Control and shutdown system for small and micro nuclear reactors

Abstract

A nuclear reactor, such as particularly a small or micro nuclear reactor, implementing an arrangement of diverse and redundant control and safety systems that eliminates the need for the conventional control rod system, replacing it with simpler, more streamlined safety mechanisms. Specifically, the control and safety system of the invention is characterized by high diversity and redundancy, wherein multiple systems are employed for reactivity control, in which the system include at least two, preferably at least three of the following systems Control absorbing plate system; Moderator block with layered absorbing material system; Absorbing cylinder system; and Gaseous absorber injection system.

Description

CONTROL AND SHUTDOWN SYSTEM FOR SMALL AND MICRO NUCLEAR REACTORSRELATED APPLICATIONS

[0001] This application claims the benefit of U.S. Patent Application Serial Number 63 / 744,146 filed January 10, 2025 titled “Control and Shut Down System for Small and Micro Nuclear Reactors” which is incorporated herein by reference.BACKGROUND OF THE INVENTION

[0002] 1. FIELD OF THE INVENTION

[0003] This invention generally relates to small and micro nuclear reactors configured with the nuclear fuel in a geometric layout which allows criticality of the reactor core to be achieved and sustained over a long period of time using nuclear fuel, and to a control and shut down system used therein.

[0004] 2. BACKGROUND INFORMATION

[0005] Nuclear reactors are the heart of a nuclear power plant. Nuclear reactors contain and control nuclear chain reactions that produce heat through a physical process called fission. The heat from fission is used to make steam that spins a turbine to create electricity. As of 2025, the International Atomic Energy Agency reported there are 417 nuclear power reactors and 226 nuclear research reactors in operation around the world. With more than 400 commercial nuclear reactors worldwide, including 93 in the United States, nuclear power continues to be one of the largest sources of reliable carbon-free electricity available.

[0006] The main job of a reactor is to house and control nuclear fission, a process where atoms split and release energy. Reactors generally use uranium for nuclear fuel. The uranium is processed into small ceramic pellets and stacked together into sealed metal tubes called fuel rods. Typically, more than 200 of these rods are bundled together to form a fuel assembly. A reactor core is typically made up of a couple hundred assemblies, depending on power level. Inside the reactor vessel, the fuel rods are, in current designs, immersed in fluid such as commonly water which acts as both a coolant and moderator. A moderator, discussed further below, helps slow down the neutrons produced by fission to sustain the chain reaction. Control rods can then be inserted into the reactor core to reduce the reaction rate or withdrawn to increase it. The heat created by fission is typically used to turn water into steam, which spins a turbine to produce carbon-free electricity.Petruzzi et al Page 1 of 16 Patent Application

[0007] A key aspect in the design of a nuclear reactor is the ability to reach and maintain criticality during the cycle of operation. The method by which the nuclear fission reaction is sustained varies among different designs. In simple terms, the ability to remain critical is in generating more neutrons than the total of the neutrons consumed by the fission reaction and other reactions occurring in the system and those neutrons that leak form the boundary of the reactor.

[0008] Broadly speaking there are different type of reactors depending on the energy spectrum of the neutrons. The thermal-neutron reactors are the one where the high energetic neutrons emerging from the fission diffuses, scatters and in the process lose energy to reach ‘thermal equilibrium’ with the surrounding. These ‘slow’ neutrons are peculiar because, in that regime, the probability of creating a fission in the fertile material present in the core (e.g., U-235) increases as the inverse of the velocity of the neutron. These are called ‘Thermal Neutron Reactors’. To thermalize the neutrons, special material called ‘moderators’ are used in the reactor. In Light Water Reactor (LWR) technology, which forms the majority of the current operating fleet of reactors, the coolant itself (water) is used as moderator and coolant at the same time as suggested above. In other technologies such as the CANDU reactors, or generically referred as Heavy Water Reactors (HWR), the cooling and moderating functions are decoupled. Heavy water (D2O) collected in a ‘calandria’ surrounding the fuel is used as moderator while light water (H2O) flow in pressurized tubes containing the fuel assembly to remove the heat generated. The term CANDU stands for Canada Deuterium Uranium and has been identified as Canadian pressurized heavy-water reactor design used to generate electric power. The acronym refers to its deuterium oxide (heavy water) moderator and its use of (originally, natural) uranium fuel. CANDU reactors were first developed in the late 1950s and 1960s by a partnership between Atomic Energy of Canada Limited (AECL), the Hydro-Electric Power Commission of Ontario, Canadian General Electric, and other companies.

