RIA Challenge Problem Connection with NRC DG-1327

Gregg Swindlehurst

LOCA & RIA Challenge Problem Integrator

GS Nuclear Consulting, LLC

CASL Industry Council Meeting

Charleston, South Carolina

April 4, 2017

2

RIA Challenge Problem Goal

3

DG-1327 Proposed RIA Regulatory Guidance

• Fuel rod segment RIA test results in the French CABRI facility and the Japanese NSRR facility in 1993 indicated that fuel rod performance during RIA degraded with fuel exposure

– Cladding failure due to pellet-to-cladding mechanical interaction (PCMI) would occur at significantly lower fuel enthalpy than the legacy 280 cal/g limit

• In 2007 NRC issued interim RIA acceptance criteria in SRP Section 4.2, Appendix B

• On November 21, 2016 NRC issued DG-1327 “PWR Control Rod Ejection and BWR Control Rod Drop Accidents” for public comment (ML16124A200)

– Includes final RIA acceptance criteria and guidance for acceptable methodology

– The technical basis is an NRC memo dated 3/16/2015 (ML14188C423)

4

DG-1327 Content

• Cladding failure thresholds

– High-temperature cladding failure (post-CHF)

– PCMI (cladding strain due to solid fuel expansion)

– Incipient fuel melting (cladding strain due to molten fuel expansion)

• Core coolability limits

– Maximum fuel enthalpy (prevent energetic fuel-to-coolant interaction)

– No melting in outer 90% of fuel pellet (prevent molten fuel from causing cladding failure and energetic interaction with coolant)

• Radiological consequence analysis guidance

• Guidance on acceptable PWR rod ejection and BWR control rod drop methodology

5

RIA PCMI Research Results

RIA fuel rod tests with higher burnup have shown reduced capability to survive power excursions due to cladding failure caused by PCMI

6

RIA PCMI Regulatory Changes

Lower RIA Regulatory Limits at Higher Burnup

7

RIA Overheating Regulatory Change

• Low initial power RIA fuel rod tests are subject to cladding failure caused by post-CHF brittle failure (thermal shock during quench), and by post-CHF ductile failure (balloon and burst). The latter is aggravated by rod internal pressure (fill gas + FGR due to exposure + transient FGR during the RIA)

• For initial power levels ≥ 5% use traditional DNBR/CPR evaluation and assume cladding fails if limit exceeded

8

Impact of DG-1327 on Industry

• The fuel vendors and licensees that perform their own RIA analyses will need to submit methodology revisions to NRC for approval

– Methodology revisions that follow DG-1327 guidance should have a straight-forward licensing process

– Methodology revisions that do not follow DG-1327 are allowed provided they have sufficient basis (more protracted licensing process)

• Licensee LARs with potential to affect RIA analyses will trigger NRC use of DG-1327 as a review standard

• New RIA analyses of record will be extensive to cover range of intermediate power levels and times-in-cycle expected by NRC

• Possibility of tight margins and impact on reload design

9

CASL RIA Modeling & Simulation

• CASL RIA M&S is a graded approach with increasing fidelity(stand-alone → iteration → coupled)

• Code development, validation, and demonstration problems are underway

10

RIA M&S Activities StatusRIA-Related Code Development

• Transient VERA-CS (coupled MPACT + CTF) (FY17)

• Transient VERA-CS (coupled MPACT + CTF + BISON 1.5D) (FY17/18)

• BISON RIA (FY17/18)

RIA Validation

• Stand-alone CTF to NSRR RIA fuel rod tests (completed 2014)

• Stand-alone MPACT to SPERT small reactor RIA tests (completed 2016)

• VERA-CS (coupled MPACT + CTF) SPERT tests (FY17/18)

• BISON RIA validation suite (FY16/17/18)

• CTF RIA validation suite (FY17)

RIA Demonstration

• CTF PWR whole-core control rod ejection accident (completed 2014)

• VERA-CS mini-core control rod ejection accident (FY17)

• MPACT AP1000 control rod ejection accident (FY17)

• VERA-CS PWR control rod ejection accident (FY18 – L1 milestone)

• BISON BWR control rod drop accident (FY19 – L1 milestone)

11

BISON Validation – CABRI REP Na

• CABRI REP Na-2,3,5, and 10 RIA tests have been selected for validation

• Validation effort will compare against available data and to EPRI Falcon code predictions

Papin, J., et al. "Synthesis of CABRI-RIA tests interpretation." Eurosafe Meeting, Paris, France. 2003.

