Small integrated modular nuclear reactor without refueling operating as vertical dry storage for irradiated fuel at the reactor end of life

Abstract

An integrated PWR nuclear reactor that requires no refueling and is cooled by natural convection. It is housed within a reactor containment unit which is transportable to a placement site and placed within an underground building that allows heat transfer from the reactor to the surrounding soil. Tire reactor containment unit encloses an integrated PWR nuclear reactor, a pressure and chemical control system, a safety system and a turbo generator. After its operational life, the reactor's water and air are replaced with inert gases, circulating by natural convection to remove decay heat to the surrounding soil. This enables the integrated PWR reactor, initially designed to generate electricity, to act as a dry storage facility for uranium fuel elements after the end of its operational life.

Description

United States Provisional Patent ApplicationTITLE: SMALL INTEGRATED MODULAR NUCLEAR REACTOR WITHOUT REFUELING OPERATING AS VERTICAL DRY STORAGE FOR IRRADIATED FUEL AT THE REACTOR END OF LIFETECHNICAL FIELD

[0001] This invention relates generally to the field of nuclear reactor technology. More particularly, this invention pertains to an integrated Pressurized Water Reactor (PWR) system designed for underground installation.BACKGROUND OF THE INVENTION

[0002] Nuclear reactor construction currently faces the challenge of unexpectedly high investment costs. Today, Small Modular Reactors (SMRs) are considered an attractive alternative to overcome such difficulties by maximizing construction activities at module manufacturing facilities and minimizing on-site construction efforts. SMRs utilize the same reactor technology as current commercial reactors, primarily Pressurized Water Reactors (PWRs) and, to a lesser extent. Boiling Water Reactors (BWRs). They operate at reduced power levels and incorporate different reactor configurations, including passive security systems and natural convection of the primary circuit water.

[0003] The concept of SMRs is not new; Westinghouse proposed a similar approach in 1955 as an alternative to large PWRs. The International Atomic Energy Agency (IAEA) held a conference on SMRs in 1960, then referred to as Small and Medium Power Reactors. These early SMRs were factory-constructed modules transported and assembled on-site. Recently, the IAEA defined a Transportable Nuclear Power Plant (TNPP) as a transportable nuclear- power plant manufactured in a factory and capable of generating power when loaded with fuel. This definition applies to a nuclear reactor with two options: with or without fuel installation at the SMR production plant. Pre-Loaded TransportableNuclear Power Plants (PL-TNPP) are described as reactors where the reactor module is pre-loaded with nuclear fuel at the same factory’ that built the reactor. PL-TNPPs of the PWR type are not new either. US Patent Nr. US3086933A, filed by Nagey et al. in 1960. described a PWR PL-TNPP reactor and the TES-3, a 1 ,5-megawatt electric reactor installed in four self-propelled tracked vehicles, began operations in 1961. It was produced at the Kirov Factory in Leningrad.

[0004] Today, various countries have introduced different models of PL-TNPPs as prototypes, including ABV-6M, UNITHERM, SHELF, and FLEXBLUE. These models, according to recent IAEA publications, are proposed with and without refueling during operational life.

[0005] PL-TNPPs require nuclear fuel refueling to achieve a long design lifespan, involving complex mechanical systems for infrequent refueling activities. An alternative is designing reactors that can be transported to refueling facilities, entailing significant initial costs in handling and transport systems. A third option is No-Refuel PL-TNPPs, where the uranium fuel remains inside the reactor vessel for its entire operational life, and decommissioning occurs simultaneously with fuel extraction at the end of service life, incurring significant costs.

[0006] PL-TNPPs, while minimizing on-site construction efforts, face significant additional costs and activities, challenging their economic viability. Designs with low power density cores incm high initial costs, while others generate high end-of-life costs. Some PL-TNPP designs propose using the reactor as dry storage for irradiated fuel after core depletion. However, these designs fail to meet safety criteria for dry’ storage of spent fuel, lacking a sealed double metallic casing for acceptable protection.

[0007] Additionally, the cost of managing spent fuel represents a significant problem for SMRs compared to conventional large PWR fleets. Due to the economy of scale in spent fuel management costs, SMRs with medium or small-sized electricity grids face higher levelized energy costs per spent fuel unit, further challenging economic viability.

