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Abstract

Due to its advantages of low critical pressure and temperature, stability, non-toxic, abundant reserves and low cost, supercritical CO₂ becomes one of the most common supercritical fluids in modern researches and industries. This paper presents an overview focusing on the researches of supercritical CO₂ in nuclear engineering and prospects its applications in the field of nuclear industry. This review includes the recent progresses of supercritical CO₂ research as: (1) energy conversion material in both recompression cycle and Brayton cycle and its applicability in Generation IV reactors; (2) reactor core coolant in the Echogen power system and reactors at MIT, Kaist and Japan, and other applications, e.g. hydrogen production. Based on the rapid progress of research, the supercritical CO₂ is considered to be the most promising material in nuclear industries.

Keywords

Nuclear reactor energy conversion material reactor core coolant recompression cycle Brayton cycle

Supercritical CO₂ used as energy conversion material

After reaching the critical temperature of 30.98°C at 73.78 bar, carbon dioxide will become a kind of supercritical fluid, called supercritical carbon dioxide (supercritical CO₂, SCO₂). Supercritical CO₂ is an intermediate state between gas and liquid phase which shows many unusual properties, as well as many dramatic changes in specific enthalpy, viscosity and density, etc.¹

Supercritical CO₂ is an ideal cycling fluid, which is considered as an ambitious competitor for moderate temperature heat utilization due to its density in nuclear reactor operation parameter range being larger and no phase change. So it can reduce the cycle temperature and provide a better thermal efficiency.² The high fluid density of supercritical CO₂ enables compact turbo machinery designs. It behaves more like liquid through the main compressor, resulting in reduced volume of compressor and realizes the modular construction, which achieve a good efficiency and high safety.^3–6 So the supercritical CO₂ power conversion system has been widely studied in various fields,⁷ and it is an attractive candidate for Generation-IV reactor designs.⁸

Supercritical CO₂ cycles

There are two primary types of supercritical CO₂ cycles. One is called recompression cycle, which is designed by Massachusetts Institute of Technology for Generation IV reactors. The other is a simplicity cycle with attractive efficiency, called simple SCO₂ Brayton cycle.^3,9,10

Supercritical CO₂ shows excellent properties in thermal aspects. Its low compressibility allows a smaller compressor and a higher thermodynamic efficiency at lower temperatures.¹¹ The most basic system of SCO₂ Brayton cycle in nuclear reactor consists of compressor, regenerator, turbine, cooler and heat source; the layout is shown in Figure 1.¹²

Figure 1.

Simple SCO₂ Brayton cycle layout. SCO₂: supercritical carbon dioxide.

According to the layout, there are two cycles. The core is the heat source of direct cycle, while the heat exchanger is the heat source of indirect cycle. The compressor inlet temperature can be controlled near fluid’s critical temperature and reduce the compression power consumption to improve the thermal system efficiency.¹³ Petr et al.¹⁴ proved that the best outlet temperature for SCO₂ Brayton cycle in nuclear reactor core is between 450°C and 650°C, while the best temperature difference for reactor core inlet and outlet is between 150°C and 200°C.¹³ Dostal et al.¹² made thermodynamic analysis of SCO₂ Brayton cycle, optimized and compound the cycle efficiency of supercritical CO₂ cycle layouts. They evaluated that the highest thermal efficiency is achievable with the reheated supercritical CO₂ cycle at 550°C.^6,14

The supercritical CO₂ recompression cycle consists of one turbine, two compressors (recompressing and main), two recuperators (high and low temperature), and one precooler. The layout is shown in Figure 2.¹

Figure 2.

Recompression cycle layout.

