Robert L. Sindelar June 2, 2010

1. Materials Performance and Aging Considerations for Power Reactor (PR) and Research Reactor (RR) Spent Nuclear Fuel in Storage Robert L. Sindelar June 2, 2010 IAEA International Conference for Management of Spent Fuel from Nuclear Power Reactors VIC - Austria, AS

2. Outline • Spent Nuclear Fuel • Research Reactor (RR) – Aluminum Alloys • Power Reactors (PR) – Zirconium Alloys • Wet Storage Considerations • Water Quality • Dry Storage Considerations • Drying • Environmental Controls • Objective: Limit DDegradation Cladding During Storage Zircaloy-Clad PR Fuel Al-Clad RR Plate Fuel RR Fuel in Wet Storage PR Fuel in Wet Storage RR Fuel Dry Storage Facility PR Fuel in Storage Casks Establish environmental conditions/controls so as to limit materials degradation during interim storage

3. Functions of a Storage System for Spent Nuclear Fuel • Thermal Performance • Radiological Protection • Confinement • Sub-Criticality • Retrievability The fuel cladding is a critical confinement barrier and structural material for fuel integrity for the storage period and also for safe fuel retrieval and handling pending final disposition

4. PR Fuel Cladding – Pre-Storage Condition • Hydride Microstructure • Hydrogen in Cladding • Radiation Damage Microstructure • Oxide Scale [Zirconia: ZrO2] UO2 Fuel Zircaloy Cladding PR fuel clad cross section showing hydride microstructure in cladding [micrograph courtesy of EPRI]

5. RR Fuel Cladding – Pre-Storage Condition • Maintains Metallurgical Fuel/Clad Bond • Up to 50 mm of Oxide Scale [Boehmite: γ-AlO(OH) or Al2O3•H2O] • Radiation Damage Microstructure MTR Design (Plate) Fuel Irradiated fuel plate MTR post-irradiation RR fuel clad cross section [ANL micrograph]

6. Zirconium Alloys – Wet Storage • Formation of passive layer of ZrO2 leads to very slow corrosion, especially at pool temperatures • US experience: 200,000 assemblies in water pool storage with no failures • Water controls are specified for water activity control

7. Various Corrosion Modes On Aluminum-Clad Plate Fuel Aluminum Alloys – Corrosion with Poor Water Quality Through-Clad Pit Blisters

8. Wet Storage – Water Quality • Water quality is defined by a set of parameters that are used to characterize the water physical and chemical conditions. It includes: • pH; • conductivity; • dissolved impurity species; • undissolved solids; • colloids; • organic substances; • biological organisms; and • temperature

9. Aluminum Alloy Water Quality Parameter Limits Recommended physical-chemical parameters, limits, and monitoring frequencies for water in fuel decay and storage basins [from IAEA Water Quality Management document] (*) Water Radioactivity level and the presence of radioisotope species should be measured each time a water sample is drawn or one time per week. A gamma scan is recommended to measure the presence of radioisotopes that would have come from failed fuel (e.g. Cs-137). No specific limits are set. The presence of radioisotope species should be evaluated on case-by-case basis. Measurement of the activity from filters and resin columns should be performed to detect the presence of leaking fuel.

10. Corrosion Surveillance for RR Fuel • Corrosion Surveillance Program • Periodic Removal of Fuel Clad Material Specimens • Evaluate Corrosion Modes and Rates of Corrosion • Compare to Basin Chemistry Records for Immersion Period • Program in INTERNATIONAL ATOMIC ENERGY AGENCY, “Recommended practices for water quality management in research reactors & spent fuel storage facilities,” IAEA Nuclear Energy Series, to be published 2010.

