CRITICALITY SAFETY ANALYSIS OF AS-LOADED SPENT NUCLEAR FUEL CASKS Kaushik Banerjee and John M. Scaglione Oak Ridge National Laboratory Contact: banerjeek@ornl.gov International Conference on Nuclear Criticality Safety September 13-17, 2015 Charlotte, NC

Acknowledgments This work was supported by the U.S. DOE, Office of Nuclear Energy, Fuel Cycle R&D Program, Used Fuel Disposition Campaign This is a technical presentation that does not take into account contractual limitations under the Standard Contract. Under the provisions of the Standard Contract, DOE does not consider spent fuel in canisters to be an acceptable waste form, absent a mutually agreed to contract modification. Prepared by Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, Tennessee 37831-6285, managed by UT-Battelle, LLC, for the U.S. Department of Energy under contract DE-AC05-00OR22725. This presentation was prepared as an account of work sponsored by an agency of the United States government. Neither the United States government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States government or any agency thereof.

Outline Background Uncredited criticality safety margin associated with as‑loaded criticality analysis and its applications As-loaded criticality analysis methodology As-loaded criticality analysis results for eight reactor sites (215 loaded casks) with and without basket degradation Conclusion

Spent nuclear fuel (SNF) is stored in a basket positioned within a canister/cask SNF is stored within a basket cell Flux-trap basket design Non Flux-trap design Neutron absorber (such as Boral®) plates are attached to the basket cells Neutron absorbers are comprised of a chemical form of the neutron absorber nuclide (such as B-10 in B4C) and a matrix (such as aluminum or stainless steel) Canisters are placed in different overpacks for storage, transportation, and disposal (if they are to be disposed of) Dry storage at Trojan MPC-32

SNF Canisters in Dry Storage SNF will eventually be disposed of in a geological repository Need to address two criticality- related issues for direct disposal of currently loaded canisters Fuel integrity during transportation after extended storage Groundwater infiltration and associated canister structural degradation (e.g., degradation of neutron absorber panels) during post-closure time period Direct disposal of loaded canisters has many potential benefits Lower cost Less fuel handling Less repackaging (facilities, operations, new canister hardware) Lower worker dose Less secondary waste (e.g., no separate disposal of existing canister hulls) SNF Canisters in Dry Storage Sometime before 2040 more than half of the commercial SNF in the U.S. will be stored in ~7,000 canisters at power plants or decommissioned sites.

Uncredited criticality margin exists in loaded SNF canisters Bounding fuel characteristics (e.g., fuel type, initial enrichment, and discharge burnup) are applied for licensing applications In practice, discharged SNFs available for loading are diverse (e.g., wide variations in SNF assembly burnup values) Licensing application keff = 0.90 As-loaded keff = 0.66 Discharged inventory 3.7 3.2 34.0 33 32.0 3..2 36.0 38.0 40.0 39.0 33.0 Uncredited margin = 0.90-0.66 = 0.24 Δkeff X Y Z Assembly average initial enrichment (wt %) Assembly average burnup (GWd/MTU) Cooling (years)

Uncredited criticality margin can be credited to offset system aging-related reactivity increases Potential changes in as-analyzed geometric configuration Fuel reconfiguration Effects of neutron absorber degradation

Tube and disk design with carbon steel disks Loss of neutron absorber Loaded canisters at eight reactor sites were evaluated to quantify uncredited margin Tube and disk design with carbon steel disks Loss of neutron absorber Loss of neutron Absorber + carbon steel disk Site C canister model Site C canister model for disposal Sites A, C, D, and F employ 24-assembly canisters (flux-trap design) Site B employs both 24- and 26-assembly canisters (flux-trap design) Sites G and H employ 32-assembly canisters (burnup credit canister) Site E employs 80-assembly boiling water reactor (BWR) canisters

A new tool has been developed to perform as‑loaded analyses Used Nuclear Fuel- Storage, Transportation & Disposal Analysis Resource and Data System (UNF-ST&DARDS) streamlines various waste management-related analyses UNF-ST&DARDS provides a comprehensive database and integrated analysis tools Data relations facilitate analysis automation Minimum user interaction assures accuracy

UNF-ST&DARDS uses ORNL’s SCALE code system for criticality calculations As-loaded analysis for SNF requires the determination of time dependent isotopic number densities—depletion and decay calculation Isotopic composition of the SNF from the depletion step is used to determine the cask neutron multiplication factor (keff)—criticality calculation Depletion and decay calculation of SNF to generate assembly- specific composition: TRITON sequence and ORIGEN module are used Isotopic compositions are generated for assembly-specific initial enrichment, burnup, and decay times for major fuel class (e.g., C1414C) Conservative depletion parameters such as constant soluble boron concentration (1000 ppm) are used Depletion calculations include the presence of burnable poison rod (PWR) and control blade (BWR) throughout the irradiation time

UNF-ST&DARDS uses ORNL’s SCALE code system for criticality calculations KENO-VI is used to perform cask criticality calculations using cask- specific fuel inventory (loading maps) Continuous energy ENDF/B-VII.0 cross section library 12 actinides and 16 fission product isotopes are credited (NUREG/CR-7108, -7109) for transportation (slightly different set is credited for post-closure criticality as recommended in the disposal criticality methodology topical report) PWR axial burnup profiles are used from NUREG/CR-6801 (18 node) Uniform profile is used for the one BWR site analyzed Specific fuel types are modeled (e.g., C1414C, C1414A, C1414W) Conservatism is maintained for the analyses Bounding assembly models (e.g., fresh design basis assembly) as determined in the FSAR/SAR are applied for irregular assemblies (e.g., assemblies with missing fuel rods, damaged fuel assemblies).

Uncredited criticality margins were quantified for loaded canisters at eight sites using as-loaded configurations 215 loaded casks at eight sites were analyzed Six canisters types including 24-assembly baskets (flux trap design), 32-assembly baskets (burnup credit canisters), and a BWR basket

215 canisters from eight sites were also evaluated for post-closure criticality using as-loaded configurations Two degradation scenarios were considered Loss of neutron absorber panels (seven sites) Loss of carbon steel components and neutron absorber panels (one site) Representative subcritical limit keff <0.98 is used in this study as a representative acceptance criterion for as‑‑‑loaded calculations With loss of neutron absorber panels With loss of neutron absorber panels With loss of neutron absorber panels and carbon steel components (Site C)

75% of analyzed canisters are below the representative subcritical limit with as‑loaded analysis (fresh water)

Uncredited criticality margin could be credited to offset postulated aging-related structural performance losses Canister-specific as-loaded criticality calculations have been performed for eight reactor sites (total of 215 loaded canisters) It was observed that most of the as-loaded canisters have substantial uncredited safety margins ranging from 0.05 to almost 0.30 Δkeff Uncredited margin can offset increase in keff (indicated to be 4%*) from potentially credible fuel failure configurations that could occur during transportation after extended storage 215 loaded casks were then analyzed with basket degradation that may occur over a post-closure time period 75% of analyzed loaded canisters were below the representative subcritical limit used in this study Aqueous species (e.g., chlorine) present in the groundwater can also be credited with as-loaded analysis As-loaded canister model (instead of design basis) provides significant criticality benefit *W. J. Marshall and J. C. Wagner, Consequences of Fuel Failure on Criticality Safety of Used Nuclear Fuel, ORNL/TM-2012/325, Oak Ridge National Laboratory, Oak Ridge, Tenn., September 2012.