Comparison of SCO₂ recompression cycle and SCO₂ Brayton cycle

Maio et al.¹⁵ compared two different SCO₂ thermal cycles applied in the conversion system of a nuclear power plant. They analyzed and predicted the design and off-design performance of power plants by General Electric software (GateCycle™ v. 6.1.1). They took ALFRED (Advanced Lead Fast Reactor European Demonstrator) as research object. ALFRED is a fast spectrum nuclear reactor which is characterized by a thermal power of 300 MW, with a coolant of liquid lead. As for thermodynamic analysis, they took both SCO₂ recompression cycle and SCO₂ Brayton cycle into account. The comparison results under different pressure levels are shown in Tables 1 and 2.¹⁵

Table 1.

SCO₂ Recompression cycle parameters at different pressure levels.

		MAX pressure: 18.25 MPa			MAX pressure: 22.25 MPa			MAXpressure: 25.25 MPa
From	To	T [K]	P [MPa]	h [KJ/kg]	T [K]	P [MPa]	h [KJ/kg]	T [K]	P [MPa]	h [KJ/kg]
RE	T	728.15	18.0	422.27	728.15	22.0	418.34	728.15	25.0	415.58
T	HT-R	626.74	7.6	316.17	603.94	7.6	290.11	589.52	7.6	273.69
HT-R	LT-R	455.66	7.5	122.22	473.81	7.5	142.91	485.13	7.5	155.77
LT-R	Splitter	351.60	7.5	−6.65	359.42	7.5	4.60	364.59	7.5	11.75
P-C	MC	304.15	7.4	−187.22	304.15	7.4	−187.22	304.15	7.4	−187.22
MC	LT-R	334.65	18.3	−169.11	342.22	22.3	−163.04	347.23	25.3	−158.63
LT-R	Mixer	438.72	18.2	53.67	456.61	22.2	65.39	467.78	25.2	72.83
AC	Mixer	434.60	18.2	47.38	463.89	22.2	76.35	483.34	25.2	96.06
Mixer	HT-R	436.98	18.2	51.02	459.46	22.2	69.71	473.59	25.2	81.61
HT-R	RE	583.39	18.2	244.97	567.20	22.2	216.90	558.28	25.2	199.54

SCO₂: supercritical carbon dioxide.

Table 2.

SCO₂ Brayton cycle parameters at different pressure levels.

		MAX pressure: 18.25 MPa			MAX pressure: 22.25 MPa			MAX pressure: 25.25 MPa
From	To	T [K]	P [MPa]	h [KJ/kg]	T [K]	P [MPa]	h [KJ/kg]	T [K]	P [MPa]	h [KJ/kg]
RE	T	728.15	18.1	422.22	728.15	22.1	418.30	728.15	25.1	415.54
T	R	625.69	7.5	602.97	603.94	7.5	289.10	588.59	7.5	272.73
R	P-C	357.03	7.5	363.23	473.81	7.5	9.89	367.20	7.5	15.30
P-C	MC	304.15	7.4	304.15	359.42	7.4	−187.35	304.15	7.4	−187.22
MC	R	334.65	18.3	342.16	304.15	22.3	−163.18	347.16	25.3	−158.77
R	RE	504.19	18.2	491.43	342.22	22.2	116.02	485.12	25.2	98.67

SCO₂: supercritical carbon dioxide.

The conclusions proved that efficiencies of SCO₂ recompression cycle are higher than those obtained from the Brayton’s one at different pressure levels.

Applicability of SCO₂ cycles to generation IV reactors

Among several different fluids, supercritical CO₂ was selected as the most promising candidate for Generation IV reactors because of its advantages on low cost, sufficient reserves, security and stability. It can be used in a direct cycle of air cooled fast reactor system to simplify the system circuit and provide higher cycle heat efficiency.^13,16 As for an indirect cycle, the supercritical CO₂ can also be adapted to a wide variety of reactor types. Representative types are shown in Table 3.¹⁰

Table 3.

Applicability of SCO₂ indirect cycle to GEN-IV reactors.

