Heat-generating nuclear reactor with liquid metal coolant and core comprising coolant-sealed tubes each housing "triso" nuclear fuel particles
Patent Information
- Authority / Receiving Office
- EP · EP
- Patent Type
- Applications
- Current Assignee / Owner
- COMMISSARIAT A LENERGIE ATOMIQUE ET AUX ENERGIES ALTERNATIVES
- Filing Date
- 2024-06-13
- Publication Date
- 2026-04-22
AI Technical Summary
Current nuclear reactors face challenges in providing high-temperature heat for industrial applications efficiently, particularly in decarbonizing industries like ammonia and sodium carbonate production, due to limitations in power density, safety risks, and complex core physics in existing solid fuel reactors cooled by liquid metal or gas.
A nuclear reactor design featuring a liquid metal coolant with TRISO particles housed in coolant-tight tubes, allowing natural convection and high fuel temperatures, enhancing power density and safety through physical separation of fuel and coolant, and enabling efficient heat transfer.
The design significantly increases power density, simplifies fuel handling and refueling, reduces accident risks, and allows for efficient heat transfer, making it suitable for high-temperature industrial applications with reduced infrastructure needs.
Smart Images
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Abstract
Description
[0001] Description
[0002] Title: Calogenic nuclear reactor with liquid metal coolant and a core comprising coolant-tight tubes each housing nuclear fuel particles, known as TRISO.
[0003] Technical field
[0004] The present invention relates to the field of solid fuel nuclear reactors cooled with one or more heat transfer fluids of the liquid metal type, in particular with liquid sodium.
[0005] The main objective of the invention is therefore to provide a calogenic nuclear reactor which operates at low pressure, typically less than 5 bars and which is intended to provide relatively high thermal power, of the order of a few tens to hundreds of MWth.
[0006] In particular, the invention aims to achieve a mainly thermal coupling between a nuclear reactor and an industrial production site, for example chemical installations for the production of ammonia or sodium carbonate.
[0007] By "heat-generating" is meant here and within the scope of the invention, a nuclear installation, a nuclear power plant or a nuclear reactor whose power can be dedicated mainly to the supply of heat. The power of a heat-generating reactor can be 100% to supply heat. In its calogenic configuration, a small part of its power can still be used to supply electricity.
[0008] Although described with reference to a nuclear reactor cooled by liquid sodium, the invention can be applied to any other liquid metal, such as lead, as a heat transfer fluid for a primary circuit of a nuclear reactor.
[0009] Also, although described with reference to a reactor operating in thermal spectrum, the invention can also be applied to reactors with fast neutron spectra.
[0010] The invention can be applied to small power reactors or SMR in English (acronym for "Small Modular Reactor"), typically with an operating power of less than or equal to 150 MWth. By "SMR reactor" is meant here and in the context of the invention, the usual technological meaning, namely a nuclear fission reactor, of smaller size and power than those of conventional PWR reactors, which is manufactured in a factory and transported to a nuclear installation site to be installed there.
[0011] Prior art
[0012] One of the current development topics for nuclear reactors concerns so-called calogenic reactors intended to provide thermal power.
[0013] Among the various existing technological solutions that provide heat from nuclear fission, it is commonly accepted that to date, pressurized water reactors (PWRs) are the most suitable for providing heat at relatively low temperatures.
[0014] In other words, REPs are those which make it possible to immediately supply a power level of a few tens of MWth, mainly for the purpose of supplying so-called urban heat, i.e. in urban networks, for towns / agglomerations of several hundred thousand inhabitants.
[0015] Boiling water reactors (BWR) are designed to produce steam in a primary circuit directly used in a turbo-alternator group, in order to produce electricity.
[0016] Industrial heat, at levels above those required for district heating, typically above 500°C, now accounts for more than 20% of global energy demand.
[0017] More specifically, the global heat market amounts to approximately 57,000 TWh, of which approximately half comes from industrial demand, i.e. from industrial production sites: [1],
[0018] It is already recognized that solutions such as SMR reactors could go some way towards replacing the 75% of industrial heat supplied by oil, coal and natural gas.
[0019] In addition to their ease of use and compactness, which would allow them to be transported by land, candidate SMR reactors must be intrinsically safe. The inventor reviewed existing mature technologies that could be implemented in SMR nuclear reactors to provide relatively high-temperature heat on site to industries that are difficult to decarbonize with electricity, such as ammonia or sodium carbonate production facilities.
