The US-APWR design is a simplified design that enhances redundancy and independency of systems. A new high-performance accumulator system is used in the emergency core cooling system design. It incorporates the functions of both traditional accumulators and low-pressure injection pumps. The US-APWR replaces the traditional diesel generators with more reliable, air-cooled, fast-start gas turbine generators. This ensures immediate backup power will be supplied to the safety and cooling systems in the case of a station blackout event or other serious accidents. All of the systems have been extensively tested and verified in operating plants in Japan. Safety is Mitsubishi’s first priority. The design fully satisfies all post-Fukushima design requirements.
The reactor vessel houses a core and core support structures. It is designed to fully serve its function as a reactor coolant pressure boundary to withstand high temperatures and high pressures during normal operation, abnormal transients, and embrittlement. In the US-APWR, large forged steel pieces are adopted to minimize the length of the welds requiring in-service inspections. Improved designs are applied for the head adaptors of the reactor vessel head to reduce residual stresses. In addition, design and material improvements in Mitsubishi’s reactor vessel heads have enhanced their corrosion-resistance, thereby improving their “preventive maintenance” function. The number of fuel assemblies in the reactor core has been reduced while output is increased. The 14’, open-lattice fuel assembly design contributes to the efficient use of neurons, resulting in excellent fuel economy and, therefore, the need for less fuel.
Within the reactor, Neutron Reflectors are provided in the US-APWR design to minimize the neutron exposure rate in the reactor vessel. This efficient use of neutrons improves reactor core internal reliability and facilitates an economical use of uranium.
Reactor Coolant Pump
The reactor coolant pump circulates the reactor coolant to remove heat from the reactor core at a constant flow. The US-APWR system features improved hydraulic performance achieving over 85% efficiency. In addition, new cartridge type seals improve durability. The service life of the seal has been extended through improvements in the design and material specification.
The Steam Generator separating the reactor system from the turbine system plays a crucial role in maintaining the reliability of the entire plant. Since producing its first unit in 1970 for the Mihama Unit 2, Mitsubishi has manufactured and installed about 100 steam generators, including replacement units. The US-APWR features advanced steam generators which utilize improved, stress corrosion crack resistant tube materials, redesigned tube support plates which allow a more steady flow of water through the holes, a single stage high performance moisture separator, and an increased number of anti-vibration bars.
The high-performance accumulators passively change the post-event core injection flow rate from high to low in Emergency Core Cooling injection operation. The new accumulator functions both as a traditional accumulator and lower-pressure injection pump.This eliminates the need for an independent low-pressure injection pump, therefore improving reliability.
Comprehensive Digital Instrumentation & Control
Highly reliable instrumentation and control (I&C) systems with extended capabilities and operability are indispensable for the safe and stable operation of a nuclear power plant.
Improved Control Rod Drive Mechanism and RCP Seal
“Canopy-seals” have traditionally been adopted in the Control Rod Drive Mechanism (CRDM) housing designs so that the latching assembly can be easily replaced on-site in the event of a failure. Over the years, however, canopy-seals have exhibited a susceptibility to stress corrosion cracks. To overcome this problem, Mitsubishi has developed a reliable design against leakage that still allows the removal of the latching assemblies on-site. This new design is currently applied to replaceable vessel heads.
This improved, cartridge type seal has been introduced to improve the maintainability of reactor coolant pumps. The large face plate design enhances sealing capability and improved materials are applied to O-rings. The service life of the seal has been extended through improvements in the design and material specification. The performance and integrity have been duly verified by full flow tests in the MHI facility.
The key elements of the turbine generator system are turbines to convert thermal energy generated at the steam generator to kinetic energy and a generator to eventually convert the kinetic energy to electrical energy. Mitsubishi’s new turbine design incorporates 74 inch turbine blades, making the turbine larger and more efficient than traditional turbines.