The topic of pellet cladding interaction (PCI) in light water reactor (LWR) fuel rods is reviewed and further explored in this paper by considering the mitigating effect of slow ramps. To that end, a number of PCI mitigating mechanisms were considered in relation to several failure criteria, including peak stress, strain energy density and other cumulative damage formulations. Towards a tentative working hypothesis for PCI mitigation by low ramp rate, the results fuel codes simulations of some slow ramps that have been part of the 3rd SCIP MWS (Studsvik Cladding Integrity Program Modeling WorkShop) are used in the paper in order to support or question the applicability of different possible mechanisms, considered as potential mitigating factors in slow ramps. In addition, the results of out-of-pile biaxial tests on irradiated cladding are presented for a loading scheme that aimed at reproducing the stress rate conditions of a slow ramp; the relatively small contribution of stress relaxation during the power increase stage of a slow ramp that was noticed in code calculations was confirmed by the mechanical biaxial tests. Thus, the main outcome of the code calculations in the 3rd SCIP MWS and related SCIP studies coupled with the literature review was that slow ramps' benefits cannot be explained by mechanical effects only and a hypothesis is proposed that is based on the protective role of the cladding inner oxide and the healing of its cracks during a slow ramp provided the corresponding fuel rod gap chemistry exists.
The stress relaxation (SR) process in zirconium-based alloys is intimately related to creep behavior as stress is diminished with time by creep. Open literature data show a complex and not fully understood relation between the primary and quasi-steady-state creep rates and the corresponding SR rates; in particular, it was observed that higher primary SR rates occur under irradiation. To further investigate this topic, in-reactor SR experiments were conducted on pre-irradiated zirconium alloy specimens. A unique four-point bend (4PB) technique was applied to measure SR of pre-irradiated and unirradiated rectangular specimens (35 by 6.5 by 0.8 mm in size) at regular intervals during their irradiation in the RBT-6 research reactor and a sibling out-reactor rig. The 4PB method had never been applied previously to highly pre-irradiated (up to 34 dpa) and prehydrided (up to 339 ppm) small specimens of zirconium alloys. The measured SR behavior can be analyzed with various creep behavior models. In addition, the alloy variants tested in this study were also a subset of those from a previous study on irradiation growth behavior. The unique loading fixture utilized can simultaneously apply 4PB loads on up to six specimens, without need for removing the specimens from the fixture to record SR versus time data. Additional fast fluence accrued during SR testing in RBT-6 was minimal. Post-test examinations on selected specimens were performed by metallography and transmission electron microscopy. This paper describes (i) the design of the loading fixture, (ii) in-reactor and out-reactor SR data, (iii) the SR data trends, especially the lack of any significant dependence of SR with c-component loop density and hydrogen, (iv) some observations on the effects of alloying additions on SR, (v) application of a simple phenomenological creep model to the SR data, and (vi) the post-test characterizations of SR test samples.
The availability of reliable tools and associated methodology able to accurately predict the LWR fuel behavior in all conditions is of great importance for safe and economic fuel usage. For that purpose, AREVA has developed its new global fuel rod performance code GALILEOTM along with its associated realistic thermal-mechanical analysis methodology. This realistic methodology is based on a Monte Carlo type random sampling of all relevant input variables. After having outlined the AREVA realistic methodology, this paper will be focused on the GALILEOTM code benchmarking process on its extended experimental database and the GALILEOTM model uncertainties assessment. The propagation of these model uncertainties through the AREVA realistic methodology is also presented. This GALILEOTM model uncertainties processing is of the utmost importance for accurate fuel design margin evaluation as illustrated on some application examples. With the submittal of Topical Report for GALILEOTM to the U.S. NRC in 2013, GALILEOTM and its methodology are on the way to be industrially used in a wide range of irradiation conditions.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
customersupport@researchsolutions.com
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.