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Safety Evaluation of GE Rept NEDE-23786-1 Amend 1 Fuel Rod Prepressurization.Concludes That Conditional Approval of Rept Is Warranted
ML20204C640
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Issue date: 11/21/1978
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Office of Nuclear Reactor Regulation
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ML20204C637 List:
References
NEDE-23786-1, NUDOCS 7811290268
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SAFETY EVALUATION OF THE GENERAL ELECTRIC TOPICAL REPORT FUEL R0D PREPRESSURIZATION AMENDMENT 1 NEDE-23786-T pg REGug ~

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Y k**M NOVEMBER 1978 CORE PERFORMANCE BRANCH UNITED STATES NUCLEAR REGULATORY COMMISSION

. WASHINGTON, D. C. 20555 78112Seacos

TABLE OF CONTENTS S

1. Abstract.......................................................iii
2. Introduction.................................................. 1 ,
3. Review Considerations......................................... 4 3.1 Fu el P e rf o rma nc e . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 ,

3.2 Safety Effects........................................... 6 .

3.2.1 Loss of Coolant Accident Analysis................. 7 -

3.2.2 Transient Analysis................................ 10 3.2.3 Core Stability..................................... 13 3.3 Level of Prepressuri zati or. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 3.4 Prepressuri zed Test Assemb1y. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

4. Conclusions................................................... . 17
5. R e f e re n c e s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 3 L

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1. Abstract This document describes the review of a General Electric licensing topical report on fuel rod prepressurization. The report describes the technical benefits and safety significance associated with prepressurizing GE BWR fuel rods with helium to three atmospheres. This design change is expected to result in improved fuel reliability and has both beneficial and detri-mental effects on plant safety analyses. Based on our review, we conclude that conditional approval of the subject topical report is warranted.

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2. Introduction The current General Electric (GE) fuel rod design consists of high density uranium dicxide (UO2 ) pellets stacked in a Zircaloy-2 cladding tube which is evacuated, backfilled with helium to one atmosphere, and sealed by welding Zircaloy end plugs in each end. GE has submitted a licensing topical report (J_) describing the technical benefits and safety significance of raising the backfill pressure from one to three atmospheres of helium. The concept of pressurized backfill, or prepressurization, is not a new one in nuclear fuel design. The manufacturers of pressurized water reactors (PWRs) currently pressurize their fuels to a high pressure to avoid creep collapse of the cladding as a result of in-reactor fuel densification. The GE boiling water reactor (BWR) fuel is not subject to creep collapse because of the l

l lower system pressure and larger cladding thickness-to-diameter ratio.

l Prepressurization in either the PWR or BWR fuel design is desirable '.,

because it improves pellet-to-cladding gap conductance and thus results in lower fuel temperatures, reduced U02 thermal expansion and reduced fission gas release.

The ratio'nale for prepressurizing the current GE fuel design is based primarily on anticipated fuel pe formance and reliability improvements rather than safety considerations. However, prepressurization results in some changes to plant safety analyses. These changes are described in the licensing topical report, NEDE-23786-P (1), and an amendment (2) to the original report. The original submittal was based on calculations performed by the fuel performance code, GESTR (3_), which is currently under review. In accordance with conclusions reached

t at a meeting between GE and the NRC staff on April 11, 1978, an amendment to NEDE-23786-P was provided in a form similar to the original report but based on calculations perfoimed by another fuel performance code, GEGAP-III (4), which is currently approved by the NRC. This evaluation is based entirely on the amendment . PEDE-23786-1-P. A chronology of

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events leading up to t'h is evaluation is given in Table 1.

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u TABLE I 1

REVIEW CHRONOLOGY. j March 22, 1978 Submittal of _ General Electric licensing topical report " Fuel Rod Prepressurization" (NEDE-23786-P). -

April 11, 1978 - Meeting with General Electric to discuss fuel rod prepressurization.

May 24, 1978 Submittal of Amendment 1 to General Electric licensing topical report " Fuel Rod Prepressurization" (NEDE-23786-1-P).

