rai responses to round 7 rai on anp-10285p. › docs › ml1231 › ml12319a408.pdfupdate: august...

RAI Responses to Round 7 RAI on ANP-10285PRockville, MDNovember 16, 2012Draf

t

2NRC Meeting – RAI Responses to Round 7 RAI on ANP-10285P – November 16, 2012

Agenda Summary of outstanding issues and fuel design change Summary of preliminary results – loads on grids Grid strength definition, un-irradiated and irradiated – RAI 68 Impact of grid strength definition upon thermal-hydraulics,

non-LOCA, and LOCA Bundle deflection amplitude – RAI 64 Impact of grid deformation on loads – RAI 70 Impact of plant seismic reanalysis on fuel assembly closure

plan Summary and Next stepsDraf

t


Near Term Interactions Impact of fuel design change on thermal-hydraulics, non-

LOCA, and LOCA Irradiated fuel assembly damping

Draft

Summary of Outstanding Issues and Fuel Design ChangeBrett Matthews Draf

t


Key Issues To Be AddressedPrior to Modified Closure Plan

Presented on May 2, 2012

Forced vibration versus pluck test Empirical fuel assembly frequency response at high

amplitudes Fuel assembly characteristics in the irradiated condition Linear versus non-linear bundle modeling Definition of grid strength and testing protocol CASAC acceptability Modeling of grid post-buckling behavior Evaluation of coolability under grid buckling Evaluation of control rod insertion under grid buckling SSE plus AOO Criteria

Draft


Key Issues To Be Addressed

Forced vibration versus pluck test Empirical fuel assembly frequency response at high

amplitudes Fuel assembly characteristics in the irradiated condition Linear versus non-linear bundle modeling Definition of grid strength and testing protocol CASAC acceptability Modeling of grid post-buckling behavior Evaluation of coolability under grid buckling Evaluation of control rod insertion under grid buckling SSE plus AOO Criteria

After Modifications

Draft


Fuel Assembly Design Change Increased strip thickness

Two grids modified Located in the middle of the fuel assembly axially Grid loads are highest for these grids

Increased weld nugget size All grids impacted Strip thickness unchanged for other grids

Impact of change – mechanical Strength of all grids increased Strength of grids located in the middle of the fuel assembly axially

increased significantly Result is that all grids have margin to the grid strength definitionDraf

t


Impact of Fuel Assembly Design Change

Mechanical No significant impact on bundle characteristics

Lateral stiffness impact is minimal since the stronger grids are close to the middle of the bundle, where the rotation due to bending is minimal

Natural frequency impact is minimal as well, since stiffness does not change Mid-span stiffer grids could affect 2nd mode, but the participation factor for this mode is

zero for lateral acceleration loading. Updated spacer grid characteristics for grids 4 & 5

Lateral analysis accounts for new through-grid stiffness and damping for grids 4 & 5 Thermal-Mechanics

Evaluate rod stress with new grids Non-LOCA

No impact Thermal-Hydraulics

Discussed later LOCA

Discussed later

Draft


Key Issues Remaining:Forced Vibration versus Pluck Test

UNCHANGED FROM MAY 2 PUBLIC MEETING:AREVA will perform comparative testing on a 14 ft HTP bundle representative of the U.S. EPR fuel design

Bundle will be in a simulated irradiated condition Both forced vibration (up to ~0.3 inches) and pluck test (up to ~1.0 inches)

will be performed

Objective is to demonstrate equivalency in the two approachesThereby validate the existing test basis for the U.S. EPR which is largely based on forced vibration testingDraf

t


Key Issues Remaining:Frequency Response at High AmplitudesUNCHANGED FROM MAY 2 PUBLIC MEETING:AREVA will perform testing on a 14 ft HTP bundle representative of the U.S. EPR fuel design

Bundle will be in a simulated irradiated condition Pluck test (up to ~1.0 inches) will be performed

This empirical information can be used to validate and reassess the range used for the frequency sweep

Draft


Key Issues Remaining:FA Characteristics in Irradiated Condition

UPDATED FROM MAY 2 PUBLIC MEETING:Irradiated Bundle Frequency

AREVA will perform testing on a 14 ft bundle representative of the U.S. EPR fuel design Bundle will be in a simulated irradiated condition Pluck test (up to ~1.0 inches) will be performed

This empirical information can be used to validate and reassess the range used for the frequency sweep

Irradiated Condition Damping The primary fuel assembly damping mechanism is independent of the lateral

stiffness of the bundle, hence, it is the same at Un-irradiated and Irradiated UPDATE: This will be discussed in greater detail later in the presentation

Draft


Key Issues Remaining:Linear versus Non-linear Bundle Modeling

UPDATED FROM MAY 2 PUBLIC MEETING:AREVA has provided information to show that the linear modeling (frequency versus amplitude) of the U.S. EPR is appropriateSensitivity to non-linear effects is reduced in absence of grid buckling

For grid impacts below buckling, the second order non-linear effects of the bundle stiffness vs. amplitude become even less significant.

With stable grid operation, the ability to capture the overall strain energy stored in the fuel assembly in a displaced configuration is the primary focus.

Importance of axial load re-distribution between grids lessens with stable grid operation.

