MAJOR TECHNICAL PARAMETERS
Parameter Value
Technology developer, country of origin
IRIS, International Consortium
Reactor type Integral PWR
Coolant/moderator Light water / light water Thermal/electrical capacity,
MW(t)/MW(e)
1000 / 335 Primary circulation Forced circulation NSSS Operating Pressure
(primary/secondary), MPa
15.5 / 5.8 Core Inlet/Outlet Coolant
Temperature (oC)
292 / 330
Fuel type/assembly array UO2/MOX/17x17 square Number of fuel assemblies in
the core
89 Fuel enrichment (%) 4.95 Core Discharge Burnup
(GWd/ton)
65 (max) Refuelling Cycle (months) 48 (max)
Reactivity control mechanism ICRDM (Internal Control Rod Drive Mechanism)
Approach to safety systems Passive
Design life (years) 60
Plant footprint (m2) 14 000 (four-unit layout) RPV height/diameter (m) 21.3 / 6.2
RPV weight (metric ton) 1045 Seismic Design (SSE) 0.3g Fuel cycle requirements /
Approach
3-year cycle with half-core reload Distinguishing features Integral primary system configuration
Design status Basic design
1. Introduction
IRIS is a LWR with a modular, integral primary system configuration. The concept was originally pursued by an international group of organizations led by Westinghouse. Current IRIS related activities, especially those devoted to large scale integral testing, are being pursued by Italian organisations (ENEA, SIET, CIRTEN). Its principle characteristics are:
- A medium power of 335 MW(e) per module;
- A simplified compact design where the primary vessel houses the steam generators, pressurizer and pumps;
- An effective safety approach of active and passive safety systems; optimized maintenance with intervals of at least four years.
2. Target Application
The primary application of the IRIS design is electricity production. However, this integrated PWR can support heat production and seawater desalination options. Coupling with renewable energy parks and energy storage systems have been addressed as well.
3. Main Design Features
Design PhilosophyIRIS is designed to provide enhanced safety, improved economics, proliferation resistance and waste minimization.
Nuclear Steam Supply System
All the major NSSS equipment i.e. reactor coolant pumps, steam generators and pressurizer are located inside the RPV, resulting in a more compact configuration and elimination of the large loss-of-coolant accident.
Reactor Core
The IRIS core is an evolutionary design based on conventional UO2 fuel enriched to 4.95%. An IRIS fuel assembly consists of 264 fuel rods with a 0.374 in. outer diameter in a 17×17 square array. The central position is reserved for in-core instrumentation, and 24 positions have guide thimbles for control rods. Low-power density is achieved by employing a core configuration consisting of 89 fuel assemblies with a 14-ft (4.267 m) active fuel height, and a nominal thermal power of 1000 MW. The core is designed for a 3–3.5-year cycle with half-core reload to optimize overall fuel economics while maximizing the discharge burnup. In addition, a 4- year straight burn fuel cycle can also be implemented to improve the overall plant availability, but at the expense of a somewhat reduced discharge burnup.
Reactivity Control
Reactivity control in IRIS is achieved through solid burnable absorbers, control rods, and the use of a limited amount of soluble boron in the reactor coolant. The reduced use of soluble boron makes the moderator temperature coefficient more negative, thus increasing inherent safety. Control rod drive mechanisms (CRDMs) are located inside the vessel, in the region above the core and surrounded by the steam generators.
Their advantages are in safety and operation. Safety-wise, the uncontrolled rod ejection accident is eliminated because there is no potential 2000-psi differential pressure to drive out the CRDM extension shafts. Operation- wise, the absence of CRDM nozzle penetrations in the upper head eliminates all the operational problems related with corrosion cracking of nozzle welds and seals.
Reactor Pressure Vessel and Internals
The IRIS reactor vessel (RV) houses not only the nuclear fuel and control rods, but also all the major reactor coolant system components: eight small, spool type, reactor coolant pumps; eight modular, helical coil, once through steam generators; a pressurizer located in the RV upper head; the control rod drive mechanisms; and, a steel reflector which surrounds the core and improves neutron economy, as well as it provides additional internal shielding. This integral RV arrangement eliminates individual component pressure vessels and large connecting loop piping between them, resulting in a more compact configuration and in the elimination of the large loss-of-coolant accident as a design basis event. It has an internal diameter of 6.21 m and an overall height of 22.2 m including the closure head.
Reactor Coolant System
The integral reactor coolant system of IRIS houses 8 helical-coil steam generators and 8 spool type primary coolant pumps. The motor and pump consist of two concentric cylinders, where the outer ring is the stationary stator and the inner ring is the rotor that carries high specific speed pump impellers. The spool type pump is located entirely within the reactor vessel, with only small penetrations for electrical power cables and for water cooling supply and return. Water flows upwards through the core and then through the riser region. At the top of the riser, coolant is directed into the upper part of the annular plenum between the extended core barrel and the RV inside wall, where the suction of the reactor coolant pumps is located. The flow from each pump is directed downward through its associated helical coil steam generator module. The primary flow path continues down through the annular downcomer region outside the core to the lower plenum and then back to the core completing the circuit.
