Nuclear Fuel Cycle Reactor Fuel Design and Fabrication

1. Nuclear Fuel CycleReactor Fuel Design and Fabrication SYE 4503 Mahmoud R. Ghavi, Ph.D. Center for Nuclear Studies

2. Nuclear Fuel Selection Factors • Influenced by characteristics of the reactor core: • Fuel enrichment • Choice of moderator and coolant • Operating temperature and pressure • Fuel burn-up • Fuel exposure time Center for Nuclear Studies

3. Reactor Fuel Requirements • Temperature below melting and reaction point (fuel and cladding) • NRC requirement 10 CFR 50 (2200 oF for cladding) • Will cause plant shut-down • Containment of fission products inside the cladding • Minor leakage is expected • Meet power cycle requirements • Satisfy design objectives including heat transfer, structural and mechanical integrity Center for Nuclear Studies

4. Important Nuclear Fuel Considerations • Nuclear reactivity and control • Acceptable heat transfer and thermal hydraulics • Containment of fission products in fuel and cladding • Very low contamination (avoid neutron absorption) • Economical issues factors: • High burn-up • Good in-core fuel management • Standard fuel design • Reliability in manufacturing (strong QA/QC) • Reliable performance in intense radiation and temperature fields Center for Nuclear Studies

5. Uranium Metal as Fuel • Not a good choice: • Highly reactive • Structural instability at elevated temperatures (excessive expansion and contractions) • Low melting point—2070 oF Center for Nuclear Studies

6. Ceramic Fuel– UO2 • Most extensively used form of fuel • Cylindrical pellets • Cold pressed and sintered • Desirable properties: • Chemical and structural stability (e.g., no reaction with high temperature water-corrosion resistance) • High melting point—5189 oF • Low σc for oxygen, high neutron utilization • Containment of fission products in UO2crystals • Compatibility with cladding • Ease and low cost of fabrication (a 1000 MWe, LWR has about 9 million pellets contained in about 150 km of fuel rods and 37-40 rods/Mwe) Center for Nuclear Studies

7. Ceramic Fuel– UO2 • Undesirable properties: • Poor thermal conductivity (high melting point compensates for low conductivity) • Inferior strength (brittle) compared to uranium metal • Poor thermal shock resistance • High temperature causes grain growth, induces stress, and cracking • Cracking causes release of fission products (OK as long as contained in cladding) Center for Nuclear Studies

8. Fuel Performance Objectives • Maximize burn-up (MWD/TU)—40,000 to 50,000 MWD/TU • Maximize average linear power density: • (total thermal power/total fuel rod length) • Related to the thermal conductivity of fuel from the temp at center to the edge • Avoid fuel failure (fission product release into coolant) • Avoid fuel growth: • Irradiation growth due to the crystal structure of uranium crystals • Swelling due to creation of fission products causing separation in lattice and lowering density • The rate of swelling (ΔV/V %) increases with burn-up (0.16% up to about 4000 MWD/TU and up to 0.7% at higher burn-up levels) Center for Nuclear Studies

9. Fuel Fabrication: Pellet Production • UO2 is crushed and ground to a fine powder and mixed with an organic binder • Powder is cold pressed into pellets and sintered • Sintering • Hydrogen • 1650° C for 24 hours • Final density: 94% theoretical density

10. Fuel Fabrication: Pellet Production • Pellet final diameter: ~.9 mm diameter • Pellets are ground to final diameters • Ends are be cupped • Each pellet is individually inspected

11. Fuel Fabrication: Pellet Production

12. Fuel Cladding • Protect the fuel from coolant (prevent corrosion and erosion) • Retain fission products • Accommodate fuel volume changes • Provide heat transfer surface • Tolerate harsh radiation, temperature and pressure conditions • Have small neutron absorption cross section Center for Nuclear Studies

13. Typical PWR Fuel Rod Parameters • Outside diameter = 10 mm • Cladding thickness = 0.57 mm • Cladding-fuel gap = 0.166 mm – gap filled with helium gas • Pellet diameter = 8.2 mm • Rod array per assembly = 17X17 • Fuel Rods per assembly = 264 Center for Nuclear Studies

14. LWR Cladding • Zirconium is the metal of choice: • Low thermal neutron absorption cross section • Good corrosion resistance • High melting point (3,353 oF) • High tensile and mechanical strength • Abundant in nature but need to separate hafnium • Zirconium is used in the form of Zircaloy (about 2% other metals) Center for Nuclear Studies

15. NUCLEAR REACTORS Center for Nuclear Studies

16. NUCLEAR REACTORS Fuel Rods • A fuel rod consists of Uranium Oxide pellets stacked to approximately 12 feet in height in a zircaloy cladding. • Cladding is sealed at the lower end by a welded plug. • Enrichment of fuel depends on core location. ( Range between 2-4.6%) • Fuel Rods are pressurized with Helium gas to 1000-2000 psi to prevent collapse of cladding due to high RCS pressure (reduce clad creep). • Helium is used because it is inert and will not chemically react with the fuel or clad. • Helium also has good heat transfer characteristics. • Zircaloyused as the cladding material because: • Good heat transfer characteristics • Low absorption of neutrons • High corrosion resistance • High melting point Center for Nuclear Studies