Concept	Reactor outlet T	SCO₂ Turbine Inlet T	Est. SCO₂ Cycle thermal efficiency
GFR	850°C (He)	800°C	53
LFR	550–800°C	530–780°C	43–52
SFR	550°C	530°C	43
MSR	700–800°C	680–780°C	49–52
SCWR	510–550°C	500°C	42
VHTR	1000°C	800°C	53

SCO₂: supercritical carbon dioxide; GEN-IV: generation IV.

Notes: 1. Nuclear News, November 2002.

2. Limited by corrosion, and to a lesser extent by dissociation.

3. IHX ΔT is 50°C for gas/gas, 20°C for liquid/gas.

4. For net plant efficiency subtract approx. 4% for gas/gas house loads and 2% for liquid/gas combinations.

Sandia National Laboratories has conducted extensive experiments concerning the supercritical CO₂ in advanced nuclear concepts and explored its effect on turbo machinery, bearings and seals.^13,17 Compared with a steam turbine, SCO₂ turbines are more efficient with simpler and single casing body.^8,17 The Sandia National Laboratories Supercritical CO₂ Brayton cycle test loops are trying to achieve commercial power levels, just like 10 MW. With their further study on SCO₂ Brayton cycle, well-developed industrial turbo machinery may establish a more efficient power conversion cycle.

Hejzlar et al.¹ assessed gas cooled fast reactor with indirect supercritical CO₂ cycle and proved that the indirect SCO₂ recompression cycle can greatly reduce the core outlet temperature, simplify the primary system and achieve a well efficiency. The researchers demonstrated that SCO₂ power conversion system allows an achievement of well plant efficiencies at moderate core outlet temperatures and makes it easier to eliminate contamination if fission product leakage.¹

Sienicki et al.¹⁸ concentrated on the SCO₂ Brayton cycle power conversion for a sodium-cooled fast reactor (SFR) and stated that recompression closed Brayton cycle can offer a reasonable temperature and significant benefits to the SFR. Moreover, they demonstrated that the SCO₂ Brayton cycle power converter can eliminate sodium–water reactions as well as can reduce the space requirements and cost.^18,19 Compared with normal shutdown heat removal system, they came up with a new heat removal system and a new design of exchanger based on a combination of a sodium Loop and a CO₂ cycle, called sodium-to-CO₂ exchanger. An overall view of the reactor, intermediate sodium, S-CO₂ Brayton cycle power converter, and SCO₂ normal shutdown heat removal components is shown in Figure 3.¹⁹ The conceptual design of an optimized SCO₂ Brayton cycle power converter and supporting systems has been developed for the Advanced Fast Reactor-100 SFR small modular reactor (SMR) which is under ongoing development at Argonne National Laboratory (ANL).¹⁹

Figure 3.

Overall view of AFR-100 reactor, S-CO₂ Brayton cycle and S-CO₂ normal shutdown heat removal components and systems.

Supercritical CO₂ as core coolant

As shown in Figure 4, the development of gas-cooled power plants has a long history. As for the fourth generation of nuclear power system, gas cooling agent has many advantages. Helium gas is the most common gas coolant in reactors, but Helium cooled reactor requires high core outlet temperature (general requirements at 800°C–1000°C) to ensure its economy.²⁰ Supercritical CO₂ is a good choice to solve this problem. It can reduce the compression power consumption and achieve high efficiency under a medium core outlet temperature. Present research has confirmed that the SCO₂-cooled GFR concept was promising to be a safe and competitive coolant in the future.²¹

Figure 4.

The development of gas-cooled power plants.

Achievement of Echogen power systems and dresser-rand

Echogen Power Systems cooperated with Dresser-Rand making a study of special power generation technologies that not only can transform heat from waste, but also renewable energy sources into electricity and process heat. Supercritical CO₂ fluid was used in a closed loop to constitute a waste heat of power cycle, which is the basis of this special thermal engine technology. Compared with organic steam-based waste heat recovery systems, supercritical CO₂ can achieve high efficiencies over a wide temperature range of heat sources with compact components resulting in a smaller system. The comparison of waste heat exchanger operation under the same thermal source and different working fluids is shown as Figure 5.¹¹

Figure 5.

