Hybrid Small Modular Nuclear Reactor with Concentrated Solar Power: Towards 4+ Reactors?

Abstract

Solar thermal energy is one of the most interesting sustainable solutions for decarbonizing the energy sector. Integrating solar collectors with other energy sources is common, as seen in domestic heating, where solar collectors are combined with common heaters to reduce fuel consumption (gasoline, electricity, gas, and biomass) and therefore, the energy cost. Similarly, this concept can be applied to nuclear energy, where the reduction in nuclear fuel consumption is very strategic for decreasing not only its cost but also the risk in handling, transportation, and storage (both the fuel and the nuclear waste as well). Nuclear energy, on the other hand, seems to be very useful in reducing the land occupation of concentrated solar power plants (CSPs) and helping a more constant production of electricity, both points being two important bottlenecks of CSP technologies. CSP and nuclear reactors, on the other hand, share common heating technologies and both can produce energy without CO₂ emissions. Solar and nuclear energy, especially with the advent of the fourth generation of small modular reactors (SMRs), present a compelling opportunity for sustainable electricity generation. In this work, we present a brief review of CSP technology, a brief review of SMR concepts and development, and a brief overview of the combination of these two technologies. The review shows that in general, combined SMR + CSP technologies offer several advantages in terms of a strong reduction in the solar field extension areas, improved dispatchability of energy, improved efficiency of the SMRs, and, in particular, lower nuclear fuel consumption (hence, e.g., with a lowered refueling frequency).

1. Introduction

The relevance of the interest in solar energy is not new; one of the first methods to preserve food was drying by exposing the food to natural ventilation and the sun’s rays. Hayashi reported that in old ages, pieces of meat were dried and preserved in this way by people living around the Don River in southern Moscow (9000 BC) [1]. Solar saltworks were common in the Greek and Roman ages, as in the “Salina di Trapani” and “Salina di Ostia”. Solar drying under direct sun irradiation has also been used in the treatment of many other types of food such as fish or grapevine, and is still used in the industrial sector, for instance, to produce solar heat for industrial processes (SHIP) for dry processes in pharmaceutical sector or for pasteurization in agrifood [2]. Nowadays, one of the main applications of solar thermal energy is the heating of water for domestic use [3]. In water heating, the system is composed of solar collectors and a water tank (for thermal energy storage, TES): the tank is coupled with a heater powered by fuels such as biomass, natural gas, or gasoline [4]. In this kind of application usually, solar collectors preheat the water (usually 40 ÷ 60 °C) while the fuel in the heater is used to reach the final required temperature (60 ÷ 80 °C) when the solar irradiation is low. The high efficiency of solar collectors (>80%) allows a strong reduction in fuel consumption (in many cases higher than 50%) [5,6]. Thanks to their efficiency, low intrinsic and operational cost, and plant reliability, solar collectors for water heating are widely used commercially [7,8]. Solar thermal systems can also be coupled with biomass to provide heat to a community using district heating plants: the Danish concentrated solar power (CSP) smart grid for district heating being an important example. A growing number of demonstrators are dedicated to the production of electric power using high-temperature solar systems. In Spain, for instance, the PS10 and PS20 solar power plants can produce 11 and 20 MW, respectively, while the Ashalim Power Station in Israel can reach up to 120 MW [7,9,10]. Many other examples of electric power production have been realized in China, Morocco, South Africa, the USA, Australia, and Greece, with a continuous reduction in the levelized cost of electricity (LCOE) (Irena 2020) [11,12]. The Noor Power Plant in Morocco, generating 160 MW of electricity since 2016 [13], along with the 210 MW Cerro Dominador hybrid CSP and photovoltaic plant in Chile and the Ivanpah CSP plant in California (consisting of three distinct units, including the 126 MW Ivanpah 1 solar tower), demonstrate the viability of large-scale CSP technology for electricity production [14,15]. In these applications, solar collectors produce heat to directly activate the heat transfer fluid for the turbine, an example of direct steam production. Due to the significant requirement of land occupation, the need for high direct normal irradiation (DNI), and the limited diffusion of CSP panels, this technology has yet to achieve widespread adoption. Nonetheless, thanks to the relatively low cost, the high efficiency, and the absence of pollutant emissions, several studies in the literature are devoted to integrating solar energy in the existing power plants, or in developing hybrid power plants, such as the integration of solar thermal energy in natural gas power plants. Al Kaseem, for instance, shows that the hybridization of a solar tower in gas turbine power plants results in enhanced annual generation and lower costs [1]. The construction of large solar thermal plants such as Miraah in Oman, Sundrops Farms in Australia, or Brønderslev Forsyning in Denmark shows that the modern CSP can deliver power comparable to that of nuclear micro and small modular reactor (MMR and SMR) power plants. This circumstance stimulated the uprising of studies dedicated to the combination of solar thermal energy with nuclear energy for hybrid energy production. Moreover, many plants with CSP technology already operate at high or medium-high temperatures with supercritical sCO₂, liquid metal, molten salt, or steam (e.g., direct steam generation) and this can facilitate, from the technological point of view, the integration of these two technologies. For instance, supercritical carbon dioxide (sCO₂) holds significant promise as a heat transfer fluid in concentrated solar power (CSP) systems, particularly for high-temperature applications, due to its resistance to thermal degradation. This same property, along with other favorable characteristics, also makes it attractive for use in gas-cooled reactors [16,17,18,19]. Virtually, solar energy can be used, similarly to the domestic heating case, to reduce the consumption of nuclear fuel, thus reducing the risk related to the production, use, and nuclear waste storage. The decarbonization of energy systems and hybrid power plants offer realistic and promising pathways toward this goal. Concentrated solar power demonstrated interesting potentialities for some industrial processes due to its high efficiency and low cost. Its primary weaknesses are land occupation and intermittency. Modern nuclear energy (e.g., IV generations) in the form of small modular reactor SMR offers complementary solutions, and integrating SMRs with CSP has the potential to unlock significant synergies. CSP and nuclear energy share, in fact, the same thermal technologies for electricity generation, making the integration of the technologies relatively straightforward. This scientific field is quite new and unfortunately, a comprehensive review of this promising integration is currently lacking in the literature. This review addresses this critical gap, systematically analyzing the existing research on hybrid power systems based on small modular reactors and concentrated solar energy. This work reveals the mutual benefit of these two technologies. For instance, increasing the SMR generation efficiency is possible (of 20%), and the cogeneration of CSP SMR induces a further reduction in the use of nuclear fuels as well as a reduction in nuclear waste generation. (reduction of 20 ÷ 40%). From the CSP perspective, a substantial land area required for solar fields can be spared. This work provides a comprehensive review of CSP-SMR hybrid systems. Section 2 provides a detailed overview of CSP working principles and various CSP technologies, including parabolic trough collectors, linear Fresnel, and solar towers. Section 3 describes the characteristics and advantages of small and micro modular nuclear reactors, focusing on the advantages of this technology like safety and efficiency. Section 4 presents a dedicated review of the existing literature on solar-nuclear hybrid systems, analyzing different configurations and highlighting the synergistic benefits of integrating CSP and SMR technologies, particularly in terms of improved dispatchability and reduced nuclear fuel consumption. The key research gaps and future directions in this promising field are also reported. Finally, a summary of the results is presented.

