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:
on (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 recommending different criteria should address cladding temperature, pressure, time duration, oxidation, and embrittlement. iv. Overheating of Fuel Pellets. Traditional practice has also assumed that failure will occur if centerline melting takes place. This analysis should be performed for the maximum linear heat generation rate anywhere in the core, including all hot spots and hot channel factors, and should account for the effects of burnup and composition on the melting point. For normal operation and AOOs, centerline melting is not permitted. For postulated accidents, the total number of rods that experience centerline melting should be assumed to fail for radiological dose calculation purposes. The centerline melting criterion was established to assure that axial or radial relocation of molten fuel would neither allow molten fuel to contact the cladding nor produce local hot spots. The assumption that centerline melting results in fuel failure is conservative. v. Excessive Fuel Enthalpy. The sudden increase in fuel enthalpy from a reactivity initiated accident (RIA) below fuel melting can result in fuel failure due to pellet/cladding mechanical interaction (PCMI) (see Subsection II, item 1.B.vii). Exceeding the DNBR for a PWR or the CPR for a BWR may result in cladding failure during an RIA. See Appendix B for criteria. vi. Pellet/Cladding Interaction. No criterion currently exists for fuel failure resulting from PCI or PCMI. The difference between PCI and PCMI is subtle, and it is sometimes difficult to differentiate the two types of failures from visual observation of the failure. PCI is generally caused by stress-corrosion cracking due to fission product (iodine) embrittlement of the cladding, while PCMI is primarily a stress-driven failure. The design basis