[0020] High-temperature reactors (or HTRs for "High Temperature Reactors") are thermal spectrum reactors, with graphite moderators and cooled by a heat transfer fluid known as "transparent" to neutrons. They offer the possibility of reaching high heat transfer fluid temperatures, typically of the order of 750°C at the core outlet.
[0021] In recent concepts, this heat transfer fluid is pressurized helium. In another recent concept, the demonstrator from the Kairos Power company, named Hermès, an ionic heat transfer fluid consisting of molten salts based on fluoride salts was considered.
[0022] In an HTR reactor, the nuclear fuel used consists of so-called TRISO particles (acronym for "TRIStructural ISOtropic"). Each of the particles is formed from a uranium oxide core coated with layers of carbon compounds that serve as a first containment barrier to retain the fission products. The particles thus have the appearance of balls approximately 1 millimeter thick. They are compacted in graphite matrices (called compacts or balls) so that the coolant is not in direct contact with the first containment barrier.
[0023] The architecture of HTR reactors is generally of the loop type, i.e. with heat exchangers between primary and secondary circuits connected to the core by tubular connections, and uses fluid circulators, i.e. pumps, blowers, and heat exchangers towards the fluids of the secondary circuit, which can be helium, pressurized water, supercritical CO2. . .
[0024] In HTR reactors, heat transfer between the first containment barrier and the coolant is ensured by conduction through the interfaces formed by the graphite matrices in which the TRISO particles are compacted.
[0025] HTR reactors have the following major advantages:
[0026] - the TRISO fuel particles used can accommodate high temperatures and pressures in the event of incidental or accidental situations. Typically, the fuel temperature can be higher than 1500°C and the internal pressure of the heat transfer fluid (helium) between 10 and 100 bars;
[0027] - a neutron spectrum can be very thermal, which results in a high moderating and Doppler coefficient, typically equal to -15pcm / °C, and therefore leads to a rapid variation in the reactor power during variations in temperature within the graphite and / or the fuel;
[0028] - the core has a high thermal inertia due to the mass of graphite and therefore limits the impacts of thermal shocks, hot or cold;
[0029] - combined with a low power density value, typically equal to 2W / cm 3, the above-mentioned advantages lead to the practical elimination of the risk of a serious accident, i.e. of a generalized meltdown of the core and relocation of the molten fuel.
[0030] The major disadvantages of HTR reactors can be summarized as follows:
[0031] - as mentioned, a maximum value of the power density which is low because it is partly imposed by respect for the integrity of the first containment barrier in the event of rapid depressurization of the gas heat transfer fluid;
[0032] - residual safety risks which are linked to the risks of dispersion of radioactive material in the event of air ingress, i.e. risks of graphite or water fires (risks of steam explosions following hydrogen production);
[0033] - a relatively large boiler (primary tank) footprint due to a low-density primary gas heat transfer fluid which requires, in particular, a significant size of components for heat extraction, and a configuration of the primary loop circuit which involves a spread arrangement of components in several separate rooms;
[0034] - a need for power electricity on the site, typically several tens of MWe depending on the power of the boiler, which is essential to ensure the operation of the installation, in particular for supplying the circulators for the primary gas;
[0035] - a ratio of thermal power produced / electrical power consumed for the circulation of the primary fluid that is less favourable for gas-cooled HTRs than for other reactor technologies, cooled by incompressible fluids (water, liquid metals, etc.). Fast neutron reactors (RNR), cooled by liquid metal, particularly sodium (RNR-Na), and more particularly those of the fourth generation also provide heat at temperature levels compatible with the requirements of industrial production sites. Typically, the temperatures of the primary sodium at the outlet of the core are around 550°C.
[0036] The fast neutron reactor sector was developed to enable better management of nuclear fuel, in particular through sustainable management of the plutonium stock and the ability to recover the inventory of the uranium isotope 238, an isotope that is not very recoverable in thermal neutron reactors.
[0037] Solid-fuel fast neutron reactors rely on a physical separation between the solid fuel and the liquid metal coolant, by a casing (rod or needle) forming a first physical containment barrier (cladding). The fuel itself is made of solid materials at the target operating temperatures (notably in the form of oxides, silicon carbides (SIC) or nitrides of fissile materials or directly in the form of a metal alloy). The casings are assembled in bundles and are immersed, along with all the other components of the primary circuit (neutron absorbing material rods, exchangers, instrumentation), directly in the liquid metal coolant which therefore ensures heat transfer.
