2.3 Combustion Engineering Model
' Combustion Engineering performed exper'iments to determine the effect of rod bowing on DNBR which included'some cases in which the effect of partial bowing as well =as bowing to contact was determined.
Again, a straight line interpolation is used.
However,. the point of zero DNBR reduction i's not at zero clearance reduction but rather, at an intermediate value of clearance reduction.
This-is'shown schematically
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The horizontal straight line, representing the initial.
1 pitch reduction ' factor is included as expla'ined previously' in Section.2.2 i
2.4 Models for Babcock and Wilcox and Exxon j
On August 17, 1975 reprbsentatives of Babcock ~and Wilcox inet with the staff to discuss this problem.
Babcock and Wilcox did not present any data on the effects of rod bowing on DNBR..They had previously presented data to the staff on the amount of bowing to be expected in Babcock and Wilcox 15x15 fuel assemblies'.
Because
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staff applied-the Westinghouse model-to calculate the effect.of rod bowing on DNBR for Babcock and Wilcox fuel. This is acceptable since f [
the conditions of operation are nearly the same in pressurized water i
reactors from both vendors and the fuel bundle. designs-are similar.
The amount of fuel rod bowing as a function of burnup was calculated using the Babcock and Wilcox 15x15 fuel bundle data.~
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Representatives of the Exxon Nuclear Corporation discussed the i
l effects of fuel rod bowing in the presence of an unh.eated rod'on DNBR
-F with the staff on August 19, 1976.
E,xxon has not performed DNB tests with bowed rods and thus has no data pertinent to this problem.
The first cycle of~ Exxon fuel has just been-removed from H. B.; Robinson.
i-and the results of measurements on the magnitude of rod bowing have--
not yet been presented to the staff. :The effects.of-fuel rod bowing;-
for Exxon fuel were evaluated on'a plant by plant basis as. discussed in Section 4.0 y
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t 2.5 Apolication of the Rod Bow /DNBR Model Using these empirical models, the staff derived DNBR reductions to be applied to both operating reactors and plants in the Operating License review stage.
The procedure in applying these empirical models is as follows:
Step 1:
Predict the clearance reduction due to rod bow as a function of burnup. An expression of the form f
=a+bTIBU o
is used where ffl = fractional clearance reduction due to rod bowing o
a,b = empirical c'onstants obtained for a given fuel ' design BU = burnup (region average or bundle. average, depending on the fuel designer).
Westinghouse showed in Reference 6 that an equation of the above form fit the rod bow data from 26 fuel regions.
The constan.t a represents the initial bow of the fuel rods due' to fabrication tolerance.
The staff has approved the above equation (Reference 8).
Also included in the constants,a, and b is a factor of 1.2'to convert from the cold conditions at which the measurements were made to the hot operating conditions and a factor of 1.645 which,- when multiplied by the standard deviation, gives an amount of bow greater than that expected from 95% of the fuel rods with a g5% confidence.-
Step 2: Apply the previously discussed empirical models of DNBR
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o calculated from step 1.
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Step 3: The staft has permitted the reduction in DNBR calculated l
in step 2 to be offset by.certain available thermal margins.
These.
may be either generic to a given fuel design or plant dependent.
An example of a generic the'rmal margin which would be used to offset the DNBR reduction due to rod bow is the fact that the DNBR limit of 1.30 is usually greater than the value of DNBR abcve which 95% of the data lie with a 95% confidence.
The difference between 1.30 and this number may be used to offset the DNBR reduction.
For Westinghouse 15x15 fuel, the value of DNBR which is greater than 95% of' the data at a 95% confidence level is 1.24 (Reference 1).
For Westinghouse 17x17 fuel this number is 1.28 (Reference 1). A review of the data used to derive these numbers shows that the use of three significant figures is justified.
An example of a plant specific therral margin would be core flow greater than the value given in the plant Tachnical Specifications.-
A discussion ~ of the application of this method to Construction Permit and Operating License reviews is given in Section 3.0.
A discussion of the application an,d,the results of this method to operating reactors is given in Section 4.0.
The application to reactors using Exxon fuel is also discussed in Section 4.0.
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3.0 Application to Plant in Construction Permit And Operating I
License Review Stage t
3.1 CP Applications No interim rod bow DNB penalties should be applied to CP applications. The rod bow data upon which the interim limits have been based should be ccnsidered preliminary.