May 30, 1978 NRC request for information on General Electric fuel' rod prepressurization. >

1 June 8, 1978 General Electric response to request' for information.

August 1, 1978 NRC request for additional information on General

lectric fuel rod prepressurization.

August 14, 1978 General Electric response to request for additid.tal information.

October 27, 1978 NRC statement on review status of General Electric fuel rod prepressurization. N f

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3. Review Considerations In our review of the General Electric report on prepressurization, only 1

a modest effort was required with respect to acceptance of the actual design change. Since it appeared that General Electric intended to reference this topical report on specific plant applicatior.s with pre-viously performed safety analyses (using the unpressurized fuel), a major part of the review effort was directed toward identifying the effects of prepressurized fuel on the safety analyses. The reference value of this document is therefore anticipated to be for existing plant applications, I

~ wherein the safety analyses have been submitted and approved on the basis of the unpressurized fuel design.

This safety' evaluation report does not address the approval of three- s atmosphere prepressurized fuel based on anticipated fuel performance and )

reliability benefits. Rather, this evaluation addresses the safety sig-  ;

nificance associated with prepressurizing GE BWR fuel rods. Prepressuri-E zation results 'i some detrimental as well as some beneficial changes in l core safety analyses. In order to gain a perspective of these changes, l I

it is of interest to describe the fuel performance benefits as well as the safety effects. I 1

3.1 Fuel Performance The primary effect of prepressurization on fuel performance is t) increase the thermal conductance between the UO2 fuel pellet cnd the Zircaloy cladding, which correspondingly reduces fuel temperatures. The improvement

I in gap conductance is partially due to the higher inventory of helium in the fuel rod, and this highe'r inventory reduces the dilution of helium with lower conductivity fission gases. In addition, because the release of fission gas from the fuel increases with increasing temperature, a feedback mechanism exists that further reduces the release of fission  !

l gases from'the fuel. l l

A second effect of prepressurization on fuel performance is to increase ]

the failure resistance of the fuel design. The specific failure mechanism l of interes_t is a result of pellet / cladding interaction (PCI). Pellet /

cladding interaction is a phenomenon that may result in failure by various mechanisms, such as stress corrosion cracking or cladding mechanical over-x strain. The phenomenon occurs when fuel rods are subjected to power in-creases that produce local cladding strains, usually in the presence of embrittling fission products. It is believed that the likelihood of PCI failure can be reduced by limiting local cladding strain and reducing the release of corrosive fission products. To an extent, both of'these ,

parameters are reduced by the lower temperatures in prepressurized rods.

General Electric has attempted to quantify the increase in fuel failure resistance due to prepressurization. The results, expressed in terms of an effective increase in linear power rating, are n'ot applied in safety analyses.

3.2 Safety Effects In addition to the fuel perfomance improvements resulting from pre-pressurization, General Electric has also indentified a number of safety-related effects thi.t result from changes in the thermal character-istics of the fuel rod. The most important of these altered characteris-tics are a reduction in fuel temperature, a reduction in the overall fuel rod themal time constant, and a change in the distribution of- fuel rod pressures. In their submittal, GE describes the effects of prepres-surization on loss-of-coolant accident analyses, anticipated transient analyses and core stability margins. The staff requested (5) that General '

Electric identify all transient and accidents where the change from one atmosphere to three atmosphere prepressurized fuel will be reflected in tihe s safety analyses. GE responded (6) with additional detail on the effects of I-prepressurizing fuel, but did not identify any new areas of significance beyond those listed in the subject topical report.

I The NRC ctaff requested Q) additional information on the safet'y effects of fuel rod prepressurization. The General Electric responses (8_) to these questions complete the documentation of the proposed design change.