Comparison of waste heat exchanger operation under the same thermal source and different working fluids.

Moreover, researchers made field testing of the Echogen 250 kW demonstration system and proved that the Echogen SCO₂ cycle holds great promise in providing a flexible, efficient, low cost system with waste heat recovery from a wide variety of applications. The results of this program can be used to guide the initial design of a multi-megawatt scale system in large industrial and nuclear power generation.¹¹

Supercritical CO₂-cooled reactor of MIT

The Massachusetts Institute of Technology (MIT) has investigated a large size (2400 MW) SCO₂-cooled fast reactor concept combined to direct SCO₂ Brayton cycle.²² MIT formed an SCO₂ recompression circulation pattern based on the early Feher cycle by removing CO₂ condensation process and replacing the pump with the compressor and other improvements. In order to improve cycle efficiency, they proposed a general scheme of SCO₂ cooling fast reactor (GFR) and set up a recompression compressor and two regenerators.^23,24 According to the design, the core inlet and outlet temperature are 485.5°C and 650°C, the system thermal efficiency can achieve 51%. MIT adopted columnar core structure and proposed a method of using high pressure SCO₂ as a radial reflector to solve the problem of cavitation in fast reactor design. What’s more, they compared the volume of SCO₂ gas turbine, steam turbine and helium turbine, and confirmed the advantages of SCO₂ gas turbine system in reducing volume.²⁵

Supercritical CO₂-cooled micro modular reactor of KAIST concepts

An SCO₂-cooled micro modular reactor (MMR) is a particularly compact and truck-transportable nuclear reactor which was designed by the Korea Advanced Institute of Science and Technology (KAIST). The thermal power of MMR is 36.2 MW with a designed lifetime of 20 years without refueling.²² The first design goal for the KAIST MMR required it to be a fully integrated system in a single reactor vessel. The other goal is achieving a super-compact and truck-loadable structure. To satisfy these two goals, MMR was directly cooled by SCO₂ coolant with a fast spectrum core adopted in it.²⁶ In order to analyze the power conversion efficiency and material compatibility, researchers analyzed system pressure and coolant temperatures. The design parameters of KAIST MMR are shown in Table 4.²²

Table 4.

Design parameters of KAIST micro modular reactor (MMR).

Parameter	Value
Rector power/lifetime	36.2 MW/20 years
Number of fuel assemblies	18
Active core equivalent radius/height	46.58 cm/120 cm
Whole core equivalent radius/height	82 cm/280 cm
Coolant pressure/speed	20 MPa/6.92 m/s
Coolant inlet and outlet temperature	655 K/823 K
Total mass of core	39.6 ton

KAIST: Korea Advanced Institute of Science and Technology.

In the KAIST MMR, the SCO₂ power conversion system driven by the coolant is a direct process without intermediate heat exchanger. Due to the design of compact modular, the SCO₂ power conversion module and the reactor core can be integrated into a single reactor vessel.²²

Supercritical CO₂-cooled reactor concept of Japan

Tokyo Institute of Technology (TIT) came up with a new view based on thermodynamic cycle analysis, which is called partial pre-cooling direct cycle. The new cycle is built from the simple Brayton cycle. It added shunting intermediate compression and intermediate cooling process to improve cycle efficiency and allowed an operation at a lower pressure. The process is shown in Figure 6.²³

Figure 6.

The new cycle layout of TIT. TIT: Tokyo Institute of Technology.

Researchers analyzed that usual spherical fuel and block fuel can still be used in this new SCO₂-cooled reactor. Moreover, TIT also completed an SCO₂ corrosion test loop to find reasonable in-pile material.²⁷ The new SCO₂-cooled reactor designed by TIT was a medium temperature gas turbine reactor, which has a partial precooling cycle with an efficiency of 45.8%. Lower temperature provides more flexibility in choosing materials. The Bird’s-eye view of the new reactor is shown in Figure 7.²³

Figure 7.

