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Thesis Format

Integrated Article

Degree

Doctor of Philosophy

Program

Mechanical and Materials Engineering

Supervisor

Klassen, Robert J.

Abstract

The recent mechanical tests on the ex-service Inconel X-750 spacers have indicated significant embrittlement and reduced load carrying capacity compared to as installed condition. This is an immense safety concern to the nuclear industry, as in-service spacer examination and their replacement within fuel channels are very costly and impractical. The primary degradation mechanism is complex, and thus provides the focus of the current investigation. Despite few previous reports about hardness of non-irradiated and irradiated Inconel X-750, thermally-activated time-dependent deformation conditions have not been studied. This dissertation has attempted to overcome this scarcity of data by providing a series of fundamental investigations involving the use of novel pyramidal nanoscale indentation test techniques to measure the time-dependent plastic deformation of He+ at 300 ºC and Ni+ at 25 ºC at different doses separately. Established theories were applied to grain by grain nano indentation testing to assess the hardness of the X-750 alloy under different irradiation doses and temperatures data. Both He+ and Ni+ ion implantation at lower doses, resulted in softening of the X-750 alloy, however at higher irradiation doses indentation hardness was increased. Constant-load pyramidal indentation tests performed at 25 ºC showed that the average indentation stress, and strain rate increases with decreasing indentation depth and increasing levels of irradiation. The apparent activation energy of the obstacles that limit the rate of dislocation glide during indentation, increased with increased He+ implantation dose whereas, it was decreased by increasing Ni+ irradiation as irradiation defects increased. On the other hand, apparent activation volume and activation area (Δa) decreased by an increased indentation stress for both He+ and Ni+ irradiated samples. This indicates that the obstacles controlling deformation process are rate sensitive “thermal” obstacles. These approaches are unique to the literature since they demonstrate the individual effect of irradiation damage on mechanical behavior and operative kinetic plastic deformation on irradiated Inconel X-750.

Summary for Lay Audience

Canadian deuterium uranium reactors (CANDU) fall into a subset of pressurized heavy water reactors (PHWR) that use heavy water coolant, D2O, as opposed to H2O in light water reactor (LWR) designs, as both a moderator and coolant. Helical spring annulus gas spacers in the fuel channels of current CANDU nuclear reactors are made from age-hardened Inconel X-750. Fuel channels are consisting of high temperature Zircalloy pressure tubes (Zr-2.5%Nb), cool Zircalloy-2 calandria tubes, and Inconel X-750 internal spacers that maintain an insulating gap filled with CO2 gas between the two tubes. These Inconel X-750 garter spring components prevent contact between the two tubes that would otherwise generate a large heat sink and compromise the thermodynamic integrity of the reactor, in addition to causing eventual hydride blistering and rupturing of the pressure tube that could ultimately result in failure and a local loss of coolant accident (LOCA). Neutron irradiation is known to cause microstructural and mechanical property changes within these spacers. Recently, it has been found that these spacers become very brittle after long exposure to neutron irradiation and this is an ongoing concern for the operation of the nuclear reactors. As such the current research aims to use a novel technique of nanoscale mechanical testing to assess the effects of irradiation, temperature and fundamental kinetic deformation parameters in Inconel X-750. The data obtained can be useful in developing models to predict life-time of existing spacers and will also be helpful for the design of the future materials used in garter spring spacers.

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