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l 3.2.1 Loss of Coolant Accident Anaysis The three LOCA analysis parameters that are significantly affected by prepressurization are: (1) calculated fuel rod temperatures or stored energy, which decrease with prepressurization, (2) the degree of fuel rod ballooning, which may increase with prepressurization, and (3) calculated number of cledding perforations or ruptures, which may increase with prepressurization. Gerferal Electric has shown that prepressurization increases the gap conductance and therefore decrease the fuel rod temperatures or stored energy for all calculational models (GEGAP-III (4), GEGAP-III with NRC correction (9), and GESTR (3)) and for all' exposures up to a peak rod exposure of 40,000 Megawatt-days per l metric ton. The dependence of cladding strain and rupture on prepres-surization is not as simple because the relative pressure difference s between each fuel type changes as a function of exposure. The precise b effect must, therefore, be determined by LOCA analyses for each plant.

General ,Ilectric has provided results; of analyses showing the effect of prepressurization on the LOCA calculations for typical plants. The effect of prepressurization is expressed in terms of a change in the maximum average planar heat generation rate (MAPLGHR), an expression of the LOCA operating limit. The results vary as a function of plant type, fuel exposure and the fuel code used in the calculation, but the MAPLGHR limit was found to remain constant or increase for all cases considered.

That is, prepressurization reduces the detrimental effects of LOCA. It is interesting to note that General Electric finds no cladding ruptures are predicted for pressurized or unpressurized 8x8 fuel designs.

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Although the GE prepressurization report proposes no change in the existing LOCA analysis methods, we have questioned the applicability 1 l

of the existing approved methods to the new fuel design. Our concerns were limited to (1) gap conductance and (2) the strain and rupture be-havior of prepressurized, as opposed to unpressurized, fuel rods. A review of the approved steady-state (4) and LOCA (_10) gap conductance has shown that these models remain valid for the proposed design change. For the case of the claddin strain and rupture behavior in prepressurized fuels, GE has shown that these analyses also remain valid and the supporting data cover the entire range of temperature and pressure expected for .

prepressurized fuel.

In sommary, we find that (1) the reported typical plant analyses were performed using approved methods, (2) no model changes were required, (3) reported results show a consistent improvement in LOCA response and (4) calculated improvement in MAPLHGR due to prepressurization will not >

be used in plant operation. A sensitivity analysis (J1) was performed assuming prepressurized fuel in a slow-flooding BWR. This plant type minimizes the effect of reduced stored energy and maximizes the degree I of cladding swelling. It was shown that calculations of core flow, transient CPP, reactor pressure, heat transfer coefficients, and water level would be unaffected and that only the fuel rod heatup calculations would be affected. The calculations for this limiting case showed a '

decrease of 20*F in peak cladding temperature and less than a 5*F change due to cladding swelling. Because calculated system behavior I i

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9 is not affected and because the decrease in initial stored energy has been shown to be a benefit for rod heatup calculations, the sensitivity analysis has demonstrated that prepressurization will result in lower calculated peak cladding temperature for any BWR design analysis in which cladding perforation during LOCA is not predicted. The staff conclude that previous LOCA analyses performed with unpressurized fuel aie more .

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limiting and are acceptable for showing compliance with 10CFR50.46.

Therefore, LOCA reanalyses for prepressurized fuel is not required for applications which reference NEDE-23786-1-P.

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3.2.2 Transient Analysis There are two dominant core transient analysis parameters affected by prepressurization. These factors are competing and each results from the improved gap conductance of the fuel. First, the reduced thermal time constant of the prepressurized design results in more rapid conversion of thermal energy within the fuel to voids within the coolant. This process terminates the nuclear transient sooner due to the negative void reactivity ,

of the coolant. The second factor is also related to the reduced thermal time constant of the prepressurized design. In this case, the increased energy transfer rate from the fuel to the ciudding results in an increased _

potential for film boiling and subsequent damage to the fuel. The potential for film boiling is expressed in terms of the critical power ratio (CPR),

which may be taken as representative of the total effect of prepressuri- .

T zation on core transient analysis. General Electric stated that a change of less than 0.01 in the'CPR margin results from prepressurizing the fuel.