Margin to buckling load further reduces the importance of secondorder effects (non-linear frequency versus amplitude)UPDATE: August 2012 RAI 64 response was been updated with new information and results, but no change to conclusion

Draft


Key Issues Remaining:HTP Grid Strength and Testing Protocol

UPDATED FROM MAY 2 PUBLIC MEETING:August 2012 Responses to RAI 68 and 70 address this issue

Response to RAI 68 defines a protocol for simulating the irradiated condition in grid strength testing Based on simulating effects of in-reactor operation known to have a primary influence

- Irradiation Hardening of the Material- Irradiation Induced Relaxation of Spacer Grid Springs

Protocol and strength definition are consistent with regulatory and industry framework Grid strength definition must give consideration to the amount of permanent deformation

inherent at the allowable load level Unique definitions of grid strength are given for both Un-irradiated and the irradiated

condition- Both definitions fit within the regulatory and industry framework

August 2012 Response to RAI 70 adds discussion of the acceptability of the grid model as part of the core row models Allowable crushing load inherently includes a finite amount of permanent deformation The use of a linear visco-elastic element accurately predicts peak impact loads and rebound

velocities after impact Gap sensitivity studies show that the small deformations inherent in the allowable crushing

load do not significantly alter the fuel bundle response or predicted loads

Draft


Key Issues Remaining:CASAC Acceptability

UNCHANGED FROM MAY 2 PUBLIC MEETING:Information previously provided:

CASAC code has been verified with sample problems with known answers, including problems that are directly representative of fuel assembly modeling

CASAC code and underlying modeling technique has been validated through full-scale testing with a row of six fuel assemblies for both in-air and in-water conditions.

Comparison of CASAC-generated load vs. deflection curves with actual test data is presented Draf

t

Summary of Preliminary ResultsBrett Matthews Draf

t


Fuel Assembly Design Change

HTP Grid

HTP Grid

HTP Grid

HTP Grid

Max Loads at HTP 4 and 5 Draf

t


Lateral Impact AnalysisModifications to the Analysis

Spacer grid modeling changes [ ] strip HTP grids at locations 4 and 5

Increase in strength Increase in through-grid stiffness Increase in through-grid damping

[ ] strip HTP grids with [ ] WNS at locations 1, 2, 3, 6, 7, & 8

Increase in strength only

Modify bundle damping parameters Damping strength is maintained the same at all frequencies

Previous analyses used reduced damping at Irradiated conditions

Updated time histories Update model for reactor coolant system to include rotation of building in

determining the translational displacements of the core plates Ongoing updates from Nuclear Island civil/structural groups to account for

NRC requests in modeling of soil structure interaction, etc.

Draft


Lateral Impact AnalysisSpacer Grid Testing

Draft


Lateral Impact AnalysisPost Processing of Results

A total of 720 seismic cases have been analyzed 8 soil cases 2 directions (X and Z) 5 rows (7, 11, 13, 15, and 17) 9 frequencies [ ]

An additional 180 LOCA cases have been analyzed for the two most limiting LOCA events (HELB-0 and HELB-100)

Each case evaluates grid impacts at each grid elevation at everyinterface across the core for every time step in the run.

There are millions of data points. Data is processed to highlight limiting margins and general trends.

Draft


Lateral Impact Analysis Max Loads in Un-irradiated Condition

Grid Location HTP 4&5 HTP 3&6

Grid Description

Un-irradiated Grid Strength (lbs)

Un-irradiated Max Load - (lbs)

% Margin on Load

Peripheral Bundles

Interior Bundles

Grid

Max Load

Irrad. Strength

Margin

Frequency sweep cases from [ ] constitute the “Un-irradiated” condition

Draft


Lateral Impact AnalysisMax Loads in Irradiated Condition

Frequency sweep cases from [ ] constitute the “Irradiated” condition

Grid Location HTP 4&5 HTP 3&6

Grid Description

Irradiated Grid Strength (lbs)

Irradiated Max Load - (lbs)

% Margin on Load

Peripheral Bundles

Interior Bundles

Grid

Max Load

Irrad. Strength

Margin

Draft


Lateral Impact Analysis Load Distribution Across Row, Un-irradiated

7 row (shortest) HTP 4 & 5 only Plotted loads

may come from different points in time

Draft



17 row (longest) HTP 4 & 5 only Plotted loads


Draft



Un-irradiated [ ]

11 row (limiting) HTP 4 & 5 only Plotted loads


Only case where 0.2% grid envelop deformation is exceeded

Draft


Lateral Impact Analysis Load Distribution Across Row, Irradiated

7 row (shortest) HTP 4 & 5 only Plotted loads


Draft



17 row (longest) HTP 4 & 5 only Plotted loads


Draft



Irradiated [ ]

11 row (limiting) HTP 4 & 5 only Plotted loads

may come from different points in timeDraf

t


Lateral Impact Analysis Impact History

Un-irradiated [ ]

11 row (limiting) HTP 4 only Only case where

0.2% grid envelope deformation is exceededDraf

t


Lateral Impact Analysis Observations

For Un-irradiated cases, comfortable margin exists to [ ] deformation on all cases

Positive margin can be shown for a more conservative criterion of 0.2% deformation on all cases, except one.