11. Fuel Dry Storage – Drying Considerations • PR Fuel • No zirconium oxyhydroxides • Drying: remove free water, care to cool slowly to avoid formation of radial-oriented hydrides (embrittled microstructure) • RR Fuel • Aluminum oxyhydroxides from reactor operation and wet storage • Need high temperatures to remove Boehmite (~ 500°C) • ASTM Drying Guide: C1553-08

12. Dry Storage – Example of a Dryness Specification • Limit Water in Sealed Canister for Hydrogen Deflagration or Pressure Concern • Corrosion reaction producing Hydrogen: • Avoid Deflagration Hazard of H2 in Air • Limit of 4% H2: Wwater (ml) = 38.7 Volume of canister (m3) • Avoid High Pressure • Limit Water in Sealed Canister for Materials Degradation Concern • Avoid Corrosion of Fuel and Internals FW is free water in ml V is canister volume in m3 PH2 is H2 pressure in atmospheres T is temperature in °C Aluminum 6061 Exposed to 250 °C Water Vapor for Six Months (Initial Relative Humidity 100%)

13. PR Fuel Dry Storage Degradation Phenomena • Normal Operation Degradation Phenomena: • Creep: Limit temperature to avoid 1% creep strain to avoid creep rupture • Hydride Embrittlement: Avoid formation of brittle hydride microstructure • Delayed Hydride Cracking: Avoid high H content and stress • Off-Normal Degradation Phenomena: • Ingress of air and/or water detrimental to cladding: • Cladding corrosion – additional wastage; additional hydrogen build-in • Fuel swelling - oxidation to U3O8 can split cladding Key Reference: R.E. EINZIGER, et. al.,“Data Needs for Storage and Transportation of High-Burnup Fuel,” Radwaste Solutions, March/April 2005

14. RR Fuel Dry Storage Degradation Phenomena • Normal Operation Degradation Phenomena: • Creep: Limit temperature to avoid plate deformation • Diffusion Through Cladding: Limit temperature to avoid breakthough • Off-Normal Degradation Phenomena: • Ingress of air and/or water detrimental to cladding: • Vapor corrosion of aluminum alloys is significant at high temperatures • Vapor corrosion of aluminum-based fuel is similar to aluminum alloys Reference: R.L. SINDELAR, et. al., “Acceptance Criteria for Interim Dry Storage of Aluminum-Alloy Clad Spent Nuclear Fuels,” March 1996; IAEA Wet and Dry Storage Guide for RR Fuel: in preparation

15. Dry Storage – Creep Analysis of RR Fuel • Parametric Analysis • Finite Element Model (ABAQUS) of MTR Plates • Coble Creep Deformation Mechanism • Grain Size/Temperature (100-350°C)/Time (to 10,000+ years) • Deformation Targets • Plate Slump of 0.1 inches • Plate Slippage of 0.1 inches from Slot Ref. WSRC-TR-95-0121, SRNL, 1995

16. Dry Storage – Vapor Corrosion Tests for RR Fuel Vapor Corrosion Testing of Al-Clad and U-Al Materials: Al1100, Al5052, Al6061, U-Al Alloys • Water Vapor/Air/Temperature (to 250°C) with & without Gamma Radiation Capsule Test Configuration Surface Oxide Morphology Al1100, 1 Week Exposure • Corrosion function of (t, T, RH, alloy) + radiation

17. Summary • Imperative for Interim Storage: Limit DDegradation to Fuel to Enable its Safe Retrieval • Initial Condition of Fuel from Reactor Operation and Post-Operation History • Wet Storage Controls • PR Fuel Storage in Water is Not Limited • RR Fuel Storage in Water Needs Water Quality Controls • Dry Storage Controls • Limit Moisture in a Dry Storage System to Limit Corrosion and Pressurization in Sealed Systems; Drying Practices for Specific Fuel Type, Fuel Condition, Canister System to Limit Remaining Water • Environmental Parameters to Limit Degradation Phenomena • With Appropriate Controls, Either Wet or Dry Storage Systems Can be Successful for Extended Interim Storage for PR and RR Spent Nuclear Fuel