2. Solar Thermal Energy Technologies

One of the most efficient methods to use solar energy is heat production by photo-thermal conversion. In the photo-thermal effect, the absorption of solar photons by an optical absorber surface (black surface) results in photoexcitation, the energy of the photons is transferred to the absorber surface (heat production), and the produced heat is conveyed using a heat transfer fluid (HTF) [20], Figure 1a. Photoexcitation can be observed in inorganic materials, such as noble metals and semiconductors, as well as organic materials such as carbon-based compounds, dyes, and conjugated polymers. The efficiency of photo-thermal conversion, in particular at low temperatures, is very high (>80%), and in some cases can reach a percentage higher than 90% [21]. For this reason, this kind of conversion is the most efficient method to convert sunrays into usable energy. To take advantage of an efficient photo-thermal conversion, it is necessary to realize devices that can reduce the infrared loss and configurations of the systems that are able to transport efficiently the heat produced to a specific thermal process, these devices are the solar collector Figure 1b.

Solar thermal collectors can capture the sunrays and retain heat by converting the solar radiation into thermal energy that is transferred to a working fluid, an HTF (heat transfer fluid). The HTF can be air, steam, or a liquid such as water, glycol, diathermic oil, molten salt, or ethanol [23]. Since the intensity of solar radiation varies during the day as well as with seasons, it is useful to track the position of the sun to maximize the utilization of the incoming solar radiation. The tracking collectors can be divided into two categories: one-axis tracking and two-axis tracking collectors. Because the sun’s position changes throughout the day and year, and concentrated solar power (CSP) systems rely on direct normal irradiance (DNI), tracking systems are essential for maximizing the capture of incoming solar radiation. This is particularly true for CSP systems employing medium- to high-temperature technologies.

To increase the working temperature, concentrators use reflective surfaces to concentrate the percentage of photons on a receiver. These mirrors convoy the direct normal solar irradiation (DNI) towards a receiver (e.g., a vacuum tube), increasing strongly the irradiation flux on a small area, which, thanks to the small dimension, also reduces thermal loss. Both effects allow a strong increase in the working temperature of solar collectors higher than 100 °C. The increase in the temperature of heat transfer fluid allows us to increase the number of applications of CSP technologies, for instance, steam production for sterilization or production of electricity.

Table 1. Overview of solar thermal technologies applied to the industrial sector considering the tracking, concentration of sun, working temperature, and applications.