[0009] In all these reactors the fuel is based on uranium dioxide (UO2). Pellets of UO2 are encapsulated into tubes (pins) made of Zirconium (Zr) alloy, and these pins are referenced herein as standard fuel which is form the basis of most commercially available nuclear fuel today. In nature, Uranium comes with an isotopic composition, primarily (99.3) formed by U-238 and about 0.7% by U-235. In thermalized reactors, fission occurs primarily with the U-235 and to achieve criticality the percentage of U-235 must be increased through a process called enrichment. For LWRs an enrichment up to 5% in U-235 is required to achieve criticality.Petruzzi et al Page 2 of 16 Patent Application

[0010] Other details on the LWR and HWR technologies are beyond the scope of this patent description, as the present invention relates to control and shut down systems in small and micro-reactors. A conventional nuclear reactor may be around 100MWe. In contrast nuclear micro-reactors are very small reactors usually generating less than 50 megawatts electric (MWe), whereas a small nuclear reactor is often around 50-300 MWe. For the purpose of this application a conventional reactor will be that generating greater than 500 MWe, while a small nuclear reactor is that generating 50-450 MWe, and a micro nuclear reactor is that generating less than 50 MWe.

[0011] Small and micro nuclear reactors are advanced nuclear reactors that often feature modularity to optimize construction schedules and reduce costs (and sometimes referenced as SMR for Small Modular reactors. SMRs are designed to create synergies between nuclear and distributed energy resources, particularly renewables, and subsequently tighten the coupling of these complementary technologies through various non-electrical applications. As an aside the International Atomic Energy Agency (IAEA) has indicated that together, these technologies are expected to contribute to climate change mitigation, for example by supporting the achievement of net zero emissions by 2050. The IAEA reports that in 2025, more than 70 design concepts from major lines of technologies are at various stages of development and deployment. At least 20 IAEA Member States are engaged in national and international activities involving SMRs, and the commercial operation of SMRs is now under way. The IAEA and other international organizations have published a wide range of resources on SMRs, addressing topics such as design, technology, engineering, economics, safety, safeguards, security and infrastructure development.

[0012] The core of nuclear reactors, including micro and small reactors, are typically designed with an excess of reactivity to ensure they can operate effectively throughout their intended lifespan. The control system is designed to compensate for the excess reactivity, to bring and to keep the reactor at critical condition. Conversely, the shutdown system or safety system is designed to introduce significant negative reactivity, driving the reactor into a subcritical state to halt the power generation and to allow for reactor shutdown. The control of the reactivity surplus is generally managed through the use of: Burnable absorbers; Absorbers diluted in water; and Control rods.