Un-irradiated loads tend to be higher than loads in the irradiated condition

LOCA loads contribute very little to the combined SSE/LOCA load

Overall peak loads and limiting margins occur on the periphery of the core Peak loads for specific soil and row cases generally occur on the

periphery of the core

Limiting loads do not result from row with largest available gapDraf

t


Lateral Impact Analysis Conservatisms

Alignment of assemblies at constant frequency maximizes impact loads Maximizes assembly response when input motions approach resonance

Effect is further maximized with frequency sweep approach Non-linear effects disperse frequency response across the row and with

respect to time

0.2% deformation on grid bounds all interior bundles and nearly all periphery bundles for Un-irradiated condition Comfortable margin to [ ] limit exists for all locations Strength definition at [ ] leaves additional margin to actual

grid buckling

2-D row model neglects losses and interference from neighboring rows Unobstructed and frictionless impacts across a row is unrealistic, but

conservative

Draft



Maximum loads occur on the periphery of the core Ultimate requirements of coolability and control rod insertability are less

challenged on the periphery

Control rod insertability is conservatively validated during grid testing with a rigid, oversized control rod assembly gauge

CASAC uses a lumped mass fuel assembly model Concentrated masses at grid nodes results in higher impact loads than

what would be experienced for the real case with distributed mass

Seismic time histories are based on enveloping ground motion spectra and a broad range of soil conditions that are intended to bound all current potential applicants for the U.S. EPR Site specific analyses would yield additional margin

Draft



The fuel seismic analysis is downstream and relies on the output of civil/structural and NSSS evaluations All upstream conservatisms provide additional margin to the fuel

evaluation

Analysis extends beyond the guidance of SRP 4.2 to explicitly addresses the effects of irradiation on both bundle and grid characteristics (NRC Information Notice 2012-09)

The Un-irradiated condition is not representative of typical operating conditions. Fuel bundles are more accurately characterized by the irradiated condition over the majority of their operation.

Cases run at “+” frequencies in the frequency sweep are unrealistic because they could only occur with small deflection amplitudes. The fuel bundle does not maintain a high frequency response as it deflects from its nominal position.

Draft



“Irradiated” grid strength testing is conservatively performed with hollow fuel rod cladding. The irradiated condition is moreaccurately represented with a solid rod to simulate the pellet-cladding contact. Testing with solid pins increases grid strength by approximately 10 to

30%, depending on the grid design

Draft

Grid Strength DefinitionUn-irradiated and Irradiated-RAI 68

Victor Hatman Draft


Grid Strength Definition - Overview

Spacer Grid Impact Strength Definition – Regulatory Background NRC Guidance Documents Review Previously Approved Definitions and Test Methods for P(crit) Conclusions of the NRC Guidance and Industry Practices Review

Spacer Grid Strength – Effects of In-Reactor Operation Effects of In-Reactor Operation on Zirconium Alloys Effects of In-Reactor Operation on Spacer Grid Strength Summary of In-Reactor Operation Effects on Grid Strength

Proposed Spacer Grid Strength Definition and Test Protocol for the U.S.EPR Fuel General Requirements Proposed Un-Irradiated Allowable Crushing Load Definition for the U.S.EPR Proposed Irradiated Allowable Crushing Load Definition for the U.S.EPR

Draft


Grid Strength Definition - Regulatory Guidance

SRP 4.2 – Appendix AThe consequences of grid deformation are small. Gross deformation of grids in many PWR assemblies would be needed to interfere with control rod insertion during an SSE (i.e., buckling of a few isolated grids could not displace guide tubes significantly from their proper location), …

In a LOCA, gross deformation of the hot channel in either a PWR or a BWR would result in only small increases in peak cladding temperature.

Therefore, average values are appropriate, and the allowable crushing load P(crit) should be the 95% confidence level on the true mean as taken from the distribution of measurements on un-irradiated production grids at (or corrected to) operating temperature. While P(crit) will increase with irradiation, ductility will be reduced. The extra margin in P(crit) for irradiated grids is thus assumed to offset the unknown deformation behavior of irradiated grids beyond P(crit).

Draft


Grid Strength Definition - Regulatory Guidance Observations on SRP 4.2 P(crit) Definition

SRP 4.2 derives P(crit) from the average value of the test sample. This shows that the original design basis intent was to ensure that grid deformation

stays low for most grids, with some grids being allowed to buckle, and perfectly elastic operation of the grids was not required.

Provision made to account for the in-reactor temperature. By testing hot, or by testing cold, and correcting for operating temperature. The actual

correction procedure is not specified. The effects of irradiation are assumed to counteract the deformation behavior

the un-irradiated strength is the controlling design parameter. The focus of SRP 4.2 Appendix A is primarily on:

the statistical definition of P(crit) from the grid sample data, on the condition of the test grid specimens, and on the test environment,

SRP 4.2 does not specify an individual grid test failure criterion (maximum indicated load vs. limitations on total deformation).

Draft


Grid Strength Definition - Regulatory Guidance NUREG/CR-1018

Discusses the actual grid failure criterion, and defines two spacer grid operation scenarios, the “no permanent deformation” and the “permanent deformation” case:

If spacer grid loading caused no permanent deformation of the spacer grid then rod to rod spacing and coolant channel flow area is undisturbed and a coolable geometry would be maintained. A small amount of permanent deformation is almost always present after spacer grid loading. Settling of the connecting strip joints and local deformation due to high local stresses are just two of the possible causes of permanent deformation. Obviously, a condition of no permanent deformation must be defined.

