Figure XV-1 shows a cross sectional view of IMR reactor. As shown in the figure, IMR employs several key concepts for the reactor design. The first one is the integrated primary system concept. Fuel assemblies, control rods, steam generators (SGs), and control rod drive mechanisms (CRDMs) are all installed inside the reactor vessel (RV), and there is no main coolant piping and no large penetration of RV.

The second one is the hybrid heat transport system (HHTS), which is a kind of two-phase natural circulation system operating under the self-pressurized saturation condition of the primary coolant. The coolant starts boiling at the upper part of the core, two-phase coolant flows up in the riser, and is condensed and sub-cooled by SGs. In order to control the amount of boiling and the system pressure, IMR has two kinds of SGs, i. e. SG in liquid region (SGL) and SG in vapor plenum (SGV). Table XV — 1 shows the major specifications and operating condition of the IMR primary system.

6m

IMR(350MW(e))

FIG. XV-1. Configuration of the IMR primary systems.

This concept allows eliminating a pressurizer and reactor coolant pumps, which simplify the reactor design and is beneficial to reduce maintenance work. This concept also increases the driving force of the coolant flow. The average void fraction in the riser (20%) is optimized to minimize the required RV height while keeping the appropriate thermal margin of the core. A small amount of boiling is allowed in the core but the core characteristics are still very similar to that of conventional PWRs, because the void fraction is relatively low and most of the core operates under sub-cooled condition. From the safety point of view, this concept realizes a reactor design free from large-scale break of the primary boundary (i. e. LOCA), control rod ejection accident, and loss of flow accident. As the results, no safety injection system and containment spray system is required for IMR. Additionally, such downsizing and safety feature allows applying very small dry containment, which greatly reduces construction cost.

The feasibility of HHTS is one of the most important subjects for IMR because of less knowledge of two-phase flow behavior under such high temperature (345deg-C) and pressure (15.5MPa). In order to verify the feasibility, two series of experiments have been performed. The first one is an air-water scale experiment and the second one is a natural circulation experiment under the actual temperature, pressure, and axial dimension.