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KAIST research team proposes micro modular nuclear reactor cooled by supercritical CO2

A KAIST (Korea Advanced Institute of Science and Technology) research team has proposed a micro modular nuclear reactor cooled by a supercritical CO2 (S-CO2) Brayton cycle. The core has long life (20 years) without refueling as well as inherent safety features. The S-CO2 Brayton cycle was proposed as a power conversion system to achieve a compact and lightweight module. The entire system can be contained in a single module and be transported via ground or maritime transportation.

Conceptual Figure of a KAIST Micro Modular Reactor. Click to enlarge.

Existing small modular reactor designs usually use water as coolant and oxidized uranium as fuel. Although these may be good choices in making large reactors economical, there are economical limits when the capacity is reduced, the KAIST team said.

Sandia and S-CO2
Sandia National Laboratories (SNL) is researching a thermal-to-electric power conversion technology in a configuration called the recompression closed Brayton cycle (RCBC) that uses supercritical carbon dioxide (s-CO2) as the working fluid, rather than steam, thereby dramatically increasing conversion efficiency compared to the steam Rankine cycle.
The use of s-CO2 as the working fluid in a Brayton cycle requires less work to convert a given thermal input to electricity. In general, increased efficiency represents increased output for the same thermal input, regardless of the thermal source (natural gas, nuclear, solar or coal).
Where fuel costs are a significant portion of overall costs (coal and natural gas fired plants), the benefit is reduced fuel costs. Where capital investments are high (nuclear and concentrating solar power), the benefit is increased output for the initial investment.

The KAIST research aims to overcome these issues by adopting as S-CO2-cooled micro modular reactor (S-CO2 MMR) concept that uses a long period core, which runs for more than 20 years without refueling, with supercritical CO2 as coolant and uranium-nitride as fuel. By actively adopting the supercritical CO2 power system, the team simplified the power conversion system and developed a new nuclear power system that can passively remove the decay heat in the case of reactor shutdown to avoid another Fukushima-type accident.

The team designed the reactor core and power generation system in one vessel. The small sizing and high efficiency advantage of the supercritical CO2 power conversion system was actively utilized to create a very simple and high efficiency system. PCHE, a next-generation heat exchanger that adopts the semiconductor manufacturing process, was used, as well as radial turbomachinery, which is not used for large reactors.

To avoid large amounts of decay heat even after shutdown—i.e., a Fukushima-type event—the proposed MMR has a passive heat removal system that can cool the reactor with natural circulation in case of reactor shutdown.


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