XVII — 1. Introduction

The PSRD (passive safe small reactor for distributed energy supply system)-100 is an integral type PWR with the thermal power of 100 MWt developed at the Japan Atomic Energy Research Institute based on the technologies developed for the marine reactor MRX (KUSUNOKI, T. et al., 2000; ISHIDA, T., et al., 2003-1; IAEA-TECDOC-1391). The reactor is designed for the electricity generating for a small power-grid, heat supply, and/or sea water desalination. The major characteristics are listed in Table XVIII-1.

Siting in the demand area is one of desired characteristics for this kind of reactors, which requires the extremely-higher level of the safety compared to that for the current generating LWRs without compromising economic competitiveness. In order to realize this, several futures are incorporated in the design, which includes:

• Elimination of the pipes connecting to the primary pressure boundary by adopting in-vessel steam generators (SG) and disconnecting volume control and purification systems during the power-operation period to limit the primary pipes only to those for the safety valve lines, which significantly reduces the possibility of a loss-of-coolant accident (LOCA);

• Adoption of a small and high pressure water-filled containment that can mitigate effects of a LOCA by terminating the primary coolant discharge before the core is exposed to the steam environment;

• Adoption of an in-vessel control rod drive mechanism (INV-CRDM) to eliminate the possibility of a reactivity insertion accident caused by a control-rod withdrawal;

• Natural circulation core cooling to eliminate the possibility of a transient caused by a circulation pump failure and simplify the system;

• Full utilization of passive cooling systems to enhance the reliability of the decay heat removal during accidental conditions and simplify the system; and

• Adoption of the core design that enables continuous full power operation for five years without refuelling.

Figure XVIII-1 shows the cross-section of the reactor pressure vessel (RPV) and the containment vessel (CV). Inside the RPV, the core is located in the lower part, the SGs in the middle part, and the INV-CRDMs in the upper part. The core is radially surrounded by the radiation shield located outside the core barrel. The RPV is not fully filled with water: the nitrogen gas occupies the top part of the RPV to absorb the primary liquid volume change. The SG is the once-through, helical coil tube type, where the secondary side coolant flows up inside the tubes. The volume control and purification systems are not connected to the primary loop during the power operation to limit the pipes composing the primary pressure boundary to those for the safety valve lines. The CV is filled mostly with water so that the RPV, and the heat exchangers for the emergency decay heat removal system (EDRS) and the containment water-cooling system (CWCS) are submerged. The water inside the CV has the role of radiation shielding. The RPV is covered with the water-tight shell (WTS) to house stainless-steel felt between the RPV and WTS for the thermal insulation. The heat loss from the RPV with this insulation is estimated less than 1 % of the rated power.