A sufficient condition to demonstrate that no permanent deformation has occurred appears to be that the spacer grid remain within manufacturing tolerances. This condition should be sufficient although possibly not necessary because the only meaningful definition of departure from a no-deformed condition would be that deformation which causes a measurable perturbation in the ECCS peak cladding temperature calculation. A manufacturing tolerance criteria should fall within this deformation definition. The quantity to be used for comparison with the calculated results could be defined as that load at which initial departure from the no deformation condition is obtained.

Draft


Grid Strength Definition - Regulatory GuidanceObservations The items of interest when assessing whether a grid has “no permanent

deformation” are the rod-to-rod pitch and the flow channel area. These two parameters control the grid performance and its ability to perform the coolability

function. Since this is the “no permanent deformation” scenario, Control Rod insertability is not the primary concern.

The physical dimension of the overall envelope of the grid is not a concern. This dimension only affects the grid gaps, which NUREG/CR-1018 deems to be of second

order importance (Table I in Section II, sub-section 4.2.1). It is recognized that a small amount of permanent deformation will always be

present The condition of “no permanent deformation” cannot be taken literally, and needs to be

defined. The “no permanent deformation” condition is defined in terms of a finite but

acceptably small deformation which does not interfere with the coolabilityrequirement.

Essentially, the “no deformation” condition is taken to mean the deformation threshold beyond which, the coolability would begin to be measurably impacted.

Limiting the grid deformation to the manufacturing tolerances would constitute a sufficient but not strictly necessary criterion for the “no deformation” condition.

Draft


Grid Strength Definition – Previously Approved Methodologies

AREVA BAW-10133PA Addendum 1

The grid strength definition is based on successive impact tests of grids at hot operating temperature of increasing kinetic energy content. The allowable grid impact load is defined as the lower bound of the 95% confidence interval on true mean of the instability load of the grids in the test sample.

Approved uses of the methodology: BAW-10172PA, BAW-10229PA, BAW-10239PA, BAW-10179PA, BAW-10186PA

EMF-93-074(P)(A) The grid strength was defined as the lower bound of the 95% confidence interval on true mean of the grid cold

condition buckling limit adjusted to operating temperature by scaling with the elastic modulus ratio

Combustion Engineering – CENPD-178 The defining criterion for grid “failure” is the channel closure. The maximum load at which there is no

significant deviation of channel dimensions from the original tolerances is deemed as the individual grid strength. The 95% confidence on true mean for the grid sample is defined as the general grid strength.

The most recently approved use of this methodology: WCAP-16500-NP-A, 2007

Westinghouse Electric Company – WCAP-8236(P) & WCAP-8288(NP)

The grid strength definition is based on successive impact tests of grids at hot operating temperature (or room temperature – corrected to hot) of increasing kinetic energy content. P(crit) is defined as the lower bound of the 95% confidence interval on true mean of the instability load of the grids in the test sample.

The most recently approved use of this methodology: AP-1000 Design Control Document, 2011

Draft


Grid Strength Definition – Conclusions of Guidance and Industry Practices Review

The grid strength allowable is a statistical value set at the lower bound of the 95% confidence interval on the true mean of a grid sample.

Setting the allowable strength at a certain confidence level on the true mean, signifies that the original design basis intent was to ensure that most grids do not buckle, but a few isolated ones may experience buckling without compromising the coolability and control rod insertability, as explicitly stated in NUREG/CR-1018.

The individual grid allowable strength or failure point can be treated under two scenarios: the “no permanent deformation”, and the “permanent deformation”scenario.

The “no permanent deformation” scenario does not imply the complete absence of permanent deformation, but the presence of a finite but acceptably small level of permanent deformation.

The true individual grid failure criterion for the “no permanent deformation” case is the deformation threshold beyond which, the flow channel departure begins to impact grid thermal performance in a measurable way. A sufficient criterion limiting the permanent deformation within the grid manufacturing tolerances can be substituted for the flow channel departure.

Draft


Grid Strength Definition – Effects of In-Reactor Operation

SRP 4.2 – Normal Operation Main concern is with ductility degradation, oxidation, and crud Effects of In-Reactor Operation on Zirconium Alloys Hydrogen Pickup Irradiation Hardening Irradiation Stress Relaxation Irradiation Induced Loss of Ductility Oxidation Effects of In-Reactor Operation on Grid Strength Identify the Main Drivers on Spacer Grid Strength Discuss Oxidation Effects of In-Reactor Operation - Summary

Draft



Effects of In-Reactor Operation on Zirconium Alloys Hydrogen Pickup – second order effect

Ductility: In irradiated state, over the range of expected hydrogen uptake levels for M5®

structural components, the effect is completely negligible. The primary effect on ductility comes from irradiation

Yield Strength: hydrogen uptake has a minimal strength increasing effect

upper boundM5 structural components hydrogenuptake

representative Zr‐4 structural components hydrogenuptake

upper boundM5 structural components hydrogenuptake

representative Zr‐4 structural components hydrogenuptake

Ultimate Elongation and Yield Strength for Zirconium Alloy vs. Hydrogen Content

Draft



Effects of In-Reactor Operation on Zirconium Alloys Yield Strength Irradiation hardening is a first order

effect Strong increase in yield strength

[

]

