MAJOR TECHNICAL PARAMETERS
Parameter Value
Technology developer, country of origin
Hitachi-GE Nuclear Energy, Japan
Reactor type Boiling water reactor
Coolant/moderator Light water / light water Thermal/electrical capacity,
MW(t)/MW(e)
840 / ~300 Primary circulation Natural circulation NSSS Operating Pressure
(primary/secondary), MPa
7.17 Core Inlet/Outlet Coolant
Temperature (oC)
186 / 287
Fuel type/assembly array UO2 pellet / 10x10 square configuration in channel box Number of fuel assemblies in the
core
400 (short fuel assembly) Fuel enrichment (%) < 5
Core Discharge Burnup (GWd/ton)
< 60 Refuelling Cycle (months) 24
Reactivity control mechanism Control rod drive and soluble boron injection
Approach to safety systems Hybrid (passive + active)
Design life (years) 60
RPV height/diameter (m) 15 / 4.8 Seismic Design (SSE) 0.45g
Distinguishing features Simple reactor design, natural circulation system, hybrid safety system, multipurpose energy use
Design status Basic design
1. Introduction
DMS stands for double MS: modular simplified and medium small reactor. The design concept is developed by Hitachi-GE Nuclear Energy under the sponsorship of the Japan Atomic Power Company (JAPC). The DMS is a small-sized boiling water reactor (BWR) that generates a capacity of 840 MW(t) or about 300 MW(e). The DMS reactor aims to optimize the design according to the power output and achieve high economy by utilizing proven technologies of existing reactors. The heat produced in the core is removed by natural circulation of the coolant, thus eliminating the recirculation pumps and their driving power sources. This feature allows for a simplified and compact reactor pressure vessel (RPV) and containment. Due to the natural circulation feature, reactor internals and systems are also simplified. The main features of the DMS reactor design are the miniaturization and simplification of systems and equipment, integrated modulation of construction, standardization of equipment layouts and effective use of proven technology. The factory-fabricated module reduces the construction period and enables the modules to be transported to the site.
2. Target Application
A small-to-medium sized BWR is suitable for remote regions with less developed grids and infrastructures.
DMS design provides a nonelectric use of energy such as for district heating, mining (oil sand extraction/steam assisted gravity drainage) and desalination.
3. Main Design Features
Design PhilosophyThe DMS is developed with the concept of high-economy small sized reactor of short construction period to
meet the diversified market needs. The design is based on a small sized reactor pressure vessel, simplified safety systems, rationalized layout and architectural design. The reactor is based on a proven technology with experience from existing BWRs, intended for using systems and equipment that requires no large-scale development. The DMS is designed to obtain a high safety level equivalent to the existing reactors with optimized operations and maintenance performance in accordance with the level of output power.
Nuclear Steam Supply System
The nuclear steam supply system (NSSS) of the DMS is a direct cycle, where steam, generated in the core, goes into the turbine directly. NSSS includes the reactor core and internals, reactor pressure vessel and the coolant/steam piping within the containment system. As for the reactor coolant flow, natural circulation is adopted. Therefore, DMS does not require any reactor coolant pump. This feature eliminates loss of flow accident (LOFA). Penetrations of large piping connected into the RPV are located in the active fuel so the reactor core will always be covered by coolant even in the most severe LOCA condition. The NSSS is designed to remove core power by natural circulation during normal operation, and the core can be also cooled by natural circulation flow even in anticipated transients or accident cases.
Reactor Core
The reactor core is loaded with 400 fuel bundles. The fuel active length is 2.0 m with the enrichment of less than 5wt%. The short active fuel length reduces the pressure drop in core and enables natural circulation. The short fuel length increases the number of fuel assemblies necessary to secure the required thermal output, which results in increasing the diameter of RPV and the number of control rod drives, but the flow rate of natural circulation can be reduced, making possible the reduction of RPV height. The core power density is about 44 MW/m3. The core can produce energy with the refuelling period of 24 months.
Reactivity Control
The DMS has two kinds of diverse reactor shutdown systems (i.e., control rod (CR)/control rod driving system (CRD) and standby liquid control system (SLCS)). CR uses B4C or Hf as a neutron absorber and it is designed to be inserted from the bottom of the core. Every CR has an independent CRD per each at the bottom of the reactor vessel. Since the CRD of DMS has a motor-controlled fine motion capability, it is also called Fine Motion CRD (FMCRD) and it controls a positioning of CR, including insertion and withdrawal. DMS has 97 CRs and FMCRDs. FMCRD has two kinds of independent operation mode, one is fine motion control by electric motor, and the other is rapid motion (scram) by hydraulic pressure. In the normal operation, the reactivity in the core is controlled by the fine motion feature of FMCRD.