[0038] The vessel of an RNR reactor is generally designed to be suspended from a metal slab itself connected to a civil engineering reactor pit.
[0039] The major advantages of liquid metal-cooled RNR reactors, particularly Na-RNRs, are that they make it possible to consider closing the fuel cycle (possibility of breeding) and the management of certain high-level, long-lived waste (americium, etc.) and that they have a high power density, typically of the order of 200 W / cm 3 .
[0040] However, known Na-NRs have the following major drawbacks:
[0041] - the need for sodium circulation in forced convection due to the high power density;
[0042] - the impossibility of eliminating any risk of core meltdown accident, by design (risks of loss of global or local primary flow, risks of untimely rise of the neutron absorber rods, etc.) - the need to provide numerous pieces of equipment to prevent a core meltdown accident or limit the associated consequences;
[0043] - complex core physics with, in particular, significant coupling between thermal effects and the geometric characteristics of the core;
[0044] - due to the fast neutron spectrum, the need for specific neutron protection (activation of structures, corrosion products, secondary fluid, etc.);
[0045] - due to the high residual fuel power, the need for specific infrastructure for handling fuel assemblies (refrigerated gas hoods) or their storage (internal storage, barrel, pools) as well as for handling other sodium components (washing wells, special treatment of certain fuel assemblies, higher neutron protection if not sealed, etc.).
[0046] There is therefore a need to improve solid-fuel nuclear reactors cooled with liquid metal or gas, intended for calogenic purposes for the purpose of supplying heat to industrial production sites, in particular in order to overcome the aforementioned drawbacks.
[0047] The aim of the invention is to meet at least part of this need.
[0048] Statement of the invention
[0049] To this end, the invention relates, in one of its aspects, to a liquid metal-cooled nuclear reactor, comprising a vessel, called the primary vessel, axisymmetric around a central axis (X), filled with a fluid containing at least one liquid metal as a heat transfer fluid for the primary circuit of the reactor, the vessel comprising a core comprising a plurality of hollow, sealed and fixed tubes, arranged parallel to the axis X so that during operation of the reactor, the liquid metal circulates in contact with their outer periphery, each hollow tube housing a stack of fuel assemblies, housing nuclear fuel particles, called TRISO, mixed with a matrix.
[0050] According to an advantageous embodiment variant, the matrix in which the TRISO particles are mixed is in the form of a compact block.
[0051] According to an advantageous embodiment, the reactor comprises a structure forming a step, with a central axis coincident with that of the primary vessel, the structure being arranged in the primary vessel to separate the interior thereof into a central zone and a peripheral zone so that during operation of the reactor, the liquid metal coolant circulates by natural convection, in a closed loop from the bottom of the central zone where the reactor core is arranged within which the fission reactions occur, from which it rises by heating, to the top of the central zone where it enters through an inlet of at least one heat exchanger outside the reactor vessel to exit through an outlet of the exchanger then towards the top of the peripheral zone to descend towards the bottom of the peripheral zone where it is diverted towards the core of the reactor.
[0052] According to a thermal spectrum reactor operating configuration, the reactor core houses at least one moderating material.
[0053] The reactor core may also include a reflector made of moderating material arranged below each sealed tube.
[0054] The core of the reactor may also include at least two blocks of moderating material arranged around each sealed tube, allowing the liquid metal to circulate around and in contact with the latter.
[0055] According to an advantageous embodiment, each sealed tube comprises a blind tubular part forming a glove finger in which the fuel assemblies are stacked prior to operation of the reactor.
[0056] The liquid metal of the heat transfer fluid can be chosen from sodium (Na), lead (Pb) or a lead-bismuth alloy (Pb-Bi).
[0057] The material of the TRISO particle matrix and / or core may be graphite.
[0058] The material used for waterproof tubes can be ceramic.
[0059] According to another advantageous embodiment, the reactor comprises a reactivity control system consisting of control rods internal to the primary vessel.
[0060] In the context of the invention, the term "moderator material" means any material that can slow down neutrons. In the usual sense, the kinetic energy of a fast neutron is greater than IMeV, while that of a thermal neutron is less than leV, typically of the order of 0.025eV. Reference may be made to publication [2], and in particular to figure 4, which indicates, for several types of reactors, the thermal fraction and the fast fraction of the neutron flux.