They also stated that the effects of prepressurized fuel on the peak system

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pressure during a transient are negligible. '

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The staff questioned (5) the completeness of the spectrum of events used in analyzi,ng the effects of the revised fuel ' design. General Electric responded (6) with a list of transients considered in their evaluation. ,

l results as typical for the prepressurized fuel design. Less than 0.01 l

change in the minimum critical power ratio (MCPR) was found for all events when prepressurized fuel was. included in the analyses.

The staff further questioned the increased gap conductance resulting from prepressurization and how it is factored into the analysis of tran-sients and accidents. In response, General Electric described no change in the constant average value of gap conductance which is used to assess cere transient response. This constant value differs from the power and burnup dependent gap conductance used in LOCA analysis. The method of using a constant value is documented in the REDY (12) and ODY'l (13) tran-sient m0'els. General Electric has stated (14, lj5) that, during limiting reactor vessel pressurization transients, smaller values of gap conductance lead to more severe power increase transients. The severity of the tran- s sient is therefore expected to be reduced by the proposed change to prepres- b l

surized fuel. They further state that' for the most limiting transient (turbine trip without bypass), " conservative values of all inputs are used" (16) and that "the net effect of a full core of prepressurized fuel is insignificant." (6) l It is the staff opinion that GE has not adequately addressed the subject of conservatism in gap conductance for transient and accident analyses. We

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also note that the REDY and ODYN codes are currently under review and consider the gap conductance issue unresolved. Although additional docu-mentation may be required to show the appropriateness of gap conductance l

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in the transient analyses, this information is not required for acceptance I

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I of the proposed fuel-design change. On an interim basis, we conclude '

that existing methods of transient analysis may be used for both un-pressurized and nrepressurized fuel designs. The spectrum of transients considered should conform with the revised evaluation procedures des-cribed by GE (JJ7_) for application of the REDY and ODYN codes. For licensing applications where the analyses have previously been sub-mitted on the basis of the REDY code exclusively, this requirement has generally not been met. In each application, reanalyses cf all limiting i events will be required to show conformance with the revised evaluation procedures. If the use of prepressurized fuel is anticipated in the licensing application, the effects of prepressurization must also be ,

N included in the reanalyses of all limiting events.

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P 3.2.3 Core Stability The. introduction'of a pressurized fuel design and the resulting decrease in the fuel themal time constant was discussed previously. Because of void reactivity and other feedback effects on core power, the themal-hydraulic stability of the system must be considered. In general, the reduction in the thermal time constant of the fuel reduces the core and ,

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channel stability margins. General Electric calculations indicate that the increase in decay ratio .is less than or equal to 0.05. Thus, fuel rod prepressurization, through an increase in core-average gap conductance, results in a decrease in stability margins. Licensing analyses of core stability by GE have traditionally used a core-average gap conductance of 1000 Btu /hr-ft *F. General Electric has calculated the actual core- x average gap conductance for the reactor conditions at which the minimum I

- stability margin typically occurs. The resulting core-average gap con-ductance values all lie below the value used in licensing analyses, but the margin in each case is decreased. Sir.ce the traditional high gap con-ductance value is assumed to be conservative, GE has proposed that changes in reactor core stability due to prepressurization are accomodated by existing conservatisms in licensing analyses.

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At the:present time the staff is reviewing the GE criteria for themal- ..

hydraulic stability. This review will also involve the methods (_18) 8 used by GE to analyze' stability. The outcome of this review will be P

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applied to both prepressurized and unpressurized fuel. Our conclusions may result in decreased operational flexibility for some plants and can be accommodated by modifications to the technical specification limits on the allowable power-flow rate operating regime.

- 1 3.3 Level of Prepressurization _

General Electric has presented analyses wherein the thermal benefits of prepressurization compete with the detrimental effects predicted for the L0CA event. A prepressurization value of three atmospheres was selected as optimum. We agree with the GE approach to selecting a design level of prepressurization. Since safety analyses were not

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presented for prepressurization levels beyond three atmospheres, we ,

limit this approval to three atmospheres.