Range of Temp.	Collector	Tracking	Concentration	Temperature	HTF	Application	Performance Analysis	Reference
LOW TEMPERATURE 50–150 °C	FPC	No	No	50–80 °C	Air or Water 50–100	Domestic water heating District heating Cleaning process Food agriculture drying Hydrogen AEM/PEM Heating of water and chemical for (50–80 °C) Air heating Domestic water heating District heating Galvanic bath	High efficiency at low temperatures; limited scalability for high-temperature applications. Efficiency range: 30–70%	[24,25,26,27,28,29,30,31]
	ETC	No	No	50–120 °C	Water 50–120		ETC insulation is better than FPC, enabling higher efficiency in lower environmental temperatures. Efficiency range: 20–50%	[32,33,34]
	CPC	No	Yes (3–10)	50–180 °C	Water 50–150		Moderate efficiency with improved acceptance of DNI, allowing for higher heat collection and operating temperature. Efficiency range: 25–55%	[35,36,37]
MEDIUM TEMPERATURE 150–400 °C	FVC	No	No	100–200 °C	Water 50–150	Solar cooling Steam production for steam Solar dry Water disinfection Desalination Heating liquid sulfur in the nickel industry Power plant Agrifood processes	Simplicity in design but no tracking leads to lower efficiency. Efficiency range: 60–80%	[38]
	PTC	Yes (1 axis)	Yes (10–100)	100–400 °C	Water, oil, or direct steam Molten salt sCO2		PTCs are effective for large-scale plants, with high-temperature operation that requires more complex heat transfer systems. Land use 45% Efficiency range: 60–80%	[39,40,41,42,43,44,45,46]
	LFR	Yes (1-axis)	Yes (10–100)	100–400 °C	Water, oil, or direct steam Molten salt		Lower efficiency than PTC but cheaper and easier to install. Higher land use with respect to other technologies in order to avoid self-shading phenomena. Land use 66% Efficiency range: 40–65%	[47,48,49,50]
HIGH TEMPERATURE: 400–1200 °C	PTC	YES (1 axis)	Yes (>100)	595 °C	Molten salt	Electricity	Specialized HTFs for high-temperature operation are required. Efficiency range: 60–80%	[51,52]
	Solar Dish	Yes (2 axes)	Yes (100–1000)	400–1200 °C	Oil, helium, or steam Molten salt	Electricity Power plant SMR steam methane reforming and (850 °C) SG steam gasification for SOEC cell Steam methane reforming Iron-oxide-based redox pair cycle (1200 °C) Sulfur iodine cycle (850 °C) Sulfur hybrid 1200 °C Water splitting Biomass Gasification Thermochemical water splitting Two-step thermochemical process based on MgO/Mg redox reaction Methane cracking (1800 °C) High-temperature electrolysis (650 °C)	High efficiency due to point-focus design, but challenging to scale up. Complexity in heat volumetric receiver Efficiency range: 45–72 %	[53,54,55,56]
	Solar Tower	Yes (2 axes)	Yes (100–1000)	400–1200 °C	Oil, helium, steam, or molten salt sCO2		Best efficiency among CSP technologies; highly scalable but complex thermal management Complexity in heat volumetric receiver Efficiency range: 70–90%	[56,57,58,59]

shows some examples of solar collectors considering the temperature ranges: low-temperature applications (80 ÷ 150 °C), medium temperature (150 ÷ 400 °C), and high temperature (400 ÷ 1200 °C). For low temperatures, the most suitable technologies are flat plate collectors (FPCs) and evacuated tube collectors (ETCs), while compound parabolic collectors (CPCs) show excellent efficiency in the temperature range of 50 ÷ 120 °C thanks to their simplicity of installation and low cost. For medium temperature, linear Fresnel reflector (LFR) and parabolic trough collector (PTC) are used.

The PTC and LFR are designed to track the sun on one axis. They can be oriented in the north–south or east–west direction and this allows more efficient use of direct normal irradiation. The absorber tube has an area 25 ÷ 35 times smaller than the aperture area. The reflecting surface often consists of an aluminum sheet or a reflective coating applied directly to the curved glass section [60]. PTCs and LFRs can usually reach a temperature of 500 °C whereas the small-sized parabolic trough can operate at lower temperatures ranging from 100 °C to 300 °C. Linear Fresnel reflectors (LFRs) are composed of several lines of flat mirrors. Each line is tilted with a specific angle to reflect the DNI along a focus line towards a vacuum tube receiver [47]. These collectors can track the sun in one direction with a north–south or east–west orientation like PTC. They can reach higher outlet temperatures than PTCs because they have a higher concentration ratio. The LFRs are easy to mount and can also be installed on flat roofs and they have fewer problems with self-shading than PTC. The typical working temperature of PTC and LFR is between 100 and 500 °C. The high-temperature systems are based on solar towers and solar dishes where the concentration factor can also be higher than 1000. The solar dishes are equipped with a two-axis system for solar tracking that allows the best use of solar irradiation during the day and across the different seasons [61,62]. In solar dish technologies, the concentration ratio is very high, and this allows reaching temperatures higher than 1000 °C. The solar tower is composed of a receiver placed on a tower and a series of mirrors focalized on the receiver, the tower concentration ratio usually is from 300 to 1000, and the temperature of the working cycle can reach values higher than 580 °C [63]. Several solar technologies show different working temperatures and different costs, being, therefore, suitable for different applications, from water heating to the support of industrial processes. An overview of their applications is reported in Table 1.

3. Small Modular Reactors

3.1. Nuclear Reactors and Nuclear Fuel

The first civil use of nuclear reactors dates to the mid-1950s [64], and has since then experienced an impressive evolution, both in terms of reactor technologies and reactor fleet expansion [65,66]. As of December 2023, 413 nuclear reactors are operational in 31 IAEA member states (capacity 371.5 GWe), with 59 more expected to be finalized and licensed in the next decade [67,68,69,70]. In brief, a nuclear reactor uses a supercritical nuclear chain reaction (i.e., a self-sustaining reaction) to generate the heat necessary for water evaporation and subsequent electricity production by a steam-powered turbine. Such a reaction can be triggered by an incident neutron onto a fissile atom (such as ²³⁵U), leading to its fission into (generally) two lighter atoms, while also freeing energy (about 200 MeV), and ejecting 2 ÷ 3 high-energy neutrons (Figure 2). These neutrons, if moderated to the suitable kinetic energy range (thermal neutrons), can impact onto other fissile nuclei, thus fostering the chain reaction. In most operating reactors, water is used both as moderator and coolant/working fluid (light water reactors, LWRs), while the chain reaction rate is managed using neutron-absorber control rods [71].

