DE4431290C1 - Safe nuclear reactor with continuous loading and unloading - Google Patents

Abstract

A nuclear reactor has a core (1) containing loosely loaded (2) polyhedral (for example cubic) shaped fuel elements (3) which can be continually loaded and unloaded. The fuel elements consist of thousands of very small, coated spherical shaped fuel particles contained within a ceramic housing. Components are provided (7, 8) for loading and unloading under gravity of the fuel elements. Pipelines (11, 12) are provided for the supply of the coolant and moderating medium (10), which can be light or heavy water or a mixture. The core shape is either cylindrical or annular with a central column. The fuel particle coating is a double layer of silicon or zircon carbide with an intermediate layer.

Description

The invention relates to a nuclear reactor in the Loose bed of fuel located in reactor core elements in a form that continuously and discharge allowed, and made of ceramic materials non-moderating way in which the fuel in Coated particles is included with Reactor internals for guiding and merging of loose bulk fuel as through Gravity moving fluid bed, with on directions for continuous loading and unloading of the reactor core with fuel elements attached to the Reactor internals connected and through which Passing reactor core enclosing are with this container and the empty space of the loose Filling of fuel filling medium as Coolant, with devices for supply and for Removal of this medium into the reactor core, and with Devices for switching off and regulating the Neutron chain reaction outside of the loose Bulk fuel and the leading Reactor internals are arranged (see. DE 26 31 237 A1).

It is the aspiration of the professional world, one if possible to design a disaster-free nuclear reactor, d. H. a reactor in which none of the conceivable Disasters, such as going through the  Neutron chain reaction due to the incorporation of burn-off excess reactivity and others destabilizing properties.

The newly designed nuclear reactor should therefore Have fuel assemblies of a shape that is continuous Allow nuety loading and unloading. With that the Incorporation of excess reactivity due to combustion and the associated hazard potential avoided.

The newly designed nuclear reactor should also where possible, positive characteristics differ unite nuclear reactor types. So it is (Oldekop, W. - Pressurized water reactors for Nuclear Power Plants, Thiemig Paperbacks, Volume 51, 2nd ed., 1974) that light water moderated systems can be designed so that they are "under-moderated" and the chain reaction cannot "go through" for operational reasons:

With light water moderated systems this results Multiplication constant in the infinitely extended System k for the ratio (H / S) of the number of Hydrogen atoms (H) of the moderator, reflector and Coolant light water (H₂O) and the number of fissile atoms (S) of the fuel uranium dioxide (U₂O) in the non-poisoned System a maximum for (H / S) = approx. 260. This is that so-called optimal H / S ratio. With an interpretation with an H / S ratio below the optimum, d. H. with so-called "sub-moderation", leads the company conditional reduction in the density of water, e.g. B. at Temperature rise and the loss due to malfunction the moderator light water to a decrease of the Multi duplication constants. This is a captive,  self-acting and stabilizing property the design for the neutron chain reaction.

An interpretation with an H / S ratio of above 260, where the system would be overmodern with operational reduction in the density of the Water, e.g. B. with temperature increase and malfunction partial loss, an increase in Lead multiplication constants. This would be one self-acting, destabilizing property. The reason for this is that with the increase in the multipli cation constant associated performance increase, the in the worst case, a runaway of the neutron Chain reaction and thus destruction of the Brenn elements and other barriers up to containment and can lead to the release of radioactive substances.

It is also known (Mamert, Ch., Wimmers, M., Ziermann, E .: The AVR experimental nuclear power plant - wich results of another reactor concept, p. 275- 284) that the technique of continuous loading and Discharge of the high temperature reactor with spherical Fuel elements has the advantage that none combustion-related excess reactivity is necessary. Of the The reason for this is that spherical fuel assemblies Have shape that continuous loading and unloading of the high temperature reactor allowed.

It is also known (Schenk, W., Nabielek, H., Pott, G., Nickel, H .: Fission product retention in Kugelbrennelement, S. 321-328) that the technology of Multi-micro inclusion of the fuel in "Coated Particles" from Development and Application of the family of high temperature reactors, especially with the high temperature reactors  spherical fuel assemblies, advantageous in terms of High temperature resistance and the high temperature permanent retention of radioactive substances up to Temperatures of e.g. B. 1600 ° C.

It is an object of the invention to provide a nuclear reactor conceive of the advantages of the lightweight water reactor and the advantages mentioned High temperature reactor connects.

This object is achieved in that the fuel elements take the form of a space-filling or have almost space-filling polyhedron and the medium Light water (H₂O) or heavy water (D₂O) is. Space filling polyhedra are cubes, stumps of octahedra and Rombendodecahedron. An almost space-filling polyhedron is the tetrahedron. The use of fuel elements in Polyhedron shape does it - with a suitable charge Fuel assemblies - possible to get to a reactor of the beneficial properties of the two reactor types connects with each other.