[0008] Therefore, there is a need for a more economical PL-TNPP capable of addressing the inherent disadvantages associated with the presence or absence of intermediate refueling during the reactor's operational life. Additionally, efforts are required to mitigate the inherent drawbacks related to the economies of scale concerning the levelized energy costs per spent fuel, especially for countries with medium and small-sized electricity grids.SUMMARY OF THE INVENTION

[0009] The main object of the present invention is to provide an integrated Pressurized Water Reactor (PWR) system that is cooled by natural convection and installed underground, designed to operate without the need for refueling throughout its entire operational life.

[0010] Another object of the present invention is to offer a reactor containment system that is fully fabricated off-site and transported as a single module to the installation site, facilitating ease of transport and assembly.

[0011] Yet a further object of the present invention is to ensure the efficient transfer of heat from the reactor containment to the surrounding cold soil, utilizing a specialized underground structure that enhances cooling performance.

[0012] A still further object of the present invention is to provide a post-operational conversion process that allows the reactor containment unit and the Reactor Pressure Vessel (RPV) to function as a long-term dry storage solution for irradiated fuel, thereby eliminating the need for significant maintenance and reducing the handling of spent fuel.

[0013] A PWR reactor system in accordance with the present invention is achieved by combining some or all of the following elements:1- An integrated PWR reactor, cooled by natural convection and installed underground, which operates without the need for refueling during its operationallife and which is housed completely within the lower area of a reactor containment unit. - A reactor containment unit formed by a cylindrical metallic vessel, the reactor containment unit including: a. an upper area including: i. an adjustment and chemical control system of the primary water circuit, ii. a pressure control system, iii. a reactivity adjustment and control system, and iv. safety systems of the reactor, b. a lower area within which the PWR reactor is frilly housed.This reactor containment unit is pre-fabricated and transported to the installation site as a single module. - A vertical underground building structure including concrete pieces and compacted heat-conducting material, designed to facilitate the transfer of heat from the reactor containment unit to the surrounding cold soil. - A post-operational conditioning process including: a. sealing all penetrations in the RPV and the reactor containment unit with welded seals, creating two successive independent sealed metallic barriers that completely surround the irradiated fuel, which can operate without maintenance for a long period of time of up to 100 years or more, b. replacing the water used by the PWR reactor system with dry inert gases, c. replacing the air used in the reactor containment unit with dry inert gases at a pressure lower than the pressure external to each of the metal barriers, and d. extracting the heat generated by the decay of the fuel elements by natural convection of the inert fill gas in each independent metallic barrier by the impulsion of the buoyant force that is generated because the hot source is under the height of the cold source and the conduction of heat through the respective metallic casingBRIEF DESCRIPTION OF THE DRAWINGS

[0014] FIG. 1 is a vertical schematic section of an embodiment of a Reactor Pressure Vessel (RPV) and the internals of an integrated Pressurized Water Reactor (PWR) in accordance with the present invention.

[0015] FIG. lb is a perspective view of the RPV depicted in FIG. 1.

[0016] FIG. 2 is a vertical schematic section of an embodiment of a containment unit of an integrated PWR in accordance with the present invention, showing an RPV and a steam turbine inside.

[0017] FIG. 2b illustrates the external geometry of an embodiment of a containment unit in accordance with the present invention.

[0018] FIG. 3 depicts different sections of an embodiment of a containment unit in accordance with the present invention, each with distinct atmospheric pressures.

[0019] FIG. 4 is a vertical section of an embodiment of a buried building in accordance with the present invention, showing the containment unit and RPV installed inside during power operation.

[0020] FIG. 5 is an exterior schematic view of an embodiment of a containment unit in accordance with the present invention, similar to that in FIG. lb, being transported on a truck trailer.

[0021] FIG. 5b is a lateral view of a truck transporting an embodiment of a containment unit.

[0022] FIG. 6 is a vertical schematic section of an embodiment of a buried reactor building in accordance with the present invention, with a containment unit and RPV inside during the initial conditioning for the start-up process.

[0023] FIG. 6b is a perspective view of the image shown in FIG. 6.

[0024] FIG. 7 is a section of an embodiment of a reactor in accordance with the present invention, highlighting the mechanical fastening system of the steam turbine.

[0025] FIG. 7b is a detailed perspective view of FIG. 7.

[0026] FIG. 8 shows the detail of the plugs that must be removed to allow gas circulation by natural convection inside the containment unit during the dry storage phase in an embodiment of the present invention.

[0027] FIG. 9 is a vertical section of an embodiment of an underground reactor building in accordance with the present invention, illustrating the plenum distributors and heat transfer systems necessary for operating as dry storage for irradiated fuel after the reactor's operational life.

