Reactor design: SMRs
The Westinghouse SMR20 April 2012
Due to its compact size, around 25 of Westinghouse’s Small Modular Reactor containment vessels could fit within the envelope of an AP1000 containment building. By Robert Fetterman, Matthew Smith, Alexander Harkness and Creed Taylor
Westinghouse Electric Company recently announced its intention to apply to the US Department of Energy (DOE) for funding to develop its 225 MW Small Modular Reactor for deployment in the United States.
Through a new programme announced in January 2012, DOE could share up to half of the engineering, design, certification and licensing costs for up to two US SMR designs. Funding of $452 million could be available over five years, subject to Congressional approval. DOE plans to consider SMR designs that incorporate passive safety features and that could begin commercial operation by 2022. Westinghouse will apply for DOE’s small modular reactor investment funds with a consortium of unnamed utilities.
In January, Westinghouse chief technology officer and senior vice president of research and technology Kate Jackson said:?“Access to this investment fund helps lower the barrier to market entry for American companies. Virtually all energy sources that feed the national grid have been developed through public investments in public-private research and development partnerships. Westinghouse is well-positioned to be the first to market with the most economic small modular reactor.”
Other contenders for the funding could include the B&W mPower reactor, Holtec International’s HI-SMUR, the Hyperion reactor and the NuScale multi-application small reactor.
The Westinghouse SMR is a light-water moderated reactor that uses control rods for load follow and plant shutdown. The integral reactor, protected by redundant passive safety systems, is housed within a compact containment vessel that is located completely below grade. The design uses many of the key features from the Westinghouse AP1000 plant, including passive safety systems.
The main reactor data for the Westinghouse SMR are listed in Table 1. The design of the main systems including the reactor core, steam generators, containment, and safety systems, is discussed in detail below.
Utilizing an integral pressurized water reactor (iPWR), the Westinghouse SMR eliminates a complete class of potential accidents typically associated with traditional nuclear steam supply systems which use large-bore piping to connect the system components (Figure 1). All of the components associated with the reactor coolant system are contained within a single pressure vessel. The maximum diameter of the reactor is held to less than 12 feet to ensure that it can be shipped via standard rail package. The reactor is an ASME Division 1, Section III, Subsection NB pressure vessel with a design pressure and temperature of 2500 psig (17.2 MPa) and 650°F (343°C).
To achieve a thermal power of 800 MWt, the core is made up of 89 17x17 Robust Fuel Assemblies (RFA) with an active length of 8 ft (2.4 m). The core sits near the bottom of the reactor vessel while the steam generator with an integral pressurizer is located above the reactor vessel flange. A bolted flange near the centre of the integral reactor assembly allows for the steam generator-pressurizer assembly to be removed, allowing access to the fuel during a refuelling outage. Water level in the reactor is controlled within the pressurizer, as in all Westinghouse PWRs, using sprays and heaters. The steam generator is a straight tube configuration with the primary reactor coolant passing through the inside of the tubes and the secondary coolant on the outside. Eight axial flow seal-less pumps mounted to the shell of the reactor vessel just below the closure flange provide reactor coolant flow through the fuel assemblies necessary to operate the plant. The upper internals of the reactor support 37 control rod drive mechanisms (CRDMs) used to control reactivity (Figure 3).
The CRDMs are a high-temperature and -pressure version of the recently developed AP1000 CRDM, which has already been tested to eight million steps, to be used within the pressure boundary of the SMR integral reactor. Elimination of control rod penetrations through the reactor pressure boundary has resulted in reduced cost of manufacture and the elimination of the normally-postulated rod ejection event.
For compactness, Westinghouse has chosen to employ an evolution of a straight-tube steam generator with a steam separating drum located outside of the containment vessel. In this design, primary reactor coolant flow is directed vertically downward through the inside of the tubes where heat is transferred to the secondary fluid. Secondary flow enters the steam generator shell as a subcooled liquid and exits as a saturated steam mixture, where it is directed to the steam drum for moisture separation. The moisture separation equipment in the drum was selected and sized to produce dry steam conditions, = 99.9% quality, for input to the turbine for power generation. Operating conditions were selected by design to operate predominantly within the efficient nucleate boiling heat transfer regime. For example, the steam drum provides a mixing volume where recirculating liquid preheats the feedwater, reducing the sensible heat addition required for onset of nucleate boiling (ONB), resulting in a more compact design. During normal full-power conditions, a secondary recirculation pump is used for head addition to offset piping losses; during low power conditions, the recirculation pump can be bypassed and steam drum elevation head is adequate to ensure natural circulation.