There is sufficient t;me.
available to review the data and assess a penalty, if any, prior to the OL stage. We will advise each CP applicant of.the nature of interim penalties being applied to OL reviews and operating reactors.
As stated above, the data used to evaluate the effects of rod
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bow on DNBR are preliminary.
They are also incomplete.
In order to.
assess the conservatism of the straight line approximation and to obtain data on designs for which no data is now avaii6ble we will require the applicant to (1) fully define the gap closure rate for prototypical bundles and (2) determine by an appropriate experiment t
the DNB effect that bounds the gap closure-from part (1). Such f
requirements will be part of our CP review effort.
3.2 0'L Applications Plants which are in the operating license review stage should consider a rod bow penalty.
This penalty should be as described in Section 2.2 for Westinghouse or Section 2.3 for Combustion Engineering.
Babcock and Wilcox plants should use the rod bow vs.
burnup curve appropriate to their fuel and the Westinghouse curve of DNBR reduction as a function of rod bow.
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All applicants may propose appropFiate thermal margins (as t
discussed in Section 2.4) to help offset the calculated DNBR 4
I reduction.
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4.0 Acalication To Operating Reactort l
This section divides the operating plants into distinct categrries and lists them according to the fuel and/or reactor manufacturer. Operating plants which cannot be so categorized (such as plants with fuel supplied by more than one vendor) are placed in a separate category. The plants assigned to each category are listed in the appropriate subsection.
'The conclusions reached in this section are in some cases dependent on conditions or analysis which are valid only for the present fuel cycle. Hence, the FAH or DNBR reductions which are I
given (or the fact that no such reduction is concluded to be I
required) is valid only for the present operating cycle.
4.1 Westinghouse LOPAR Fuel The designation LOPAR stands for low parasitic and refers to the fact that the guide tubes in the fuel bundle are piade of Zircaloy.
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Table 4.1 gives a list of the operating plants which fall 'into this I
classification.
TABLE 4.1: PLANTS WHICH CURRENTLY USE THE WESTINGHOUSE.LOPAR FUEL ASSEMBLY
_i 15 x 15 17 x 17 l
Zion 1 Cycle 2 Trojan Cycle l' l.
Zion 2 Cycle 1 Beaver Valley 1 Cycle 1 l.
Indian Point 3 Cycle ~1 Turkey Point 3 Cycle 4 L
i Turkey Point 4. Cycle 3 Prairie Island 2 Cycle 2 Prairie Island 1 Cycle 2
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f 15 x 15 Surry 1 Cycle 4 i
Surry 2 Cycle 3 Kewaunee Cycle 2 Point Beach 1 Cycle 5 Point Beach 2 Cycle 3 The reduction in DNBR due to fuel rod bowing is assumed to vary linea 'y with the reduction in clearance between the' fuel rods (or fuel rod and thiinble rod) according to the model discussed in Section 2.2.
The maximum value of DNBR reduction (at contact), obtained from
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the experimental data was used to calculate the DNBR reduction
. vs. bow for the 15x15 LOPAR. fuel. This DNBR contact reduction was adjusted for the lower heat flux in the 17x17 LOPAR fuel.
The clearance reduction is conservatively assumed to be given by the following equation for the 15x15 (and 14x14) fuel, h =
a + b5 l
where AC.
is ti.e reduction in clearance
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Co Bu is the region average burnup and a,b are empirical constants fitted to Westinghouse 15x15 rod bow dat'a
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j For the 17x17 LOPAR fuel, the clearance reduction was calculated f
from the equation:
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X 15x15 17x17 where L = the distance between grids i
I = moment of inertia of fuel rod On December 2, 1976, Westinghouse informally showed the staff new data pertaining to the magnitude of rod bow as a' function of region average burnup in 17x17 fuel assemblies. This data show that the above correction is probably conservative and that the magnitude of fuel rod bowing in 17x17 fuel rods can better be represented by an empirical function..This review is now underway.
The calculated DNBR reduction is partially offset by existing thermal margins in the core design.
For the Westinghouse LOPAR fuel design some or all of the following items were used in calculatina the thermal margin for the operating plants:
. design pitch reduction
. conservatively chosen TDC used in design *
. Critical heat flux correlation statistics (assumed in thennal analysis safety calculations) are more conservative than required.
. Densification power spike factor included although no longer required (Reference 4)
After taking these factors into account, the reductions in FaH shown in Table 4.2 were found necessary. All operating plants listed.