Figure 3a schematizes the two most common LWR types: in boiling water reactors (BWRs) the steam/water cycled in the turbine is directly the one cooling and moderating the core, while in pressurized water reactors (PWRs, the most common type) the steam is generated in a secondary loop while the water in the core is pressurized to prevent boiling. The most common chemical form of current nuclear fuel is uranium dioxide (UO₂), with the natural uranium composition being enriched in fissile ²³⁵U or mixed with fissile ²³⁹Pu depending on the type of reactor. Upon usage, a small fraction (<1% weight) of the fuel transforms into transuranic elements (Pu and minor actinides Np, Am, and Cm) that have very long half-lives and are responsible for the long-term thermophysical evolution [72,73,74] and radiotoxicity [75,76,77] of spent nuclear fuel (SNF).

More innovative designs involve other innovative coolants such as gas (CO₂ or He, gas-cooled reactors, GCRs), liquid metal (liquid metal-cooled nuclear reactor, LMR), or molten salts (molten salt reactors, MSRs). Working fluids other than water are being studied for their improved thermal properties and lower moderation effects. Unmoderated neutrons (i.e., fast) are less likely to cause fission in uranium, but they can efficiently fission plutonium isotopes. Fast reactors, therefore, employ fuel containing enough plutonium to achieve a self-sustaining chain reaction. This is possible because a ²³⁹Pu fission generates 25% more neutrons than a ²³⁵U fission, thus providing enough neutrons not only to sustain the chain reaction but also to convert ²³⁸U into ²³⁹Pu [78]. Fast reactors can, therefore, use the fuel up to a much higher yield, reducing refueling operations and, in general, greatly decreasing fuel consumption [79,80]. Both gas- and liquid metal (sodium, lead, and lead–bismuth eutectic LBE)-cooled reactors can employ fast neutrons and be operated also as breeder reactors (gas-cooled fast reactors, GFRs; liquid metal fast reactors, LMFRs). A breeder is a reactor type that produces more fissile Pu than it consumes, and it can be fed fuel containing small amounts of minor actinides (Np, Am, and Cm) to transmute them into fissile isotopes (such as, for example, ²³⁷Np becoming ²³⁸Pu, with a much larger fission cross-section) [81]. This allows both to produce useful fuel from nuclear waste in view of a closed fuel cycle and/or to decrease the radiotoxicity of spent nuclear fuel (SNF) in sight of the final repository [82,83]. Indeed, long-term radioactivity in SNF is dominated by the presence of minor actinides and plutonium, requiring up to 10⁵ years to fall below the radioactivity threshold of the pristine unirradiated UO₂ fuel [57]. Fast reactors can be used to both transmute minor actinides and consume plutonium, resulting in SNF of much lower radiotoxicity over time [84,85,86]. Figure 3b shows a scheme of an LMFR, including the breeding blanket, constituted of fuel containing minor actinides to be transmuted (positioned in specific locations of the core-periphery to maximize the neutrons absorption). The example of GCR represented in Figure 3c instead is a thermal pebble bed-type reactor, a design that employs spherical fuel (TRISO, TRi-structural ISO-tropic particle fuel [87]) that can be replaced in continuous. Molten salt reactors (MSRs) are an even more innovative design that employs liquid fluoride/chloride fuel that also serves as a working fluid (Figure 3d). Such configuration yields some intrinsic advantages in terms of reactor safety: core meltdown and loss of coolant accidents are practically impossible; the working pressure is close to atmospheric pressure; the liquid fuel is easier to chemically reprocess with respect to solid fuel. Moreover, some MSRs can be operated also as fast reactors to reprocess LWR SNF. Currently, MSRs and LMRs have not been commercially deployed yet, while GCRs represent about 3% of the commercial reactors worldwide [70]. In general, from the dawn of nuclear energy to the latest developing prototypes, reactor types can be grouped into four generations:

• I generation: Early exploratory research prototypes.
• II generation: Most of the currently operational nuclear plants (upscaled versions of gen. I, mostly LWRs).
• III and III+ generation: How most nuclear plants are currently being built (evolutionary designs improving II generation reactors in terms of safety and reliability).
• IV generation: Future reactors (revolutionary designs aimed to improve safety, sustainability, efficiency, and cost), including some MSRs, LMFRs, and GFRs (and more) [88].

3.2. Small Modular Reactors

The concept of small modular reactors (SMRs) plunged its roots in the 1980s, when the expansion of civil nuclear power was halted by the change in public opinion following dramatic nuclear-related accidents (Three Miles Island and Chernobyl) [89]. This period saw little to no construction of new LWRs, while safer (III/III+ generation) and more efficient (IV generation) reactor designs were being developed. Besides exploring innovative aspects such as fast neutrons, exotic coolants, and novel fuel designs, IV generation rector concepts often also involved the combination of small power modules to achieve power outputs comparable to those of the existing LWRs [90]. Although such designs cannot be classified as SMRs according to current definitions, the first reports on the potential benefits of SMRs were published in these years by IAEA [91] and NEA [92]. The modern SMR concepts [93] share the aims outlined in these early reports and are earning increasing public acceptance thanks to (1) the proven safety and reliability of already existing reactors; (2) the increasing demand for electric power; (3) the growing concern over fossil fuel.