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3.4 Prepressurized Test Assembly In support of the proposed design change to prepressurized fuel, General Electric submitted a description of prototype in-reactor testing in 1976 (19). In April 1977, a pressurized test assembly (PTA) began operation l in the Peach Bottom Unit 3 reactor. The pressurized test assembly I

. contains twenty-four fuel rods prepressurized to three atmospheres of helium. The fuel rods are identical to those proposed by the pre-pressurization report except nonstandard end plugs were used to fac li-tate prepressurization with nonproduction equipment.

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In light of the substantial information available on prepressurized fuel  :

operation in PWRs, it is not considered necessary that the pressurized test assembly surveillance be concluded before the proposed design N change is implementt.d. Surveillance information will be obtained, how-

.l ever, and this pressurized test assembly will thus serve as a lead )

irradiation test of the new fuel design.

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__ In reviewing the subject of the pressurized test assembly, we note that a somewhat higher MCPR operating limit was required for the pressurized test assembly compared with the standard fuel assemblies utilized in the reload batch. Since standard 8x8, retrofit 8x8, and prepressurized 8x8 fuel assemblies may exist in any given reload core, an analysis procedure must be identified for mixed reload cores. General Electric has responded to this concern by stating that the beneficial impact of the prepressurized assemblies will not improve the calculated core transient response in

l initial reload cores, in which relatively small numbers of prepressurized i fuel bundles exist. This is because the gap conductance used in core l transient analysis for mixed cores is determined for the dominant fuel type.

In such a case, the transient critical power of the pressurized fuel assem-blies is still decreased and will require a slightly higher MCPR operating  ;

1 limit. I l

We conclude that reference,to NEDE-23786-1-P in a reload safety analysis is not sufficient to meet the requirements for the use of prepressurized j l

fuel. General Electric has stated that additional analyses will be submitted for each reload application and that these analyses will reflect the three atmosphere prepressurized fuel. In mixed-core analyses, the corewide transient response will be determined by the N

dominant fuel type. However, the MCPR operating limits will be L calculated for each fuel type in the core. k-l i

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4. Conclusions The General Electric report, " Fuel Rod Prepressurization, Amendment 1,"

(NEDE-23786-1-P) describes the technical benefits and safety significance associated with prepressurizing GE BWR fuel rods with helium to three  ;

atmospheres. Although this report and subsequent information provided by GE have identified a number of areas impacted by prepressurization in safety analyses, the changes to calculated plant safety parameters ~are small. In recognition of the fact that the beneficial changes in reactor l

core safety due to prepressurization outweigh the detrimental aspects of )

the change, we have concluded that conditional approval. of the subject i topical report is warranted. Based on our review, we conclude that:

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1. The LOCA analyses submitted in support of the plant safety 'l I

analysis must include the effects of prepressurization to -

three atmospheres. For license applications where unpres-surized fuel designs have been reviewed by the staff and found acceptable, and the introduction of prepressurized fuel is anticipated, the staff conclude that previous LOCA i

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analyses performed with unpressurized fuel are more limiting l

and are acceptable for showing compliance with 10CFR50.46.

Therefore, LOCA reanalyses for prepressurized fuel is not required for applications which reference NEDE 23786-1-P.

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2. -For anticipated transients, reanalysis of all limiting events will be required to show confonnance with revised i evaluation procedures described by General Electric. This

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requirement will be made regardless of the fuel design used.

If the use of prepressurized fuel is' anticipated'in the l_icensing application, the-effects of prepressurization must be included in the reanalyses.

3. At the present time, the staff is reviewing the GE criteria for thermal hydraulic stability. This review will also involve the methods used by GE to analyze stability. The outcome of this review will be applied to both prepressurized and unpres- g3 surized fuel. Our conclusions may result in decreased opera-tional flexibility for some plants and can be acconsnodated by modifications to the technical specification limits on the allowable power-flow rate operating regime. _
4. Referencing of the topical report in reload applications will, in general, be accepted to the same extent as that accepted

, for new plant applications. It should be noted, however, that technical aspects of the safety analysis for full and partial cores of prepressurized fuel are different.