3 in Table d.1 will be required to incorporate tiiese reductions in
'FaH into their present operating limits.
- TDC (thermal diffusion coefficient) is a meas'ure of the amcunt of-mixina.between adjacent sute.hannels.
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FaH REDUCTION FOR WESTINGHOUSE LOPAP. FUEL f
CYCLE REDUCTION IN FAH (%)
'15x15 17x17 ZION 182 1st Cycle (0-15 Gwd*/MTU) 0-2 ramp 0-9.5 0-6. ramp 2nd Cycle (15-24 Gwd*/MTU) 4 12 8
3rd Cycle (24-33Gwd*/MTU) 6 12 10 i
These ' reductions in FaH may be treated on a region ~ by region l
basis.
If the licensee chooses, credit may be taken for the margin between the actual reactor coolant flow rate and the flow rate used in safety calculations.
Credit may also be taken for a difference between the actual core coolant inlet temperature and that assumed in safety l
analyses.
In taking credit for coolant flow or inlet temperature margin, I
the associated uncertainties in these quantities must be taken into account.
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4.2 Westinghouse HIPAR and Stainless Steel Clad Fuel 1
1 The designation HIPAR stands for high parasitic and refers to the
' fact that the guide tubes in the fuel bundle are made of stainless steel.
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These twc fuel types, MIPAR and Stainless Steel clad, are grouped toge,ther because the amount of bowing expected (and observed) is significantly less than that in the observed Westinghouse LOPAR fuel. The plants -
which fall under this classification are listed in Table 4.3.
- Gwd Mwd
= 1000 MTU MTU
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i TABLE 4.3:
HIPAR AND STAINLESS STEEL PLANTS I
Ginna Indian Point 2 San Onofre Connecticut Yankee The model for the reduction in DNBR due to fuel rod bowing is assumed to be identical to that used for the LOPAR fuel. This is i
acceptable since cladding material should have no effect on CHF (critical heat flux) and the same DNB correlation applies to both HIPAR and LOPAR grids.
For reactors in this category, the peak reduction in DNBR i
(corresponding to 100% closure) was adjusted to correspond to the peak overpower heat flux of that particular reactor, The amount of rod bowing for the plants listed in Table 4.3 which use HIPAR and stainless steel fuel, was calculated by means of.
an adjustment to the 15x15 LOPAR formula.
This adjustment took the form of the ratio
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I amount of bow for assembly typ2'
= (L/IE) assy type amount of bow for LOPAR fuel (L/IE). LOPAR l
where L is the span length between grids I is the moment of inertia of the fuel rod E is the modulus of eiasticity of the fuel rod cladding I
i Ginna-
, Cycle 6 The Ginna plant is fueled with 121' fuel assemblies. Two of these l
are Exxon assemblies, and two are B&W assemblies. The remainder are Westinghouse HIPAR fuel assemblies..The-experimental value of DNBR reduction was adjusted for heat flux and pressure from peak experimental to actual plant conditions.
Ginna took credit for the thermal margins l
due to pitch reduction, design vs. analysis values of TDC and
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fuel densification power spike.
These themal margins offset the i
calculated DNBR reduction so that' no reduction in FaH is required.
i San Onofre' Cycle 5 San Onofre is fueled with 157 bundles of 15x15 stainless steel clad fuel. An FAH of 1.55 was used in thermal design and in the Technical Specifications. To offset the reduction in FAH due to rod i
bowing San Onofre has proposed.taking credit for margin available from i
the assumed worst case axial power distribution used in the thermal I
i analysis for San Onofre and that which would be possible during I
I operation..This proposal is now being reviewed by the staff.
l Indian Point 2 Cycle 2 Indian Point 2 is fueled with HIPAR fuel bundles. The experimental
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value of DNBR reduction was adjusted for heat flux and pressure to actual plant conditions.
Indian Point Unit 2 had themal margin to
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offset this DNBR reduction in pitch. reduction, design vs. analysis i
values of TDC, fuel densification power spike and a value of FAH of 1.65 used in the design (vs.1.55 in the Tech Spec). Therefore, no reduction of FAH is required for Indian Point Unit 2.
Connecticut Yankee Cycle 7 Connecticut Yankee is fueled w'ith 157 stainless steel clad fuel l
assemblies. The DNBR reduction at contact was assumed to be that l'
a used for the Westinghouse LOPAR 15x15 fuel.
