Development of Performance Criteria for High Burnup Cladding Materials, 20-R9565

Printer Friendly Version

Principal Investigators
Yi-Ming Pan
Kwai S. Chan
David S. Riha
Wuwei Liang

Inclusive Dates:  10/03/05 – 04/09/07

Background -  Fuel rod cladding failure caused by hydride-induced embrittlement is a reliability concern for spent nuclear fuel after extended burnup. High burnup increases the thickness of the oxide layer on zirconium alloy cladding, the amount of absorbed hydrogen in cladding, and the internal fuel rod pressure. The mechanical integrity of zirconium alloy cladding degrades as burnup increases because of a higher susceptibility to premature fracture resulting from hydride-induced embrittlement and wall thinning by oxidation. Although a significant quantity of the future spent nuclear fuel from nuclear power plants will reach high burnup (greater than 45 GWd/MTU), performance criteria for predicting radial hydride fracture of high burnup cladding have not been established.

Approach - This project included three research and development activities: cladding database assessment; high burnup cladding modeling; and model evaluation and verification. Micromechanical models have been developed for treating oxide fracture, blister fracture, delayed hydride cracking, and cladding fracture. Uncertainties in fuel rod characteristics have been modeled by treating the relevant cladding properties as random variables and describing them in terms of probabilistic distribution functions. Using the existing SwRI-developed NESSUS® software, these models have been integrated into a methodology for assessing the probability of hydride-induced failure in zirconium alloy cladding.

Accomplishments - A probabilistic micromechanical methodology has been developed for assessing the probability of hydride-induced failure in zirconium alloy cladding. The developed methodology includes micromechanical models, as incorporated into the computer model ZRCRACK, for describing the cladding failure processes and probabilistic modules for treating uncertainties in cladding properties. ZRCRACK has been used to compute the times to leakage and failure by delayed hydride cracking in Zr-2.5%Nb. The integrated probabilistic micromechanical methodology was evaluated by conducting case studies to examine the performance of fuel cladding during the vacuum drying and dry storage processes. A critical burnup level about 68 to 70 GWd/MTU was found to be the most important criterion for cladding performance prediction.

Figure 1. Computed times to leakage (tl) and times to failure (tf) by delayed hydride cracking in Zr-2.5%Nb as a function of temperature. The computation was performed at an internal rod pressure (P) of 5 MPa [0.7 ksi], a mechanical stress (σ) of 275 MPa [39.9 ksi], and a threshold stress intensity factor (K1H) of 5 MPa·m1/2 [4.6 ksi·in1/2].

Figure 2. Fuel rod burnup and cladding thickness interaction plot for 200 MPa [29 ksi] mechanical stress and failure after 20 years of dry storage.

2007 Program Home