Nowadays, SMRs are defined as nuclear reactors with a capacity of up to 300 MWe per unit (700 MWe for medium-sized modular reactors), requiring a fraction of the size of a conventional nuclear power plant (small), and capable of being assembled and transported as a unit to the operational location (modular) [94]. Their characteristics intrinsically yield some key advantages with respect to the already existing LWRs [95]:

Their small size makes them more flexible, and therefore more suitable for locations where it would be impossible to install a conventional nuclear power plant [96].

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In addition, smaller reactors imply more efficient passive safety systems, as the operating power and pressure are significantly lower than in conventional reactors. Reaction times for operators to counter accident situations are also largely extended due to the decreased operational power [97].

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Most SMRs (including MSRs; LMFR; and high-temperature gas-cooled reactors, HTGRs) have negative void and/or temperature coefficients of reactivity. This implies that in case of loss-of-coolant accidents (LOCAs), the chain reaction self-extinguishes, meaning they are intrinsically safer than the already existing LWRs (whose chain reactions must be shut down by the operators).

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The shipment of single working modules allows for quicker and cheaper implementation. It also opens the possibility of reaching the power output of a conventional plant in a stepwise investment rather than a more difficult all-in-one solution [98,99].

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Less consumed fuel also means a lower radioactive inventory inside spent fuel, meaning lower radioactivity, lower worker exposure, and smaller emergency planning zones (EPZs). SMR SNF is less radioactive when discharged and remains radioactive for less time after discharge.

Several classifications of SMRs have been proposed, generally related to size/power (SMRs vs. MMRs, micro modular reactors), coolant fluid (AMRs, advanced modular reactors), operating temperature, or support type (land- or marine-based). Depending on the purpose, one criterion might be preferred over the others.

Table 2. Main characteristics of the SMRs under development around the world (as summarized in the IAEA latest report [100]).

Coolant	Base	Reactor Types	Developing Countries (Operational/Licensing Phase)	Max Output [MWe]	Output T [°C]	Fuel Type
Water	Land	PWR/PHWR/BWR	ARG, CAN, CHN, CZE, FRA, JPN, KOR, RUS, CHE, GBR, and USA	450/300/250	280–330	UO2 pellets
	Water	PWR	CHN, KOR, and RUS	325
Gas	Land	HTGR/GFR	CAN, CHN, JPN, RUS, ZAF, and USA	300/265	600–950	TRISO (U/Th) and UC pellets
Liquid metal	Land	LMFR	CAN, ITA, JAP, KOR, RUS, SWE, and USA	450	500–650	MOX, U-Zr, and (U, Pu)2N3
Molten salt	Land	MSR/FHR	CAN, CHN, DNK, JPN, NLD, GBR, and USA	300/140	600–800	TRISO and (U, Th)F4 + salts

Note: Country names in bold indicate the presence of one or more operational reactors, while in those underlined one or more reactors are undergoing the licensing phase. (PWR: pressurized water reactor; BWR: boiling water reactor; PHWR: pressurized heavy water reactor; HTGR: high-temperature gas-cooled reactor; GFR: gas-cooled fast reactor; LMFR: liquid metal fast reactor; MSR: molten salt reactor; FHR: fluoride salt-cooled high-temperature reactor; TRISO: TRi-structural ISO-tropic particle fuel.)

summarizes some of the key features of SMRs, grouped according to the IAEA classification [100].

At the time of writing, more than 80 SMR designs are being developed in 18 IAEA member countries [100]. However, only a few countries have already passed the design stage, with SMRs being under construction in Argentina, China, Russia, and the United States. Operational SMRs exist in China (two HTGRs, one in operation and the other one soon operable), Japan (HTGR), and Russia (PWR), with a few more expected to complete the licensing stage in the coming years [99]. It is worth remarking on two important milestones in nuclear power generation history that were achieved by SMRs: the Russian PWR is the first ever floating civil nuclear power plant, while the Chinese HTGRs represent the first examples of IV generation reactors. The deployment of SMRs in the next decades is, therefore, expected to have the twofold advantage of strongly contributing towards decarbonization while also fostering further development of even more mature nuclear technologies. The compatibility between CSP technologies and SMRs is evident by looking at the reactor working temperature ranges reported in Table 2. LWR (PWR and BWR) working temperatures span between 280 and 330 °C, while for more advanced reactors (LMFRs, GFRs, and MSRs) they exceed 500 °C (up to 950 °C). The former temperature range is compatible with consistent with PTC and LFR solar technologies, and the latter with solar tower and solar dish technologies. It comes, therefore, as no surprise that growing interest has sparked around the possibility of integrating solar and nuclear technologies into a hybrid power plant, with a variety of studies investigating the benefits and downsides of such combined systems.