[0028] FIG. 9b represents an embodiment of an underground concrete reactor building in isolation, in accordance with the present invention, showing the columns linking the upper and lower concrete structures.

[0029] FIG. 10 is an external view of an embodiment of a plant in accordance with the present invention, with a cutaway showing the layout of an underground reactor building and the dry air condensing tower.DETAILED DESCRIPTION AND BEST MODE OF IMPLEMENTATION

[0030] Disclosed is a small integrated modular PWR reactor system without refueling, operating as vertical dry storage for irradiated fuel at the reactor end of life.

[0031] Some general aspects of the present invention have been summarized so far in the first part of this detailed description and in the previous sections of this disclosure. Hereinafter, a detailed description of the invention as illustrated in the drawings will be provided. While some aspects of the invention will be described in connection with these drawings, it is to be understood that the disclosed embodiments are merely illustrative of the invention, which may be embodied in various forms. The specific materials, methods, structures and functional details disclosed herein are not to be interpreted as limiting. Instead, the intended function of this disclosure is to exemplify some of the ways -including the presently preferred ways- in which the invention, as defined by the claims, can be enabled for a Person of Ordinary Skill in the Art. Therefore, the intent of the present disclosure is to cover all variations encompassed within the spirit and scope of the invention as defined by the appended claims, and any reasonable equivalents thereof. Before proceeding with the detailed description, it is important to note that the implementation described here is not limited to a specific type of reactor. Therefore, although this implementation is described as applied to an “integrated PWR” type reactor for explanatory convenience, it can be implemented in other types of reactors.

[0032] This invention presents a new type of reactor that provides an economical nuclear reactor with a different concept regarding the arrangement of spent fuel, as well as a different lifespan concept that allows for substantial improvement in the economics of the investment.

[0033] Referring to the drawings in more detail, Fig. 1 illustrates a vertical schematic section of the proposed integrated PWR type reactor 1, where the Reactor Pressure Vessel (RPV) 2 contains the reactor's core 3. This core is composed of several Uranium fuel elements 4, a Steam Generator (SG) 5, a Steam Re-heater 6, and the Upper Head 7 of the RPV 2. A truncated cone plate 8, made from steel, facilitates the removal of neutrons over lOMeV that cause activation in the secondary water circulating in the SG 5. This leads to the activation of N16and N17(radioactive isotopes of nitrogen) due to the short distance between the core 3 and the SG 5’s tubes. Tn the center of Fig. 1, a cylindrical rising conduit known as the Core Riser 9 is visible, through which the primary circuit water 10a circulates,filling the entirety of the RPV 2. At the Upper Head 7 of the RPV 2, the General Control Rod Drive Mechanisms (CRDMs) 11 and the Fine-Tuning Control Rod Drive Mechanisms (CRDMs)12 are visible, which are essential for raising and lowering the General Absorbent Control Rods 13 and Fine-Tuning Absorbent Control Rods 14 respectively. General Absorbent Control Rods 13 are used for the safe startup and shutdown of reactor 1's core 3. These rods are described as absorbent because they are made from materials that absorb neutrons. When an absorbent control rod is inserted into the Uranium core of a nuclear reactor, it absorbs the neutrons responsible for sustaining the fission reaction. This absorption alters the criticality condition of the reactor from critical to subcritical, effectively shutting down the reactor.

[0034] In this invention, the General Control Rod Drive Mechanisms (CRDMs) 11 operate solely in two positions: either completely withdrawn or completely inserted; intermediate positions do not exist. This is because the stability of the criticality conditions is naturally and inherently established by the reactor’s feedback coefficients. The extraction speed of the General Absorbent Control Rods 13 is extremely low, while their insertion speed, driven by gravity and aided by springs, is very high. When all of the General Control Rods Drive Mechanisms (CRDMs) 11 are positioned at the upper part, it means that all of the General Absorbent Control Rods 13 are withdrawn from the Uranium fuel core. Conversely, when the CRDMs 11 mechanisms are lowered, all of the General Absorbent Control Rods 13 are inserted into the uranium fuel. Since these rods are made from a neutron-absorbing material, the insertion leads to the shutdown of the nuclear reactor. Control of integrated PWR reactor 1 at all power levels is achieved by responding to the steam demand from the turbo generator (17 of Fig. 2), primarily through adjustments in the temperature of the primary circuit water 10a. This is facilitated by a strong negative primary temperature feedback coefficient, with the reactor pressure regulated by a Pressurizer (as the ones normally found in PWRs), in the operating mode known as 'reactor follows turbine' and under the concept of 'Minimum Attention Plants' (MAP). The reactivity change due to fuel depletion is managed by incorporating burnable poisons into the primary water, along with the use of a few Fine-Tuning Control Rods Drive Mechanisms (CRDM) 12. These Fine-Tuning Control Rods Drive Mechanisms (CRDM)12 perform gradual, slow, and controlled movements, extracting the Fine-Tuning Absorbent Control Rods 14 throughout the reactor’s operational life. This means that the Fine-Tuning Absorbent Control Rods 14 are completely inserted into the fuel element at the start of reac tor operation and are gradually extracted over time to compensate for fuel depletion.

