Which Resolution can be Achieved in Practice in Neutron Imaging Experiments? – A General View and Application on the Zr - ZrH 2 and ZrO 2 - ZrN Systems

Current methodical developments improve the spatial resolution of neutron imaging facilities. Objects with dimensions down to several microns should be detectable. However, the minimum object size detectable depends not only on the facility hardware like detector resolution or neutron optics, but also on the attenuation contrast. In this paper the relation between illumination time needed, neutron contrast of the objects and their minimal size detectable is derived and an analysis of the minimal dimension of an object can be detected in neutron radiography and tomography is discussed at two examples zirconium hydride ZrH2 in Zircaloy 4 as a high contrast system and zirconium nitride ZrN in zirconium oxide ZrO2 as a low contrast system. It is concluded which minimal sizes of the precipitates can be detected in realistic time

In Situ Neutron Radiography Investigations of Hydrogen Related Processes in Zirconium Alloys

In situ neutron radiography experiments can provide information about diffusive processes and the kinetics of chemical reactions. The paper discusses requirements for such investigations. As examples of the zirconium alloy Zircaloy-4, the hydrogen diffusion, the hydrogen uptake during high-temperature oxidation in steam, and the reaction in nitrogen/steam and air/steam atmospheres, results of in situ neutron radiography investigations are reviewed, and their benefit is discussed

Experimental program QUENCH at KIT on core degradation during reflooding under LOCA conditions and in the early phase of a severe accident

The most important accident management measure to terminate a severe accident transient in LWR is the injection of water to cool the uncovered degraded core. In order to detailed investigation of the reflood effect on bundle degradation the QUENCH program was initiated in 1996 followed-up the CORA bundle tests and is still on-going. So far, 17 integral bundle QUENCH experiments with 21–31 electrically heated fuel rod simulators of 2.5 m length using zirconia as fuel substitute have been conducted. Influence of following parameters on bundle degradation were investigated: degree of pre-oxidation, temperature at reflood initiation, flooding rate, effect of neutron absorber materials (B4C, Ag-In-Cd), air ingress, influence of the type of cladding alloy, formation of a debris bed in the core. Integral bundle experiments are supported by separate-effects tests. The program provides experimental data for the development of quench-related models and for the validation of SFD code systems. In seven tests, reflooding of the bundle led to a temporary temperature excursion. Considerable formation, relocation, and oxidation of melt were observed in all tests with escalation. The temperature boundary between rapid cool down and temperature escalation was typically 2100–2200 K in tests without absorber. Tests with absorber led to temperature escalations at lower temperatures. Although separate-effects tests have shown some differences in oxidation kinetics of advanced cladding materials, the influence of the various cladding alloys on the integral bundle behaviour during oxidation and reflooding was only limited. The two bundle tests with air ingress phase confirmed the strong effect of air on core degradation especially when pre-oxidation in steam is limited and oxygen starvation occurs during the air ingress phase. Oxidation in a nitrogen-containing atmosphere accelerates the kinetics by the temporary formation of zirconium nitride and causes strongly degraded and non-protective oxide scales. The latest QUENCH-LOCA tests investigated influence of secondary hydriding of ruptured cladding on mechanical properties of cladding tubes

Oxidation and Quench Behavior of Cold Spraying Cr-Coated Zircaloy Fuel Cladding Under Severe Accident Scenarios [in press]

The oxidation performance and quench behavior of cold spraying Cr coated Zircaloy-4 cladding tubes were investigated from 1100°C up to 1500°C in steam. The coated samples displayed significantly improved oxidation resistance, good thermal shock resistance and high post-quench ductility during oxidation at 1100°C and 1200°C for 1 hour accompanied by subsequent quench in water. However, substantial bending of the coated tubes was observed during oxidation at 1100°C, which led to loss of protective effect of the coatings on the tensile (convex) side. The underlying mechanisms for the occurrence of bending phenomenon at such temperature seems to be related with the residual stress state and high-temperature creep behavior of the coated cladding. Once the oxidation temperature exceeds the Cr-Zr eutectic temperature (~1330°C), oxygen drives the Cr diffusing inwardly (owing to the low solubility of Cr in ZrO2) and the formation of liquid phase contributes to rapid degradation of the coated cladding, connected with a much faster oxidation rate compared to the uncoated reference sample. I