4. Hybrid SMR + CSP

The integration of solar thermal energy with hybrid solutions involving the use of fossil fuels represents a crucial and significant opportunity for innovation and sustainability aspects. Indeed, in the realm of steam generation and water heating, hybrid technologies are effective solutions to support the transition from reliance on fossil fuels [101,102,103]. Ruth et al. observed in 2014 that renewable energy can be integrated with nuclear power plants, proposing a sort of hybrid nuclear system integrated with CSP technologies [104]. Petrovic, in 2014, proposed the integration of solar thermal energy by using a concentrated solar power plant in a nuclear power plant and by also integrating thermal energy storage (TES) (NuRenew). The system was designed using molten chloride and fluoride salts as heat thermal fluid and storage media [105]. The first complete work on hybrid nuclear and CSP power plants has been proposed by Popov and Borissova, and was dedicated to the viability of combining solar thermal energy produced by solar towers with nuclear power (Nuclear Solar Tower Plant—NSPP) to enhance their combined system performance. In particular, the work was oriented toward the optimization of the performance of the superheater [106], because many of the current designs of SMRs are based on light water reactor technology. In this kind of reactor, the pressure and temperature of steam are far from the optimal conditions typically found in fossil fuel power plants, and this implies that the efficiency of SMRs remains limited. The work is based on modeling the integration of the solar tower heat flux in the superheater after the pressurized water reactor. The combination of these two technologies leads to an increase in the combined system performance, improving the nuclear plant’s electrical efficiency to 37.5% and the CSP efficiency to 56.2%. The increase in efficiency led to a reduction in solar field installed capacity by up to 25% compared to conventional CSP. A similar study was applied to a parabolic trough collector (PTC), instead of the solar tower, and in this case, the whole efficiency of the nuclear power plant reaches an interesting value of 33% with a levelized cost of electricity of 0.18 €/kWh. Zhao et al. estimated the performance by modeling the fluoride salt-cooled small modular advanced high-temperature rector (SmAHTR) combined with the molten salt solar receiver (200 MW) [107]. The results showed that the SMR integrated into the CSP power plant system could mitigate the fluctuations in heat generation and reduce the start-up and shut down times. The optimal proportion in terms of power peak of SMR and CSP is around 50% of a 100 MW nuclear and 100 MWe of a CSP plant, combined with 14.8 h of thermal storage capacity [107]. Soto et al. obtained similar results for a solar tower and a 950 Wth LFR nuclear plant [108]. The modeling analysis showed that a hybrid system can suitably produce more energy when the price of energy is high, and moreover, it was possible to increase the efficiency of the system close to the turbine design point and cost savings thanks to the economy scale of the turbine. Also in this work, the author observed that the best benefit is possibly obtained when nuclear power and solar power are comparable (100 MWth for CSP and from 150 to 259 MWth for nuclear). In this study, the CSP operated in parallel with the nuclear power plant. The solar thermal energy produced by the CSP was subsequently integrated into the nuclear reactor in order to provide additional support for the superheater device, similar to the Popov et al. approach [106]. Woo Son et al. studied the feasibility of a solar-nuclear hybrid system for a microgrid application in insular or remote areas. Their system did not need frequent refueling [109]. The designed system was composed of a solar tower and a micro modular reactor (MMR) using a supercritical CO₂ power cycle. The hybrid system showed a better performance than CSP alone as there was an increase in the fulfillment of the demand ratio and a strong reduction in the solar field extension (2.35). Anyway, with thermal storage of 800 MWth and a ratio between MMR and CSP of 1:1.3, it was not always possible to satisfy fully the electrical demand. But increasing the ratio of the heat source from MMR to that of CSP up to 2:1 or 3:1, it seemed feasible to increase the electricity production and to satisfy fully the electricity demand; anyway other configurations of grid integration need to be explored [110,111,112]. Tauveron et al. simulated the behavior of supercritical CO₂ SMR with the Brayton cycle instead of the Rankine one, integrated with a CSP system based on a solar tower [113]. The higher operating temperature provided by the CSP allows an increase in the inlet turbine temperature and overall efficiency. The Brayton cycle seems to be particularly appealing for this kind of hybrid system due to its compactness and environmentally friendly characteristics. White et al. developed the thermodynamic modeling of the supercritical steam Rankine cycle for an integrated CSP-lead-cooled fast reactor system [114]. In this case, the author considered a 950 MW nuclear reactor. They observed that the integration of CSP can improve the dispatchability of the energy produced. Naserbegi et al. designed a system composed of CSP and Nuscale SMR able to support, with the heat at a lower temperature, a desalination system [115]. This kind of application is based on a solar tower and a Nuscale reactor and seems very useful for areas where there is a lack of water and energy. They simulated a system composed of a nuclear power plant (150 MWth) and a solar power plant for water desalination on the Makram coast [115]. In addition to a 3% efficiency increase, such a combination allowed the desalination of high amounts of water (82 kg/s) without the release of greenhouse gases in the atmosphere. The main works on hybrid nuclear-CSP power plants are reported in

Table 3. Overview of literature studies on hybrid nuclear and concentrated solar power plants.

