The thermal storage tank in a district heating system is a component that stores excess heat during normal operation and releases the stored heat to increase the efficiency of the system, when the heat source is stopped or additional demands occur. Recently, an obsolete thermal storage tank was dismantled for the first time since it began operation 30 years ago. In this work, the corrosion integrity of the obsolete thermal storage tank was evaluated by examining its appearance, thickness thinning, corrosion products, microstructure, and mechanical properties. Samples were taken at various locations (roof, shell, bottom) of the thermal storage tank, which enabled diagnosis of the respective environmental degradations. Severe corrosion was found in the roof edge plate due to corrosion under the insulation, and exhibited thinning exceeding ~49% of the designed thickness. In this location, the ferrite-pearlite band structure disappeared and deteriorated microstructures, such as decarburization and spheroidized pearlite, were measured, which resulted in a ~27% decrease in hardness. The inner surfaces of the bottom and shell plate were well covered with a magnetite film, and the degradation of the microstructure and mechanical properties showed a permissible limit in terms of ASTM A285/A516. In addition, no particular drop in hardness was found in the weld zone of each plate.

EXECUTIVE SUMMARYThe use of thorium in current or advanced light water reactors (LWRs) has been of interest in recent years. These interests have been associated with the need to increase nuclear fuel resources and the perceived non-proliferation advantages of the utilization of thorium in the fuel cycle. Various options have been considered for the use of thorium in the LWR fuel cycle including: (1) its use in a once-through fuel cycle to replace non-fissile uranium or to extend fuel burnup due to its attractive fertile material conversion, (2) its use for fissile plutonium burning in limited recycle cores, and (3) its advantage in limiting the transuranic elements to be disposed off in a repository (if only Th/U-233 fuel is used). The possibility for thorium utilization in multi-recycle system has also been considered by various researchers, primarily because of the potential for near breeders with Th/U-233 in the thermal energy range.The objective of this project is to evaluate the potential of the Th/U-233 fuel multi-recycle in current LWRs, with focus this year on pressurized water reactors (PWRs). In this work, approaches for ensuring a sustainable multi-recycle without the need for external source of makeup fissile material have been investigated. The intent is to achieve a design that allows existing PWRs to be used with minimal modifications. In all cases including homogeneous and heterogeneous assembly designs, the assembly pitch is kept consistent with that of current PWRs (21.5 cm used). Due to difficulties encountered with keeping the PWR design intact, the potential modifications (other than assembly pitch) that would be needed for PWRs to ensure a sustainable multi-recycle system have been investigated and characterized. Additionally, the implications of the use of thorium on the LWR fuel cycle are discussed.Investigations have been conducted to assess the feasibility of both homogeneous and heterogeneous PWR assemblies to achieve Th/U-233 fuel multi-recycle. The conclusions from the study are:1. A practical sustainable fuel cycle required by this study cannot be achieved with a homogeneous PWR assembly within the parameter space of initial U-233 content and reasonable moderator to fuel volume ratio (MR) obtained by varying the fuel pin size.2. A 17-by-17 heterogeneous assembly design achieved the following attributes and partially met the requirements defined in this study for sustainable Th/U-233 multi-recycle: a. A derated core power of 1000 MWt.b. A 2.7-year cycle length with sustainable fissile inventory (assuming 3-batch fuel management).c. Discharge burnup of 18 GWd/t, which is significantly lower than the burnup of current PWRs.d. The Th/U-233 system requires hard neutron spectra to support high conversion of Th-232. The neutron spectra of both the seed and the blanket region are harder compared to the standard thermal systems.e. Compared to the used nuclear fuel (UNF) of the standard PWR uranium-dioxide (UOX) fuel system, the high level waste (HLW) of the Th/U-233 multi-recycle system has...