Micro Modular Reactor moves forward in Canada

30 October 2019



The first example of Ultra Safe Nuclear Corporation’s advanced micro modular reactor (MMR) technology is getting closer to becoming reality as an initial unit remains on track for construction at Chalk River Laboratories, Ontario, Canada. Francesco Venneri introduces the design.


ULTRA SAFE NUCLEAR CORPORATION (USNC) proposes to construct and operate a 15MW thermal small modular reactor (SMR) using micro modular reactor (MMR) technology, with its partners Global First Power and Ontario Power Generation (OPG). This consortium is the first in the Canadian Nuclear Laboratories SMR programme to advance to reach the commercial arrangements stage of a thorough review process (see box, p28).

Seattle-based USNC’s strategy is to make nuclear power usable by as many people, and in as many places, as possible, by developing MMRs that can be mass-produced, while enhancing nuclear security and nonproliferation. Mass production will lead to cost reduction.

It will start at the Chalk River Laboratories site in Ontario, Canada.

Improved safety requires better fuel

Big nuclear accidents happen when a reactor’s fuel gets damaged. This occurs if the circuit is unable to remove sufficient heat from the reactor core and the temperature of the fuel becomes too high. The fuel can degrade, melt or even vaporise, and radioactive fission products are released.

Traditional nuclear reactors operate at very high power densities and use fuel that can be easily damaged. So they operate close to failure limits — too close. Designers have to engineer elaborate safety systems to prevent or contain fuel melting. These systems aim to cool or contain the reactor during an accident and usually involve intricate networks of piping, water reservoirs and pumps, as well as containment structures. These massive and expensive additions increase the cost and complexity without assuring fundamental safety.

In the USNC reactor proposed at Chalk River, the objective of safety under all circumstances is achieved through the use of extremely robust fuel and low power density, rather than additional external countermeasures. It is a bottom-up approach that ensures the reactor cannot damage itself. The safety mechanism is within the fuel packaging, instead of the intricate external emergency systems. This is a safer and far simpler approach.

The fuel packaging starts with a uranium fuel particle measuring less than 1mm across. The particle is coated with layers designed like tiny pressure vessels. The layers contain the fission products and ensure mechanical and chemical stability during irradiation and temperature changes. This is called a Triso particle. Developed in the 1960s for gas-cooled reactors, Triso has been subject to continued development, including extensive support from the US DOE and national laboratories in advanced gas-cooled reactor and other programmes.

USNC’s breakthrough is encasing the Triso particles in a dense ceramic matrix, called fully ceramic microencapsulated (FCM) fuel. The ceramic matrix provides extremely strong structural support to the Triso particles that will protect them for the duration of power production, during accidents and into long-term storage.

Creating this matrix is challenging because the ceramic’s melting temperature is greater than the Triso particle’s damage threshold. Using earlier research conducted in the US national laboratories, Ultra Safe has developed and patented a technique for ceramic microencapsulation of Triso particles through proprietary processing and additive manufacturing.

Normal fuel will fracture and leak, releasing radioactive fission products. But FCM fuel is uniquely stable under irradiation and capable of withstanding temperatures well in excess of all postulated accident conditions, ensuring total containment of radioactivity.

FCM fuel pellets are arranged into pellet stacks, and these stacks are placed inside graphite blocks. Graphite is the moderator, slowing down neutrons so they cause fission. Cooling channels are built into the graphite blocks with cold helium flowing through them to pick up the heat.

The reactor core, inside the MMR vessel, consists of approximately 200 such fuel blocks — roughly two tons of fuel. Built-in openings and channels are used for control rods. The reactor is physically small and has very low power density. It can withstand accidents without emergency cooling systems because the fuel remains intact and safe even at very high temperatures. The reactor core is sensitive to temperature. As the temperature rises, its reactivity decreases. This is an inherent physical property of the core that ensures that heat production is stable at all times. Extracting heat The MMR energy system is separated into two parts: the nuclear plant and the adjacent plant. A typical MMR energy system will have two reactors, each producing 15MWt, with a power density of 1.24MW/m3. A molten salt loop (40% KNO3 + 60% NaNO3) connects the two plants. For sites requiring more energy, multiple MMR reactors can be linked.