[0061] Thus, the invention essentially consists of a nuclear reactor with a liquid metal coolant, in particular liquid sodium, and a solid fuel assembly in the form of TRISO particles compacted in a matrix stacked with other assemblies, in sealed tubes, which separate the fuel from the flow of liquid metal.
[0062] Thus, the flow of liquid metal circulates outside a sealed barrier. This guarantees the absence of contamination during normal operation of the liquid metal, the primary coolant, by fission products because two independent sealed barriers separate it from the fissile material contained in the TRISO particles.
[0063] The design of the reactor according to the invention essentially makes it possible to combine the advantages of HTR reactors and those of liquid metal-cooled RNR reactors.
[0064] More particularly, the design of the reactor according to the invention makes it possible to significantly increase the power density of existing HTR reactors. Indeed, the use of a low-pressure liquid metal, typically 1 to 2 bars, replacing pressurized helium, typically at 10 to 100 bars, allows normal operation of the reactor with higher fuel temperatures than those of existing HTRs, the TRISO particles being able to withstand these higher temperatures.
[0065] Furthermore, the physical separation between the TRISO particle fuel and the liquid metal flow offers significant advantages.
[0066] First, this allows for simple handling of the fuel (insertion into the fixed sealed tubes), since the assembly is not in contact with the liquid metal and, therefore, there is no need for cleaning operations. More generally, the fuel, the control rods and most of the instrumentation consisting of "consumable" components or those requiring regular replacement or inspection are positioned in a fluid zone separate from that of the primary liquid metal coolant. This feature allows for handling of components not polluted by the liquid metal coolant in order to protect most of the consumable components from any interaction with the metal.
[0067] Typically, the inventor estimates this power density to be about 10 MW / m 3with liquid sodium, a value 5 times higher than that of an HTR reactor. Considered in parallel with the thermal robustness of the TRISO particles, this allows for radiation cooling during handling of spent fuel, even with high residual power, immediately after the fission reaction has stopped. Given both the high heat transfer coefficient of liquid sodium and the power density (much lower than that of liquid metal-cooled fast reactors), heat transfer between the fuel and the liquid metal can be achieved by simple thermal conduction.
[0068] The neutron weight of a fuel assembly is low, which allows for simple online refueling, i.e. at full power, without significant neutron disturbances. This online refueling allows for increased reactor availability and reduces the risks inherent in refueling operations by reducing the time pressure on this critical operation.
[0069] The characteristics of TRISO particles also allow dry storage in racks, cooled only by ventilation, then by radiation once placed in drums, a few weeks after unloading the core.
[0070] Ultimately, the reactor according to the invention can operate with a primary liquid metal under low pressure and at high temperature, typically of the order of 750°C at the outlet of the core for liquid sodium, which ultimately guarantees the reactor's calogenic vocation for the purpose of supplying heat to industrial production sites, for example chemical installations for the production of ammonia or sodium carbonate.
[0071] Other advantages and characteristics of the invention will become more apparent upon reading the detailed description of examples of implementation of the invention given by way of illustration and not limitation with reference to the following figures.
[0072] Brief description of the drawings
[0073] [Fig 1] Figure 1 is a schematic longitudinal sectional view of a liquid metal-cooled nuclear reactor according to the invention, with a loop exchanger configuration.
[0074] [Fig 1 A] Figure 1 A is a detail view of Figure 1, at the level of a sealed tube housing TRISO particle assemblies between two blocks of moderator material.
[0075] [Fig 2] Figure 2 is another cross-sectional and perspective view of a liquid metal-cooled nuclear reactor according to the invention, with a loop exchanger configuration.
[0076] [Fig 2A] Figure 2A is a perspective view showing the insertion of a TRISO particle assembly at a sealed tube of the core of Figure 2. [Fig 3] Figure 3 is a cross-sectional view of a core of a nuclear reactor according to the invention, showing the relative arrangements between fuel assemblies, sealed tubes, and blocks of moderator material.
[0077] Detailed description
[0078] Throughout the present application, the terms "vertical", "lower", "upper", "bottom", "top", "below" and "above" are to be understood by reference to a primary vessel filled with a liquid metal of a nuclear reactor according to the invention, as it is in the vertical operating configuration.