[0013] Burnable absorbers (BAs), also known as burnable neutron poisons, are materials inserted into a nuclear reactor core that contain non-fissile nuclei with large neutronabsorption cross sections. The BAs absorb neutrons, decreasing their population significantly over the course of the core’s operational cycle. As the BA abundance Petruzzi et al Page 3 of 16 Patent Applicationdecreases, so too does the BA’s impact on reactivity. BAs can provide a variety of benefits, including reactivity control for extended fuel cycles. While BAs can vary widely based on economics, thermal hydraulics, manufacturing, response to radiation damage, and reprocessing / disposal, BAs have a strong negative reactivity worth, which of course decreases by design as they are depleted due to neutron absorption. Burnable absorbers are often used to reduce power peaking in the core, either for a fresh fuel assembly (typically in light water reactors (LWRs)) or locally within a fuel pin or plate (typically in research and test reactors). The negative reactivity of a BA offsets the positive reactivity of fresh fuel. Depletion of fuel is accompanied by depletion (burnout) of the BA material; ideally, the BA will effectively burn out at the same rate as the fuel in such a way that the net reactivity remains constant with time. BAs are considered distinctly separate from control rods. Control rods in nuclear reactors can be withdrawn from the reactor core during operation and can be used for several years to decades before replacement is necessary. Other neutron poisons that are present in the core that are released as fission products are also not considered BAs; instead, they are an unavoidable consequence of nuclear fission reactions. The most common BA materials in commercial LWRs are boron and gadolinium. Although not as common, hafnium-based materials have been proposed as attractive BA candidates, not only because of their ability to readily absorb neutrons above thermal energies (a phenomenon known as resonance energy absorption), but also because it’s worth changes slowly with irradiation. Cadmium-based BAs have been proposed due to asserted attractive neutronic properties for extended fuel cycles. The use of Cd-based BAs, however, presents fabrication and disposal complications due to significant health concerns. Both Cd and CdO are ranked as a health hazard Level 4 according to the National Fire Protection Association. Very short exposure to small amounts of either of these materials can be lethal, as they are carcinogenic, can cause lung oedema and death if inhaled, and are extremely toxic if ingested. Further, the 350 °C melting point of cadmium metal limits its applicability, particularly during a possible design-basis accident.

[0014] The reactor safety or shut down system, on the other hand, is typically ensured by a SCRAM system generally implementing control rod insertion, and injection of a liquid neutron absorber.

[0015] A SCRAM system is generally an emergency shutdown of a nuclear reactor effected by immediately terminating the fission reaction. It is also the name that is given to the manually operated kill switch that initiates the shutdown. In commercial reactor operations, this type of shutdown is often referred to as a "scram" at boiling water reactors, a Petruzzi et al Page 4 of 16 Patent Application"reactor trip" at pressurized water reactors and "EPIS" at a CANDU reactor. In many cases, a scram is part of the routine shutdown procedure which serves to test the emergency shutdown system. Scram is sometimes cited as being an acronym for “safety control rod axe man’’ or “safety cut rope axe man" and was supposedly coined by Enrico Fermi when he oversaw the construction of the world's first nuclear reactor.

[0016] In any reactor, a scram is achieved by inserting large amounts of negative reactivity mass into the midst of the fissile material, to immediately terminate the fission reaction. In light-water reactors, this is achieved by inserting neutron-absorbing control rods into the core, although the mechanism by which rods are inserted depends on the type of reactor. In pressurized water reactors the control rods are held above a reactor's core by electric motors against both their own weight and a powerful spring. A scram is designed to release the control rods from those motors and allows their weight and the spring to drive them into the reactor core, rapidly halting the nuclear reaction by absorbing liberated neutrons. Another design uses electromagnets to hold the rods suspended, with any cut to the electric current resulting in an immediate and automatic control rod insertion. In boiling water reactors, the control rods are inserted up from underneath the reactor vessel. In this case a hydraulic control unit with a pressurized storage tank provides the force to rapidly insert the control rods upon any interruption of the electric current. In both the PWR and the BWR there are secondary systems (and often even tertiary systems) that will insert control rods in the event that primary rapid insertion does not promptly and fully actuate.

