Simulation Used to Spur Design Development for Nuclear Power

Nuclear power is a key player in the future of clean energy, and multiple companies are pursuing new technologies to maximize nuclear’s contribution to the clean energy space. Founded in 2011 and based in Cambridge, MA, Transatomic Power is an advanced nuclear technology startup developing and commercializing a molten salt reactor (MSR), or a nuclear reactor whose fuel is in liquid, rather than solid, form. This technology, originally developed at the Oak Ridge National Laboratory (ORNL) in the 1960’s, offers multiple safety and cost benefits over traditional nuclear reactors, in which the fuel is in the form of solid pellets cooled by water.

Tranatomic’s MSR design builds on the original work at ORNL and adds a few innovative new features that reduce the reactor’s size and, as a result, it’s cost – a huge factor in building new nuclear power plants. Though the development process is a long one, the world needs a larger capacity for clean energy generation, and it’s this ultimate goal that drives the Transatomic team forward.

Nuclear Power Basics

Before outlining the role that ANSYS has played in the Transatomic development process, I’d like to briefly outline some nuclear power basics, as it is a technology with which a lot of people aren’t necessarily familiar. Essentially, nuclear energy depends on atoms of uranium, which absorb neutrons and split apart in a process called fission. This splitting both releases a great deal of energy, which is converted into heat, and more neutrons, which go on to split other uranium atoms. If you can cause enough of these fission reactions in a contained space (a nuclear reactor), you create a self-sustaining chain reaction – or, in nuclear terms, the reactor is critical.

In addition to the uranium, which acts as the reactor’s fuel, there are two other key materials involved in nuclear energy. The first is a coolant, which is a liquid such as water or molten salt that carries the heat energy away from the fissioning uranium to some sort of power cycle (typically a steam turbine). The coolant is extremely important, because without it the reactor would continue heating until the fuel melts (this is the phenomenon called a “meltdown,” which occurred at Three Mile Island and Fukushima).

The second material that many nuclear technologies have is a moderator. After a uranium atom fissions, the neutrons it produces are traveling at very high speeds, and as a result are less effective at causing fission to occur in the uranium isotope most reactors use for fuel. A moderator is a material, such as water or graphite, which helps to slow down the reactor’s neutrons to a speed at which they can cause uranium atoms to fission. Some advanced technologies are attempting to use the original, faster neutrons to cause fission (and there are reasons why this could be possible), but Transatomic’s uses a moderator to slow neutrons down.

Finally, a note on Transatomic’s technology. Because the Transatomic reactor’s fuel is in liquid form, it actually flows through the reactor past a solid moderator as fission occurs. This is essentially the opposite of a traditional reactor, in which water (acting as both coolant and moderator) flows past solid fuel. By flipping the relationship between these materials, Transatomic’s technology both avoids safety issues like meltdowns (because the fuel is already molten) and presents new challenges from a design perspective.

Cooling the Moderator

While developing our technology, Transatomic’s engineers identified numerous areas in which CFD simulations would prove essential, one of which involved the reactor’s moderator. The moderator is in the form of zirconium hydride rods clad in silicon carbide (a ceramic which is both extremely durable and works well in nuclear reactors).

As we developed the design, the team had to ensure that these moderator rods were not overheating from direct exposure to the fissioning fuel. If overheating were to occur, the hydrogen in the zirconium hydride could escape, which would in turn destroy the rod’s ability to moderate neutrons. ANSYS Fluent was used to accurately predict the hottest rod’s temperature profile (below) taking into account an axial heat generation profile and the flowing salt environment.

A visualization of the mesh and geometry used (left) as well as the resulting velocity (middle) and temperature profiles (right)

Overcoming Friction

Another area that Transatomic engineers assessed with ANSYS simulations was frictional loss and flow shaping within the reactor. As the fuel flows through the core, it winds its way around structures and components, and this tortuous path can cause friction that overpowers the fuel’s ability to flow. If this occurred, pumping requirements would increase, which both raises the reactor’s cost and can introduce design integration issues.

To simulate frictional losses in the reactor’s lower plenum, or the bottom part of the core where fuel enters the moderator zone, Transatomic engineers built a 3-D model of one quarter of the plenum. Simulation results allowed us to iterate over different flow shaping considerations, resulting in less frictional loss for the optimized product.

An axial cross-section of the fuel velocity profile in several iterations
of a representative lower plenum.

The ANSYS Startup Program has been essential to completing this work quickly and efficiently at low cost, and Transatomic is in a much stronger position as a result. We look forward to what the future holds!

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