RU2524681C2 - Fuel element for nuclear reactor - Google Patents

Abstract

FIELD: physics, atomic power.

SUBSTANCE: invention relates to nuclear engineering and can be used to produce fuel elements for slow and fast neutron reactors. A fuel element for a nuclear reactor, having a sealed shell in which there is a fuel kernel and voids, further includes a working fluid with a melting point and a boiling point which correspond to the operating temperature range on the periphery of the shell and at the centre of the fuel kernel, and the fuel kernel contains nuclear fuel of a capillary structure with interconnected porosity.

EFFECT: high heat removal in a fuel element of a nuclear reactor, which significantly improves performance of fuel elements of nuclear reactors.

7 cl, 3 dwg

Description

The invention relates to the field of nuclear engineering and can be used to create fuel elements (fuel elements) for nuclear reactors using thermal and fast neutrons.

Known rod fuel rod for a nuclear reactor, consisting of a tubular shell made of corrosion-resistant steel or zirconium alloy, sealed at the ends with plugs, a fuel core made of fissile material in the form of granules, tablets, bushings (tablets with a central channel), lower and upper gas collector filled inert gas helium.

Insufficiently efficient heat removal when using a helium sublayer, which leads to an increase in the temperature of the fuel core and cladding in unregulated situations, is the main disadvantage of this type of fuel elements, which are widely used both in our country and abroad.

Known rod fuel rod for a fast neutron reactor, consisting of a shell, muffled at the ends, a fuel core in the form of rods or tablets of UPuN, UPuC, UPuZr and liquid metal filling the gap between the fuel and the shell, the lower and partially upper free volumes [Reshetnikov F .G., Bibilashniili Yu.K., Golovnin I.S. and others. Development, production and operation of fuel elements of power reactors: in 2 books. / Ed. F.G. Reshetnikova. M .: Energoatomizdat, 1995]. The use of a liquid metal sublayer improves heat transfer only at the initial stage of operation. In the future, due to swelling - gas swelling - the gap between the fuel and the cladding disappears, the liquid metal is displaced into free volumes and does not significantly affect the heat removal from the surface of the fuel element.

The objective of the proposed technical solution is the implementation of increased heat removal in the fuel rod of a nuclear reactor, which will significantly improve the operational characteristics of the fuel rods of nuclear reactors.

To solve this problem, a fuel rod of a nuclear reactor, including a sealed shell with a fuel core placed in it and free volumes, additionally contains a working fluid with melting and boiling points corresponding to the temperature range at the periphery of the shell and in the center of the fuel core, and the fuel core contains a nuclear fuel capillary structure with interconnected porosity.

Uranium, thorium, plutonium in the form of a metal, alloy, or chemical compound selected from the series: intermetallic, oxide, nitride, carbide, in tablets with a central channel and axial grooves on the outer surface to create a capillary structure, is used as nuclear fuel.

As the working fluid, alkali metals are used, as well as silver, calcium, antimony, strontium, barium, thallium, indium, lead, bismuth, or their alloys.

The inner surface of the upper free volume of a fuel rod contains a capillary structure in the form of a screen, mesh, porous body, etc.

The fuel cladding can be made entirely of corrosion-resistant austenitic or ferritic-martensitic steel, or composite: in the region of the upper free volume, from austenitic, and in the region of the lower free volume and fuel core, from ferritic-martensitic steel.

A fuel rod contains cold and hot cleaning filter sorbents located in its lower and upper free volumes, respectively.

The manufacture of a fuel element in which the core of nuclear fuel (uranium, plutonium in the form of a metal, alloy or chemical compound) is made with a system of communicating porosity (granulate, tablets with a central channel and a gap between the tablets and the shell), and the lower free volume , the porous core and part of the upper free volume are filled with liquid metal, for example sodium, to achieve the goal, namely to realize effective heat removal by the mechanism of a closed heat pipe when it is operated in a vertical position in the core of a nuclear reactor.

In the high-temperature part of the fuel core, sodium turns into steam (T _boil. Na = 880 ° С), moves along the central channel (or pores in the granulate) to the upper gas collector (T _gas. = 600-700 ° С), condenses on the walls of its shell and under the influence of gravity (and capillary forces) it flows through the gap to the lower part of the core, after which the process repeats.

To regulate the temperature in a fuel rod with a heat sink by the mechanism of the heat pipe, corrugation or finning of the upper free volume sheath is used, as well as the addition of non-condensable gas to the working fluid. The gas is displaced by the steam flow into the condensation zone, where a relatively sharp interface is established, above which the heat sink is practically absent. Thus, by moving the interface, varying the portions of the introduced gas, it is possible to change the heat transfer surface in the upper free volume, and therefore, to control the temperature of the fuel in the fuel element.

