IEEE Power & Energy Magazine - March/April 2022 - 59

designs that use known and tested balance-of-plant components,
such as steam turbines, condensers, generators, switchgear,
protection devices, and control platforms. This modular
design approach can also simplify licensing and regulatory
compliance to help shorten the time to project completion.
The multimodule concept allows for the scaling of an SMR
system to match the demand of a specific application. A multimodule
SMR facility can either operate together to meet
demands greater than the rating of each unit and/or operate in
a staggered or redundant fashion to provide continuity of supply.
As demand grows, additional modules could be added at
the same premises. For example, the NuScale system design
includes a reactor building that can hold up to 12 reactor modules.
By incrementally adding capacity as needed, the up-front
cost and construction time can be optimized, so a return on the
investment can be realized more quickly than for a larger sitebuilt
nuclear power plant with a similar output capacity.
The modular design also makes it possible to load the
nuclear fuel at a factory to produce a sealed transportable unit,
which is sometimes referred to as a " nuclear battery. " In this
case, all handling of radioactive materials would occur in the
controlled, secure environment of the factory. This type of unit
would be deployed for a fixed operating life span. At the end
of that period, the entire reactor system could be removed and
returned to the factory for refurbishment and refueling. Some
designs target a 30-year refueling interval. This feature reduces
the operational complexity, particularly for deployments at
remote sites, and the contamination risk of handling radioactive
materials on site. Also, it reduces the proliferation risk of
nuclear materials being diverted into unauthorized hands.
Another key feature that SMR designs have in common
with the latest fourth-generation large-scale reactor designs
is the use of passive safety systems. In particular, several proposed
designs include a convective primary cooling loop that
eliminates the need for pumps, resulting in a simpler design
and eliminating potential points of failure. SMR cores are also
physically smaller than those of larger reactors, with lower
thermal power ratings and core power densities. In some
designs, the compact cores also place the fuel closer to the exterior
of the reactor vessel. This simplifies the task of decay heat
removal once the reactor is shut down. This can reduce the risk
of a core meltdown. The goal of these passive safety design features
is to create a reactor facility that is walk-away safe even in
the event of a total loss of auxiliary electrical power at the site.
Four major types of SMRs and their key characteristics and
design parameters are shown in Table 1. The integrated pressurized
vessel designs are based on well-established watercooled
technologies. They have reactor output temperature
levels suitable for both seawater desalination or district heating
applications in addition to electricity generation. The gascooled
SMRs, which are typically cooled with helium, offer
even higher temperatures for the process heat. (See " One Module
of an SMR Plant. " ) This opens up more possibilities for
industrial applications, including steam methane reforming,
biomass gasification, and high-temperature steam electrolysis.
march/april 2022
The molten-salt- and liquid-metal-cooled reactors include several
fast reactor designs with the potential to use spent fuels
from other commercial reactors. This contributes to the more
efficient use of nuclear fuel.
The most common balance-of-plant systems for SMRs use
a traditional steam Rankine cycle turbine and synchronous generator
pair, leveraging well-understood and widely available
technologies. However, some designs are based on direct-cycle
helium gas and supercritical CO2 Brayton cycle turbines. These
designs can reduce the size of balance-of-plant components
dramatically, although with tradeoffs in cost, working pressure,
and potentially shaft speed, necessitating the use of a gearbox
in some cases.
In remote communities and industrial sites, the cost of diesel
fuel-generated electricity can reach over US$.50/kWh due
to their isolation and dependence on seasonal roads or water
access for delivery. Given such high cost, and the CO2, NOx,
and particulate emissions from combustion, these applications
could represent a viable market for SMRs. They have been
identified as a key target of deployment by the Canadian government
in a recently released SMR roadmap. However, the
electrical power demands for remote communities are modest,
typically between 2 and 10 MWe, so only designs on the low
power end of the SMR scale would be applicable. These small
SMRs are sometimes referred to as micromodular reactors,
micro-SMRs, or very small modular reactors.
For remote industrial applications, the power requirements
can be significantly higher than for remote communities, ranging
from 4 to 125 MWe (though typically 25-30 MWe) for
mining, and 300+ MWe for oil and gas extraction and processing.
In addition to electrical power, these applications also need
high-temperature process heat. For mining sites, the nuclear
battery concept is particularly attractive since the deployment
is meant to be limited to the life span of the project, after which
the infrastructure can be " picked up " and the site remediated.
Another important consideration for both industrial and remote
sites is the lack of available water for cooling in some locations.
This factor favors SMRs that can potentially use air cooling
instead of relying on lake, river, or ocean water.
There are over 50 different SMR designs at various stages of
development worldwide, according to the International Atomic
Energy Agency. The earliest planned commercial deployment
is targeted for 2026. Given the number and variety of these
designs, there remains a significant amount of work in modeling,
instrumentation, and control to effectively optimize the
designs to complement modern power systems. This includes
microgrid applications with renewable energy resources.
Applications of SMRs in Microgrids
In all power systems, the electricity supply must match the
load demand at all times to achieve stable operation. However,
this is a particular challenge in standalone renewable
microgrids due to the intermittent nature of wind and solar
resources, the lack of rotating inertia in inverter-interfaced
sources, and the large load variations (i.e., large with
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