Key features of the Westinghouse SMR steam generator and of the recently-developed drum design include:
- Alloy 690 tubes with an outer diameter of 0.625 inch (15.8 mm) on triangular pitch
- 25-foot active heated tube length
- Tube supports to reduce tube flow-induced vibration
- Flow baffles for secondary flow distribution
- Large secondary liquid volume for decay heat removal in most accident scenarios
- Proven best-in-class moisture separation equipment
- Secondary blowdown provisions for chemistry control
- Secondary loose parts trap.
The Westinghouse SMR containment vessel is compact (Figure 4). Only 32 feet in diameter, approximately 25 of these vessels will fit within the containment vessel of AP1000. The compact design significantly reduces the cost of the vessel, facilitates modular construction and increases the allowable design pressure. Designed to be submerged in water during normal operation, the containment also withstands relatively high internal and external pressure loadings. The containment vessel is an ASME Division I, Section III, Subsection NE MC Vessel.
The Westinghouse SMR nuclear island contains all systems that are critical to safety. Systems required to bring the plant into safe shutdown condition are located below grade. Physical separation between redundant safety related divisions is incorporated into the design. Above-grade floors will house equipment that is not subject to radionuclide releases. The spent fuel pool is located below grade to protect against external threats.
Balance-of-plant systems such as the turbine-generator set, condenser and feedwater heaters are located on the turbine island. No safety-related systems or components are housed within the turbine island.
For simplification of design and ease of licensing, Westinghouse has chosen to power its SMR with a derivative of the successful 17x17 Robust Fuel Assembly (RFA) design. This fuel assembly design consists of a square lattice of 264 fuel rods with an outer diameter of 0.374 inches (9mm), 24 guide thimble tubes and a central instrument tube.
This design was chosen as the basis for the SMR fuel assembly due to its significant operating experience and excellent performance. Over 14,000 assemblies have operated in 252 fuel cycles at 50 plants worldwide since 1997. Fuel assemblies have operated with lead rod burnups closely approaching the USNRC licensed lead rod burnup limit of 62 GWD/MTU and lead test assemblies have operated to approximately 70 GWD/MTU. This operating experience has been beneficial in the development of the AP1000 PWR fuel assembly as well as in the SMR fuel assembly design. Key features of the Westinghouse SMR fuel assembly design include:
- 8-foot active core length
- Optimized ZIRLO fuel rod cladding
- Alloy 718 top and bottom grids
- Optimized top grid spring designs to reduce rod bow
- Thick-walled ZIRLO thimble tubes with tube-in-tube dashpots for enhanced dimensional stability and improved resistance to assembly bow
- Removable top nozzles
- Debris filter bottom nozzles, protective grids and oxide-coated cladding for resistance to debris-related failures.
The size of the reactor core is an important design consideration in the Westinghouse SMR. The diameter of the core is directly related to the radial size of the reactor pressure vessel, which itself is limited by the requirement to ship the vessel by rail car. The height of the core directly influences the length of the control rods and their drives and the reactor internals; each foot of core height adds three feet of height to the reactor pressure vessel.
Another important consideration in the size of the reactor core is the customer expectation for 24-month operating cycles. A total of 89 fuel assemblies can fit within the SMR pressure vessel, with room remaining for a heavy radial reflector. Given this maximum core radial size, the active core height was then selected as eight feet for total core uranium loading of approximately 27 MTU. This core size supports the 24-month cycle energy requirement with a reasonable number of feed assemblies each cycle, while also maintaining a relatively modest core height. A typical loading pattern feeds 36 fuel assemblies each cycle at 4.95 % by weight U-235, providing a cycle energy output of 700 effective full power days at 800 MWt.
The design of the Westinghouse SMR enables inherently safe power operation while also allowing for a safe transition from normal operating conditions to a passive shutdown condition. The design includes the three main barriers of protection of traditional PWRs (fuel cladding, RCS pressure boundary and containment) with the added benefit of an external pool of water to filter out radionuclides that might escape from the containment pressure vessel. Additionally, the Westinghouse SMR’s underground placement reduces the likelihood of external events affecting the safety of the plant.
The integral design of the RCS contains no large-bore piping, which significantly reduces the flow area of postulated loss of coolant accidents. The use of CRDMs internal to the pressure boundary eliminates the possibility that the ejection of a control rod will occur. The pump-driven RCS flow during power operation results in a large thermal margin of safety that is predictable to a high level of confidence. The vertical arrangement of the plant allows for a safe transition to natural circulation in the event of a disruption to the forced reactor coolant flow. Also, the vertical arrangement of the plant inherently places the majority of the RCS water directly above the core for use in cooling of the reactor during an event.
