ML19317F012
| ML19317F012 | |
| Person / Time | |
|---|---|
| Site: | Oconee  |
| Issue date: | 05/17/1973 |
| From: | Stello V US ATOMIC ENERGY COMMISSION (AEC) |
| To: | Deyoung R US ATOMIC ENERGY COMMISSION (AEC) |
| References | |
| NUDOCS 8001080716 | |
| Download: ML19317F012 (6) | |
Text
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MAY 171973
$~}bf R. C. DeYoung, Assistant Director for Pressurized Water Reactors, L OCONEE 1 FUEL DENSIFICATION REPORT Enclosed is a summary report on the effects of B&W fuel densification techniques for Oconee 1.
A full and detailed report will be provided to you for use on Three Mile Island 1.
N*'Idaal signet sy a, y,,,
Victor Stallo, Jr., Ass stant Director for Reactor Safet Directorate of Licensing cc:
T. Novak J. Hendrie F. Schroeder D. Ross A. Schwencer I. Peltier H. Schierling N. Lauben R. Lobel M. Tokar S. Kim D. Davis Distribution:
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3 4.1 STAFF EVALUATION 4.1.1-General The staff reviewed the effects of fuel densification for Oconee 1 on the basis of the staff's guidelines and the technical evaluation of the applicant's safety analysis of steady state operation, operating transients and postulated accidents.
In the evaluation'the applicant appropriately considered the staff guidelines including the effects of instantaneous and anisotropic densification (initial density minus 2 6', final density 96.5% TD), the assumption i
of no clad creepdown as a function of core life, and the assumption of an axial gap leading to a power spike. The staff reviewed the effects of fuel manufacturing and reactor operating parameters on the fuel densification mechanism. The staff reviewed the applicant's assumptions, methods, and computer codes used in evaluating the fuel densification effects. The mechanical integrity of the fuel cladding and the thermal performance of the fuel were considered in the analyses of steady state operation, operating transients, and postulated accidents as discussed in the following sections.
4.1.2 Mechanical Integrity of Cindding Clad creepdown during the ccre life is not considered by the applicant in the calculation of gap conductance. This is a con-servative assumption since the reduced gap size due to clad creep-down would result in a higher gap conductance and thus in a lower stored energy in the fuel. The staff reviewed the B&W method for calculating the clad collapse time, which is the time required 1
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c for an unsupported clad tubing to flatten into the axial gap volume caused by fuel densification. On the basis of independent staff
.alculations and from experience of fuel performance in other reactors, the staff concurs with the applicant that clad collapse is not expected for the Oconee 1 fuel during the first cycle of 7500 effective full power hours (EFPil). However, the staff concludes that the evaluation model for collapse time calculations contains several_ deficiencies in its application to Oconee 1.
The staff I
has informed the applicant that a final resolution of the B&W model for collapse time calculations is necessary for subsequent fuel cycles of Oconee 1.
4.1.3 Fuel Pin Thermal Analysig The applicant uses the B&W computer code, TAFY, to calculate gap conductance, fuel temperature, and stored energy for the Oconee 1 fuel, used as a basis for the safety analysis. To demonstrate the applicability of the TAFY code for the evaluation of the Oconee 1 fuel thermal behavior, the applicant compared TAFY predicted fuel temperatures and gap conductances with experimental data.
The staf f reviewed the TAFY code and concludes that realistic and/or conservative assumptions have been used for the modeling of the physical phenomena incorporated into the code (thermal expansion, fuel swelling, sorbed gas release, fission gas release), with two exceptions:
(1) partial contact between the clad and fuel and (2) formation of a central void due to fuel restructuring on the 4
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. basis of columnar grain growth $tatemperatureof3200*F. These
. assumptions are discussed below.
'The~ assumption of a partial contact between fuel and clad is l
based on the work by Kjaerheim and Rolstad who determined the UO2 l
thermal conductivity from measured fuel temperctures using a fuel-clad geometry with cold diametral gaps ranging from 1.85 mil to 6.61 mil. 'In order to predict measured temperatures Kjaerheim and Rolstad developed an analytical model which assumes heat transfer i
not'only by radiation and conduction through the gap but also
. I assumes conductive heat transfer at a partial contact area, CA, betw2en fuel and clad (conduction through gas in contact area and conduction at solid-to-solid interface), which is attributed to fuel cracking. The CA model predicts a minimum contact area of 10 percent regardless of the initial diametral gap for a particular fuel diameter. For the Oconee 1 fuel-clad geometry with a cold diametral gap of 12.8 mil the partial contact area is 11 percent, on the average. The TAFY calculated gap conductance for Oconee 1 at BOL and with a linear heat rate of 16 kW/ft is 1052 Btu /hr-ft -F*
of which approximately 1/3 is due to heat transfer across the partial contact area. The comparison of TAFY predicted temperatures and gap conductances with corresponding experimental values shows that TAFY is conservative for small diametral gaps (f 6.61 mil), but for gap
-sizes comparable to the Oconee 1 gap, and larger, the code predictions are not consistent and can be either conservative or not conservative.
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l The staff concludes that the use of the partial contact area mode j
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may be very useful for predicting temperatures of an exper men i
j f rom which the model was derived, but should not be used when disassociated from other features in the model and when extrapolated 1
to other gap sizes.
The TAFY code uses a fuel restructuring model based on the fuel assumption of columnar grain growth at 3200*F with a change in density from 96.5% TD which leads to the formation of a central i
l temperature, void in the fuel, and results in a reduction of maximum fue The staff reviewed this assumption on the basis and stored energy.
&W fuel, of photomicrographs of cross sections of exposed typical B formation and concludes that fuel restructuring associated with the histories of a central void can take place under certain operating However, these parameters are not necessarily' and conditions.
known for the Oconee 1 reactor operating conditions and it would be possible that.the irradiation induced fuel densification has been f
completed before a temperature of 3200 is achieved and, there ore i
th the possibility of fuel rectructuring due to columnar gra n grow would be foreclosed.
4.1.4 Conclusion there is a reasonable assurance that The staff has concluded that fuel clad collapse will not take place during the first fuel cycle The staff has informed the (7500 EFPil) of the Oconec Unit 1.
applicant that the method to calculate clad collapse time is not l
acceptable and the applicant has committed to develop a mode j
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acceptable to the staff prior to a second fuel cycle of the Oconce j
Unit 1.
The staff has concluded that the assumptions in the TAFY code (1) of partial contact between fuel and cladding and (2) fuel restructuring leading to a central void in the fuel has not been justified by B&W and are not acceptable.
In absence of a timely of the two assumptions for Oconec Unit 1, the staff resolution i
requested the applicant to remove the assumption of restructuring i
from the TAFY code and to reduce the TAFY calculated gap conductance by 25%. With this reduction TAFY predictions for experimental data will be conservative for gap sizes comparable to Oconee 1.
The staff concludes that the use of as-built dimensions for the fuel and clad in the TAFY code is acceptable with the exception that the initial fuel density with a minus 20 value is.to be assumed, consistent with the staff fuel densification report. The applicant has-recalculated gap conductance and fuel temperatures with these
. assumptions and used these values to establish maximum linear heat rates.
In order to prevent fuel melting at a temperature of 5080*F, the maximum allowable linear heat rate is calculated to be 20.1 kW/ft and in order to not exceed a clad temperature of 2300*F.
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(ECCS criteria) a maximum linear-heat rate of 18.6 kW/ft has been calculated.
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