[0017] Liquid neutron absorbers (neutron poisons) are also used in rapid shutdown systems (Scram Systems) for heavy and light water reactors. Following a scram, if the reactor (or section(s) thereof) are not below the shutdown margin (that is, they could return to a critical state due to insertion of positive reactivity from cooling, poison decay, or other uncontrolled conditions), the operators can inject solutions containing neutron poisons directly into the reactor coolant. Neutron poison solutions are water-based solutions that contain chemicals that absorb neutrons, such as common household borax, sodium polyborate, boric acid, or gadolinium nitrate, causing a decrease in neutron multiplication, and thus shutting down the reactor without use of the control rods. In the PWR, these neutron absorbing solutions are stored in pressurized tanks (called accumulators) that are attached to the primary coolant system via valves. A varying level of neutron absorbent is kept within the primary coolant at all times, and is increased using the accumulators in the event of a failure of all of the control rods to insert, which will promptly bring the reactor below the shutdown margin. Petruzzi et al Page 5 of 16 Patent ApplicationIn the BWR, soluble neutron absorbers are found within the standby liquid control system, which uses redundant battery-operated injection pumps, or, in the latest models, high pressure nitrogen gas to inject the neutron absorber solution into the reactor vessel against any pressure within. Because they may delay the restart of a reactor, these systems are only used to shut down the reactor if control rod insertion fails. This concern is especially significant in a BWR, where injection of liquid boron would cause precipitation of solid boron compounds on fuel cladding, which would prevent the reactor from restarting until the boron deposits were removed. In most reactor designs, the routine shutdown procedure also uses a scram to insert the control rods, as it is the most reliable method of completely inserting the control rods, and prevents the possibility of accidentally withdrawing them during or after the shutdown.

[0018] There is a need in the art to design small and micro nuclear reactors configured with diverse and redundant control and safety systems.SUMMARY OF THE INVENTION

[0019] The various embodiments and examples of the present invention as presented herein are understood to be illustrative of the present invention and not restrictive thereof and are non-limiting with respect to the scope of the invention.

[0020] One aspect of the present invention provides a nuclear reactor, such as particularly a small or micro nuclear reactor, implementing an arrangement of diverse and redundant control and safety systems that eliminates the need for the conventional control rod system, replacing it with simpler, more streamlined safety mechanisms. Specifically, the control and safety system of the invention is characterized by high diversity and redundancy, wherein multiple systems are employed for reactivity control, in which the system include at least two, preferably three of the following systems control absorbing plate system; moderator block system; absorbing cylinders system; and gaseous absorber injection system.

[0021] A notable characteristic of the control absorbing plate system; moderator block with layered absorbing material system; and the absorbing cylinders system is that these control and safety mechanisms are housed within the stainless-steel vessel. The gaseous absorber injection system, however, is located externally to shield the absorbing material from neutron flux, thereby preventing material consumption.

[0022] The small or micro nuclear reactor according to one aspect of the invention provides wherein the reactor core includes a plurality of conductive solid moderator assemblies within Petruzzi et al Page 6 of 16 Patent Applicationa vessel and each conductive solid moderator assembly with a solid moderator with embedded nuclear fuel pins comprising nuclear material and cladding. The solid moderator of each conductive solid moderator assembly may be formed of Beryllium (Be), Beryllium Oxide (BeO), Graphite (C) and combinations thereof.

[0023] One aspect of the present invention provides that the control and safety system includes a control absorbing plate system having a plurality of horizontally radially movable plates. A portion of the control absorbing plate may actively regulate reactor operations during normal functioning, ensuring stable and controlled performance and wherein another portion of the plate is kept in reserve and selectively deployed, specifically designated for safely shutting down the reactor in the event of an accidental or emergency condition. The control absorbing plates may be designed to move along dedicated guides associated with a horizontal linear insertion mechanism allowing precise adjustment of their proximity to the components of the core.