For example, the ceramic Fuel assemblies the ratio (H / S) of hydrogen Atoms (H) of the moderator, reflector and coolant Light water (H₂O) and the fissile atoms (S) of Uranium dioxide (UO₂) fuel in a fuel assembly the relatively small value, measured in the form Water volume / (cube volume + water volume) of 20% can be set so that with decreasing density or no loss of reactivity when water is lost entry.

The direct transfer of the technique of the spherical Fuel element with coated particles  Moderation-neutral ceramic materials on the Moderation by light water means because of the emptiness volume share of the loose ball filling of 39% despite high loading with coated particles always "over- Moderation ".

The use of cube-shaped fuel elements - instead of the spherical fuel elements - enables that Reduction of the empty volume portion to over 20%, so that the resulting improvement factor for the The moderation ratio is 2.56. As a result of this factor and high loading overall "sub-moderation".

When using heavy water, for example (D₂O) as moderator, reflector and coolant optimal D / S ratio at higher numerical values than 260. Accordingly, the empty volume portion of the loose fill to higher numerical values and / or the Load density set to low numerical values will. In summary, this means that for difficult water-moderated systems spherical fuel assemblies according to the type of fuel elements of the high-temperature reactor fulfill the task.

From DE 27 22 508 A1 or from US 5 015 437 is the Use of block-shaped or prism-shaped Fuel elements known. The block-shaped structure after However, DE 27 22 508 A1 serves no special purpose. US 5 015 437 teaches that a prism shaped structure of the fuel assemblies a special arrangement tion or structure of the reactor core enables. The subject one can therefore not infer from these publications Polyhedral fuel assemblies to the solution according to the requirements use the task of the invention.  

Reak has an expedient embodiment of the reactor built-in gates for guiding the loose bulk of kiln elements in the form of a cylinder or a cylin with a central column. It is known (Barnert, H., Singh, J .: Design Evaluations of a Small High-Tempera ture Reactor for Process Heat Applications, Nuclear Engineering and Design 109 (1988), pp. 245-251) that derar shaped cores for automatic removal of the Lead post-heat.

It is also known (Schenk, W., Gontard, R., Nabielek, H .: Performance of HTR Fuel Samples under High-Irradiation and Accident Simulation Conditions, with Emphasis on Test Capsules HFR-P4 and SLPI, Jül-2992, November 1994 ; Kugeler, K., Neis, H., Ballensiefen, G. (ed.): Advances in energy technology for an economical, environmentally friendly and damage-limiting energy supply, Prof. Dr. Rudolf Schulten for the 70th year, monographs of the Research Center Jülich , Volume 8, 1993) that the coating of the coated particles in the best fully developed form of the so-called TRISO particle consists of four layers: the buffer layer and three sealing coatings, these are the inner layer made of pyrocarbon, the gas-tight Layer of silicon carbide and the outer layer of pyro-carbon. As is known, the manufacturing-related leakage of the gas-tight coating layer made of silicon carbide (Schenk, W., Nabielek, H., Pott, G., Nickel, H .: The fission product retention in the spherical combustion element, pp. 321-328) is approximately 10 ^-5 , ie among 100,000 particles produced, one is not gas-tight due to the manufacturing process.

To improve the production-related gastightness, it is advantageous to carry out the coating layer made of silicon carbide of the known technology of the coated particle now as a "double layer with intermediate layer", that is to say as a gas-tight coating layer made of silicon carbide, as an intermediate coating layer made of pyrocarbon and as a gas-tight coating layer made of silicon carbide. The advantage is that by the production of the double layer with intermediate layer, the probability of occurrence of the production-related gas leakage is further reduced to values far below 10 ^-5 , according to simple mathematical calculation, for. B. to 10 ^-10 . Another advantage is that the production-related contamination of the coating located further outside is further reduced compared to the prior art. It is also advantageous that due to the execution as a double layer with an intermediate layer, the high-temperature-resistant retention of radioactive substances is further improved and at higher temperatures than e.g. B. 1600 ° C is shifted. The reason for this is the narrowing of the scatter by the mean failure temperature of e.g. B. 1900 ° C.

Instead of using silicon carbide for the gas-tight coating layers it may be appropriate one or all two layers of zirconia to produce carbide.

Embodiments of the reactors, the polyhedral Fuel elements and the coated particles are in the drawing is shown schematically and are in the following:

Show it  

Fig. 1 dividend a light-water reactor fuel assembly,

Fig. 2 shows a cube-shaped fuel assembly,

Fig. 3 shows a reactor of higher thermal performance, and

Fig. 4 shows a coated particle in section.