During an event, the Westinghouse SMR relies on the natural forces of gravity and convection to shut down and maintain the plant in a safe condition. The protection system of the Westinghouse SMR will diagnose that reactor protection is necessary and send a signal to unlatch the control rods. The control rods will fall by gravity to shut down the nuclear reaction, just like in the AP1000 plant. In the unlikely event that the control rods do not fall into the core or an event occurs while at a shutdown condition, diverse shutdown will be performed through the gravity-fed injection of highly borated water from the core makeup tanks.
The RCS pressure boundary of the Westinghouse SMR will be maintained through the use of self-actuating, spring-loaded pressurizer safety valves that will automatically open to limit any RCS pressure increase.
The large water inventory of the steam drum is automatically available for decay heat removal during most accident scenarios; however feedwater flow can also be isolated from the steam generator tubes and containment if a break occurs on the secondary side of the steam generator. If available, natural circulation from the steam drum to the steam generator tubes and back again to the steam drum provides residual heat removal via steam dump. The capability to add additional inventory to the steam drum and the capability to pump flow to the steam generator tubes are also available; however, neither are required to demonstrate that a safe shutdown condition is achieved for all design-basis scenarios. An additional benefit to the large water inventory in the steam drum is improved water level control and extended response time to level transients.
If steam generator heat removal is unavailable or inadequate, multiple independent residual heat removal heat exchangers are automatically actuated by the protection system. Through the automatic opening of a single valve associated with each heat exchanger, natural circulation of RCS water through the heat exchanger is established to provide residual heat removal. The inlet flow of the cooling side of the heat exchanger is cooled via natural circulation by the plant’s ultimate heat sink tanks. The configuration and heat transfer capacity of the heat exchangers addresses design-basis accident considerations related to the availability of the equipment. The capability to pump flow through the heat exchangers in the loop connected to the RCS and the cooling loop is also available; however, pumped flow is not required to achieve a safe shutdown condition for all design-basis scenarios.
An additional layer of residual heat removal capability is available through the containment wall to the external pool of water surrounding the containment vessel. Filling of the external pool of water is accomplished automatically via a connection to the ultimate heat sink tanks. In total, there is sufficient water inventory available to remove residual heat for a minimum of seven days without the need for offsite AC power sources. Connections are available to add additional water to indefinitely cool the plant.
The RCS water volume, which is large in proportion to the thermal power level, provides an increased energy sink during anticipated operating occurrences and postulated events. Should additional inventory be required during a postulated loss-of-coolant accident, multiple tanks of water will automatically inject additional inventory. In the unlikely scenario that additional inventory is required and the RCS is at an elevated pressure relative to containment, the RCS will be automatically depressurized to equalize the pressures. Once equalized, flow from the in-containment pool will automatically inject into the RCS to provide additional inventory.
Since the outside of the containment wall is cooled with water, the steam from the break will condense along the inside of the containment wall and collect in the containment sump. A path from the containment sump to the in-containment pool and into the RCS can be automatically opened to allow water collecting in the sump to re-enter the RCS. This establishes an internal natural convection and recirculation loop to provide liquid inventory to the RCS that will continue as long as the outside wall of containment continues to be cooled.
In an unlikely postulated severe accident such as core melt, the reactor vessel wall will be cooled by the water in the shield tank, which can be manually actuated to cool the outside of the reactor vessel. The cooling of the reactor vessel wall is sufficient to prevent molten core debris in the lower head from melting the steel vessel wall and spilling into the containment.
This article is based on a paper presented at the ASME 2011 Small Modular Reactors Symposium SMR2011 September 28-30, 2011, Washington, DC, USA. Robert Fetterman, Matthew Smith, Alexander Harkness, Westinghouse Electric Company Cranberry Township, PA, USA. Creed Taylor, Westinghouse Electric Company Chattanooga, TN, U.S.A. The authors would like to thank Ed Cummins, Stephanie Harsche and the SMR design team for their support.
This article was published in the March 2012 issue of Nuclear Engineering International magazine.Related ArticlesWestinghouse forms SMR alliance with utilities Westinghouse signs up Ameren for SMR bid
|The SMR's market advantages|
The Westinghouse SMR will provide a practical non-greenhouse gas producing replacement option for the USA's ageing fossil fueled generation capacity. The reactor's size (225 MWe) will allow for the use of the infrastructure associated with existing non-nuclear generating stations, providing a nuclear option for non-nuclear utilities. The small capacity also provides an option for small markets such as national laboratories and military installations.