Reactor Coolant System
The DMS reactor primary cooling mechanism under normal condition and shutdown condition is by natural circulation of coolant. In order to enhance the driving force, divided chimney of about 3m height is installed above the core. The reactor coolant system (RCS) is designed to ensure adequate cooling of reactor core under all operational states, and during and following all of the postulated off normal conditions. The increased height and diameter of the RPV ensure the availability of large coolant inventory. Like the conventional BWR, steam separation is performed inside the RPV. In DMS however, this mechanism is done through free surface separation (FSS) in which the steam is separated from water by gravity force. Hence, no physical separator assembly is required.
Reactor Pressure Vessel and Internals
The size of the RPV is a main factor that determine the size of primary containment vessel (PCV) and influences construction cost of the reactor building. The small size RPV is made possible by simplifying core internals through smaller core and natural circulation, and by eliminating steam separator. The flat core concept with short-length and large core diameter is adopted to reduce the power density to about 44 MW/m3. The low power density results in a moderate evaporation rate and lower steam velocity in the upper plenum of the RPV.
This let the design to adopt the FSS system.
4. Safety Features
DMS has a larger coolant inventory compared to forced circulation type LWRs of the same output. This is because RPV’s height is increased to secure the driving power for natural circulation. The RPV’s diameter is also increased to accommodate more fuel assemblies. These features eliminate the need of high-pressure injection system.
Engineered Safety System Approach and Configuration
As a defence-in-depth measure, enhanced hybrid safety systems that combine passive and active methods are adopted. The safety system configuration of DMS has been rationally simplified compared to a conventional large BWR. There are four main distinctive features: (1) High pressure core flooder (HPCF) equipped in conventional BWR is eliminated due to the larger coolant inventory in the DMS; (2) Isolation condenser (IC) and passive containment cooling system (PCCS) were added to the active system as a countermeasure against long-term SBO. IC and PCCS can passively remove the decay heat during at least 10 days; (3) Gas turbine generator (GTG) was adopted instead of conventional diesel generator (D/G). GTG includes less auxiliary
equipment than D/G, so maintenance load decreases and reliability increases. Though required time for start-up of GTG is longer than that of D/G, DMS can adopt GTG because DMS has large time margin until water level in the RPV reaches below the top of an active fuel; and (4) Reactor core isolation cooling (RCIC) system and low pressure core flooder (LPFL) system were rationally integrated as hybrid RCIC. RCIC can inject water into the RPV by using steam generated in the RPV under high RPV pressure and LPFL can inject water by motor-driven pump under low RPV pressure. The hybrid RCIC can inject water by using steam power under high RPV pressure, and by using electric power under low RPV pressure. Long-term SBO and design basis accident (DBA) were preliminary analysed and it was confirmed that the core could be cooled for 10 days against SBO and peak cladding temperature (PCT) was kept less than 1200oC even against the most severe DBA.
Cut-away view of DMS power plant Decay Heat Removal System
The residual heat removal system (RHR) includes a number of pumps and heat exchangers to cool the reactor or the suppression pool (S/P) in the PCV. The RHR can remove residual heat not only during normal shutdown plant’s outage but also during an accident. The PCV cooling is accomplished by extracting and cooling the S/P water and injecting cooled water back to either the S/P injection line or the containment spray lines. The IC and PCCS are passive safety systems without using any AC power. Both systems can condense steam generated in the RPV and steam released from the RPV to the PCV via ruptured piping during LOCA. The heat exchangers of the IC and PCCS are cooled by the water pool located above the PCV, which is filled with an amount of water enough to remove decay heat at least 10 days passively.
Containment System
Steel containment is used to achieve the design pressure of 427 kPag equivalent to that of Mark-I type containment, and the quantity of material is reduced by reducing the diameter and height of PCV by adopting dish shape drywell and eccentric RPV arrangement. As in BWR and ABWR, the pressure suppression containment is applied while compactness was aimed at by eliminating steam separator, thus reducing the height of RPV and the number of main steam pipes. The decrease in PCV height is achieved by reducing the active fuel length of the DMS core, which is about 2 m compared with 3.7 m in the conventional BWR. The PCV is inserted by nitrogen during normal operation, therefore, hydrogen combustion in the PCV in early timing is practically eliminated. For a long-term accident, a few passive autocatalytic re-combiners (PARs) are planned in the PCV to react hydrogen and oxygen generated due to water radiolysis.
water Main Steam Lines
Feedwater lines Primary containment
vessel (PCV)
RPV
Control rod drive
mechanism T o main
condenser
Condensate filter Condensate demineralizer Condensate pump Generator
Main condenser High-pressure
turbine Low-pressure
turbine
Main feedwater pump
Sea Moisture separator
T urbine bypass line
High-pressure feedwater heater
Low-pressure feedwater heater
～
Apprication of passive systems
Apprication of GTG
5. Plant Safety and Operational Performances
The performance of the plant is improved by applying the main steam isolation valve of low pressure loss developed for large sized reactors. DMS uses only two main steam lines with diameter equivalent to that of conventional BWR thus minimizes the size of the primary containment vessel.
