FAST Code Development and Applications

Office of Nuclear Regulatory Research

Web site: https://fast.labworks.org

What Is It?• FAST (Fuel Analysis under Steady-State

& Transients) calculates the

thermal-mechanical response of nuclear fuel

as a function of burnup under steady state,

anticipated operational occurrences (AOOs),

and design-basis accident (DBA) transients.

• Calculated parameters include temperature,

stress/strain, fission gas release, and oxidation.

Regulatory UsesNRR/NRO

• Assess SAFDLs (NUREG-0800,

Chapter 4), assess vendor codes

and methods, and provide initial

conditions DBA analyses.

NMSS

• Perform spent fuel analyses,

considering cladding hoop stress

and creep under drying conditions.

Assessment• FAST is built on more than 30 years of

assessment stemming from the FRAPCON /

FRAPTRAN codes, as well as experience with

fuel vendor codes and data.

• It offers more than 200 assessment cases that

cover relevant specified acceptable fuel

design limits (SAFDLs) for the UO2/Zirconium

fuel system, and new cases added for

metallic fuels.

Users• Used by more than 75 domestic

and international organizations,

including other regulatory

bodies, technical scientific

organizations, and utilities, for

safety and core reload

applications.

• Runs on Windows and Linux, with

typical run times of ~3–5 seconds.

FAST =

FRAPCON +

FRAPTRAN +

ATF + non-LWR fuels +

modern software

engineering

SCALE Code Development and Applications

Office of Nuclear Regulatory Research

• The SCALE code system is a modeling and simulation suite for nuclear safety analysis and design:‒ SCALE is used by the U.S. Nuclear Regulatory Commission (NRC) and in 60 countries (about 9,000 users and 33 regulatory bodies).‒ SCALE provides a comprehensive, verified, well validated, and user-friendly tool set for criticality safety, reactor physics, radiation

shielding, radioactive source term characterization, and sensitivity and uncertainty analysis, including the latest nuclear data.‒ SCALE is a modernized code with a long history of application in the regulatory process.‒ The new Monte Carlo code Shift, developed to support the U.S. Department of Energy (DOE) Office of Nuclear Energy’s Consortium

for Advanced Simulation of Light-Water Reactors (LWRs), is being incorporated into the SCALE suite. Shift will combine the features of KENO (criticality) and MAVRIC (shielding) and perform depletion calculations and output nodal data to a core simulator (PARCS).

‒ Products are unified under the Fulcrum graphical user interface and the NRC’s SNAP interface.

• Some recent applications of SCALE in support of NRC licensing reviews:‒ Office of Nuclear Reactor Regulation (NRR) (e.g., MELLLA+, spent fuel pools)‒ Office of Nuclear Material Safety and Safeguards (NMSS) (e.g., consolidated interim storage facilities, burnup credit)‒ Office of New Reactors (NRO) (e.g., previously reviewed and planned technologies such as Canada Deuterium Uranium (CANDU),

high-temperature gas-cooled reactors (HTGRs), sodium-cooled fast reactors (SFRs), and molten salt reactors (MSRs))

MELCOR Code Development and Applications• Fully integrated, engineering-level code

‒ Thermal-hydraulic response in the reactor coolant system, reactor cavity, and containment buildings‒ Core heatup, degradation, and relocation‒ Core-concrete attack; hydrogen production, transport, and combustion‒ Fission product release and transport behavior

• Traditional application‒ User builds models from basic constructs (e.g., control volumes, flowpaths, heat structures)‒ Multiple core designs (i.e., pressurized-water reactor (PWR), boiling-water reactor (BWR), spent fuel pool,

HTGR) ‒ Adaptability to new reactor designs, including non-LWRs and small modular reactors

Office of Nuclear Regulatory Research

Code Development & Regulatory Applications

• Validated physical models‒ International standard problems, benchmarks,

experiments, and accidents

• Uncertainty analysis‒ Relatively fast-running

• User convenience‒ Windows/Linux versions‒ Utilities for constructing input models‒ Capabilities for postprocessing and visualization‒ Extensive documentation

Integrated Models Required for Self-Consistent Analysis

International Collaboration

Application to Accident Tolerant Fuels (ATFs)

Office of Nuclear Regulatory Research

Development Process for Near-Term Concepts Calculation Capabilities

The NRC’s ATF Project Plan (ADAMS Accession Number ML18261A414)