[0079] The arrows symbolize the circulation of primary liquid sodium in the reactor vessel and in the exchanger between the primary and secondary circuits.
[0080] Figures 1 to 3 show a nuclear reactor cooled with liquid metal and fuel in the form of TRISO particles according to the invention.
[0081] Such a reactor 1 comprises a primary vessel 2 or reactor vessel filled with liquid sodium, which is a straight cylinder with central axis X and inside which is present the core 3 where a plurality of fixed sealed tubes 4 are installed housing removable fuel assemblies 5 with particles 50 of TRISO fuel, which generate thermal energy by the fissions of the fuel. The liquid sodium is therefore the heat transfer fluid of the primary circuit: it stores and transports the heat of the core 3.
[0082] As shown in Figure 1A, each fixed sealed tube may comprise a blind tubular part forming a glove finger in which the fuel assemblies 5 are stacked prior to operation of the reactor. If necessary, they can be replaced during operation of the reactor. The arrangement of each sealed tube 4 is such that its opening through which the fuel assemblies are stacked is at a distance from the primary liquid metal.
[0083] Solid nuclear fuels are assemblies 5 housing TRISO 50 particles compacted in a matrix of moderating material 51, preferably graphite. Each of these particles is formed of a uranium oxide core coated with layers based on carbon compounds which serve as a first containment barrier to retain the fission products.
[0084] Optionally, a sheath 52, in particular made of graphite, can surround the matrix 51. As also visible in this figure 3, a ring configuration of the compact 50, 51 can be provided and in the center of the latter a volume 53 empty or filled with inert gas, preferably helium, at low pressure. This volume 53 makes it possible to limit the core temperature of the fuel and also makes it possible to accommodate any expansions of the matrix 51.
[0085] Each sealed assembly 5 may have a hexagonal (Figure 2A) or cylindrical (Figure 3) outer cross-section.
[0086] The outer coating of the TRISO particles constitutes the primary containment barrier for the radioactive materials contained in core 3.
[0087] A support slab 6 supports the primary tank 2 as well as the weight of the liquid metal of the primary circuit and the internal components. This slab 6 is arranged directly above the core 3 and closes the primary tank 2 to contain the liquid metal, acting as a barrier between said liquid metal and the external environment.
[0088] The reactor vessel 2 is separated into two distinct zones by a separation structure consisting of at least one shell 7 arranged inside the reactor vessel 2. This separation device is also known as a redan.
[0089] As symbolized by the arrows in Figure 1, the redan 7 is arranged in the primary vessel 2 by forming a central chimney, to separate the interior of the primary vessel 10 into a central zone and a peripheral zone so that in operation of the reactor, the liquid metal circulates by natural convection in a loop from the bottom of the central zone where the reactor core 3 is arranged above a base 8, from which it rises by heating to the top of the central zone where it enters through an inlet 90 of at least one heat exchanger 9 outside the reactor vessel 2 to exit through an outlet of the exchanger 91 then towards the top of the peripheral zone to descend towards the bottom of the peripheral zone where it is diverted towards the core 3.
[0090] Thus, in normal operation, the primary liquid metal circulates only by natural convection in the reactor vessel 2 in a closed loop through at least one exchanger 9 and exchanges its heat within it with a secondary fluid which enters cold through the inlet 92 and leaves hot through its outlet 93. Typically, the temperature of the secondary fluid at the outlet 93 can be equal to 700°C. The shape of the step 7 makes it possible to improve the circulation by natural convection of the liquid metal.
[0091] Reactor 1 includes a reactivity control system (not shown) which may consist of control rods internal to primary vessel 2.
[0092] Reactor vessel 2 includes a canopy, usually called a pile canopy, which can be filled with an inert gas, such as argon or helium, above the liquid metal. This canopy allows, on the one hand, to absorb the thermal expansion of the liquid metal within the reactor vessel, when it undergoes a level variation, and on the other hand, to recover the gaseous fission products generated by nuclear fissions within the fuels.
[0093] An advantageous arrangement of a sealed tube 4 housing a stack of fuel assemblies 5 is shown in Figure 1A. The sealed tube 4, preferably made of ceramic, is arranged between at least two blocks of moderator material 20, preferably graphite. This arrangement allows the primary liquid metal to circulate around and in contact with the sealed tube in a well-defined space E.