[0024] One aspect of the invention provides that the control and safety system includes a moderator block system with a portion of the block system formed as neutron reflectors and the moderator block system includes absorbing material. The moderator block system may be configured, in accidental conditions, to move in a coordinated manner with actuators whereby the neutron reflector material of moderator block is moved farther away from the core, reducing the reactivity, and the absorbing material of the block system is moved toward the core, wherein the absorbing material of the block system is configured to stop at the core's center, where it introduces the maximum possible negative reactivity, and which is enough to effectively suppress the nuclear chain reaction. The absorbing material of the block system may be effectively formed of control plates of a control plate absorbing system.

[0025] One aspect of the invention provides that the control and safety system includes a gaseous absorber injection system, wherein the gaseous absorber injection system includes safety channels configured to allow for the injection of a gaseous absorbing material from a source into the reactor core, and wherein the gaseous absorber source is a pressurized vessel outside of the core. The control and safety system may include an absorbing cylinders system including a plurality of cylinders each of which includes chemical elements that are capable of absorbing neutrons that can be directed into the core, wherein the wherein absorbing cylinders are configured to move along the safety channels.

[0026] These and other advantages of the present invention are described below in connection with the attached figures in which like reference numerals represent like elements throughout.Petruzzi et al Page 7 of 16 Patent ApplicationBRIEF DESCRIPTION OF THE FIGURES

[0027] FIGURE 1 is a schematic radial section view of a reactor core for a small or micro nuclear reactor implementing a control absorbing plate system according to one aspect of the present invention.

[0028] FIGURE 2 is a schematic view of the view of the integrated moderator block system and control absorbing plate system for a small or micro nuclear reactor according to one aspect of the invention.

[0029] FIGURE 3 is a schematic view of the view of the absorbing cylinders system; and gaseous absorber injection system for a small or micro nuclear reactor according to one aspect of the invention.BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0030] One aspect of the present invention provides a nuclear reactor, such as particularly a small or micro nuclear reactor, comprising a reactor core 10. The reactor core 10 in one embodiment may have a plurality of conductive solid moderator assemblies 20 each with a solid moderator with embedded nuclear fuel pins comprising nuclear material and cladding, wherein the solid moderator is formed of Beryllium (Be), Beryllium Oxide (BeO), Graphite (C) and combinations thereof. The conductive solid moderator assemblies 20 may be arranged within a second moderator 30 embedded or formed of material different from the solid moderator material of the conductive solid moderator assemblies. The nuclear material may, preferably be uranium with uranium isotopic enrichment limited to less than 5% in U-235. The nuclear reactor according to the invention may be designed for power cycles that exceed ten years.

[0031] One key feature of the present invention provides a nuclear reactor, such as particularly a small or micro nuclear reactor, implementing an arrangement of diverse and redundant control and safety systems that eliminates the need for the conventional control rod system, replacing it with simpler, more streamlined safety mechanisms. Specifically, the control and safety system (shown in figures 1-3) of the invention is characterized by high diversity and redundancy, wherein multiple systems are employed for reactivity control, in which the system include at least two of the following systems control absorbing plate system 40; moderator block 80 with layered absorbing material system; absorbing cylinders 60 system; and gaseous absorber injection system 70.Petruzzi et al Page 8 of 16 Patent Application

[0032] Power control according to the present invention in a small or micro nuclear reactor is effectively managed through the use of a control-absorbing plate system shown in figure 1. Control-absorbing plates 40 play a crucial role in regulating the reactor's neutron flux and maintaining safe operating conditions.

[0033] These plates 40 are designed with a straight forward horizontal linear insertion mechanism 50 that operates through horizontal movement radially toward and away from the core 10 components, offering a simplified yet reliable approach in executing the associated control function. The insertion system 50 is shown schematically in figure 1 and a single linear actuator 50 may independently operate a pair of plates 40. The plates system 40 may be integrated with the moderator blocks 80 discussed below. Unlike traditional systems that require substantial vertical space above the reactor core 10 to house control rods, the control-absorbing plate system 40 design of the present invention eliminates that necessity. Instead, the entire absorbing plate 40 is strategically positioned within the reactor vessel itself, optimizing spatial efficiency. This configuration not only reduces the overall size of the reactor structure of the core 10 but also enhances its compactness and design flexibility, making it well-suited for applications where space constraints or modularity are critical.