The implementation of effective heat removal from fuel rods contributes to:

- improving the safety of a nuclear reactor due to a decrease in heat content in its core;

- improving the compatibility of fuel with a shell made of corrosion-resistant steel and zirconium alloy;

- reduction of vacancy swelling of the shells of austenitic chromium-nickel steels due to a significant decrease in their operating temperature in the region of maximum neutron fluence;

- creating conditions for the use of weakly swollen ferritic-martensitic steels as the cladding material of the active part of a fuel rod (the cladding material of the upper gas collector is austenitic chromium-nickel steel, for example ChS-68).

Figure 1 shows a longitudinal section of a fuel rod reactor for fast

neutrons, where:

1 - a protective sheath of the active part of a fuel element;

2 - cladding of the upper fuel rod gas collector;

3 - adapter;

4 - the top cap;

5 - the bottom cap;

6 - fuel core with a communicating porosity system;

7 - lower volume free of fuel;

8 - upper free volume intended for the collection of gaseous fission products;

9 - a porous plug fixing the fuel core;

10 - working substance - liquid metal filling the lower free volume, a fuel core with a system of communicating porosity and partially upper free volume;

11 - sorbent filters for cleaning the working substance from impurities;

12 - the direction of flow of the working fluid.

Figure 2 shows the types of capillary structures: 13 - wall material, 14 - multilayer mesh or porous body, 15 - porous screen.

Figure 3 presents the macro- (a) and microstructure (b) of thin sections of experimental fuel elements with helium (1) and sodium (2) filling (center A3).

The fuel element for a nuclear reactor contains a shell 1, sealed from the ends of the upper 2 and lower 3 plugs, in the inner cavity of which are placed the fuel core 4 with communicating porosity, the lower 5 and upper 6 are fuel-free volumes. The fuel column is fixed by a porous plug 7, welded by spot welding to the shell. The working substance - liquid metal 8 is filled with the lower free volume, the fuel core and part of the upper free volume. The upper and lower free volumes contain sorbent filters 9, for example, based on metal chips, zeolite, activated carbon, which purify the working substance from impurities by the “cold trap” mechanism and chemical “hot cleaning”.

Figure 3 presents the results of cermet studies of two adjacent fuel elements with granular oxide fuel (effective density 8.84 g / cm ³ , O / M = 2.001-2.003) and an austenitic steel sheath 0X16H15M3B (pipe section 6.0 × 0, 3 mm) with helium and sodium filling. The fuel rods were irradiated in the experimental fuel assemblies of the BOR-60 reactor at a maximum specific heat load of 420 W / cm.

In contrast to a fuel element with a helium sublayer (Figs. 3a, 1), a reformation of the initial structure of granular fuel did not occur in the sodium-filled fuel element, the columnar zone and the central cavity were not formed. This indicates that heat removal by the mechanism of the heat pipe lowered the temperature of the fuel center from more than 2200 ° C to less than 1600 ° C.

The sodium-filled fuel rod showed no signs of fuel interaction with the cladding. In fuel elements with a helium sublayer, the corrosion of steel from the fuel side was significant. Several helium-filled fuel rods in the fuel assembly have failed due to the appearance of through-shell defects of a corrosion origin. The profilometric studies showed a ~ 3-fold lower value of the vacancy swelling of the cladding of fuel elements with sodium filling in comparison with fuel elements with a helium sublayer.

A decrease in temperature opens up a tempting prospect for using as a cladding material the active part of a fuel element of a low-swelling ferritic-martensitic steel, which is currently not used due to its low long-term strength at temperatures realized in a BN-600 fast-neutron energy reactor.

Claims (7)

1. A fuel rod of a nuclear reactor, including a sealed shell with a fuel core placed in it, upper and lower free volumes, characterized in that it further comprises a working substance that is in operation in a two-phase state: liquid at the shell and vaporous in the center of the fuel core, made with interconnected porosity, allowing to realize effective heat removal by the closed heat pipe mechanism 2. A fuel rod according to claim 1, characterized in that uranium, uranium with plutonium, or thorium with uranium in the form of a metal, alloy, or chemical compound selected from the series are used as nuclear fuel: nitride, carbide, oxide. 3. The fuel rod according to claim 1, characterized in that silver, calcium, antimony, strontium, barium, thallium, indium, alkali metals, lead, bismuth or their alloys are used as a working substance. 4. The fuel rod according to claim 1, characterized in that tablets with a central channel and axial (vertical) grooves on the outer surface or granules are used as the fuel core of the capillary structure. 5. The fuel rod according to claim 1, characterized in that the inner surface of its upper free volume contains a capillary structure in the form of a screen, mesh or porous body. 6. The fuel rod according to claim 1, characterized in that its shell is made of composite: from ferritic-martensitic and austenitic corrosion-resistant steels in the area of the fuel core and in the region of the upper free volume, respectively or integral, completely from each of these steels. 7. A fuel rod according to claim 1, characterized in that its lower free volume contains sorbent filters based on metal chips, zeolite or activated carbon, which purify the working substance from impurities by the "cold trap" and chemical "hot cleaning" mechanism.

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