Document: NUREG-0800
Document ID: 28c03fd2-3542-41ec-bfd7-5df2087c4ee6
Document Type: srp
Title: FUEL SYSTEM DESIGN
Source: NUREG-0800
Source URL: https://www.nrc.gov/docs/ML0520/ML052070407.pdf
Revision Date: 2023-06
Chapter: 4
Section ID: 4.2
CFR Part: 
CFR Title: 

Content:
failure criteria should be given for all known fuel rod failure mechanisms. Fuel rod failure is defined as the loss of fuel rod hermeticity. Although we recognize that it is not possible to avoid all fuel rod failures and that cleanup systems are installed to handle a small number of leaking rods, it is the objective of the review to assure that fuel does not fail due to specific causes during normal operation and anticipated operational DRAFT Rev. 3 - April 1996 4.2-6 occurrences. Fuel rod failures are permitted during postulated accidents, but they must be accounted for in the dose analysis. Fuel rod failures can be caused by overheating, pellet/cladding interaction (PCI), hydriding, cladding collapse, bursting, mechanical fracturing, and fretting. Where applicable, the fuel rod failure criteria should consider high burnup effects based on irradiated material properties data. Fuel failure criteria should address 27 the following to be complete. (a) Internal Hydriding: Hydriding as a cause of failure (i.e., primary 28 hydriding) is prevented by keeping the level of moisture and other hydrogenous impurities within the fuel very low during fabrication. 29 Acceptable moisture levels for Zircaloy-clad uranium oxide fuel should be no greater than 20 µg/g (20 ppm) . Current ASTM specifications 30 (Reference. 730) for U0 fuel pellets state an equivalent limit of 2 µg/g 31 2 (2 ppm) of hydrogen from all sources. For other materials clad in 32 Zircaloy tubing, an equivalent quantity of moisture or hydrogen can be tolerated. A moisture level of 2 mg H 0 per cm of hot void volume 2 3 within the Zircaloy cladding has been shown (Reference. 832) to be 33 insufficient for primary hydride formation. (b) Cladding Collapse: If axial gaps in the fuel pellet column occur due to densification, the cladding has the potential of collapsing into a gap (i.e., flattening). Because of the large local strains that accompany this process, collapsed (flattened) cladding is assumed to fail.