Document: NUREG-0800
Document ID: 3ea2f0ac-4d7e-464a-b1c4-390c3970f642
Document Type: srp
Title: provides specific thermal-hydraulic criteria.  The available radioactive fission product
Source: NUREG-0800
Source URL: https://www.nrc.gov/docs/ML0707/ML070740002.pdf
Revision Date: 2023-06
Chapter: 4
Section ID: 4.4
CFR Part: 
CFR Title: 

Content:
s impurities within the fuel is kept very low during fabrication. Acceptable moisture levels for Zircaloy-clad uranium oxide fuel should be no greater than 20 micrograms per gram (µg/g) (20 parts per million (ppm)). Current specifications of the American Society for Testing and Materials (ASTM),1989 edition, Standard C776-89, Part 45, for uranium oxide fuel pellets state an equivalent limit of 2 µg/g (2 ppm) of hydrogen from all sources. For other materials clad in Zircaloy tubing, an equivalent quantity of moisture or hydrogen can be tolerated. A moisture level of 2 milligrams of water per cubic centimeter of hot void volume within the Zircaloy cladding has been shown to be insufficient for primary hydride formation. External hydriding is caused by waterside corrosion in which the water reaction with the zirconium alloy results in zirconium hydrides as well as zirconium dioxide. ii. Cladding Collapse. If axial gaps in the fuel pellet column result from densification, the cladding has the potential to collapse into a gap (i.e., flattening). Because of the large local strains that accompany this process, collapsed (flattened) cladding is assumed to fail. iii. Overheating of Cladding. Traditional practice assumes that failures will not occur if the thermal margin criteria (DNBR for PWRs and CPR for BWRs) are satisfied. SRP Section 4.4 details the review of these criteria. Violation of the thermal margin criteria is not permitted for normal operation and AOOs. For postulated accidents, the total number of fuel rods that exceed the criteria has been assumed to fail for radiological dose calculation purposes. Although a thermal margin criterion is sufficient to demonstrate that overheating from a deficient cooling mechanism can be avoided, it is not a necessary condition (i.e., DNB is 4.2-9 Revision 3 - March 2007 not a failure mechanism) and other mechanistic methods may be acceptable. At present, there is little experience with other approaches, but new positions