Supercritical CO₂-cooled micro modular reactor

KAIST research team (Prof. Jeong Ik Lee, Prof. Yonghee Kim, and Prof. Yong Hoon Jeong) suggested an innovative concept of a reactor cooled by supercritical state carbon dioxide (S-CO₂). 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. Due to the compact core and power conversion system, the entire system can be contained in a single module and be transported in ground or maritime transportation.

Article  |  Spring 2016

Throughout the world, interest is moving towards small modular reactor systems. However, if we observe the currently proposed designs, most have succeeded in decreasing the size, but haven’t been able to completely modularize the whole reactor. To break the limits, the supercritical CO₂ Brayton cycle was chosen over the steam Rankine cycle for the power generation cycle.

Also, the proposed reactor is a gas cooled reactor which can run without refueling for 20 years. The power generation cycle is integrated with the reactor to further modularize it. To guarantee its safety over long operational time, new designs that haven’t been used in conventional large reactors were adopted in the power generation system. The development of the proposed reactor will allow nuclear power to contribute not only in power generation but also in energy dependent industries like offshore plants, heavy chemical industry, steel industry, and etc. which would ultimately reduce green house gas emission. Furthermore, energy supply for extreme environments (polar areas, deserts, space, and etc.) will be available for long periods of time.

The existing small modular reactor designs mostly use water as the 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. This research aims to overcome these issues by adopting a supercritical CO₂ cooled micro modular reactor (KAIST MMR) concept that uses a long period core(can be run for more than 20 years without refueling) with supercritical CO₂ as coolant and uranium-nitride as fuel was developed. Also, by actively adopting the supercritical CO₂ power system, designated as one of top thirteen industry engine project, we have simplified the power conversion system and developed a new nuclear power system that can passively remove the decay heat in case of reactor shutdown to avoid another Fukushima accident.

As our main research achievement, the feasibility of downsizing and modularization of nuclear power plant was confirmed by designing the reactor core and power generation system in one vessel. Also, in reactor design, uranium-nitride, suitable for long period low enriched uranium fast reactors, was selected; the dynamic properties and variables for nuclear reactor control were designed and evaluated. In the power generation system, the small sizing and high efficiency advantage of supercritical CO₂ power conversion system was actively utilized to create a very simple and high efficiency system. Moreover, PCHE, a next generation heat exchanger that adopts semiconductor manufacturing process, was used and a technical accomplishment of using radial turbomachinery, which are not used for large reactors, was achieved. As seen in the Fukushima accident, nuclear reactors produce large amounts of decay heat even after shutdown and if external power source is not provided, catastrophes can happen. To avoid such events, The proposed KAIST　MMR (Micro Modular Reactor) consists of a passive heat removal system that can cool the reactor with natural circulation in case of reactor shutdown.

be transported in ground or maritime transportation.