The reactor shown in Fig. 1 has a reactor core 1 with therein a loose bed 2 of fuel elements 3 in a form which allows a continuous loading and unloading. The fuel assemblies are made of non-moderating ceramic materials, in which the fuel is enclosed in coated particles 4 according to FIG. 2. The reactor also has reactor internals for guiding 5 and for merging 6 of the loose bed of fuel elements as a fluid bed which is slowly moved by gravitation and devices for continuous loading 7 and unloading 8 of the reactor core with fuel elements which are connected to the reactor structures and through which the reactor core enclosing container 9 are passed. The reactor further consists of this container and the empty space of the loose bed of fuel filling liquid 10 , for. B. light water, heavy water or mixtures thereof, which is also a moderator, reflector tor and coolant, and from facilities for supplying 11 and discharging 12 of this liquid into the reactor core and from facilities for switching off and controlling 13 the neutron chain reaction, which is outside the bulk material of fuel elements and the leading reactor internals are arranged.

The polyhedron-shaped fuel element 3 in Fig. 2, Darge represents the example of the cube-shaped fuel element, in which the fuel is included in coated particles 4 , consists of the inner matrix 14 of a mixture of ceramic, non-moderating material and coated particles and from the outer layer 15 , which contains no coated particles and consists of the same non-moderating ceramic material as the matrix. The dimension of the cube-shaped fuel element is z. B. Cube size 50 mm. With a cube size of z. B. 44 mm some 10,000 to 100,000 coated particles. The coated particle with TRISO coating of the known technology ( FIG. 2) consists of the fuel core 16 and the coating 17th This consists of 4 coating layers: the buffer layer made of carbon 18 , the inner coating layer 19 , made of pyro carbon, the gas-tight coating layer 20 made of silicon carbide and the outer coating layer 21 made of pyrocarbon.

The reactor of higher thermal power shown in FIG. 3 is constructed essentially like the reactor of low thermal power according to FIG. 1, but has — instead of the simple cylindrical shape “cylindrical shape with central column” 22 . The center column consists of reactor internals 23 for guiding the loose bed in the central region of the reactor core, as well as devices 24 for regulating and switching off the neutron chain reaction and heat accumulator internals 25 in the central region.

The coated particle 4 according to the invention shown in FIG. 4 is distinguished from the prior art in that the gas-tight coating layer 20 (according to FIG. 2) is made of silicon carbide as a "double coating device with an intermediate layer" and thus from the inner gas-tight coating layer 201 consists of silicon carbide, the intermediate layer 202 and the outer gas-tight coating system 203 made of silicon carbide. It may also be advantageous to use both or one of the two, e.g. B. the inner coating layer, zirconium carbide instead of silicon carbide. The coating 17 on the fuel core 16 thus consists of the following coating layers with the typical layer thicknesses, listed from the inside out: buffer layer: 80 μm, inner coating layer: 30 to 40 μm, inner gas-tight coating layer: 20 to 40 μm, intermediate layer : 10 to 20 µm, outer gastight coating layer: 20 to 40 µm, and outer coating layer: 30 to 40 µm. For low operating temperatures, e.g. B. in a light water reactor of about 300 to 400 ° C, the smaller dimensions of the coating layers come into question for higher operating temperatures, such as. B. in a high temperature reactor of z. B. 600 ° C, rather the higher values of the dimensions of the coating layers.

Claims (4)

1. nuclear reactor with a loose bed of fuel elements located in the reactor core, in a form which permits continuous loading and unloading, and of non-moderating ceramic materials in which the fuel is enclosed in coated particles,
with reactor internals for guiding and merging the loose bed of fuel elements as a fluid bed moving slowly due to gravity,
with devices for the continuous loading and unloading of the reactor core with fuel elements which are connected to the reactor and are passed through the container surrounding the core of the reactor,
with this container and the empty space of the loose bed of fuel filling medium as coolant,
with facilities for feeding and discharging this medium into the reactor core and
with facilities for switching off and regulating the neutron chain reaction, which are arranged outside the loose bed of fuel elements and the leading reactor internals,
characterized in that the fuel elements ( 3 ) have the shape of a space-filling or almost space-filling polyhedron and the medium is light water (H₂O) or heavy water (D₂O). 2. Nuclear reactor according to claim 1, characterized, that the reactor internals to guide the loose Fill cylindrical shape or cylindrical shape with Have center column. 3. Nuclear reactor according to claim 1 or 2, characterized, that the gas-tight coating layer of the Coating the coated particle as Double coating with intermediate layer is. 4. Nuclear reactor according to claim 3, characterized, that the two or one of the two gas-tight Coating layers of the double coating with Intermediate layer consists of zirconium carbide.