Steam Generator
The IRIS adopts once-through steam generators (OTSGs) with helical-coil tube bundle design with the primary fluid outside the tubes. Eight (8) OTSG modules are installed in the annular space between the core barrel and the RV. Each IRIS OTSG module consists of a central inner column which supports the tubes, the lower feed water header and the upper steam header. The enveloping outer diameter of the tube bundle is 1.64 m. Each OTSG has 656 tubes. The tubes are connected to the vertical sides of the lower feedwater header and the upper steam header. The SG is supported from the RV wall and the headers are bolted to the vessel from the inside of the feed inlet and steam outlet pipes. The steam and feed lines as well as the emergency heat removal system (EHRS) are designed for the full primary pressure of 15.5 MPa. The EHRS does not inject water, but only removes heat from the reactor via the SGs.
Pressurizer
The IRIS pressurizer is integrated into the upper head of the reactor vessel. The pressurizer region is defined by an insulated, inverted top-hat structure that divides the circulating reactor coolant flow path from the saturated pressurizer water. This structure includes a closed cell insulation to minimize the heat transfer between the hotter pressurizer fluid and the subcooled primary water. Annular heater rods are located in the bottom portion of the inverted top-hat which contains holes to allow water insurge and outsurge to/from the pressurizer region. These surge holes are located just below the heater rods so that insurge fluid flows up along the heater elements. By utilizing the upper head region of the reactor vessel, the IRIS pressurizer provides a
very large water and steam volume, as compared to plants with a traditional, separate, pressurizer vessel. The IRIS pressurizer has a total volume of ∼71 m3, which includes a steam volume of 49 m3.
4. Safety Features
IRIS adopts passive safety systems and the safety by design philosophy including the risk informed approach.
Due to the integral configuration, by design, with no intervention of safety systems, a variety of accidents either are eliminated or their consequences and/or probability are greatly reduced. In fact, 88% of class IV accidents are either eliminated or downgraded. The auxiliary building is fully seismically isolated. This provides a high level of defence in depth (DID) that may allow IRIS to claim no need for an emergency response zone. The IRIS pressure suppression containment vessel has a spherical configuration and is 25 m in diameter. In case of small break loss of coolant accident (SB LOCA), the RPV and containment become thermodynamically coupled. The pressure differential across the break equalizes quickly and LOCA is stopped. The core remains covered for all postulated breaks during the whole transient. The heat sink is designed to provide cooling for 7 days without operator action or off-site assistance for replenishing.
Engineered Safety System Approach and Configuration
IRIS has passive EHRS made of four independent subsystems, each of which has a horizontal, U-tube heat exchanger connected to a separate SG feed/steam line. These heat exchangers are immersed in the refuelling water storage tank (RWST) located outside the containment structure. The RWST water provides the heat sink to the environment for the EHRS heat exchangers. The EHRS is sized so that a single subsystem can provide core decay heat removal in the case of a loss of secondary system heat removal capability. The EHRS operates in natural circulation, removing heat from the primary system through the steam generators heat transfer surface, condensing the steam produced in the EHRS heat exchanger, transferring the heat to the RWST water, and returning the condensate back to the SG. The EHRS provides both the main post-LOCA depressurization of the primary system and the core cooling functions. It performs these functions by condensing the steam produced by the core directly inside the reactor vessel. This minimizes the break flow and actually reverses it for a portion of the LOCA response, while transferring the decay heat to the environment. The safety strategy of IRIS provides a diverse means of core shutdown by makeup of borated water from the emergency boration tanks (EBT) in addition to the control rods; also, the EHRS provides a means of core cooling and heat removal to the environment in the event that normally available active systems are not available. In the event of a significant loss of primary-side water inventory, the primary line of defence for IRIS is represented by the large coolant inventory in the reactor vessel and the fact that EHRS operation limits the loss of mass, thus maintaining a sufficient inventory in the primary system and guaranteeing that the core will remain covered for all postulated events.
Containment System
IRIS integral RV configuration eliminates the loop piping and the externally located steam generators, pumps and pressurizer with their individual vessels. Hence, the footprint of the containment is greatly reduced. This size reduction, combined with the spherical geometry, results in a design pressure capability at least three times higher than a typical loop reactor cylindrical containment. The current layout features a spherical, steel containment vessel (CV) that is 25 m (82 ft.) in diameter. The CV is constructed of 1-¾ in. steel plate and has a design pressure capability of 1.4 MPa (∼190 psig). The pressure suppression pool limits the containment peak pressure to well below the CV design pressure. The suppression pool water is elevated such that it provides a potential source of elevated gravity driven makeup water to the RV. Also shown is the RV flood- up cavity formed by the containment internal structure. The flood-up level is 9 m and ensures that the lower section of the RV, where the core is located, is surrounded by water following any postulated accident. The water flood-up height is sufficient to provide long-term gravity makeup, so that the RV water inventory is maintained above the core for an indefinite period of time. It also provides sufficient heat removal from the external RV surface to prevent any vessel failure following beyond design basis scenarios.