Effect saturates at ~ 2.5e+25 n/m^2 Yield strength reaches the

asymptotic value after 1 year of full power operation

Draft


Grid Strength Definition – Effects ofIn-Reactor Operation

Effects of In-Reactor Operation on Zirconium Alloys Ductility

Irradiation induced loss of ductility is a strong effect for Zirconium Alloys [

] Effect saturates at ~ 4.5e+25 n/m^2 Ductility loss reaches the asymptotic

value after ~2 years of full power operation Draf

t



Effects of In-Reactor Operation on Zirconium Alloys Irradiation Stress Relaxation Irradiation stress relaxation is

first order effect – more details to follow

Effect saturates at ~ 3e+25 n/m^2

Ductility loss reaches the asymptotic value after ~1 year of full power operation

Asymptotic value is 100% the Irradiated grid cell insertion load is almost nil. Draf

t


Grid Strength Definition – Effects of In-Reactor Operation Zirconium Alloys Oxidation

M5® has reduced oxidation compared to Zr-4 For irradiated M5® grid strips, the oxide layer is [ ] Oxidation has a positive effect on grid strength – details to follow

M5® Cladding Oxidation vs. Burnup

Draft



Testing of AREVA Irradiated Grids 2003 submittal: Closure of Interim Report 02-002: Spacer Grid Crush

Strength – Effects of Irradiation Tests on full spacer grids operated in a reactor [

] Strength of irradiated grids was proven to be lower than that of fresh

production grids The irradiated grid failure mode is the same as for un-irradiated Inspection after crush test showed no signs of brittle fracture. The grid

material retained enough ductility Grid cell relaxation is the main factor contributing to the loss of grid

buckling strength Irradiated grid strength was shown to be strongly influenced by the grid

elevation in the fuel assembly – the center grids has the highest strengthDraf

t



Irradiated Grid Failure Mode The irradiated grid failure mode

is the same as for un-irradiated Racking is concentrated in a few

rows Indicative of buckling failure Inspection after crush test

showed no signs of brittle fracture.

The grid material retained enough ductility even in irradiated condition Draf

t



Irradiated Grid Strength vs. Grid Cell Relaxation

The main driver of reduced grid strength in irradiated condition is the cell relaxation

Test Base: tested new, [ ] irradiated grids – plotted vs.

cladding insertion force (proxy for cell relaxation

Plot lot shows the grid buckling load vs. cladding insertion force

Strong correlation between grid buckling load and cell relaxation

Cell spring relaxation disrupts the fuel rod cladding straight line pattern, thus perturbing the load path through the cladding, and lowering the buckling strength

Draft



Oxide Layer Effect on Grid Strength Irradiated grid strength was found to increase with grid elevation Oxide layer has a higher elastic modulus higher grid strip stiffness Strength increases by up to [ ] - not used in the U.S.EPR evaluation

Draft



Effects of In-Reactor Operation - Summary Irradiation Induced Stress Relaxation

This is a first order effect. As shown on the irradiated grid tests, there is a very strong correlation between the grid buckling load and the average cell cladding insertion load. Cell spring relaxation reduces the stability of the fuel rod array, and reduces the buckling load.

Irradiation Induced Hardening This is a first order effect. The irradiated grid tests have indicated that the deformation to buckling of the

tested grids is less than for the new material grids. This indicates that the effective grid stiffness increases with increased material strength due to the fact that localized yielding of the grid strip material is reduced.

Oxidation The increased modulus of elasticity of the oxide layer contributes to the grid buckling capability.

Consequently, a grid strength evaluation without taking credit for oxidation is conservative. Irradiation Induced Loss of Ductility

This is a second order effect. Even if the loss of ductility is not negligible, the material retains enough ductility to prevent brittle fracture, as indicated by the irradiated condition grid tests

From a grid mechanical strength point of view, the irradiation induced loss of ductility is a second order effect. Hydrogen pick-up

This is a second order effect. The effects on ductility and strength are marginal when compared to the effects of irradiation.

Irradiated grid testing showed that the material retains sufficient ductility to avoid brittle fracture.Draf

t


Grid Strength Definition – Proposed P(crit) Definition

P(crit) Definition – Overview Establish the General Requirements for P(crit) Definition

Stability of deformation on the loading curve Limited by either peak load or maximum allowable deformation Control rod insertability Deformation distributed uniformly between fuel rod rows negligible T/H impact

Formulate the Un-Irradiated P(crit) Definition Deformation controlled

Review the Test Basis for Un-Irradiated P(crit) Definition Loading curves / Control Rod Insertability

Formulate the Irradiated P(crit) Definition Load controlled

Formulate the Test Protocol for Irradiated Condition P(crit) Address material yield strength increase and stress relaxation

Review the Test Basis for Irradiated P(crit) Definition

Draft



P(crit) Definition – General Requirements The Limit Point on the Stable Side of the Loading Curve

On the stable side of loading curve higher impacts are required to produce higher permanent deformation Contrast with the un-stable side, where decreasing impacts can produce increasing permanent deformation Consistent with the general philosophy of the ASME Code – use of load or stress limits to control deformation

Limit dictated by maximum allowable deformation (B) or buckling strength (A) NUREG/CR-1018 establishes that the real

P(crit) criterion is the threshold of deformationbeyond which there is a measurable impact on the thermal-hydraulic performance