6. Instrumentation and Control Systems
The DMS adopts digital I&C systems that include microprocessors and the field programmable gate arrays (FPGAs) making use of fault detection and fault tolerance. A diversity is important in providing a countermeasure against common cause failure (CCF). Hardwired back-up safety system based on analogue technology is planned to be installed to the DMS for diversity to mitigate influence of CCF of the digital I&C system. The I&C system includes the safety system logic and control (SSLC), the plant control systems, the hardwired back-up safety system, the auxiliary control system, and the plant computer system. The reactor protection system (RPS) which initiates ECCS are included in the SSLC.
7. Plant Layout Arrangement Reactor Building
The reactor building is minimized by both system simplification and PCV compactness. The number of system component is reduced by adoption of large capacity equipment, common use of single equipment for different system, and adoption of passive system. PCV compactness is achieved by dish shape drywell and eccentric RPV arrangement, i.e., the RPV is installed not at the real centre but at an eccentric centre of the PVC. Compact PCV lets the number of floor levels to reduce from six in current ABWR’s to four, which contribute to saving in the construction period. The building is divided into fixed standard area, where hardly influenced by site conditions and variable flexible area which may depend on site conditions. The main power block surrounding the PCV or the secondary containment is designed to be the standard area. On the other hand, the circumferential area such as the electrical room, plant make up facilities, etc. are designed as flexible areas.
By this approach of rationalized layout, it is possible to realize that the building volume per unit output power is equivalent to ABWR.
Balance of Plant
The advanced BOP system allows the utilization of produced heat for non-electrical applications such as process heat, mining (oil sand extraction) and desalination.
i. Turbine Generator Building
The structure of turbine systems is simplified by applying the single casing design that uses 41-inch turbine, which is proper for the output level as well as by using single shell condenser and single train (4-stage heating) feed-water heater. The number of equipment is reduced by integrating high- and low-pressure condensate pumps and optimizing the configuration of systems.
ii. Electric Power Systems
The DMS plant is connected to the external grid via a main connection and a standby connection. The main connection is the connection between the generator transformer and the external grid. The standby connection is the connection between the auxiliary standby transformer (AST) and the external grid.
8. Development Milestones
2000-2004 Conceptual design
2014 Basic design (pre-licensing)
2017~ Design review or design certification 2020~ Proposal to customer or commercial bid 2030~ Commercial operation
MAJOR TECHNICAL PARAMETERS
Parameter Value
Technology developer, country of origin
Mitsubishi Heavy Industries, Ltd. (MHI), Japan
Reactor type Integral PWR
Coolant/moderator Light water / light water Thermal/electrical capacity,
MW(t)/MW(e)
1000 / 350 Primary circulation Natural circulation NSSS Operating Pressure
(primary/secondary), MPa
15.5 / 5.0 Core Inlet/Outlet Coolant
Temperature (oC)
329 / 345
Fuel type/assembly array UO2 pellet / 21x21 square Number of fuel assemblies
in the core
97
Fuel enrichment (%) 4.8
Core Discharge Burnup (GWd/ton) > 40 Refuelling Cycle (months) 26
Reactivity control mechanism Control rods drive mechanism Approach to safety systems Hybrid (Passive + Active)
system
Design life (years) 60
Plant footprint (m2) 4900 RPV height/diameter (m) 17 / 6 RPV weight (metric ton) --
Seismic Design (SSE) 0.3g
Fuel cycle requirements / Approach Similar to existing PWR plants Distinguishing features Integral PWR with natural
circulation; employs two types of in-vessel steam generator
Design status Conceptual design completed
1. Introduction
The Integrated Modular Water Reactor (IMR) is a medium sized power reactor with a reference thermal output of 1000 MW(t) producing an electrical output of 350 MW(e). The IMR is developed for potential deployment after 2025. IMR employs the hybrid heat transport system (HHTS), a natural circulation system for the primary heat transport. The in-vessel control rod drive mechanism (CRDM) is the primary means of reactivity control.
These design features allow the elimination of the emergency core cooling system (ECCS).
2. Target Application
The IMR is primarily designed as a land-based modular power station to generate electricity. Because of its modular characteristics, it is suitable for large-scale power stations consisting of several modules and also for small distributed-power stations, especially for small grids. IMR can also be used for cogeneration of electricity and district heating, seawater desalination, process steam production and so forth. IMR adopts structures, systems and components that require no large-scale infrastructure. This facilitates regulatory authority’s approval for the construction and operation of power plant.
3. Main Design Features
Design PhilosophyThe IMR is an integral PWR where the primary system components are installed within the reactor pressure vessel (RPV). Main coolant piping and primary coolant pumps are eliminated by adopting natural circulation