[0035] The nuclear reactor proposed here will be assembled in a factory that will also be responsible for loading the Uranium fuel into the RPV 2 vessel. Once the vessel is sealed and filled with light water , which constitutes the primary circuit water 10, under the appropriate chemical conditions, it will not be opened again during the reactor's operational life. This is because minor water losses from radiation interaction are compensated by venting and a simple water makeup injection system. Consequently, this reactor does not require mid-life refueling, eliminating the need to disassemble the Upper Head 7 from the RPV 2 during reactor operation. Consequently, unlike traditional PWRs, BWRs, and integrated PWRs, the design proposed here does not require a heavy lifting crane to disassemble the RPV 2’s Upper Head 7. Additionally, there is no need for complex electromechanical systems to replace spent fuel, nor for extensive water and fuel management systems. This significantly reduces the required space and size of the buildings.

[0036] In FIG. lb, a cross-sectional and perspective view of FIG. 1 is presented, where the Uranium fuel 4 is represented by rectangular prisms to simplify the illustration. This figure showcases the integrated PWR type Reactor 1, the Reactor Pressure Vessel (RPV) 2, the core 3, the Steam Generator (SG) 5 composed of helical tubes, the Steam Re-heater 6 composed of helical tubes, the Upper Head 7, the truncated cone plate 8, the Core Riser 9, the General Control Rod Drive Mechanisms (CRDMs) 11, the Fine-Timing Control Rod Drive Mechanisms 12, the General Absorbent Control Rods 13 and the Fine-Tuning Absorbent Control Rods 14.

[0037] FIG. 2 shows the reactor capsule unit 15, which consists of a reactor containment unit 16 housing the integrated PWR type reactor 1 , as described in FIG. 1 , along with theturbo generator 17 located in the upper zone 18a of the containment unit 16. This turbo generator 17 comprises a high-pressure turbine 23 and a low-pressure turbine 26, connected via a shaft 19 to the electric generator 20. Additionally, the feedwater inlet pipe 21 can be seen passing through the Upper Head 7 and connecting with the SG 5, which is made up of several metal tubes in a helical shape, immersed in the primary circuit water 10a.

[0038] The primary circuit water 10a is heated by the fission of the Uranium fuel elements 4, and this heat is transferred to the SG 5 tubes, within which the secondary circuit water 10b circulates, turning into steam inside the SG 5 tubes. This steam is then conveyed through a first pipe 22 to the inlet of the high-pressure turbine 23, causing it to spin. In this design configuration, a Steam Re-heater (6 in FIG. 1) has been introduced, similar to the SG 5, consisting of several tubes immersed in the RPV (2 in FIG. 1). The expanded steam exiting by the outlet of the high-pressure turbine 23 is carried by a second pipe 24 to the re-heater 6, where the steam is reheated, increasing its temperature and pressure. This reheated steam is then carried through a third pipe 25 and injected into tire inlet of the low- pressure turbine 26, causing it to spin and enhancing the efficiency of the electric power generation system, which includes the electric generator 20. At the outlet of the low- pressure turbine 26, an outlet pipe 27 of larger diameter carries the expanded steam toward a condensation tower system, shown as (85 in FIG. 9), where the steam condenses into liquid water. This liquid water returns to the system through the feedwater inlet pipe 21 forming a closed water and steam circuit called the secondary circuit of water 10b. In FIG. 2, the middle zone 18b and the lower zone 18c of the containment can also be clearly seen.

[0039] In FIG. 2b, an exterior image of the reactor capsule unit 15 is presented, in which the three steel gates 28 and the steel external ring 29 can be observed.

[0040] FIG. 3 presents a vertical schematic section of the proposed reactor capsule unit 15, featuring the reactor containment unit 16, which encloses the integrated PWR type reactor 1 described in FIG. 1, along with the turbo generator 17. This figure illustrates the external vessel compartment 30, isolated from inspection compartment 31 by flange 32. Additionally, within the reactor containment unit 16 is a dedicated steam generatorinspection compartment 31 , necessary for performing inspections, maintenance, and sealing of the steam generator SG 5 and the Steam Re-heater 6. This compartment also provides maintenance access through steel gate 28.