In the case of more compliant grids, thedeformation limit is reached first

Draft



P(crit) Definition – General Requirements (cont’d) Same grid design can exhibit different behaviors, and can trigger different limits in un-irradiated

and irradiated condition Irradiation induced material hardening makes the grid less compliant, which triggers the buckling load limit first Depending on the inherent grid stiffness, and the actual value of the deformation threshold, the allowable crushing

load at un-irradiated can be lower or higher than the Irradiated buckling strength. Control Rod insertability must be

ensured for the deformation limited scenario

For the U.S.EPR this is demonstrated througha prototypical gage insertion test

The deformed grid shape at the deformation limit must be approximately uniform

The uniformity of the deformed grid lattice isensured by visual inspection on grids tested only up to the deformation threshold

It is also confirmed by the coefficient of restitutionplots vs. impact kinetic energy – the coefficient of restitution is constant for the entire range of impactenergy up to buckling

Draft


Grid Strength Definition – Proposed P(crit) Definition – Un-Irradiated Case

U.S.EPR Grid Arrangement HMP Grids – at the top and bottom of the assembly

– these grids do not experience impacts. HTP [ ] – at the mid-elevation

locations. HTP [

] – at the remaining locations

Draft


Grid Strength Definition – Proposed P(crit) Definition – Un-Irradiated Case

HTP [ ] Strip – Un-irradiated Case The HTP [ ] grid allowable crushing load at un-irradiated is defined as the load

corresponding to a permanent deformation of [ ] on the standard loading curve – 95% confidence on true mean

[

] Test base:

[

] Observations:

[

]

Draft


Grid Strength Definition – Proposed P(crit) Definition – Un-irradiated Case

HTP [ ] – Uniform Deformation Distribution

Two grids stopped slightly above [ ] permanent deformation for the purpose of providing test evidence of the uniform character of grid deformation

The deformed shape of the grid is indistinguishable upon visual inspection

The grid cells maintain the regular array No concentration of the deformation in a

given area of the grid (as opposed to buckled grids, where the deformation is concentrated in a few rows)

These findings are consistent with the fact that the coefficient of restitution is constant over the entire stable side of the loading curve Draf

t



HTP [ ] – Control Rod Insertability

All grids tested at [ ] permanent deformation using a prototypical control rod gage

Gage passed through all grids without any applied force

Sources of Conservatism: Gage pins were sized at [

]

Gage pins are rigid – test does not take advantage of the inherent flexibility of the control rods Draf

t


HTP [ ] Strip – Un-irradiated Case The HTP [ ] grid allowable crushing load at un-irradiated is defined as the load

corresponding to a permanent deformation of [ ] on the standard loading curve – 95% confidence on true mean

[

] Test base:

[

] Observations:

[

]


Draft



HTP [ ] Strip – Control Rod Insertability and Uniform Deformation

All grids tested at [ ] permanent deformation using a prototypical control rod gage

Gage passed through all grids without any applied force

Gage pins were sized at [ ] OD –prototypical U.S.EPR control rod

Gage pins are rigid – test does not take advantage of the inherent flexibility of the control rods test is conservative

HTP [ ] Strip – Uniform Deformation

The uniform deformation proven for the [ ] grids,

which are stiffer and stronger These findings are consistent with the fact that

the coefficient of restitution is constant over the entire stable side of the loading curve

Draft



U.S. EPR – P(crit) Definition - Un-irradiated Case - Summary Production grids tested at [ ] 95% confidence on true mean at [ ] permanent cumulative deformation

Draft


Grid Strength Definition – Proposed P(crit) Definition – Irradiated Case

Characteristics of the Irradiated Condition

Main drivers for grid strength [

] Hardening vs. relaxation

[

]

Test Protocol [

]

Draft



[

][

]

[

]

Test Protocol for the Irradiated Condition

Draft



Test Protocol for the Irradiated Condition – Test BasisTest performed in 2002 on Zr-4 AFA2G irradiated full grids

[

]Test Protocol

[

]Test Results

[

]

Draft



U.S. EPR – P(crit) Definition - Irradiated Case - Summary [ ] 95% confidence on true mean of the buckling load Permanent Deformation is very low (less than ½ manufacturing tolerances)

Draft


Grid Strength Definition – Proposed P(crit) Definition – Conclusions

Comparison irradiated vs. un-irradiated – HTP [

] The un-irradiated strength at

[ ] permanent deformation is higher than the Irradiated strength

Permanent deformation in simulated Irradiated condition is very low – not a concern

Draft


Grid Strength Definition – Proposed P(crit) Definition - Conclusions

Comparison irradiated and un-irradiated – HTP [

] The un-irradiated strength at

[ ] permanent deformation is higher than the irradiated strength

Permanent deformation in simulated irradiated condition is very low – not a concern

Draft


Grid Strength Definition – Proposed P(crit) Definition – Conclusions

Conclusions The grid strength is defined based on the following considerations (per NUREG/CR-1018):

The objective criterion for establishing grid failure is the cumulative permanent deformation threshold beyond which the thermal-hydraulic performance of the grid would begin to be measurably impacted.