AUX. T.B.
BLDG.
Main Steam Line (1 of 4) Isolation Valves
Main Feed Line (1 of 4) Isolation Valves SG
Make up Tank
P/H P/H
P/H P/H
EHRS
Heat Exchanger Refueling Water Storage Tank (1 of 1)
Start Up FeedWater Steam Generator
(1 of 8)
FO FO
Suppression Pool (1 0f 6)
ADS/PORV (1 of 1)
Long Term Core Makeup from RV Cavity
(1 of 2)
RCP (1 of 8)
SG Steam Lines (2 of 8)
SG Feed Water Lines
(2 of 8)
FO FO
Safety Valve
Safety Valve
RV Cavity Suppression
Pool Gas Space
Integral Reactor Vessel
Emergency Heat Removal System (EHRS) 1 of 4 Subsystems
DVI EBT (1 0f 2)
5. Plant Safety and Operational Performances
The IRIS design provides multiple levels of defence for accident mitigation, resulting in extremely low core damage probabilities. In addition to the traditional DID levels (barriers, redundancy, diversity, etc.) IRIS introduces a very basic level of DID, i.e. elimination by design of accident initiators or reduction of their consequences/probability. Even though the reference design features a two-batch, 3-year fuel cycle, IRIS is capable of eventually operating in straight burn with a core lifetime of up to 8 years. IRIS has been designed to extend the need for scheduled maintenance outages to at least 48 months. With the 4-year maintenance cycle, the capacity factor of IRIS is expected to exceed the 95% target, and personnel requirements are expected to be significantly reduced. Both considerations will result in decreased O&M costs.
6. Instrumentation and Control Systems
The instrumentation systems and components for IRIS are in principle similar to those for Generation III PWRs. Innovative solutions have been developed specifically to address: level measurement in the RPV upper head volume (pressurizer), primary flow rate measurement in annular space geometries, primary fluid temperature measurements at the outlet of steam generator modules. For safety purposes IRIS eliminated all the lower head RPV penetrations for instrumentation guide tubes.
The control system envisaged for IRIS is based on an Autonomous and Hierarchical Control Functional Architecture, adopting Model-predictive, Multivariate robust and Fault-tolerant Controllers. Moreover, a Multi-modular IRIS Control and Operational Reconfiguration for a Flow Control Loop has been developed.
That approach was used both to control a multi-module IRIS NPP as well as to monitor and control a hybrid system, adopting IRIS units as electricity and heat supply, to feed both the grid and cogeneration units, like desalination plants.
7. Plant Layout Arrangement
Almost half of the IRIS containment vessel is located below ground, thus leaving only about 15 m above the ground (i.e., several times less than the containment of a large LWR). This very low profile makes IRIS an extremely difficult target for aircraft flying terrorists; in addition, the IRIS containment is inconspicuously housed in and protected by the reactor building. The cost of putting the entire reactor underground was evaluated; it was judged to be prohibitive for a competitive entry to the power market and unnecessary since the IRIS design characteristics are such to offer both an economic and very effective approach to this problem.
8. Design and Licensing Status
The IRIS team has completed the design of the large-scale test facility, currently under construction, to prepare for future design certification. R&D activities in the field of design economics, financial risk and SMR competitiveness are under way.
9. Fuel Cycle Approach
IRIS longer-term objective is to further enhance its economic and proliferation resistance characteristics by extending the reloading interval to 4 years and beyond. Therefore, a multiprong approach is adopted including a range of fuel options:
1. rely on proven and licensed fuel technology to enable the near-term deployment objective;
2. perform research on advanced core designs with higher discharge burnup and longer cycle for longer-term deployment;
3. additionally, different needs and preferences of different countries should be addressed, such as emphasis on proliferation resistance, or use of MOX or thorium fuel.
10. Waste Management and Disposal Plan
Waste management and disposal plan are similar to those adopted by in Generation III PWR reactors. About decommissioning, systematic dose reduction to personnel in decontamination and decommissioning (D&D) activities was planned. A specific feature of IRIS is the radial water layer of 1.7 m between the edge of the core and the RV. This natural shielding decreases the fast neutron fluence on the RV by a factor of 105, essentially eliminating vessel embrittlement and the need for surveillance coupons and for periodic in-service inspection of the RV. Hence, reduced vessel activation, which simplifies D&D, with potential to dispose of significant portion of the vessel as nonradioactive materials.
11. Development Milestones
2001 Conceptual design completion 2001 Preliminary design start-up 2002 Pre-licensing process activities 2009 Integral testing facility construction
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