12

BISON Validation – CABRI REP Na-3

Preliminary BISON results are compared to REP Na-3 test data and to the EPRI Falcon code (enabling comparison of more parameters)

– Comparison of BISON to Falcon thermal results show good agreement such as radial average enthalpy agree within 2%, and temperatures within 4%

– Comparison of mechanical results (BISON to data and BISON to Falcon) show larger differences (perhaps attributable to using frictionless contact between fuel and cladding)

Property BISON FALCONExperimental

Reported Values

Energy Deposition (J/g) [cal/g] 510.9 [122.1] - 511 [122.2]

Peak Fuel Radial Averaged Enthalpy (J/g) [cal/g]

501.5 [119.9] 493.7 [118] 516 [123.5]

Max. mean hoop strain (%) 1.2 1.5 2.2

Residual Hoop Strain (%) 0.5 1.1 -

Fission gas released (%) 9.7 - 13.7

Max clad radial displacement (µm) 55.4 81 -

Clad radial displacement residual (µm) 14.5 51 55

Clad max axial Displacement (mm) 1.5 5.1 6

Clad axial diplacment residual (mm) 0.4 3.4 3.5

Fuel max axial displacement (mm) 12.1 5.4 -

Fuel axial displacement residual (mm) 3.7 0 3

Max fuel temperature (K) 2569 2480 -

Fuel max centerline temperature (K) 1905 1960 -

Clad max inside temperature (K) 926 935 -

Corrosion thickness (µm) ~50 average - 35-60

13

BISON Validation – CABRI REP Na-3

Comparison of preliminary BISON and Falcon mechanical strain predictions

– Predicted BISON and Falcon residual cladding radial displacement compared to post-test measurements

– Predicted BISON and Falcon cladding hoop strain transient behavior

14

BISON Validation – CABRI REP Na-3

Promising fission gas release results with recently developed transient FGR model in BISON, which considers the effect of fuel micro-cracking

• Predicted fission gas release of 9.7% (compared to 13.7%)

15

VERA-CS RIA Demonstration: Mini-core test problem

• Coupling MPACT with CTF enables the use of more advanced T/H models for a wider range of conditions (e.g. transient convection, boiling, CHF, dynamic gap conductance, etc).

• Initial condition: End-of-cycle at hot full power

– EOC is 12 GWd/MT exposure, β is 0.00565

• Assume central rod is ejected in 0.08 s ($2.42)

• Computational Details:

– Ran with quarter core symmetry for 0.3 seconds

– 4 ms constant time step, 75 total time steps

– Default discretizations used for MPACT and CTF

• MOC ray spacing is 0.05 cm, 16 azimuthal angles, 2 polar angles

• Previous demonstrations of WB1 used very coarse discretization

• CTF simulating cross flow (for initial steady state condition)

– Using new 51-group ENDF-VII.1 cross section library

– Simulations run on Titan computer cluster at ORNL with 704 processors

16

VERA-CS RIA Demonstration: Mini-core Test Problem

Runtime = 5.5 hrs 704 processors

Tem

pera

ture

(C

)

Rea

civi

ty($

)

Time (s)

Pow

er (

%)

Den

sith

y(g

/cc)

Relative Power

17

Industry Response to DG-1327

EPRI FRP Reg-TAC (and predecessor Working Group #2) has been interacting with NRC since unexpected RIA test results emerged in 1990s

• Evaluate RIA test data for applicability to operating fleet

– RIA test conditions are not directly applicable (Na coolant, low temperature, short pulse width)

– Anomalous / outlier test data (pre-spalled cladding, MOX)

• Develop and apply EPRI FREY and Falcon codes

– Understand physics and characterize effects of exposure of fuel rod response during RIA

– Proposed critical strain energy density (CSED) cladding failure criterion

– Use M&S to fill in RIA test data gaps

• Conduct RIA-related cladding and fuel rod tests

– Rapid heating and loading test of cladding material

– Modified burst test (surrogate RIA-like test) of irradiated cladding

• Coordinate industry technical and licensing response (through NEI) to NRC proposed regulations (public comment period for DG-1327 closes April 21, 2017)

– Propose alternate PCMI cladding failure thresholds

– Strive for reasonable regulations and implementation process

18

CASL M&S Role – Licensee Compliance with DG-1327 Guidance

Licensee compliance with DG-1327 will be triggered by future LARs with RIA-related content

– Power uprates

– Transitions to new fuel designs

– Burnup extensions

– Other causes for PWR control rod ejection and BWR control rod drop reanalysis

Loss of margin due to new acceptance criteria and broader scope of initial conditions may create a need for higher fidelity RIA M&S

– CASL codes offer opportunities for margin recovery via higher fidelity and better accuracy, and as the best analytical tool to address unresolved and emerging issues (e.g. CABRI Water-Loop RIA tests starting in 2018)

Because the final DG-1327 will be “guidance” and not “rules” NRC will allow alternate/advanced technical approaches to licensing. This opens the door for innovation.

19

www.casl.gov