Control rods are inserted from the top of the reactor. The primary control of power production is provided by the helium circulator, which forces cold helium through the reactor where it removes heat from the core.

More or less power can be produced at constant temperature by changing the helium flow through the circulator. The reactor stabilises to a safe equilibrium in all conditions as helium extracts heat from the core. The helium is transported into and out of the reactor through a double-walled tube with cold helium on the outside and hot helium (630°C) from the reactor on the inside.

Heat is transferred from the helium to the molten salt by a heat exchanger, which sits in a vessel below the helium circulator. The hot molten salt travels across the nuclear boundary to the adjacent plant where the heat is stored and used.

Hot molten salt can be transported from multiple reactors to a heat reservoir, as in concentrated solar power plants. Hot molten salt can be used to generate steam to produce electricity via a turbine and generator. It can also be used for process heat and district-heating applications. Exhausted steam is sent to the air-cooled condensers and cold water returns to the steam generator.

After 20 years, the spent core can be removed and stored in a spent fuel casket. If the site is to continue operating, a fresh core cartridge can be installed. Decommissioning the plant requires complete removal of the entire power plant.

Initial market uses

Of developed countries, Canada has the largest undeveloped land mass, offering a perfect opportunity to open sites for development in an environmentally-friendly way. Energy demand is increasing in off-grid mining and remote communities, with aggressive growth expected over the next 10 years. By 2030 4-5GWe of new capacity will be needed at 200 mines and 280 remote communities in Northern Canada alone. Off-grid power in Northern Canada requires reliability and versatility. Demand tends to be diurnal, seasonal and slowly varying, but the harsh environment means loss of power can be life threatening. Communities have high residential and commercial power needs and heat is just as important as electricity, especially in winter.

These sites are too remote to be connected to the grid and are currently powered using transported fossil fuel that is expensive and damaging to the local environment. They generally involve an ageing fleet of diesel generators,
usually multiple redundant units in the 1-5MWe class. Diesel is supplied by truck or ship when conditions are favourable, and by plane when they are not. As a result, prices fluctuate widely and are high, between 50¢ and 100¢/kWh (typical US cost is 10¢/kWh) during 2014. In mining, this cuts into profitability and prevents expansion. The cost of power can make up about half of a mining project’s budget.

Mining operates 24/7 with little seasonal variation in power consumption. Power needs include ventilation, heating and cooling, drilling machines, hoists, conveyors, shovels, heavy and light trucks, mobility vehicles, crushing, mills, cyclones, dryers, pumps and smelters. Loads can swing wildly, and power generators must match demand.

There are similar off-grid markets in Africa, the Middle East, and South-East Asia. Other use cases include applications that require extreme reliability and grid independence such as campus, airport or military base power.

Once the USNC concept is proven at Chalk River, its MMRs will be available to off-grid mines and remote communities in Northern Canada. Reliable, carbon-free energy and process heat from MMRs is an economical and ecologicallysound solution for remote mines and their surrounding communities.

Next steps

USNC and its partners have submitted an application for a Licence to Prepare Site at Chalk River. The process that begins three years ahead of power-up and involves analysis of environmental data and discussions with local communities. A licensing decision from the Canadian Nuclear Safety Commission (CNSC) could be made in 2022, with construction beginning shortly thereafter.

There is also potential for the reactor outside of Canada. In 2018, USNC was invited by the UK Department for Business, Energy & Industrial Strategy (BEIS) to participate in the Advanced Modular Reactor (AMR) Feasibility and Development Project. The project focuses on the application of MMRs as a nuclear cogeneration system for multipurpose applications, including hydrogen production, which the UK is considering as a replacement for natural gas. USNC is in the process of evaluating how its technology can help satisfy the UK’s zero-carbon energy goals.


Author information: Francesco Venneri is CEO of Ultra Safe Nuclear Corporation
 



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