[0041] Continuing upwards in the reactor containment unit 16 is the control systems compartment 33 in which the pressure and chemical control system 60 are included together with the reactor's security systems 50. The control systems compartment 33 is isolated from compartment 31 by a lower metal floor 34 and a metallic cylindrical tube 35 with a removable top cover 36. This pressure and chemical control system 60 manages the primary circuit water 10a and facilitates the removal of resins and filters, which are designed to operate maintenance-free between the periods of filter and resin regeneration and replacement. It also handles the replacement of the gas accumulation tank for gases generated by the radiolysis of water in the primary circuit water 10a and by the accumulation of fission gases that may be released during core operation. Additionally, this system allows for the replacement of the purge and drainage tank when it reaches its maximum capacity. Moving further upwards in FIG. 3, the compartment 38 of the turbo generator 17 is found, isolated from control systems compartment 33 by an upper metal floor 37.

[0042] The internal air of the reactor containment unit 16 circulates through an external air conditioning system, not shown in the image, which ensures the circulation of air across different compartments 30, 31, 33 y 38 inside the reactor containment unit 16 while maintaining differential pressure between compartments. This setup is designed to ensure an adequate pressure gradient, such that the air in contact with more radioactive components is at a lower pressure. Consequently, the atmospheric pressure in external vessel compartment 30 is lower than in inspection compartment 31, inspection compartment 31’s pressure is lower than in control systems compartment 33, and the pressure in compartment 33 is lower than in turbo generator compartment 38. Moreover, the pressure in the turbo generator compartment 38 is slightly lower than the external atmospheric pressure. This cascade of differential pressures prevents the escape of any significant radioactive material from the reactor containment unit 16.

[0043] Any primary circuit water 10a leak enables early identification of the leak area by measuring pressure, temperature, and dose rate inside the reactor containment unit 16. In the event of a leak, the reactor is immediately shut down, and the primary circuit water 10a is depressurized to stop the leak. Simultaneously, the external air conditioning system in the reactor containment unit 16 is isolated by automatically closing air internal isolation valve 39, external isolation valve 40, and manual isolation valve 41 on the air inlet and outlet lines.

[0044] FIG. 4 schematically presents a vertical sectional view of the underground reactor building during its operational life. The reactor capsule unit 15, containing all its equipment, including the Uranium fuel elements 4, is installed below ground level 42 within a pre-constructed underground reinforced upper concrete structure 43 and reinforced lower concrete structure 44. Above the reinforced lower concrete structure 44, a steel support ring 45 is installed, and the entire reactor capsule unit 15 is mounted into this ring. The steel external ring (29 of FIG.2b) of the reactor containment unit 16 features several holes to fix it to the steel support ring 45 using steel bolts. The space between the reinforced upper concrete structure 43 and the reinforced lower concrete structure 44 is filled with a granulated compound 46 (such as soil) that conducts heat, which surrounds the reactor capsule unit 15. Four steel walkways 47 provide access for maintenance personnel to the various compartments inside the reactor containment unit 16 and to the external compartment 51. The same figure shows that the three steel gates 28 are connected to three steel walkways 47.

[0045] FIG. 4b shows the underground building 90, including a reinforced upper concrete structure 43 and the underground reinforced lower concrete structure 44, which are joined by concrete columns 61. The same figure also presents the steel support ring 45, installed above the underground reinforced lower concrete structure 44.

[0046] FIG. 5 presents an exterior schematic view of the reactor capsule unit 15, including all its internal equipment, such as the Uranium fuel elements, being transportedhorizontally by a trailer 53. The RPV 2 is completely dry, free of primary circuit water 1 Oa, free of secondary circuit water, degassed of unwanted gases, and filled with an inert gas at a slight negative pressure relative to the external air. The transport vehicle is equipped with a vibration isolation system 54, ensuring that the fuel elements and the turbo generator 17 maintain their manufacturing and operational mechanical qualifications during transport.

