Conference Agenda

This study introduces the core design philosophy and fuel optimization strategy of an innovative Small Modular Reactor (SMR) currently under development, which adopts a boron-free reactivity control approach. Unlike conventional Pressurized Water Reactors (PWRs), this SMR eliminates the use of soluble boron for core reactivity management, thereby simplifying chemical control systems and enhancing inherent safety.

The initial core is designed to accommodate 69 fuel assemblies, while 39 fresh assemblies are loaded during each equilibrium cycle. To support a long operating cycle of 24 months, the core utilizes low-enriched uranium fuel with an enrichment of 4.95 wt% U-235. The absence of soluble boron poses a significant challenge in maintaining spatial power stability and reactivity control over the cycle.

To address this, the reactor design incorporates multiple in-core detectors distributed throughout the core to monitor spatial power distribution in real time. Control rods are utilized exclusively to ensure sufficient shutdown margin, with no dedicated rods assigned for axial power shaping. Consequently, the axial power profile must be regulated through the physical design of the fuel itself. This necessitates an inherently asymmetric axial fuel configuration to mitigate local peaking and manage power distribution effectively throughout the core life.

Traditionally, the design of such asymmetric fuel assemblies has relied heavily on expert intuition and labor-intensive trial-and-error methods, often with limited predictability of in-core performance. To overcome these limitations, this study implements a Simulated Annealing (SA) algorithm for the systematic optimization of fuel assembly configurations. SA enables the exploration of a wide design space by probabilistically navigating toward optimal solutions, thereby improving prediction accuracy for power distribution and core behavior.

By incorporating SA into the fuel design workflow, the development process becomes more efficient, reducing both time and computational resources. Moreover, this approach enhances overall core reliability and performance consistency, laying the groundwork for robust, boron-free operation in future SMR applications

An integral pressurized water reactor is a type of small modular reactor (SMR) whose operational safety is based on the use of passive safety systems. The SMRs are currently one of the most important energy options in the introduction of new reactor systems. The analyses, presented in the paper, will be carried out for the IRIS (International Reactor Innovative and Secure) reactor, an integral, medium power (1000 MWt), light water reactor. IRIS consists of eight internal cooling loops. It has eight small, spool type, reactor coolant pumps and eight modular, helical coil, once through steam generators. The pressurizer is located in the reactor pressure vessel (RPV) upper head.

There are several passive safety systems and features in the IRIS reactor used for mitigating consequences of an accident: the emergency heat removal system (EHRS), the automatic depressurization system (ADS), emergency boration tanks (EBT), the pressure suppression system (PSS), the long-term gravity make-up system (LGMS) and the injection from the reactor cavity after it is filled with discharged water. They are used for decay heat removal, controlling the reactor pressure, the high pressure and the low pressure water injection. These systems do not require electrical power, but in order to be operable, it is necessary that natural circulation conditions are ensured, that the pipelines are correctly aligned, and that other components, such as check valves, are reliable. The ADS, EHRS and EBT systems are actuated at the very start of the transient and they represent the primary safety measure to mitigate the consequences of an accident. Water injection from the containment tanks (LGMS tanks, PSS pools, reactor cavity) is actuated later, after primary pressure drops below the containment pressure. The pressure difference between the containment and the RPV acts as a driving force for the water discharge. The goal of the paper is to evaluate importance of each safety system by performing calculations with accurately adjusted boundary conditions. The results can be used in determining the optimal accident management strategy. Using the coupled code RELAP5-GOTHIC, the analyses will be conducted in a way that some of the passive safety systems will be considered unavailable. In the worst case, the consequences will lead to a severe accident, when it will no longer be possible to successfully cool the core. The impact of early and late safety measures will be specifically addressed.

Interest in heat-only small modular reactors (SMRs) is growing due to the urgent need for economically viable decarbonization of residential and industrial heating sectors. Heating accounts for more than half of global energy consumption, with the majority still reliant on fossil fuels. While renewable energy sources such as solar, geothermal, and biofuels have demonstrated promising techno-economic potential to replace outdated fossil-based heating systems, their availability is often limited—especially in regions distant from energy consumption centers. Moreover, these renewable sources are subject to natural limitations and generation uncertainties, making them less reliable for meeting the continuous and high-demand heating needs of both residential and industrial applications. This highlights the pressing need for carbon-neutral, reliable, economically competitive, and location-flexible heat supply technologies—needs that heat-only SMRs are well-positioned to meet. From this perspective, the present study investigates the heating energy requirements of both residential and industrial sectors, categorizes the temperature ranges involved, and evaluates the potential deployment of heat-only SMRs, along with the challenges they may face.