Recent events have sparked renewed interest in nuclear energy, particularly in Small Modular Reactors (SMRs). These reactors are much smaller in size, ranging between 30 and 340 MWe, compared to the large-scale pressurized water reactors (PWR) that dominate the world's nuclear fleet, with capacities ranging from 900 to 1650 MWe.

As stated by the International Atomic Energy Agency (IAEA), "Small and medium-sized or modular reactors are an option to fulfill the need for flexible power generation for a wider range of users and applications. Small modular reactors, deployable either as single or multi-module plants, offer the possibility to combine nuclear with alternative energy sources, including renewables."

In France, under the "Innovative Nuclear Reactors" program of France 2030, eight innovative projects were selected in 2023, in addition to the Nuward project initiated a few years earlier. It is worth noting that one of these projects, the Renaissance Fusion project, aims to achieve power levels on the order of gigawatts (GW).

This initiative aims to revitalize the nuclear industry by prioritizing innovation through the introduction of new small-scale nuclear reactors and providing training in nuclear professions.

This effort is particularly responsive to the need for energy decarbonization while maintaining the objective of achieving carbon neutrality by 2050.

The selected projects correspond to various types of reactors, including Pressurized Water Reactors (PWRs), sodium-cooled fast reactors (SFRs), very high-temperature reactors (HTRs), gas-cooled fast neutron reactors (GFRs), lead-cooled fast reactors (LFRs), molten salt reactors (MSRs), and Stellarators for fusion power generation.

The table below summarizes the main characteristics that influence the thermodynamic cycles that can be coupled to these SMRs: thermal and electrical power in MW, core temperature in °C.

Much has been written about these new reactors, most of which, as we can see, offer core technologies that depart from those of pressurized water reactors. However, very little information is available on the thermodynamic cycles that will be coupled to them, as well as on the design and optimization challenges of these cycles.

This is why we believe it is justified to summarize on this page the main contributions that Thermoptim can provide for both training and study needs regarding thermodynamic cycles coupled to nuclear reactors.

In most cases, these cycles constitute the secondary circuit of the system, with the primary circuit being used to cool the reactor core. The separation of the two circuits is generally adopted, primarily for containment of radioactive materials and safety reasons.

For over 25 years, Thermoptim has enabled the modeling of numerous thermodynamic cycles coupled to nuclear reactors.

The book "Les cycles thermodynamiques des centrales nucléaires" edited by Thibaud Normand, Jessica Andreani, and Vincent Tejedor, and published by Presses des Mines, already provided many examples of such models in September 2010

• second-generation power plants known as CP0, Fessenheim type

• so-called P4 nuclear power plants, Paluel type

• N4 nuclear power plants (Westinghouse license)

• EPR third-generation nuclear power plants, Flamanville type

• Fourth-generation nuclear power plants, Superphénix type and supercritical steam

• High Temperature Gas Direct Cycle GT-MHR

• VHTR indirect Brayton cycle

• supercritical CO2 cycles to heat, pre-compression, inter-cooling

A wide array of educational resources is available on this portal to provide training in thermodynamics applied to energy systems, with a particular focus on nuclear cycles. To gain an overview of the recommended approach, we suggest starting by reading the presentation page of our pedagogical method, which significantly diverges from traditional approaches.

Just over a year ago, we published the 2022 course on Energy Systems, which provides a balanced approach for individuals seeking to learn this discipline by minimizing theoretical aspects and focusing on the practical application of powerful exergy analysis methods.

Although this course does not exclusively focus on the thermodynamic cycles of nuclear power plants, the majority of them are covered here.

After completing the entirety of the first and second parts of the course, as well as the beginning of the third part on steam power plants, where PWR reactors are studied, you can proceed directly to the module on High-temperature nuclear cycles in the fourth part, which covers Brayton and supercritical CO2 cycles.

Of course, if you wish, you can explore the numerous other cycles presented in the other modules of the third and fourth parts, which are not specifically related to nuclear power.

You can then delve deeper into specific topics by referring to the pages listed below.

The 2022 online course on Energy Systems offers the advantage of utilizing guided explorations of pre-built Thermoptim models to introduce students to all the studied cycles, facilitated by the user-friendly Thermoptim Browser tool.

Additionally, there is a training module (available only in French) specifically focused on various thermodynamic cycles for generating electricity from current or future nuclear reactors. This module, developed around 2010, employs a different pedagogical approach compared to the previous course. It features online course sessions with soundtracks called Diapason, aiming to instruct students on how to independently create Thermoptim models, which is more challenging than utilizing guided explorations.

Models of thermodynamic cycles of nuclear power plants are available on several pages of this portal :

• Advanced Supercritical Steam Cycle. A thematic page focuses on this type of cycle. It should be noted that it is not specific to nuclear applications, as supercritical cycles are also used in fossil fuel power plants.

• Pressurized Water Nuclear Power Plants (PWR). A  thematic pagepresents this type of power plant, and a guidance page allows you to model one using the available resources

• High Temperature Nuclear Cycles (HTR). This thematic page presents some of the main types of high-temperature nuclear power plants recently considered, including combined cycles.

• Supercritical CO2 power generation cycles. This thematic page presents the main types of supercritical CO2 cycles currently being considered for power generation.

Recently, new models have been developed, specifically aimed at studying the operation of such cycles under off-design conditions.

A driver for a pressurized water reactor steam generator operating in off-design conditions enables the investigation of a relatively complex issue: the adaptation of a pressurized water reactor steam generator to variations in its feed conditions.

This driver relies on an external class called TechnoSteamGenerator, which has been developed to model a steam generator under off-design conditions by distinguishing between zones corresponding to the economizer, the vaporizer, and the superheater.

To achieve this, it incorporates the calculation of pressure losses and the adjustment of pressures inside the steam generator, while the remaining calculations are conducted in the usual manner. Additionally, it employs a generic method for considering nucleate boiling.

Thermoptim offers two complementary categories of cycle optimization tools: the thermal integration optimization method and the exergy methods

Thermal integration optimization method

If we set aside the relatively complex theory underlying this method, as explained in previous sections (derived from the pinch method with the distinction of components and system irreversibilities), the Thermoptim optimization method is relatively straightforward to present and utilize.

A guidance page and a guided exploration can assist in familiarizing yourself with it.

It's important to note that this is a variant for energy systems of the Linnhoff method, with the latter being applied to the design of complex heat exchanger networks involving a large number of fluids, as commonly encountered in chemical engineering. This method is highly effective when designing a heat exchanger network, such as to reduce internal irreversibilities in a combined cycle or cogeneration plant.

Exergy methods

The performance analysis of technologies typically involves calculating their energy balances in a conventional manner. Additionally, when attempting to optimize a system, establishing its exergy balance is of significant interest, as it quantifies the irreversibilities.

Preparing an exergy balance doesn't inherently pose particular difficulty but requires careful attention to detail, as errors can occur due to the non-conservative nature of these balances. Various tools for preparing exergy balances are provided in this portal: a methodological guide and a spreadsheet for simple systems, as well as the use of productive structures, which automate the construction of these balances for both simple and complex systems.