[0047] FIG. 6 schematically presents a vertical sectional view of the underground reactor building at the time of its first start. An additional startup module 55, temporarily installed in one of the rooms of the vertical compartment adjacent to the underground reactor building, can be seen. This additional startup module 55 is crucial for feeding and chemically controlling the quality of the primary circuit water 10a. It connects to the feed hoses 56 and to the extraction hoses 57, collects purges through purge hose 58 and drainages through drainage hose 59 from the pressure and chemical control system 60. This setup helps eliminate dissolved gases, extract oxygen adequately, and adjust the pH and conductivity of the water in the primary circuit water 10a. The startup module 55 is an external piece of equipment that is temporarily connected with hoses and is used only during the first startup of the reactor.

[0048] FIG. 6b presents a perspective view that shows the reactor capsule unit 15 while the additional startup module 55 is visible in the reinforced concrete compartment 52, adjacent to the hoses temporarily installed during the reactor startup. These hoses enter by passing through two of the walkways 47.

[0049] FIG. 7 schematically shows a vertical section of the underground reactor building, similar to FIG. 6, but includes the outlet pipe 27 of larger diameter that carries the expanded steam toward a condensation tower system (85 in FIG. 9). The outlet pipe 27 is equipped with a condenser isolation valve 62, which allows for the isolation of the reactor capsule unit 15 in the event of the turbo generator or its pipes breaking. The same figure also illustrates the feedwater inlet pipe 21 with its corresponding automatically actuated isolation valve 63 and its manual actuated isolation valve 64.

[0050] The turbo generator 17 is supported by a structure with tensioners 65 that pass through the reactor containment unit 16. These tensioners are connected to a plurality of dampers 66, which are mechanically linked to a metal beam structure 67. This structure transmits the weight and vibrations of the turbo generator 17 to the underground reinforced upper concrete structure 43. It is important to note that the tensioners supporting the turbo generator 17 pass through the reactor containment unit 16 using double corrugated tubular metal bellows 68. These bellows maintain the tightness of the reactor containment unit 16 while compensating for differential expansions between the turbo generator 17 and the reactor containment unit 16.

[0051] In FIG. 7b, the detail of the metal beam structure 67 that supports the turbo generator 17 is shown. It can be observed that the tensioners 65 contain dampers 66 used to prevent vibrations from the turbo generator 17 from being transmitted to the reactor containment unit 16. Additionally, the double corrugated tubular metal bellows 68 can be seen; these prevent the internal atmosphere of the turbo generator compartment 38 from coming into contact with the external atmosphere.

[0052] In the global nuclear power generation industry, when uranium fuel is depleted, it must be removed from the reactor building and stored to allow for the radioactive decay of uranium fuel elements. Typically, spent fuel is placed in a decay pool where it undergoes "wet storage” for a minimum of three years. This process allows the radioactive materials to gradually release excess heat as the radioactivity of the uranium decreases over time. Following this period, if the fuel is not defective, it should be placed in a sealed metallic containment; if the fuel is defective, it should be housed in two concentric sealed metallic steel containments. The fuel can then be transferred to a 'dry storage' facility, where it can remain for up to a hundred years. In dry storage, the fuel is kept in a mixture of inert gases atmosphere at a pressure lower than the external pressure of the sealed metallic containment housing it. The mixture of inert gases acts as a cooling medium, allowing residual heat to dissipate through conduction and natural convection.

[0053] However, in the design of the reactor proposed here, it is not necessary to remove the uranium fuel elements 4 from the reactor or transfer them to other storage facilities. Once the reactor has completed its operating life, the reactor capsule unit 15, which includes the reactor pressure vessel (RPV) 2, assumes the function of wet storage utilizing for approximately three to five years the natural convection effect of the primary circuit water 10a, facilitated by the residual heat of the fuel element. After the wet storage phase, the containment air and primary circuit water 10a and the secondary circuit water 10b are replaced by a mixture of inert gases, transforming the reactor containment unit 16 and the RPV 2 into a dry storage system for the spent fuel elements 4. This means that the same reactor capsule unit 15 serves as dry storage for a hundred years.

[0054] By eliminating the need to transfer radioactive fuel to an external wet storage facility and subsequently to a dry storage facility, this design avoids substantial installation and operational costs. It also reduces the complexity of the electromechanical equipment and eliminates the risks associated with moving and transporting radioactive fuel elements outside of the nuclear reactor.

[0055] This nuclear reactor proposed, initially designed to generate electric power, becomes its own spent fuel elements wet storage facility after the end of its operational life and then becomes its own fuel elements dry storage facility.