[0034] The control-absorbing plate system 40 is designed with a strategic approach that utilizes a portion of the control-absorbing plates 40 (safety plates) required to shut down the reactor under the most reactive conditions. In this configuration, a portion of the plates 40 actively regulate reactor operations during normal functioning (which portion may be considered control plates 40), ensuring stable and controlled performance. The other portion (also known as safety plates) is kept in reserve and selectively deployed, specifically designated for safely shutting down the reactor in the event of an accidental or emergency condition. The control plates 40 of the system 40 may have less range of motion by actuators as in normal operating conditions they will be in a regulating position rather than a removed position of the safety plates 40. This dual-purpose design enhances the system's reliability and safety by maintaining a dedicated set of safety absorbing plates 40 solely for contingency scenarios, ensuring prompt and effective reactor shutdown when necessary.

[0035] The plates 40 (both safety and control plates 40) are designed to move along dedicated guides associated with the horizontal linear insertion mechanism 50, allowing precise adjustment of their proximity to the hexagonal vessel and components of the core 10. Any precision moving mechanism for moving the control-absorbing plates 40 may bePetruzzi et al Page 9 of 16 Patent Applicationimplemented, however simple linear actuators can provide the precise radial horizontal movement required for mechanisms 50.

[0036] When the plates 40 are in the fully inserted position, they are at their closest distance to the hexagonal reactor vessel shell (RVS) of the core 10, maximizing their neutron absorption effect. Conversely, the fully extracted position represents the farthest distance of the plates 40 from the hexagonal vessel of the core 10, minimizing their interaction with the reactor core and the neutron population. This arrangement provides fine-tuned control over the reactor's neutron flux and operational stability.

[0037] Error! Reference source not found, illustrates the moderator blocks system 80, and in the preferred design each block 80 is equipped with two control-absorbing plates 40 strategically positioned around their perimeter. This configuration serves as an example of the system's design, demonstrating one possible arrangement incorporating a dual functioning control. It should be noted that various geometries can be employed beyond the specific representation shown here, offering flexibility to adapt to different reactor designs and operational requirements.

[0038] In nuclear engineering, a neutron moderator is a medium that reduces the speed of fast neutrons, ideally minimizing the capture of any, leaving them as thermal neutrons with only minimal (thermal) kinetic energy. These thermal neutrons are immensely more susceptible than fast neutrons to propagate a nuclear chain reaction of uranium-235 or other fissile isotope by colliding with their atomic nucleus. Classically, moderators include precision-machined blocks of high-purity graphite, which can be utilized for portions of the blocks 80. A neutron reflector in the art is any material that reflects neutrons. This refers to elastic scattering rather than to a specular reflection. Neutron reflectors include graphite, beryllium, steel, tungsten carbide, and gold. A neutron reflector can make an otherwise subcritical mass of fissile material critical or increase the amount of nuclear fission that a critical or supercritical mass will undergo. A reflector made of a light material like graphite or beryllium will also serve as a neutron moderator reducing neutron kinetic energy, and the light weight materials are preferred for the moderator blocks 80.

[0039] The central part of the section houses the moderator block 80, which plays a critical role in the reactor's operation. At the topmost section of this block 80 there is a layer of absorbing material. These blocks 80 can be designed in various shapes, depending on the reactor's specific requirements. The fundamental principle behind their operation lies in their dual function as neutron reflectors and absorption materials. As discussed above, reflectors are components that bounce escaping neutrons back into the reactor core, enhancing the Petruzzi et al Page 10 of 16 Patent Applicationlikelihood of further fission reactions and thus increasing the system's reactivity. When these reflector portion of the blocks 80 are removed, the neutron economy is negatively affected, resulting in a decrease in the system’s reactivity.