Type of Nuclear Reactor	Nuclear Power	Solar Collector	Solar Power	Point Integration	Solar Field	TES	Main Results	REF
Lead Cooled Reactor (LFR)	950 MWth varying PWR 20–1900 MWth	Solar tower	670 MWth TES	CSP parallel with NP Charging Molten Salt	1.27 × 106 m2 (127 ha)	YES	-Hybrid has better ability to dispatch electricity at times when electricity prices are high. -Improves the capability to work neatly with the design point. -Cost saving from turbine economy of scale. −10% benefit on electricity price -Best impact when solar and nuclear are in the same order (250 MW nuclear and 160 MW solar).	[108]
Nuscale	160 MWth	Solar tower	0–71 MWth	NP series with CSP Steam generation	0.36 km2 (Reflective area)	YES	-Solar heat to electrical efficiency 56%. -System can deliver flexible power ranging from 55 to 100% -The LCOE is 78 USD/MWh less than CSP and comparable with the LCOE of nuclear energy.	[110,111].
KAIST MMR Supercritical CO2	36.2 MWth (basic unit) Nuclear to solar ratio 1.3 ÷ 5.7	Solar tower and PTC	63.35 MWth + Reheat 27.15 MWth (CSP + TES)	CSP in parallel to MMR integration in recompression reheating Solar salt –570 °C	0.52 km2	YES	-Reduced solar field area by 2.35 times. -High-capacity factor of MMR than MMR alone; of 2.35 of the solar field. -For remote applications- reduction in transportation and installation. -Does not always satisfy 100% of the energy demand but it is possible to overcome this problem by increasing the ratio of MMR power.	[109]
NuScale SMR	160 MWth	Solar tower Tonopah Solar energy	213.168 MWth Salt at 565 °C	CSP in series with Nuclear CSP integration after the reactor Exchanger Superheater and Reheater Of steam gas turbine	0.41 km2 (9247 heliostats)	YES	-The superheating of steam after the reactor allows an increase in electrical efficiency by 37.5.%. - CSP reaches 56.2% efficiency in the gas turbine. -Reduction in cost and land occupation.	[106]
NuScale SMR	160 MWth	PTC	86 MWth	CSP in series with Nuclear CSP integration after the reactor Exchanger Superheater and Reheater Of steam gas turbine	62.26 ha collector field area (23.36 ha reflective area)	YES	−13.56 cent/kWh. -The superheating of live steam by PTC increases the nuclear efficiency by 33%. -CSP is more efficient for cycle reduction in land occupation and cost reduction.	[112]
Nucsale SMR	160 MWth	Solar tower	86 MWth	Solar tower CSP in parallel		YES	-Brayton cycles instead of Rankine cycles. -Increased efficiency by 11%.	[113]
SmAHTR Fluorinated salt cooled small modular advanced temperature reactor	125 MWth 50 MWel Parametric analysis for 0,50,100 MWe SMR	Solar tower	Parametric analysis for 200, 150, 100 MWe CSP	Parallel SMR and CSP with PCM thermal storage		YES Storage with PCM	-Fluctuation of heat generation, reduced start/stop of generation. -Large TES smaller CSP field. -Higher efficiency. -Optimal conf 200 Mwe 50% nuclear and 50%CSP and 14.8 thermal storage.	[107]
Lead Fast reactor	950 MWth	CSP	750 MWth				-CSP + LFR increases dispatchability.	[114]
NuScale	157 MWth 70.2 MWel -	Solar tower	62.574 MWth	Parallel SMR	93.32 ha helostat field 0.1913 ha reflective area Storage 948 MWhr (15 hr) (nitrate salt) Desalination system MWth 28.47	YES 948 MWhr	-Plant efficiency from 27.03% to 30.18%. -The heat at a lower temperature allows the desalination of water (82.11 kg/s).	[115]

.