The Bird’s-eye view of the new reactor designed by TIT. TIT: Tokyo Institute of Technology.

Supercritical CO₂ for hydrogen producing

Nuclear energy is a kind of power for hydrogen production industry. High-temperature reactors especially gas-cooled reactor have great potential. Yildiz and Kazimi²⁸ stated that high-temperature steam electrolysis (HTSE) combined to a supercritical CO₂ cooled reactor which equipped with a supercritical CO₂ power conversion cycle may be a good alternative to provide higher energy efficiency at a lower temperature. Hydrogen can be produced by using the high-temperature heat and electricity from the nuclear plant through HTSE processes. The advanced gas reactor (AGR) is a commercial thermal reactor; if it increases the operating pressure of it and combined it to a direct cycle supercritical CO₂ power conversion system, the temperature of the reactor coolant may be driven up to 750°C. This new group will be a more efficient and economical way to produce hydrogen under a medium temperature.²⁸

Researchers have demonstrated that the combination system of AGR–SCO₂ and HTSE is better than the integration of gas turbine modular helium reactor (GT-MHR) and HTSE; AGR–SCO₂-HTSE will provide higher efficiency due to its both relatively high operating temperature and the higher thermal power cycle efficiency. The comparisons are shown in Figure 8.²⁹

Figure 8.

Comparisons of the GT-MHR-HTSE and AGR-S-CO₂-HTSE efficiencies. GT-MHR-HTSE: gas turbine modular helium reactor-high-temperature steam electrolysis.

Other research progress

Supercritical CO₂ has great value because of its special characters. In addition to the applications above, many other institutions made researches on the supercritical CO₂ for its applications in nuclear industry.

Linares et al.³⁰ proved that supercritical CO₂ cycles may be suitable converters of thermal energy to power in fusion reactors. They presented an analysis of supercritical CO₂ Brayton cycle for low-temperature diverter fusion reactors which can be cooled by helium. Czech Technical University demonstrated the turbine systems in SCO₂ cycles.¹¹ Korea Atomic Energy Research Institute analyzed the feasibility of the combination of SCO₂ cycle and sodium cold fast reactor optimized the PCHE that used in the SCO₂ cycle.³¹

SCO₂ cycle has a higher efficiency and more beneficial than other cycles because of its higher efficiency, tiny machineries, simple configuration, and strong heat transfer capability.⁵ Nami et al.³² and Wang and Dai³³ elaborated advantages of SCO₂ by the researches made on exergy, economic and environmental impact assessment of gas turbine. They compared the economy of waste heat recovery between the SCO₂ cycle and the organic Rankine cycle. Besides, other advantages of SCO₂ have been stated by Angelino, including compact, stable and economy.^34,35

Issues for the application of SCO₂ in nuclear engineering

As the working fluid in thermodynamic cycle, SCO₂ has many advantages and potential for various applications, but there are also challenges for its application. For example, due to the special characteristics of SCO₂, the design of the fired heater and other structures will be a new issue.³⁶ Near the critical point, CO₂ has many thermodynamic characteristics, its highly non-linear properties also create new challenges for the heat exchangers, and the near critical point operation usually causes technical challenges, so it still need many improvement.^31,37–40 Jahn and Iverson have done researches on SCO₂ Brayton cycles for solar-thermal energy; they quantified the cooling requirements and associated cooling issues created by a small-scale solar thermal SCO₂ power plant. They proved that cooling and condensing the fluid from the supercritical region to the liquid region are difficult.^38,41 We can predict that the issue will also arise in Nuclear Engineering. According to the research of Kim et al.,⁴² SCO₂ compressor performance prediction tool will be constructed with test data of high pressure ratio compressor test facility in the future.