[0040] In accidental conditions, these blocks 80 are moved in a coordinated manner with actuators like actuators 50. This simultaneous movement achieves two objectives: the reflector material of moderator 80 is removed or moved farther away from the core 10, reducing the reactivity, and the absorbing material like plates 40 is inserted into the core 10. The absorber portion (plates 40) of the blocks 80 is designed to stop at the core's center, where it introduces the maximum possible negative reactivity, which is enough to effectively suppress the nuclear chain reaction.

[0041] The system's safety is enhanced by requiring fewer components than those available to shut it down, ensuring redundancy. This means that even if some components fail, the system can still be safely brought into a shutdown state.

[0042] Additionally, the safety shutdown system preferably incorporates two extra diverse mechanisms to enhance its reliability shown in figure 3, each of them with some redundancy. The first mechanism involves absorbing material cylinders 60 positioned at the top of the reactor. The compositions of material cylinders 60 include chemical elements such as boron, cadmium, silver, hafnium, or indium, that are capable of absorbing many neutrons without themselves decaying. These cylinders 60 function in a manner similar to traditional control rods but are designed with a reduced length to optimize their deployment and efficiency and minimize space requirements. During a shutdown event, these cylinders 60 are released into the reactor's fuel assemblies via dedicated safety channels 72. These channels 72 are carefully aligned to terminate at the core's center — the region with the highest reactivity. This precise positioning is critical, as the core center is where the neutron flux is most intense, making it the most effective location to disrupt the nuclear chain reaction. When the absorbing material of cylinders 60 is inserted into this central zone, it captures free neutrons that would otherwise sustain the chain reaction. By reducing the availability of these neutrons, the reaction is rapidly suppressed, bringing the reactor to a safe shutdown state. This mechanism ensures a quick and reliable response, effectively neutralizing the core's reactivity. As with other safety features, this system is designed to achieve full reactor shutdown using fewer components than those available, ensuring redundancy. This layered approach minimizes the risk of a failure compromising reactor safety, reinforcing the overall robustness and dependability of the shutdown system.Petruzzi et al Page 11 of 16 Patent Application

[0043] These safety channels 72 offer enhanced versatility, as they are also designed to allow for the injection of He3, a gaseous absorbing material from source 70, into the reactor core 10 if needed. This capability provides an additional layer of diversity (and at the same time redundancy), ensuring the system's ability to shut down the reactor even if the absorbing cylinders 60 are obstructed or fail to deploy properly. Enabling the same channels 72 to accommodate both solid and gaseous absorbers, the system maximizes its efficiency and reliability without the need for separate infrastructure.

[0044] The gaseous absorbing material from external source 70 through piping 74 works by rapidly diffusing into the safety channel 72 region and interacting with neutrons, significantly reducing the neutron population and effectively halting the nuclear chain reaction. This approach is particularly advantageous in emergency scenarios, as the gas can quickly fill the reactive zone, ensuring a prompt and thorough shutdown. To enhance operational safety and reliability, the absorbing gas is stored in high-pressure tanks 70 located outside the reactor vessel or core 10. This placement serves two critical purposes: first, it ensures the gas remains shielded from the reactor environment, protecting it from radiation exposure and potential degradation of its neutron absorption capability during normal operation; second, it prevents accidental consumption or loss of the gas. The high-pressure storage at source 70 facilitates the fast injection into the core when needed, enabling the system to respond effectively to emergency situations. As noted, figure 3 schematically illustrates an example of these dual-function safety systems, highlighting their design and operational flexibility within the reactor's safety framework.

[0045] The above description is representative of the present invention but not restrictive thereof. The full scope of the present invention is set forth in the appended claims and equivalents thereto.Petruzzi et al Page 12 of 16 Patent Application

Claims

WHAT IS CLAIMED IS:

1. A small or micro nuclear reactor comprising a reactor core and a control and safety system which is characterized by including at least two of the following systems a control absorbing plate system; a moderator block system; absorbing cylinders system; and gaseous absorber injection system.

