Mitsubishi, Japan

XII — 1. Introduction

The Integrated Modular Water Reactor (IMR) is one of the integrated primary system reactors (IPSRs) with the reference output of 1,000 MWt (350 MW(e)). The design targets of IMR are to achieve the electricity generating cost comparable to that of a large-scale nuclear reactor and higher-level safety by removing the sources that cause fuel failures by design. To achieve these targets, IMR employs integrated design with in-vessel CRDM (control rod drive mechanisms), hybrid heat transport system (HHTS) which employs two-phase natural circulation for the primary heat transportation, and stand­alone direct heat removal system (SDHS) for accident heat removal from the primary system.

IMR started its conceptual design study in 1999 at Mitsubishi Heavy Industries (MHI) reflecting changes of business environment such as less growth of economy and electricity demand, and deregulation of electricity market in Japan. An industry-university group led by MHI (Kyoto University, Central Research Institute of Electric Power Industries (CRIEPI), the Japan Atomic Power Company (JAPC), and MHI) has been developing related key technologies funded by Japan Ministry of Economy, Trade and Industry from 2001 to 2004. In this project, the concepts and the feasibility of HHTS and SDHS have been built and tested through three series of experiments. They are (1) air — water scale test to confirm void distribution and void behavior in the reactor, (2) high temperature natural circulation test to study two-phase natural circulation in the reactor with the actual temperature, pressure, and axial dimension of IMR, (3) SDHS test to study passive heat transport with the actual temperature, pressure, and axial dimension of SDHS. These test facilities were built and set at MHI Takasago R&D centre. In-vessel CRDM technology is based on marine reactor (MRX) development by Japan Atomic Energy Agency (JAEA) and MHI.

Here, the concepts and the feasibility test results of HHTS and SDHS are summarized.