Moreover, according to the process of the data post-processing, Lee et al.^43,44 found out that the SCO₂ compressor performance experiment has an issue of uncertainty on the performance measurement. They believed performance measurement uncertainty issues in a low pressure ratio compressor should be appreciated.^43,44 The same issues have also been discussed by Pham et al.⁴⁵ Sandia National Laboratories provided an overview of corrosion mechanisms in SCO₂, as well as in molten salts for nuclear applications.⁴⁶ They found that the uncertainty in the long-term reliability of various components has become more visible, when more hours of operation applied to the Recompression Closed Loop Brayton Cycle power loop. Besides, the reasonable materials are also issues for the application of SCO₂ in Nuclear Engineering.⁴⁷ Because of its special characteristic, SCO₂ can transport little, dissolved particles throughout the loop, the combination may cause erosion at some high gas-velocity locations.⁴⁸ Though some strategies have been come up to solve the erosion in SCO₂ power cycles, the erosion resistant materials and coatings for the turbines and nozzles are still in need of more researches.⁴⁹

Conclusions

Supercritical CO₂ is an emerging working medium for the nuclear power industry, it has the potential of improving the power efficiency, simplify the system volume and optimize the system structure. Using supercritical CO₂ as core coolant, power conversion working medium and the replacement of helium for hydrogen producing are important contents for international nuclear reactor researches.

Supercritical CO₂ Brayton cycle is important for supercritical CO₂ applications in the field of nuclear power industry. When the export temperature of nuclear reactor systems is between 450 and 650°C, supercritical CO₂ cycles can reduce the requirements of core outlet temperature and improve the economic benefit. Supercritical CO₂ will show its advantages of efficiency and low costs when used in power conversion system for air cooled reactor and liquid metal cooling reactor. Moreover, it will greatly improve hydrogen production efficiency if it is used in the temperature hydrogen production industry as a replacement of helium. Besides, supercritical CO₂ cycle can solve many nuclear power system issues, such as metal-water reaction in the sodium cooled fast reactor, the harmful effects for material under excessive temperature and difficulties for high power compressor manufacturing in the helium cooled reactor.

In general, supercritical CO₂ has important significance for the optimization of nuclear power systems, and it may accelerate the process of nuclear power industry. At the same time, it still has many issues to be solved before its application in nuclear engineering, and more testing and analysis are needed to complete a rigorous assessment.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The authors are grateful for the support of this research by the National Natural Science Foundations of China (Grant No. 51576211), the National High Technology Research and Development Program of China (863) (2014AA052701), and the Foundation for the Author of National Excellent Doctoral Dissertation of P.R. China (FANEDD, Grant No. 201438).

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The application of supercritical CO 2 in nuclear engineering: A review

Abstract

Keywords

Supercritical CO2 used as energy conversion material

Supercritical CO2 cycles

Comparison of SCO2 recompression cycle and SCO2 Brayton cycle

Applicability of SCO2 cycles to generation IV reactors

Supercritical CO2 as core coolant

Achievement of Echogen power systems and dresser-rand

Supercritical CO2-cooled reactor of MIT

Supercritical CO2-cooled micro modular reactor of KAIST concepts

Supercritical CO2-cooled reactor concept of Japan

Supercritical CO2 for hydrogen producing

Other research progress

Issues for the application of SCO2 in nuclear engineering

Conclusions

Footnotes

Declaration of conflicting interests

Funding

References

Supercritical CO₂ used as energy conversion material

Supercritical CO₂ cycles

Comparison of SCO₂ recompression cycle and SCO₂ Brayton cycle

Applicability of SCO₂ cycles to generation IV reactors

Supercritical CO₂ as core coolant

Supercritical CO₂-cooled reactor of MIT

Supercritical CO₂-cooled micro modular reactor of KAIST concepts

Supercritical CO₂-cooled reactor concept of Japan

Supercritical CO₂ for hydrogen producing

Issues for the application of SCO₂ in nuclear engineering