17. Improved LWR Fuel Rods • PWR: • Vantage 5: • Zirlo (new Zircaloy cladding) • Higher temperature tolerance (850) with no corrosion • Lower susceptibility to hydrogen embrittlement • Lower creep related issues • Can exceed 60,000 MWd/t burnup • BWR: • Beta quenched: • Lower growth • Lower corrosion • Longer burnup Center for Nuclear Studies

18. Advanced Cladding Materials • Silicon Carbide (SiC): • Used in industry as abrasive, refractories, etc. • High melting point (2730 oC) vsZircaloy (1850 oC) • Resist chemical attacks in acid alkaline and salt environment up to 800 oC • Forms a protective silicon oxide coating at 1200 oC • Lower neutron absorption and greater radiation damage resistance • Higher temperature tolerance • Disadvantage: manufacturing difficulties due to mechanical strength (tensile stresses induced during operations) Center for Nuclear Studies

19. Reactor Burnable Poisons (BP) • Neutron absorbing material that are consumed (burn) to control K factor as opposed to non-burnable such as Hf • Fresh core is super critical to compensate for: • Fuel depletion • Fission products buildup • Negative temperature reactivity effects • Excess reactivity needed to ramp up the reactor • Burnable poisons are used in addition to control rods and chemical shims • Amount of BP decrease with the life of the reactor • Most used materials are compounds of boron (gadolinium is also used some) • Integral fuel burnable absorber Center for Nuclear Studies

20. NUCLEAR REACTORS HOW A SINGLE FUEL PELLET IS SUPPORTED • All fuel pellets support each other and distribute their weight to the rod/ cladding. • The rod is supported by the grid assembly. • The grid assembly is supported by the thimble tubes.. • The thimble tubes are supported by the upper and lower nozzle blocks. Center for Nuclear Studies

21. NUCLEAR REACTORS GRID STRAPS • All 264 fuel pins in each assembly are held together to the guide thimbles by eight grid straps • Three intermediate flow mixers increase turbulence in the coolant thus heat transfer is improved. The flow mixers are located in each assembly between the upper three grid straps. Center for Nuclear Studies

22. NUCLEAR REACTORS NUCLEAR FUEL ASSEMBLY Rod Cluster Control Assembly • Each fuel assembly is 17 X 17 array consisting of: • 264 fuel rods that house the fuel pellets • 24 RCCA guide thimble that provides guidance for RCCA when inserted in the fuel assemblies • RCCA guide thimble attachments • a) Upper nozzle block • Removable – allows fuel re-constitution. • b) Lower nozzle block Hold Down Spring Upper Nozzle Block Grid RCCA Guide Tubes Lower Nozzle Block Center for Nuclear Studies

23. PWR Fuel Assembly Center for Nuclear Studies

24. NUCLEAR REACTORS TOP NOZZLE BLOCK Center for Nuclear Studies

25. NUCLEAR REACTORS LOWER NOZZLE BLOCK • The RCCA thimble are reduced in size near the bottom in order to create a hydraulic “dash pot”. • The dashpot slows control rods following a trip in order to minimize impact energy. Center for Nuclear Studies

26. NUCLEAR REACTORS Center for Nuclear Studies

27. NUCLEAR REACTORS Center for Nuclear Studies

28. BWR Fuel Assembly Typical 1100 MWe Plant: • Approx 750 assemblies • About 70,000 fuel rods • Some 25 million pellets • Assembly about 4.4 meters • Assembly weighs about 320 kg • Typical burnup: 50,000 MWd/t Center for Nuclear Studies

29. BWR Fuel Assembly Center for Nuclear Studies

30. PWR or BWR Fuel? Center for Nuclear Studies

31. NUCLEAR REACTORS Center for Nuclear Studies

32. NUCLEAR REACTORS CORE LOAD PATTERN • The Reactor Core consist of 193 fuel assemblies with various enrichments ranging from about 2.5 - 4.5 % • The fuel is arranged in a “low leakage” loading pattern. • Old fuel and enriched new fuel is mixed around the periphery of the core. • Higher enrichments toward outside of core decreases chances of neutrons leaking out of the core. • Results in a more uniform axial flux profile. Center for Nuclear Studies

33. Nuclear Fuel Problems • Fuel Swelling • Fuel densification • Thermal expansion/deformation due to non-uniform heating • Pellet/Cladding interaction (stress corrosion cracking-iodine)—use of barrier fuel • Hydride formation: • Zr + 2 H2O ZrO2 + 2H2 Absorption of hydrogen by Zr can lead to formation of embrittlement causing zirconium hydride ZrH2 Center for Nuclear Studies

34. Nuclear Fuel Problems • Fretting failure of cladding • Coolant chemistry: • PWR coolant contains: • Boric acid • Lithium hydroxide (to control PH) • Hydrogen to remove oxidizing species created by radiolysis of water • Cladding corrosion Center for Nuclear Studies