[0094] A reflector 21 made of moderating material, preferably graphite, can be arranged below a sealed tube 4.
[0095] An advantageous arrangement is shown in Figure 3 for tight tubes 4 and fuel assemblies 5 with a straight cylindrical cross-section. A tube 4 is arranged between two blocks 20 of moderating material, preferably graphite, arranged side by side leaving a space e between them and a larger space E in which the liquid metal circulates during operation of the reactor.
[0096] As also visible in this figure 3, it is possible to provide a filling volume of an inert gas, preferably helium, between a fuel assembly 5 and the interior of the sealed tube 5 in order to improve the thermal interface for the thermal conduction of the heat released by the fission of the TRISO particles.
[0097] The invention is not limited to the examples which have just been described; in particular, it is possible to combine characteristics of the illustrated examples within non-illustrated variants.
[0098] Other variants and embodiments may be envisaged without departing from the scope of the invention. Liquid metals other than sodium may be considered for the primary fluid: this may be lead (Pb) or a lead-bismuth alloy (Pb-Bi).
[0099] Materials other than graphite can also be considered for the matrix of the TRISO particles and / or the core, for example aluminum. As shown in Figure 2, the reactor 1 can include a sealed enclosure 10, made of concrete to house the primary vessel 2.
[0100] List of cited references
[0101] [1]: “High-temperature gas-cooled reactors and industrial heat applications” , NEA No. 7629, OECD, Paris, 2022. [2]: Jiri Krepel et al. “Self-Sustaining Breeding in Advanced Reactors: Characterization of
[0102] Selected Reactors”, Encyclopedia of Nuclear Energy 2021, Pages 801-819. https: / / www.sciencedirect.com / science / article / pii / B97801281972570012397via%3Dihub
Claims
Claims 1. Nuclear reactor (1) cooled by liquid metal, comprising a vessel (2), called primary vessel, axisymmetric around a central axis (X), filled with a heat transfer fluid with at least one liquid metal as heat transfer fluid of the primary circuit of the reactor, the vessel comprising a core (3) comprising a plurality of hollow, sealed and fixed tubes (4), arranged parallel to the axis X so that during operation of the reactor the liquid metal circulates in contact with their outer periphery, each hollow tube (4) housing a stack of fuel assemblies (5), housing nuclear fuel particles (50), called TRISO, mixed with a matrix (51).
2. Nuclear reactor (1) according to claim 1, the matrix in which the TRISO particles are mixed being in the form of a compact block.
3. Nuclear reactor (1) according to one of claims 1 or 2, comprising a structure forming a step (7), with a central axis coincident with that of the primary vessel, the structure being arranged in the primary vessel to separate the interior thereof into a central zone and a peripheral zone so that in operation of the reactor, the liquid metal coolant circulates by natural convection, in a closed loop from the bottom of the central zone where the reactor core is arranged within which the fission reactions occur, from which it rises by heating, to the top of the central zone where it enters through an inlet (90) of at least one heat exchanger (9) outside the reactor vessel to exit through an outlet of the exchanger (91) then towards the top of the peripheral zone to descend towards the bottom of the peripheral zone where it is diverted towards the reactor core.
4. Nuclear reactor (1) according to one of the preceding claims, the reactor core housing at least one moderating material.
5. Nuclear reactor (1) according to claim 4, the core of the reactor comprising a reflector made of moderating material arranged below each sealed tube.
6. Nuclear reactor (1) according to claim 4 or 5, the core of the reactor comprising at least two blocks of moderating material arranged around each sealed tube, allowing the liquid metal to circulate around and in contact with the latter.
7. Nuclear reactor according to one of the preceding claims, each sealed tube comprising a blind tubular part forming a glove finger in which the fuel assemblies are stacked prior to operation of the reactor.
8. Nuclear reactor according to one of the preceding claims, the liquid metal of the heat transfer fluid being chosen from sodium (Na), lead (Pb) or a lead-bismuth alloy. (Pb-Bi).
9. Nuclear reactor according to one of the preceding claims, the material of the matrix of the TRISO particles and / or of the core being graphite.
10. Nuclear reactor according to one of the preceding claims, the material constituting the sealed tubes being a ceramic.
11. Nuclear reactor (1) according to one of the preceding claims, comprising a reactivity control system consisting either of control rods internal to the primary vessel.