Introducing the EU-APWR

Mitsubishi Heavy Industries, Ltd. (MHI) has designed a 1700MWe class advanced pressurised water reactor for European utilities. The plant, called EU-APWR, has a basic plant configuration similar to that of conventional PWRs, but various advanced technologies have been adopted to achieve enhanced safety, reliability, and economy.

EU-APWR is designed as a larger-scale version of the Japanese APWR, aiming at higher electrical outputs to satisfy European utility requirements (see Table 1). The design concept is similar to the US-APWR for which a design control document (DCD) was submitted to the US Nuclear Regulatory Commission on 31 December 2007 and docketed on 29 February 2008.

Operational technology

The EU-APWR is a four-loop pressurised water reactor with a power rating of 4451 megawatt thermal (MWt) and a nominal gross electrical output of 1700 megawatt-class electric (MWe). Its core consists of 257 fuel assemblies, with an active fuel length of 14ft. The fuel assemblies are surrounded by a stainless steel radial reflector, designed to improve neutron utilisation, which reduces fuel cycle costs and reactor vessel irradiation. Both the EU-APWR and the Japanese APWR have much larger thermal outputs than the conventional PWRs. The introduction of 14-foot fuels to the EU-APWR achieved, without changing the reactor vessel, 24-month operation with 2-batch fuel management as an equilibrium cycle under constraints of U-235 enrichment at the maximum of 5wt% and also at the well-proven maximum fuel rod burn-up of 62GWd/t.

One HP turbine and three LP turbines are provided in the EU-APWR secondary system with a moisture separator and reheater to remove the moisture and to reheat the steam between the HP turbine and the LP turbines. Longer blades – 70in class last stage blades – for the LP turbines contribute higher turbine efficiency. The instrumentation and control system for both safety and non-safety systems is fully digital. It includes redundancy, defence-in-depth and diversity, self-diagnosis and online maintenance capabilities.

Safety features

Safety injection system

The engineered safety features of the EU-APWR use a four-train configuration. Required safety functions are performed following an accident assuming a single failure in one train with a second train out of service for maintenance. The four-train direct vessel injection (DVI) system is simple and compact and reduces the capacity of each train from 100% to 50%.

Accumulator system

The accumulator system consists of four advanced accumulators (ACCs) and the associated valves and pipings, one for each reactor coolant system (RCS) loop. The system is connected to the cold legs of the reactor coolant piping and injects borated water when the RCS pressure falls below the accumulator operating pressure. It is a passive system.

The advanced accumulator incorporates an internal passive flow damper. Its function is to inject a large flow of water to refill the reactor vessel, and then reduce the flow as the water level in the accumulator drops. When the water level is above the top of the standpipe, water enters the flow damper through both the top of the standpipe and the side of the flow damper, thus it is injected at a high flow rate. When the water level drops below the top of the standpipe, water enters the damper through the side inlet only, reducing the flow rate. Reduced flow injection is performed during core re-flooding, in association with the safety injection pumps. The combined performance of the accumulator system and the high head injection system eliminates the need for a conventional low head injection system.

Electrical system

The safety-related electrical systems consist of four 50% systems. Two of four trains are required for safe shutdown of the plant, a configuration which allows maintenance to be done online. Gas turbine generators (GTGs) are used instead of diesel engines for the emergency power supply system, as they are easier to maintain, have a smaller footprint and have fewer auxiliary systems. The gas turbine generators take longer to start up than diesels, but use of the advanced accumulator technology (see above) allows a start up time of 100 seconds.

RWSP

The refuelling water storage pit (RWSP) of the EU-APWR is located at the lowest part of containment, and four recirculation sumps are installed at the bottom of the pit. This configuration provides a continuous suction source for the safety injection pumps (SIP) and the containment spray/residual heat removal pumps (CS/RHRP), thus eliminating the conventional connection from the refuelling water storage pit (outside containment) to the containment recirculation sump.

Core damage frequency

Estimated EU-APWR core damage frequency meets the European Utility Requirement (EUR) goal of 1×10^-5.

Severe accident mitigation

The EU-APWR is designed to be able to mitigate the consequences of hypothetical severe accidents such as a reactor vessel failure. The fundamental concept of the EU-APWR for severe accident mitigation is to flood the reactor cavity with coolant water and to keep the molten fuel within the reactor cavity. In order to achieve this, the EU-APWR is provided with a very reliable reactor cavity flooding system, which consists of two diverse independent coolant water supply systems..

Building configuration

The layout of equipment within the EU-APWR buildings is designed to facilitate plant operation and maintenance, and minimise personnel

radiation exposure. Provisions including redundant train separation and segregation barriers have been made to ensure that the functions of the safety-related systems are maintained in the event of postulated incidents such as fires, floods, and high-energy pipe break events.

The containment facility is comprised of the pre-stressed concrete containment vessel (PCCV) and an annulus enclosing the containment penetration area. It provides an efficient leak-tight barrier and radiation protection under all postulated conditions, including loss of coolant accident (LOCA). The PCCV is designed to withstand the peak pressure under LOCA conditions. Access galleries are provided for a periodic inspection and testing of circumferential and axial pre-stressing tendons. The EU-APWR safety-related structures, systems, and components are designed to withstand the effects of natural phenomena, including earthquakes, without jeopardising plant safety.

The standard seismic design is based on the safe shutdown earthquake and the operating-basis earthquake. Seismic design response spectra (SDRS) define the site-independent safe shutdown earthquake for the EU-APWR standard plant. The peak ground acceleration (PGA) of the SDRS is 0.3g for the two horizontal directions and the vertical direction. The PGA for the operating-basis earthquake is set at 1/3 of the safe shutdown earthquake SDRS, so no design analysis is required to address the operating-basis earthquake loads.

The SDRS for the EU-APWR is based on from the US RG1.60 spectra. RG1.60 spectra control points have been modified by shifting the control points at 9Hz and 33Hz to 12Hz and 50Hz, respectively, for both the horizontal and vertical spectra . SDRS for the EU-APWR cover the EUR design basis earthquake, which is valid for the majority of potential nuclear sites in Europe.

Future direction

The EU-APWR technology, with its emphasis on improved safety, economics and reliability should have broad appeal in Europe. An important future milestone will be the completion of the European Utility Requirements assessment of the standard design which will help it to become accessible to as large a fleet of power stations as possible. In the longer term, there may be minor variations in the design driven by specific site conditions.

Author Info:

Makoto Kanda, Kazumasa Tanaka, Hiroto Kawahara and Seiji Terada, Nuclear Engineering Strategy Planning Section, Mitsubishi Heavy Industries Ltd, 16-5, Konan 2-chome, Minato-ku, Tokyo, 108-8215, Japan