Fuel Performance (FAST) Neutronics (SCALE)• Completed work

‒ Initial gap analyses of SCALE and PARCS capabilities against ATF fuel forms and review of available validation data

• Ongoing work‒ Development of detailed research plan for ATF fuel‒ Development of lattice-level (SCALE/Polaris) and

core-level (PARCS) sensitivity models that will quantify how the changes presented by ATF fuel forms affect core-level safety parameters in PARCS

‒ Review of gap conductance schemes in the NRC’s PARCS core simulator code

• Future work needed‒ Assessment of validation basis

• Completed work‒ Library of ATF materials: FeCrAl, U3Si2, SiC, Cr-coating‒ Multilayer cladding deformation model (coatings)

• Ongoing work‒ Updates and assessment for doped fuels, high burnup, and

higher enrichment‒ Three-dimensional solvers for noncylindrical fuel, such as

ENFISSION’s LightbridgeTM design‒ Coupling to the NRC’s TRACE code and Oak Ridge National

Laboratory’s SCALE and CTF codes for new operational regimes and materials

• Future work needed‒ Material data and model development under irradiation‒ Assessment for licensing conditions (normal operation,

AOOs, DBAs)

• Completed work‒ Initial gap analyses of MELCOR capabilities against ATF

fuel form

• Ongoing work‒ Integrate FeCrAl module into latest version, complete

benchmark, and begin sensitivity studies

• Future work needed‒ Develop preliminary source term for a representative

BWR and a PWR using FeCrAl for selected accident scenarios and compare the results to the current source term technical basis in NUREG-1465 (Regulatory Guide 1.183) and to similar analysis for high burnup/ mixed oxide fuel

Severe Accident & Source Term (MELCOR)

Application to Non-Light-Water Reactors

Office of Nuclear Regulatory Research

MELCOR

• HTGR‒ Extensive capabilities for core and source term analysis‒ Requires data for fission product diffusivity and fuel failure ‒ Need code assessment

• SFR‒ Implemented equation of state for sodium and containment phenomena

(e.g., sodium fire, atmosphere chemistry) ‒ Modeling gaps exist for core modeling and fission product release‒ Requires data for fission product release and kinetic parameters

• MSR‒ Implemented equation of state for LiF-BeF2.‒ Modeling gaps exist for core modeling and fission product release. There

are some current capabilities for both fixed fuel and liquid fuel geometries. Fission product interaction with coolant, speciation, vaporization, and chemistry can leverage basic infrastructure developed for SFRs.

‒ Data for fixed fuel geometry similar to HTGR. Requires specific data for fission product release and kinetic parameters.

Development Timeline

FAST

• Two fuel forms under development for non-LWRs, TRISO and metallic fuels, will require traditional thermal-mechanical analyses. Dissolved fuels are not under consideration.

• TRISO fuels• Can model spherical particles using FAST’s new parallel solvers• Basic properties for PyC from DOE’s TRISO code PARFUME• Limited data exist for Uranium OxyCarbide (UCO) fuels; need basic thermal

properties, irradiation induced changes, and fission gas release (FGR)• Unclear exactly what type of analyses are needed—single TRISO kernel

level or compact level with smeared kernels

• Metallic fuels• Extensive capabilities—UPuZr, HT9, sodium• Can model any geometry using FAST’s new parallel PDE solvers• Initial assessment has been performed against EBR-II rods under normal

operating conditions• Need model for Zr redistribution, improvements in FGR model• Need code assessment

• General needs• Understanding of operational conditions/transients• Safety limits that need to be analyzed• Assessment of material properties and integral behavior under all

conditions

SCALE

• SCALE offers capabilities across all non-LWR technologies, providing isotopic depletion, power distributions, kinetics parameters, radioactive source terms, decay heat, and more.

• HTGR‒ Extensive TRISO fuel modeling capabilities with TRITON (deterministic) and

Shift (Monte Carlo)‒ Shift Monte Carlo-based nodal data generation in testing to model

heterogeneous burnup distributions and external control elements‒ Coupling to MELCOR and MACCS2 in progress

• SFR‒ Shift Monte Carlo-based nodal data generation in testing to model core

leakage, core expansion, and external control elements

• MSR‒ Depletion capabilities with decay neutron precursor drift and chemical

feed and removal

Additional validation and code assessments are needed.