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5. References

'1.

R. B. Elkins, " Fuel Rod Prepressurization," General Electric Licensing Topical Report NEDE-23786-P, March 1978.

2.

R. B. Elkins, " Fuel Rod Prepressurization, Amendment 1," General Electric Licensing Topical Report NEDE-23786-1-P, May 1978.

3.

E. G. Johansson, G. A. Potts and R. A. Rand," GESTR: A Model for the i Prediction of GE BWR Fuel Rod Thermal / Mechanical Performance,"

General. Electric Licensing Topical Report NEDE-23785-P, March 1978.

4. "GEGAP-III:

A Model for the Prediction of Pellet-Cladding Thermal '

Conductance in BWR Fuel Rods," General Electric Licensing Topical Report NEDC-20181, Supplement 1 (Proprietary), November 1973. .

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5. ~

O. D. Parr, Nuclear Regulatory Commission, letter to G. G. Sherwood, General Electric, on " Review of General Electric Topical Re '

l NEDE-23786-1-P, Amendment 1, ' Fuel Rod Prepressurization,'"May port, 30, 1978.

6.

Commission, on "NRC Request for Additional Informa Prepressurization," MFN-228-78, June 8, 1978. N '

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O. D. Parr, Nuclear Regulatory Commmission, letter to G. G. Sherwood, '

August 1, Electric, General 1978. on "NEDE-23786-P, ' Fuel Rod Prepressurization,'"

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Commission, on "NRC Request for Additional Inform Prepressurization," MFN-338-78, August 14, 1978. '

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R. O. Meyer, C. E. Beyer, and J. C. Voglewede, " Fission Gas Release from Fuel atMarch NUREG-0418, High1978.

Burnup," U.S. Nuclear Regulatory Commission Report 10.

J. Duncan and P. W. Marriott," General Electric Company Analytical Model Appendix forKLoss-of-Coolant

- Volume Analysis in Accordance with 10 CFR 50, NEDE*20566P, December 1975. 1,"~ General Electric Licensing Topical Report 11.

H. Pfefferlen, Regulatory General Commission Electric, letter to R. L. Tedesco, Nuclear

- on "NRC Request for Additional Information on Fuel Rod Prepressur,ization," MFN-409-78, October 27, 1978.

12.

R. B. Linford, " Analytical Methods of Plant Transient Evaluations for the General. Electric Boiling Water Reactor," General Electric Licensing Topical. Report NED0-10802, February 1973. '

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References (Cont'd)

13. E. W. Fuller, General Electric, letter to D. F. Ross, Nuclear Regulatory 1 l

Commission, on " Transmittal of ODYN Computer Code Model Description, Revision 1," MFN-300-78, July 21, 1978.

14. R. B. Linford, " Analytical Methods for Plant Transient Evaluations for the GE BWR, Amendment No.1," General Electric Licensing Topical Report NED0-10802-01, April 1975. -
15. E. W. Fuller, General Electric, letter to D. F. Ross, Nuclear Regulatory -

Commission, on " Response to NRC Request for Information on ODYN Computer  ;

Model (Sixth Submittal)," MFN-351-78, September 1, 1978. l

16. " General Electric BWR/6' NSSS Standard Safety Analysis Report" Section 15.1.2.2.2 (8).
17. E. D. Fuller, General Electric, letter D. F. Ross, Nuclear Regulatory ',

Commission, on " Application Submittal for ODYN Transient Model," s.

MFN-136-78, March 31, 1978.

18. " Stability and Dynamic Performance of the General Electric Boiling N Water Reactor," General Electric Licensing Topical Report NED0-21506, January 1977. [
19. " Pressurized Test Assembly: Supplemental Information for Reload-1 'I Licensing Submittal for Peach Bottom Atomic Power Station Unit 2," l General Electric Licensing Topical Report NED0-21363-1, November 1976.

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