[0056] FIG. 8 schematically shows a vertical section of the reactor capsule unit 15 at the reactor's end of life, following a reasonable period of full power operation (for example, twenty years) and an estimated decay time of three to five years after the end of full power operation. After this waiting period, the transition to dry operation begins. During this transition, the primary circuit water 10a in the RPV 2 is removed, the secondary circuit water 10b is removed and the internal atmosphere of the Reactor Pressure Vessel (RPV) 2 is dehumidified and replaced with a mixture of inert gasses (for example, 50% nitrogen and 50% helium) at a pressure lower than atmospheric, following the removal of the filters and resins from the containment air purification and internal recirculation system. All pipe penetrations to the Reactor Pressure Vessel (RPV) 2 are sealed with welded covers. Inaddition, the air inside the reactor containment unit 16 is filtered, changed, and replaced with another mixture of inert gasses (nitrogen and helium) at an intermediate pressure between the Reactor Pressure Vessel (RPV) 2 and the external atmosphere. All pipe penetrations to the containment are also sealed with welded covers. Furthermore, to initiate dry operation, it is necessary' to unify the atmospheres of all compartments within the reactor containment unit 16. To achieve this, plugs 69, 70, and 71 must be removed. Once these plugs are removed, the gas mixture will be unified and maintained at a consistent pressure throughout the reactor containment unit 16. The primary circuit water 10a and the secondary circuit water 10b are extracted by a conventional hydraulic pump. The air from the reactor containment unit 16 is extracted using a conventional vacuum pump. The inert gas mixture is injected into the RPV 2 and the reactor containment unit 16 using external pressurized gas tanks.

[0057] During the stage in which the reactor transitions to its own dry / storage, cooling is achieved through natural convection of the inert gas mixture within the Reactor Pressure Vessel (RPV) 2 and the reactor containment unit 16. In FIG. 8b, arrows illustrate the circulation effect of natural convection inside the RPV 2 and the reactor containment unit 16, as well as heat conduction through the walls of both the RPV 2 and the reactor containment unit 16.

[0058] Inside the Reactor Pressure Vessel (RPV) 2, the decay heat from the Uranium fuel elements 4 rises through the Core Riser 9 until it reaches the upper area of the RPV 2, indicated by reference number 72. It then descends along the wall of the RPV 2, as shown by reference number 73, cooling through heat transfer to the exterior of the RPV 2, represented by reference number 74. The gas mixture, under the effect of natural convection, cycles back into the Uranium fuel core 3 comprises by the fuel elements 4, as depicted by reference number 75.

[0059] The heat transferred from the wall of the Reactor Pressure Vessel (RPV) 2 to the atmosphere of the reactor containment unit 16 comes into contact with the vertical flow separator 76, which directs the heated gas upward, as indicated by reference number 77,solely by the effect of buoyant force. Due to the metallic cylindrical tube 35, the nitrogen and helium atmosphere can only ascend, as indicated by reference number 79, until reaching the turbo generator compartment 38. From there, the gas mixture descends, as depicted by arrows 80, and passes through the perforations 81. The gas continues to descend, represented by reference number 82, and is cooled by the heat transfer to the exterior of the buried reactor containment unit 16, outlined by reference number 83, transferring the heat to the surrounding soil. It is worth mentioning that from two meters deep, the soil remains cooler than the outside air even at depths greater than twenty meters, which makes convection effective. The granulated compound (46 of FIG.4) ensures the transfer of decay heat to the external soil mass. The gas reaches the lower part of the reactor containment unit 16 and returns, as shown by reference number 84, toward the heated zone of the external wall of the RPV 2, reheating and generating natural convection.

[0060] The balance between buoyant forces and thermal resistances, along with the ability to extract heat from the available ground surface, is achieved by an appropriate waiting period between the end of power operation and the transition to dry operation (for example three to five years). During this phase, the reactor serves as its own dry storage, effectively facilitating the removal of decay heat from the fuel elements. The decay heat from the core of fuel elements decreases significantly during the wet storage phase and continues to diminish gradually and steadily during the dry storage phase.

[0061] By waiting an appropriate amount of time between the end of frill power operation and the transition to dry operation, temperatures of the fuel elements can be lowered sufficiently to be safely cooled for a very long time in an inert gas atmosphere. The design proposed here includes two hermetic metal barriers (one being the RPV 2 and the other the Reactor Containment Unit 16), thus meeting the requirements for dry storage of irradiated Uranium fuels 4. This form of dry operation, which facilitates the transfer of heat to the external soil, allows for the reactor capsule unit 15 to be reconfigured with simple, straightforward, and low-cost modifications as a long-term solution for storing irradiated fuels. Irradiated Uranium fuels, being the main high-activity radioactive remnants produced by a PWR reactor, are consequently a focus of public scrutiny. The designproposed here eliminates the need for large capital investments typically required in PWRs to build facilities for storing fuels for a hundred years. At the design proposed here, once the electric generation operation is completed, only tasks of low complexity are required to adapt the system, as the reactor capsule unit 15 is already designed with the necessary dimensions and materials to function as dry storage.

