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A number of the test programs currently in progress at the Halden Project are aimed at evaluating Zircaloy cladding performance in terms of corrosion behavior, particularly at high burnup. A pressurized water reactor (PWR) facility has been used to determine the effects of high (4 to 4.5 ppm) lithium on the corrosion behavior of Zircaloy-4 cladding material exposed to nucleate boiling (1% void) and to one-phase cooling conditions. The test employed four fuel rod segments, base-irradiated to an average burnup of 28.5 MWd/kg UO2 in a commercial power plant, and with average initial oxide layers of either 10, 20, or 40 μm, respectively. Four sets of oxide layer thickness measurements were performed on the four rods during the course of the investigation. Pre-test eddy current measurements were performed on the as-received segments and two interim sets of measurements were made after 80 and 245 full power days of irradiation. Final oxide measurements were made after 425 full power days, when maximum rod burnup was 45 MWd/kg UO2. Average oxide layer increases of 30 and 55 μm were observed for the rods exposed to one-phase cooling and nucleate boiling conditions, respectively. Although maximum oxide thickness exceeded 95 μm, no evidence of spalling was apparent. The measured oxide layer thicknesses compared favorably with predicted values based on model calculations derived from power plant data, with little evidence of enhanced corrosion rates in the presence of increased lithium concentrations.

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The study of the Loss Of Coolant Accident (LOCA) in a nuclear reactor has been under investigation since the beginning of the nuclear civil technology. The latest research revealed several new phenomena associated with fuel rod behaviour and the work presented in this dissertation is an experimental research focused on high burnup fuel fragmentation during a LOCA. The research primary objectives are: 1) the design and execution of a LOCA single effects tests to be performed in hot cell environment; 2) the study of fuel fragmentation and fragment size distribution under selected boundary conditions; 3) the study of the fission gas release and 4) the development of a semi-empirical correlation to obtain a first order description of the resulting fragments size distribution. The experimental test was designed based on a comprehensive review of past research LOCA tests and observations on fuel fragmentation. A particular attention was dedicated to the study of the IFA-650 LOCA test series performed at the OECD Halden Reactor Project. The experiment employed the use of Light Water Reactor (LWR) irradiated fuel with a burnup of 52 MWd/kgU and the heat-up was simulated with heat provided by an electric oven. The selected boundary conditions were: Rod segment 1: removal of the constraint provided by the cladding (rod ballooning without burst); Rod segment 2: removal of the constraint provided by the cladding and by the rod internal pressure (rod ballooning with burst); Rod segment 3: retain the constraint provided by the cladding (no rod ballooning). The parameters of the rod segments required to obtain the above mentioned boundary conditions were calculated with the fuel performance code TRANSURANUS. The experiment was successfully performed with the three selected boundary conditions achieved. The post experiment investigations included non-destructive investigation such as neutron radiography, visual inspection and axial diameter measurements by profilometry and destructive investigations such as ceramography of an archive pellet, puncturing, fragments sifting and analysis of the fission gas release. The neutron radiography showed a fragmentation of a limited number of pellets in correspondence to the burst region of rod segment 2 while the rod segment 1 that only ballooned presented cracks along the entire length. The rod segment 3 was shown to be mostly intact. The fission gas release during the transient was higher for the rod segment 2 (7.3%) followed by the rod segment 1 (6%) and rod segment 3 (3.5%). An effort was made to find a law to reproduce the fragment size distribution measured by sifting following the LOCA test. A fractal law correlation was derived starting from the study on the compressing mechanism behavior of concrete. The fractal coefficients were determined as a function of the fuel burnup, and of the individual contribution of the effects studied in this work: the cladding strain and the burst pressure. The experiment's results and the first modeling approach provide new insights and a be...