2. The small or micro nuclear reactor according to claim 1 , wherein the reactor core includes a plurality of conductive solid moderator assemblies within a vessel and each conductive solid moderator assembly with a solid moderator with embedded nuclear fuel pins comprising nuclear material and cladding.

3. The small or micro nuclear reactor according to claim 2, wherein the solid moderator of each conductive solid moderator assembly is formed of Beryllium (Be), Beryllium Oxide (BeO), Graphite (C) and combinations thereof.

4. The small or micro nuclear reactor according to claim 3, wherein the control and safety system includes a control absorbing plate system having a plurality of horizontally radially movable plates.

5. The small or micro nuclear reactor according to claim 4, wherein a portion of the control absorbing plate actively regulate reactor operations during normal functioning, ensuring stable and controlled performance and wherein another portion of the plate is kept in reserve and selectively deployed, specifically designated for safely shutting down the reactor in the event of an accidental or emergency condition.

6. The small or micro nuclear reactor according to claim 5, wherein the control absorbing plates are designed to move along dedicated guides associated with a horizontal linear insertion mechanism allowing precise adjustment of their proximity to the components of the core.

7. The small or micro nuclear reactor according to claim 3, wherein the control and safety system includes a moderator block system with a portion of the block system formed as neutron reflectors.

8. The small or micro nuclear reactor according to claim 7, wherein the moderator block system includes absorbing material.

9. The small or micro nuclear reactor according to claim 8, wherein the moderator block system is configured, in accidental conditions, to move in a coordinated manner with actuators whereby the neutron reflector material of moderator block is movedPetruzzi et al Page 13 of 16 Patent Applicationfarther away from the core, reducing the reactivity, and the absorbing material of the block system is moved toward the core.

10. The small or micro nuclear reactor according to claim 9, wherein the absorbing material of the block system is configured to stop at the core's center, where it introduces the maximum possible negative reactivity, and which is enough to effectively suppress the nuclear chain reaction.

11. The small or micro nuclear reactor according to claim 8, wherein the absorbing material of the block system is formed of control plates of a control plate absorbing system.

12. The small or micro nuclear reactor according to claim 3, wherein the control and safety system includes a gaseous absorber injection system.

13. The small or micro nuclear reactor according to claim 12, wherein the gaseous absorber injection system includes safety channels configured to allow for the injection of a gaseous absorbing material from a source into the reactor core.

14. The small or micro nuclear reactor according to claim 13, wherein the gaseous absorber source is a pressurized vessel outside of the core.

15. The small or micro nuclear reactor according to claim 13, wherein the wherein the control and safety system includes an absorbing cylinders system including a plurality of cylinders each of which includes chemical elements that are capable of absorbing neutrons that can be directed into the core.

16. The small or micro nuclear reactor according to claim 15, wherein the wherein absorbing cylinders are configured to move along the safety channels.

17. The small or micro nuclear reactor according to claim 3, wherein the wherein the control and safety system includes an absorbing cylinders system including a plurality of cylinders each of which includes chemical elements that are capable of absorbing neutrons that can be directed into the core along safety channels.

18. The small or micro nuclear reactor according to claim 17, wherein the control and safety system includes a gaseous absorber injection system, wherein the gaseous absorber injection system is coupled to the safety channels to allow for the injection of a gaseous absorbing material from a source into the reactor core.

19. The small or micro nuclear reactor according to claim 18, wherein the control and safety system includes a moderator block system with a portion of the block system formed as neutron reflectors and wherein the moderator block system includes absorbing material.Petruzzi et al Page 14 of 16 Patent Application20. The small or micro nuclear reactor according to claim 1 wherein the nuclear reactor is designed for power cycles that exceed ten years.Petruzzi et al Page 15 of 16 Patent Application

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