3.1 Safety concepts and safety systems

A large LOCA cannot occur in the MRX, since only small size pipes (^50mm) exist in the primary system. The emergency water injection systems are not provided in the MRX. The engineered safety system consists of the water-filled containment vessel (CV) system, the emergency decay heat removal system (EDRS) and the containment water cooling system (CWCS). During a small LOCA, the engineered safety system keeps the core flooding and removes the decay heat without emergency water injection. Figure 2 shows a basic idea of the engineered safety system. The decay heat in a LOCA is transferred to the atmosphere by natural convection of the primary water in the EDRS, the CV water and the CWCS working fluid. According to the PSA, this engineered safety system has a high reliability. The probability of the functionally disordered trouble is 2xl0-e at first one month after starting the operation of this system. The residual heat removal system (RHRS) is not necessarily essential in the emergency core cooling system of the MRX. The RHRS is used for controlling the temperatures of water in the RPV and the CV for long term cooling after a LOCA and is also used for long term decay heat removal during a scheduled reactor shutdown.

(1) Containment vessel (CV)

The design pressure of the water-filled CV is 4MPa to withstand a high pressure at LOCA. The size of the CV is designed to satisfy the requirements of radiation shielding and the maintenance of instruments installed in the CV. The RPV is surrounded with a watertight shell for thermal insulation. The shell can stand against the static pressure at anticipated transients. Pressure relief valves are installed to mitigate the rise of pressure in the space between the RPV and the watertight shell due to pipe breakage accidents in this area.

(2) Emergency decay heat removal system (EDRS)

The EDRS is a closed system which transfers decay heat from the core to the CV water. It includes three trains, each of which has a capacity to remove the core decay heat. Each train consists of a hydrogen reservoir tank, a cooler, two valves and piping. In the case of accidents, the valves of each train are opened actively by the signal of battery, then the primary coolant circulates by natural convection removing decay heat from the core and is cooled in the cooler placed in the CV water.

(3) Containment water cooling system (CWCS)

The CWCS is a heat pipe system for long term decay heat removal transferring the heat in the CV water to the atmosphere. It includes four trains. In the event of an accident, the water temperature in the CV will be kept lower than the design value by the arbitrary three trains operated using natural convection. For its working gas in the CWCS, anti-freezing gas such as

R22 (CHCIF2) will be used taking into account of low temperature condition in ice-sea atmosphere.