[0062] FIG. 9 shows a cross-sectional view of the entire nuclear power plant operating at full power, where the reactor capsule unit 15 and the outlet pipe 27, which carries the expanded steam toward a condensation tower system 85, as well as the feedwater inlet pipe 21, can be seen. The reactor capsule unit 15 is installed below ground level and inside an underground reinforced upper concrete structure 43 and an underground reinforced lower concrete structure 44 linked by columns (61 of FIG.4b) also made of reinforced concrete. Between these concrete structures, granulated compound 46 is added to enable the reactor capsule unit 15 to dissipate heat to the surrounding soil. The concrete columns 61 allow the soil around the containment to be in contact with the rest of the surrounding soil, thus favoring heat diffusion by thermal conduction.

[0063] The concrete room 86, attached to the vertical reinforced concrete compartment 52, contains a minimal set of equipment providing auxiliary services for the reactor. These services include an external air conditioning system for containment, support for electronics and actuators, remote communication services, and a very simple fallback console. In the upper part of the vertical reinforced concrete compartment 52 and at ground level, there is a removable concrete cover 87 that allows for the entry of heavy equipment with the aid of an external crane. Maintenance personnel can access the interior of the reactor via the staircase 88, pass through the double security door 89, and then descend using the ladder 48.

[0064] All the auxiliary equipment found in the concrete room 86 does not affect the security of the reactor, which, in the event of an accident, is cooled passively, without requiring any support system or electrical power supply.

[0065] FIG. 10 shows the reactor operating at foil power, where it can be seen that the reactor capsule unit 15 is buried underground, and where the outlet pipe 27, which carries the expanded steam toward a condensation tower system 85 composed of cooling cells consisting of radiator panels (91 of FIG.10) and fans (92 of FIG.10), forces the air to pass through the radiator panels, thus condensing and cooling the steam to turn it into liquid water. The condensed water in liquid form returns through the feedwater inlet pipe 21 to the secondary circuit water 1 Ob of the steam generator, where it is converted back into steam inside the RPV 2 to continue the closed system cycle. After the end of life of the reactor and the transition to dry operation, the condensation tower system 85 can be either removed or disconnected to be used in another identical reactor.

[0066] The description as set forth is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the teachings above without departing from the spirit and scope of the forthcoming claims.

Claims

CLAIMS1. A nuclear reactor comprising a reactor containment unit housing: a. an integrated PWR type reactor comprising uranium fuel elements; b. a pressure and chemical control system for primary circuit water; c. a safety system; and d. a turbo generator; wherein said nuclear reactor is integrally transported to a placement site and is situated within an underground building, therefore allowing heat from the reactor containment unit to be evacuated toward cold soil surrounding said reactor containment unit; wherein after a conditioning and waiting period of less than five years from when the nuclear reactor ends its operational life, primary circuit water, secondary circuit water, and any air inside the reactor containment unit are replaced with a mixture of dry inert gases; and wherein said reactor containment unit comprises metallic walls, so to allow that heat generated by decaying of the fuel elements be extracted, through natural convection of the mixture of dry inert gases, through the metallic walls of the reactor containment unit to the cold soil surrounding said reactor containment unit, thus initiating a dry storage period.

2. The nuclear reactor of Claim 1, wherein a turbo generator having a steam outlet is located inside the reactor containment unit, with said steam outlet directed to a condensation tower system located outside the reactor containment unit.

3. The nuclear reactor of Claim 1, wherein the reactor containment unit is transported horizontally and then verticalized upon installation inside the underground building.

4. The nuclear reactor of Claim 1, wherein the reactor containment unit is transported to a placement site in separate parts, which are then assembled onsite at the placement site.

5. The nuclear reactor of Claim 2, wherein the condensation tower system is cooled by circulating dry air drive by fans, such the steam coming from the turbo generator is condensed as liquid water returning from the condensation tower system to the reactor containment unit by a feedwater inlet pipe to supply a steam generator SG of liquid water.

6. The nuclear reactor of Claim 2, wherein a steam outlet from a high-pressure turbine re-enters an RPV, within the reactor containment unit, to be superheated into a re-heater located inside the RPV and wherein the superheated steam feeds an inlet of a low-pressure turbine.

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