No adjustment was 1
The value of pressure was adjusted to the overpressurd made for heat flux.
trip set point value of 2300 psi.
Fuli closure will not occur in stainless steel fuel out to the design burnup.
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Connecticut Yankee has sufficient thermal margin'in variable _
overpressure and overpower trip set points to. accommodate the
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calculated DNBR reduction. Therefore no penalty is required.
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i 4.3 Babcock and Wilcox 15x15 i
The reactors listed in Table 4.4 are fueled with B&W fuel.
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TABLE 4.4:
REACTOR USING B&W FUEL Oconee 1 Cycle 3 Oconee 2 Cycle-2 Oconee 3 Cycle-1 Rancho Seco Three Mile Island 1 Cycle 2 Arkansas 1 Cycle 1 Babcock and Wilcox met with the staff on September 8, 1975 and presented data on the amount of rod bow in B&W fuel. The staff j
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derived a model for B&W 15x15 fuel based on this data. This model
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has the form:
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= a + by Bu where is the fractional amount of closure Bu is the bundle average burnup'
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and a,b are empirical constants fitted to B&W' data The reduction in DNBR due to fuel rod bowing is' assumed to vary linearly with the reduction in clearance between the : fuel rods (or fuel rod and thimble rod) but can never be lower than that due to the pitch.
reduction factor used in thermal analysis, as explained in Section 2.2.
I Babcock and Wilcox claimed and the staff approved credit for the followin'g thermal margins:
Flow Area (Pitch) reduction Available Vent Valve credit Densification Power Spike removal Excess Flow over that used in safety analyses.
Higher than licensed power used for plant safety analyses 3
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Based on this review and the thermal margins presented by B&W to offset the new Westinghouse data; Rancho Seco is the only plant i
for which a reduction in DNBR is required.
Table 5 gives the values for the reduction of DNBR required at this time.
TABLE 5:
DNBR REDUCTIONS FOR B&W PLANTS Burnup DNBR Reduction Rancho Seco Gwd Cycle 1 (0-15 M1U )
0 Gwd Cycle 2 (15-24 }{Ri )
1.6%
Cycle 3(24-33 Gwd )
3%
MTV Plans must be submitted to the staff to establish how these reductions in DNBR will be accomo' dated.
4.4 Combustion Engineering 14x14 Combustion Engineering has presented data to the staff on the
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i amount of rod bowing as a function, of burnup. (Reference 5) The staff used this data to derive the following model for CE 14x14 fuel (Reference T I
h*a+ b5 aC/Co = fraction of closure for CE fuel Bu is the bundle average burnup and a,b are empirical constants fitted to CE data
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CE was given credit for thennal margin due to a multiplier of 1.065 on the hot channel enthalpy rise used to account for pitch reduction due to manufacturing tolerances.
Table 4.6 presents the i
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required reduction in DNBR using the model described above, after j
accounting for this thermal margin. Table 4.7 is a list of the reactors to which it applies.
A licensee planning to operate at a burnap greater than 24000 Mwd /MTV should present' to the staff an a'cceptable method of I
accommodating the thermal margin reduction shom in Table 4.6.
This may be done as part of the reload submittal if this burnup will not be obtained during the current cycle.
TABLE 4.6:
EFFECT OF ROD B0 WING ON DNBR IN REACTORS WITH COMBUSTION ENGINEERING 14x14 FUEL BURNUP REDUCTION IN DNBR' Cycle 1 (0-15 N )
0 Cycle 2 (15-24 %)
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Cycle 3 (24-33 y) 3%
i TABLE 4.7:
PLANTS FUELED BY CE FUEL TO WHICH VALUES OF TABLE i
4.6 APPLY St. Lucie 1 Cycle 1
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Ft. Calhoun
, Cycle 3 Millstone 2 Cycle 2 Maine Yankee Cycle 2.
Calvert Cliffs 1 Cycle 1 k
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4.5 Plants Fueled Partially With Exxon Fuel Palisades. H. B. Robinson, Yankee Rowe and D. C. Cook are partially fueled with Exxon fuel.
.A discussion of these reactors follows:
Palisades Cycle 2 The Palisades reactor for Cycle 2 is fueled with 136 Exxon fuel assemblies and 68 Combustion Engineering fuel assemblies.