The load/deformation allowable point must be on the stable side of the loading curve. An HTP grid can be limited by either deformation, or buckling load, depending on the

irradiated condition. Control rod insertability must be proven at the limiting deformation. The deformed pattern of the grid lattice must be approximately uniform at the allowable

deformation level. The un-irradiated condition grid strength is defined as:

95% confidence on true mean load corresponding to a [ ] permanent deformation of the grid envelope, [

]Draft


Grid Strength Definition – Proposed P(crit) Definition – Conclusions (continued)

Conclusions The irradiated condition grid strength is defined as:

95% confidence on true mean load at buckling. [

] Test protocol is based on [

]

Sources of conservatism: The un-irradiated [ ] deformation load limit is [ ] less than the actual grid

buckling capability. The [ ] deformation limit is conservative in terms of thermal-hydraulic acceptability. The irradiated grid test protocol ignores the potential benefit of oxidation for M5® grids

[ ]. The irradiated grid test protocol ignores the potential benefit of testing with solid fuel rod

pins (as opposed to hollow cladding segments). Potential benefits are in the same order of magnitude as oxidation.

Draft

Impact of Deformation Upon Thermal-Hydraulics, Non-LOCA and LOCARichard HarneLisa Gerken Draf

t


Impact of Fuel Assembly Deformation:T-H Considerations

Deformation Basis

All HTP grids reach a [ ] deformation

Uniformly distributed in one direction, leading to a slight loss of squareness, by [

]

Individual pin pitches could be reduced by [ ] in the direction of the deformation.

The lateral displacement of the fuel assembly would occur during the seismic event, however, the fuel assembly would shortly return to its nominal central position in its deformed state.

Draft



[ ] Deformation Impact on Subchannel Flow Areas

Draft



ACH-2 CHF Correlation Applicability in the Deformed State [

]

Conclusion The ACH-2 CHF correlation remains applicable for normal analyses and the post-

event state of a uniformly distributed [ ] deformation.Draft



DNBR Prediction Impact in the Deformed State DNBR predictions are performed with accommodations for uncertainties,

including an uncertainty for manufacturing variability.

The U.S. EPR DNB analyses were performed utilizing a [ ] uncertainty in the pin pitch for the limiting rod. Applied to the limiting subchannel

Produces a flow area differential with adjacent subchannels of nominal flow areas which conservatively accentuates the coolant flow expulsion from the limiting subchannel with the minimum DNBR prediction.

The ACH-2 CHF correlation is used to determine the DNBR worth of the manufacturing flexibility [ ]

Draft



DNBR Prediction Impact in the Deformed State Conclusion

The magnitude of the local pin pitch manufacturing flexibility, is greater than the local effect of the [ ] grid deformed state for subchannels in which the limiting hot rod is used to assess DNBR margin. Therefore, the ACH-2 CHF correlation remains applicable in the deformed state.

Verification will be made that the existing DNB impact of manufacturing flexibility is equal to or larger than the local deformed state DNB impact.

This conclusion is independent of core location or extent of deformation across the coreDraft



HTP Grid Hydraulic Resistance Impact in the Deformed State The [ ] reduction in the pin pitch, for interior cells, is within the

manufacturing variability observed in the pin pitch for HTP spacer grids.

Such variability would have been expected to be present within the pressure drop test fuel assembly due to HTP test grid fabrication.

Conclusion The uncertainty for manufacturing variability sufficiently accounts for the potential

impact of a [ ] deformed state within the spacer grid independent of core location or extent of deformation across the core.Draf

t


Impact of Modifications: Fuel Assembly Deformation

Non-LOCA impact for 1 mm deformation The grid deformation is [ ] uniformly distributed over the entire grid

With 17 fuel rods in a row, the fuel rod pitch would reduce by [ ] in lateral direction Assuming no change in longitudinal direction, the change in assembly and fuel rod flow areas is

less than [ ] Minimal impact on core pressure drop No impact on non-LOCA system response Thermal-Hydraullc impact has been assessed separately

Draft


Impact of Modifications: Fuel Assembly Deformation

LOCA impact for 1 mm deformation The grid deformation is [ ] uniformly distributed over the entire grid

With 17 fuel rods in a row, the fuel rod pitch would reduce by [ ] in lateral direction Assuming no change in longitudinal direction, the change in assembly and fuel rod flow areas is

less than [ ] PCT occurs in between spacer grids

1 mm deformation has negligible impact on PCT [ ]Draf

t

Bundle Deflection Amplitude-Revised RAI 64

Victor Hatman

Draft


RAI-64 Follow-up:Fuel Assembly Deflection Amplitude

Follow-up Question from November 2011 Audit This section is a follow-up to one of the questions raised during the November 2011 Audit

regarding the predicted in-core lateral displacements of the U.S.EPR fuel assembly comparedto previous designs

The following discussion must be considered as an addition to the RAI 64 response submitted in 2011

November 2011 Audit Summary RAI-64 questioned the applicability of the BAW-10133 linear bundle model to the U.S.EPR In the November 2011 audit, and the first draft of the RAI-64 response, AREVA presented

arguments based on the sources and strength of the non-linear effects in the U.S.EPR bundle, and concluded that these are weaker than in the case of previously approved designs

At the time of audit, the U.S.EPR design included weaker grids in the center two positions, which were predicted to exceed P(crit).

In May 2012, AREVA presented to the NRC a new closure plan which included a design change, consisting of stronger grids in the center two positions, which prevents grid buckling.

In view of the new U.S.EPR design change, the linearity of the bundle model becomes less critical.