As shown in Table 3, it appears evident that the main effort is focused on SMRs and MMRs, and, in particular, on fast reactors, while the main CSP plant considered for the integration is the solar tower. A solar tower, in fact, reaches high temperatures and produces high power at a relatively low cost. The average nuclear power considered is the order of 180 MWth and it is generally combined with a CSP of comparable power. The land occupied by the CSP is usually around 50 ha (hectares), but this can change significantly as a function of TES, coordinates, and percentage of integrated heat. All the studies showed a beneficial effect of the integration of CSP in nuclear power plants, allowing in many cases to improve their efficiency and to increase the production of energy when the price of electricity is high. From the CSP point of view, the nuclear power plant can reduce in a sensible manner the land occupation of the solar field and help the dispatchability of energy. Moreover, the integration of an SMR proves to be crucial to dampen the daily and yearly fluctuations in energy production by CSP, making the hybrid plant far more reliable and integrable in an already existing electric grid. The levelized cost of electricity appears to be, in general, only marginally affected by CSP. Considering the results of this study, it is particularly notable that the increase in efficiency thanks to the integration of 50% of energy by CSP technology allows fulfilling the same energy demand while reducing the nuclear fuel consumption of the SMR system (i.e., extending the refueling time of the reactor). Lower fuel consumption also implies a strong reduction in nuclear waste, posing a significant advantage both in terms of environmental sustainability and safety during handling and reprocessing. We remark that many gaps are still present in this field, and one of them is the need to also study other CSP technology integration with SMR such as, e.g., solar dishes. Further gaps exist, and additional research is needed to determine the optimal stage for integration. For instance, at the preheating stage, it would it possible to use cheaper solar collectors for steam preheating. The study of TES dimensioning in view of several energy demand profiles could be another aspect. In any case, we would like to stress that the combination of CSP with SMR seems to exhibit a wide number of advantages using currently available CSP technologies. It is of paramount importance that the increase in efficiency and reduced consumption of nuclear fuels induces a sensible reduction in SNF, thereby also reducing the risk in the transportation and management of SNF. Finally, SMRs help in the stabilization of energy production by CSP. The integration of SMRs and CSP technologies seems to be especially intriguing for contexts in which the deployment of conventional nuclear reactors is halted for economic reasons. Hybrid plants require a smaller initial investment and leave all possibilities of a scale-up open. This solution can be especially appealing for developing countries where solar energy is particularly convenient, but the economy still relies largely on fossil fuels. In such conditions, the integration with SMRs would allow a smooth transition of solar energy into the already existing electricity grid. Intermittent energy sources would, in fact, need costly and time-consuming adaptations of the civil power infrastructure, an investment that can be significantly lower with the installation of a hybrid SMR-CSP plant. Conversely, for a country willing to invest in nuclear energy but lacking the infrastructure for waste processing and eventually storage, a hybrid concept allows minimizing the investment needed for such aspects thanks to the further reduction in SNF amount and radiotoxicity. It is worth noting that (as highlighted in Table 2) SMRs are significantly more versatile than conventional nuclear power plants in terms of installation sites (a floating water-based SMR is already operational in Russia), mainly thanks to their smaller size and intrinsic safety (negative void and reaction coefficients, smaller emergency planning zones EPZs). This, combined with the limited SNF production, makes their installation range much broader than that of conventional nuclear power plants, especially in combination with CSP technologies. It is necessary to underline that CSP has demonstrated an interesting potential for many industrial processes due to its high efficiency and low cost in drying processes, sterilization, water heating, steam generation, and desalination. The hybrid system CSP + SMR system presents a compelling opportunity for industrial processes, as the generated heat can be efficiently utilized. Modern nuclear energy offers complementary solutions and integrating SMRs with CSP has the potential to unlock significant synergies. CSP and nuclear energy share, in fact, the same thermal technologies for electricity generation, making the integration of the technologies relatively straightforward. We remark that similar hybrid systems can be extended to other renewable sources such as geothermal, wind, photovoltaic, and biomass, theoretically offering interesting mutual benefits and opening the way for a new generation of power plants.

5. Conclusions

The primary focus of this overview is on hybrid power plants that integrate small modular nuclear reactors (SMRs) with concentrated solar power (CSP) technologies. SMRs offer several advantages compared to regular nuclear power plants, including smaller size, higher efficiency, and passive safety systems with negative void and/or temperature coefficients of reactivity. In a loss-of-coolant accident (LOCA), the chain reaction self-extinguishes, making SMRs inherently safer than traditional light water reactors (LWRs). Moreover, the improved efficiency of SMR reduces nuclear fuel consumption, increasing refueling time, and lowering nuclear waste production (both in terms of spent fuel quantity and radiotoxicity). Integrating SMRs with CSP technologies, such as solar towers or parabolic trough collectors, can significantly further amplify these advantages. From the SMR perspective, the combination with a CSP system enhances the reactor dispatchability and increases the overall efficiency of the power plant, resulting in better performance of the steam turbine. The improvement in the reactor efficiency results in even lower nuclear fuel consumption and waste production. The benefits appear to be mutual between SMR and CSP, with the SMR enabling a significant reduction in the required occupation area of the CSP solar field, while also contributing to mitigate the intermittence in the energy production. Currently, the studies are primarily focused on SMRs with a thermal power of 130 MWth integrated with energy provided by solar towers with an average thermal power of 90 MWth. Significant differences in the required size of the solar field were observed, which can be attributed to several factors such as latitude, available DNI dimensions, and percentage of integration. Overall, this review highlights that a balance of about 40 ÷ 50 % of energy output (CSP to SMR) maximizes the benefits of the integration of the two technologies. Nevertheless, further research is necessary to identify the optimal configuration for a wider range of conditions considering the type of integration, the specific application of the hybrid plant, the site latitude, and the availability of solar energy resources. Future research should investigate integrating other heat transfer fluids (e.g., nanofluids) and solar collector types, such as linear Fresnel reflectors (LFRs) and Compound Parabolic Concentrators (CPCs), for HTF preheating. These technologies offer promising integration potential within hybrid systems. Harmonizing the heat transfer fluid across both systems could further simplify plant structure and integration. Another key area requiring further research is the need for comprehensive techno-economic analyses of various integration configurations. Such analyses are essential to determine the optimal economic scenarios for diverse energy demand profiles. Addressing these points is strategic for identifying the most effective options for these innovative technologies. We emphasize the significant role of solar heat in industrial processes, specifically Solar Industrial Process Heat (SHIP) applications, where the produced heat sustains various processes. In these sectors there is great potential because also the heat generated by the CSP + SMR hybrid system can be utilized for diverse applications, including water heating, pasteurization, drying, desalination, distillation, washing, sterilization, curing, and steam production, thus maximizing the use of the energy produced. Furthermore, solar heat can contribute to the production of sustainable fuels like hydrogen and hydrocarbons. Despite the maturity and cost-effectiveness of solar heat technologies, their widespread industrial adoption remains limited, integrating these technologies with small modular reactor (SMR) technologies offers a promising pathway to further decarbonization by optimizing the utilization of heat generated by both processes, which can also be used for industrial processes. Importantly, the hybrid systems combining SMRs and CSP technologies represent a significant advancement in the evolution of nuclear power, potentially marking a new era beyond fourth-generation nuclear technologies.