The Combustion Engineering fuel was treated according to the i
Combustion Engineering model.for both extent of rod bow as a function of burnup and DNBR reduction due to clearance reduction.
i The Exxon fuel was assumed to bow to the same extent as the Combustion Engineering fuel. This assumption is acceptable since the Exxon fuel has a thicker cladding and other design features j
which should render the amount of bowing no greater than in the l
Combustion Engineering fuel, i
The DNBR reductiten wa.s assumed to be linear with clearance reduction according to the Westinghouse type curve of Figure 2,1, The DNBR reduction at contact was based on the Westinghouse experimental data adjusted for the peak rod average heat flux-in Palisades t
i and for the coolant pressure in Palisades.
2 The variation of the DNBR reduction with coolant' pressure is given f
in Reference 1.
The DNBR reduction decreases.as the coolant pressure decreases. The overpressure trip set point. in Palisades;is set at 1950 psi. At this pressure, according.to the data presented in Reference 1, the penalty is greatly reduced compared.to the penalty at high pressures.
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The limiting anticipated transient in the Palisades reactor I
results in a DNBR of 1.36. The themal margin between this value I
and the DNBR limit of 1.3 results 'in adequate themal margin to l
offset the rod bow penalty.
l Yankee Rowe Cycle 12 Yankee Rowe is fueled with 40 Exxon fuel assemblies and 36 Gulf l
l'aited Nuclear Corporation fuel assemblies, The fuel assemblies consist of 16x16 Zircaloy clad fuel rods, l
The reduction in DNBR due to fuel rod bowing was assumed to vary linearly with the reduction in clearance between fuel rods, The peak j
experimental conditions used in the Westinghouse test were used to j
fix the penalty at full closure, The calculated reduction in DNBR is still less than that which would produce a DNBR less than 1,3 for l
I the most limiting anticipated transient (two pump out of four pump loss-l l
of-fl ow). Thus, no penalty is required.
H. B, Robinson Cycle 5 H
B, Robinson is fueled with 105 Westinghouse fuel assemblies and 52 Exxon Nuclear Corporation fuel assemblies,,The Westinghouse 15x15 DNBR penalty model was applied to the Westinghouse fuel with a l
correction for the actual heat flux rather than the peak experimental values.
The Exxon fuel was consMered to bow to the same extent as the Westinghouse 15x15 fuel so that the Westinghouse bow vs. burnup equation was also applied to the Exxon fuel.
This assumption is conservative since the Exxon fuel has a thicker cladding and other design features which should render the amount of bowing no greater than in the Westinghouse fuel.
The DNBR redoction calculated by this method was offset by the fact that the worst anticipated transient for H. B. Robinson results ~
in a DNBR of 1.68.
i 1,,
23 D. C. Cook. Cycle 2 t
t D. C. Cook contains 128 Westinghouse fuel assemblies and 65 Exxon i
fuel assemblies. The limiting transient for D C. Cook is the Loss l
i of Flow (4 pump coastdown) which has a minimum DNBR of 2.01.
This j
value of DNBR is sufficientiy high to accommodate the rod bow penalty for Cycle 2 without reducing the DNBR below the safety limit value of 1.3.
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l 5.0 References 1.
Letter to V. Stello, Director, Division of Operating Reactors, I
USNRC from C. Eicheldinger, Manager, Nuclear Safety Department, Westinghouse Electric Corporation, NS-CE-N61, August 13, 1976.
2.
Hill, K. W. et., al, " Effects of a Bowed Rod on DNB", Westinghouse Electric Corporation", WCAP 8176.
3.
Standrad Review Plan - Section 4.4, II.l.A.
4.
Letter to R. Salvatori, Manager, Nuclear Safety Department, Westinghouse Electric Corporation from D. Vassallo, Chief, Light Water Reactors Project Branch 1-1, Directorate of Licensing, December 4, 1974.
5.
Letter to V. Stello, Director, Division of Operating Reactors, USNRC, from P. L. McGill, Combustion Engineering Company, i
December 15, 1975.
6.
Reavis, J. R., et. al., " Fuel Rod Bowing" WCAP 8691 (Proprietary)
Westinghouse Electric Corporation, December,1975.
7.
Letter to Mr. Sherer, p
}.
8.
Interim Safety Evaluation Report on Westinghouse Fuel Rod Bowing I
Division of System Safety, USNRC, April,1976.
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