Ways in which to substantially enhance the efficiency of LWRs have been studied for some time. Efficiencies as high as 44% are possible by operating in a thermodynamically supercritical regime. Supercritical high performance reactors are one of the candidates of the Generation IV initiative (The US Generation IV Implementation Strategy, 2003) for medium term deployment. The European Commission is also currently assessing the merits and feasibility of such an approach in a project involving European institutes and industry in collaboration with the University of Tokyo (Squarer et al., 2001). A review of supercritical reactors has been carried out by Oka (Proceedings of the First International Symposium on Supercritical Water-Cooled Reactors, 2000) and the EC project is assessing the available technology against a reference design (Dobashi et al., 1998). There have been considerable advances in this technology in Japan.

Supercritical water reactor (SCWR) systems are principally aimed at electricity production. Their high thermal efficiency offers a potential for improved economics compared with current generation LWRs. An important issue in regard to these systems is the need to develop materials and structures that can serve in the high temperature and supercritical pressure regimes of these plants. A sample of designs currently under consideration is given in Table 12.2.

The concept is based on a once-through cycle, operating in excess of the water critical pressure of 22.1 MPa. Water enters the reactor core and then exits without change of phase. This system has the advantage that no steam-water separation is necessary, which in principle leads to a simplified (and therefore more economic design). Heat is removed from the system via a coolant of very high temperature and because the system is single phase, the turbines are driven directly by the primary coolant. Typically, water enters the core at about 280°C and exits at 500°C or higher, yielding efficiencies of the order of about 44%.

Supercritical systems have been considered at various times over the past 50 years, initially by Westinghouse and GE and in the last decade by Kurchatov Institute and AECL, based on a CANDU system. The early Westinghouse and GE designs were light water cooled. The Kurchatov and AECL designs were graphite moderated and heavy water cooled respectively; however, these required larger reactor volume and complicated systems. This resulted in less favourable economics. The Russian design, based on an integrated supercritical PWR design, was cooled via natural circulation but was more limited in scale and power. Heavy water super critical systems are considered below.

There have been various other types of supercritical reactor designs considered, including fossil plant systems, the GE nuclear super-heater, a steam cooled FBR (FZK), a B&W design, and a University of Tokyo steam-cooled FBR.