A comparative table is shown on the next page

Draft


Fuel Assembly Deflection Amplitude Comparison of “Stretched” Fuel Assembly Designs

12’ Assemblies 14’ Assemblies

Design AP-600 CE-14 AREVA

17x17

AP-1000 CE-16 U.S. EPRre-design

APWR

Lattice 17x17 14x14 17x17 17x17 16x16 17x17 17x17

Core Heightcold active fuel

144” 132” 144” 168” 156” 165.4” 168”

Stretch 12’ 14’

N/A N/A N/A 24” 24” 21.4” N/A

FA Model Linear Linear Linear Linear Linear Linear Non-linear

Grid Model Gap-Linear

Gap-Linear

Gap-Linear

Gap-Linear

Gap-Linear

Gap-Linear

Non-linear

Grid Loading <P(crit) <P(crit) <P(crit) <P(crit) <P(crit) <P(crit) >P(crit)

Topical Report WCAP-8288

CENPD-178

BAW-10133

WCAP-8288

CENPD-178

BAW-10133

MUAP-08007

Last Topical Revision

1973 1981 2000 1973 1981 2000 Under review

Draft


RAI-64 Follow-up:Fuel Assembly Deflection Amplitude

November 2011 Audit Remaining Question Request for a comparison of predicted FA amplitudes for the U.S.EPR and previous designs Table 64-1 in the ANP-10285Q10P document was revised to include transient deflections for the

U.S.EPR, for the Advanced Mk-BW and Mk-B-HTP Differences due to higher seismic excitation (0.3 g maximum ground acceleration for the U.S. EPR

versus 0.15 – 0.18 g for the other designs), and row length. A comparison of these designs, based on consistent seismic inputs, would yield deflections that are

more comparable than what is shown in Table 64-1. The total transient compression of the spacer grids is dominated by an elastically recoverable

component, which accounts for the additional deflection above the collapsed core gap space. Since the buckling limit of the grid is not exceeded, the ability of the core model to accurately predict

the bundle incoming kinetic energy, the peak impact force, and the rebound velocity, is maintained and is sufficient for the evaluation of fuel under external loads.

Draf

t

Impact of Grid Deformation on Loads-RAI 70Victor Hatman Draf

t


Spacer Grid Linear Model and Core Gap Sensitivity

Applicability of the current linear visco-elastic element to model grid impacts

RAI-70 questions the applicability of a linear visco-elastic model as a grid impact element

At the time RAI-70 was written, the maximum grid impact load exceeded the grid strength

Following the design change communicated during the May 2, 2012 Public Meeting, all grid impacts are below allowable

Grid deformation is acceptably small and bounded (peak impact on the stable side of the loading curve)

Focus shifts from accurately predicting the bundle deflection during post-buckling to accurately predicting the peak impact load and the rebound velocity.

The linear visco-elastic element captures the peak impact load and rebound velocity very accurately for permanent deformation levels above the imposed limit.

Only question remaining is the effect of the small grid deformation on subsequent impacts addressed via a core gap sensitivity study.

Draft


Spacer Grid Linear Model and Core Gap Sensitivity

Core Gap Sensitivity Study based on the limiting time history and limiting row model 4 scenarios:

[

] Most representative scenario: FA-FA gaps nominal and FA-Baffle gaps increased In this case the sensitivity to gaps is less than [ ] Consistent with NUREG/CR-1018 assessment general conclusion that core gaps have a

second order effect on impact loads

Draft

Impact of Direct Method in Upstream Analyses on Fuel Closure PlanBrett Matthews Draf

t


Direct Method ImpactBackground

Core plate motion time histories used in fuel qualification result from upstream seismic analyses of Nuclear Island basematstructures (e.g., containment building) and the Reactor Coolant System

SASSI computer program is used to model the response of civil structures SASSI uses the “subtraction method” in accounting for soil structure

interaction between the building and its supporting soil Defense Nuclear Facilities Safety Board (DNFSB) issued a letter on April 8,

2011 highlighting issues related to the subtraction method NRC requested that AREVA justify the use of subtraction method

AREVA has decided to re-perform analyses using NRC-approved “direct method” in place of subtraction method

Draft


Direct Method ImpactExpected Effects

AREVA’s preliminary evaluations from Nuclear Island civil/structural teams indicate: Change to direct method appears to affect soft soils more than firmer

cases Medium soil cases are limiting for fuel evaluation

Primary effects of the direct method appear to be in higher frequency content Low frequencies (i.e. below 10 Hz) are of primary concern for fuel evaluation

Based on preliminary evaluations, expectation is that the change to the direct method will not significantly alter resultsof the fuel evaluation Draf

t


Direct Method ImpactPlan for Reconciliation in Fuel Analysis

Expectation is that current analyses being performed will remain bounding

Work on Technical Report for fuel seismic evaluation will proceed based on current input core plate motions derived from the subtraction method

Technical Report will be submitted according to the schedule in the current closure plan using current core plate motions

Updated core plate motions, based on the direct method, will be made available for fuel evaluation in early 2013

Complete seismic evaluations of the fuel will be performed to confirm results presented in the Technical ReportDraf

t

Summary and Next StepsJerry Holm

Draft


Summary AREVA Seismic Methodology is Conservative Grid Strength Definition Consistent with NRC Historical

Guidance Grid Strength Definition has Negligible Impact on LOCA and

Thermal-Hydraulics Grid Deformation has Negligible Impact on Maximum Load Minimum Margin to Grid Strength is [ ]

Minimum Margin Occurs in Peripheral Fuel AssembliesDraft