FUEL AND FUEL CYCLE | HTR FUEL temperature SMRs Fuelling high High-Temperature Reactor Fuel Technology is needed for upcoming High-
Temperature Small Modular Reactor projects to replace fossil fuels and to tackle climate change. Reviving HTR fuel technologies that were first developed decades ago opens the door to a new generation of low-carbon industrial heat processes
Dr. Marina Sokcic-Kostic, Principal Engineer; Dr. Georg Brähler, Technology Adviser; Karl Froschauer, Deputy Director Business Development & Sales; and Christopher Reiser, Physicist NUKEM Technologies
Below, figure 1: Top: Structure of TRISO fuel containing spherical fuel elements, Bottom: cylindrical fuel compacts of different lengths (left) and NUKEM’s pebble bed spherical fuel elements (right)
IN THE CURRENT CENTURY, BUILDING up a reliable fleet of High-Temperature Small Modular Reactors (HT-SMRs) could be a key strategy to tackle climate change by replacing fossil fuel generated process heat. Due to the continuously increasing demand for energy in line with the rise of the world-wide average temperature, nuclear power is again being taken under serious consideration as a clean energy source, especially in relation to international efforts in reducing overall CO2
emissions.
As of now, High-Temperature Reactor (HTR) technology is being promoted by several countries and companies, especially due to its unique inherent safety features. These features are based on the reactor concept and the fuel design itself. Accordingly, many of the small modular reactor (SMR) designs that are emerging are also based on the HTR technology.
HT-SMRs The dream of reducing the complexity of a nuclear power plant while increasing its safety is reflected in the concept of the Small Modular Reactor (SMR). The key focus lies on reducing the size of the reactor (compared with established designs). When this requirement is fulfilled, modularity, standardisation and increased design integration can be addressed. A more integrated design results in reduced complexity and number of components. Standardisation includes the ability to deploy the reactor more flexibly with less site and grid restrictions. Nearly all known reactor concepts from the established
ones to Generation IV designs can be designed as SMRs. The International Atomic Energy Agency (IAEA) defines them as reactors of up to 300 MWe per module. Similar considerations apply to microreactors, which cover the power requirements below the SMR range. The Generation IV design, which is meant to be the
5mm fuel free zone
Coated prticles embedded in graphite matrix
Outer pyrolytic carbon layer Silicon carbide layer
Inner pyrolytic carbon layer Porous carbon layer
fuel kernal UO2 coated particle approx. 500µm diameter approx. 920µm diameter TRISO
successor of the high-temperature gas-cooled reactor (HTGR), is the Very-High-Temperature Reactor (VHTR). Both HTGR and VHTR designs use graphite as a moderator and reflector while helium acts as the primary coolant. The main application of the VTHR is synchronous hydrogen and electricity generation. This is enabled by the higher possible outlet temperature: the HTGR reaches temperatures up to 750 °C, while VHTRs are expected to reach about 1000 °C. The comparably high temperature of HT-SMRs opens the door to a variety of chemical processes, which are not feasible at lower temperatures. One example is the production of hydrogen using high-temperature water electrolysis. This allows fossil fuels substitution for process heat and tackles a large source of current CO2
emissions.
However, the technical advantages of HTRs and SMRs are worthless if safety concerns are not addressed. In addition to the inherent safety features of the SMR concept itself, there are HTR-specific advantages. SMR-specific inherent safety features are, generally speaking, based on the scaled- down design with less fissile material and less complexity. Inherent safety features of the HTR design also come
into play. One is the retention of fission products which is already ensured by the strict requirements to the HTR fuel specifications. TRISO fuel has the key feature that all fissile material is encapsulated in layers of durable silicon carbide (SiC) as well as pyrolytic carbon. Most importantly,
22 | August 2023 |
www.neimagazine.com
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