Fuel element

 

(57) Abstract:

The invention relates to the field of nuclear technology and can be applied primarily for nuclear reactors of different types of fuel elements (cartridges), with free fuel composition, gas-filled volume for collection of gaseous fission products. Fuel element container of the type having free fuel composition gas-filled volume, intended for collection of gaseous fission products, equipped with fusible safety valve, made in the form of a pin or rivet at the top or bottom blank of metal, the melting point of which is below the melting temperature of the sheath of the fuel element and selected from the operating conditions and analysis of emergency situations of the reactor installation. The technical result is to prevent swelling of the fuel rod rupture of a fuel rod during a severe accident, preventing contact of the fuel composition with the fluid and uncontrolled release of radioactive products in the first circuit. 2 Il.

The invention relates to the field of nuclear technology and can be applied primarily for reactor facilities using Talala collection of gaseous fission products.

There are various designs of fuel elements of nuclear reactors (VVER, RBMK, PWR, BWR, bn, BGR and others ) /1/ with terrenceville the fuel rods, water cooled, liquid metal or gas coolants. Common to them is that the fuel composition in the form of pellets of uranium dioxide or plutonium is enclosed in a metal sheath. Century most designs this shell is sealed and is designed to retain gaseous fission products released during the combustion of fuel.

Under certain conditions the internal pressure of the gaseous products can grow to critical values and lead to the destruction of the shell.

To avoid this, for fast reactors with sodium and gas cooling was proposed design "vented" TVEL, i.e., fuel that is leaking and allows you to equalize the pressure between the coolant circuit and its internal volume. This fuel element was used, for example, on the American reactor EBR-II /2/.

The use of leaky membranes resulted in the need to solve the problem of trapping fragmentation fission products to prevent them from falling into the contour of the heat is, however, not able to retain all the glass particles, such as neutral gases and "vented" rods these fragments are continuously output from the first loop.

As a prototype of the present invention describes the design stereoimage of TVEL VVER-440 fuel in the form of pellets of uranium dioxide, enclosed in a sealed envelope from an alloy of zirconium, filled with helium, with gadobenic at the top.

The disadvantages of the prototype are:

1. In emergency situations, loss of coolant (leak in the primary circuit) the shell of a fuel rod is heated. The fuel temperature rises, too, in this case there are additional fragmentation gases. In the result of the internal pressure of the gases in the fuel rod exceeds the external pressure and increases up until the most hot place, and therefore less durable, shell begins to deform first elastically and then plastically. This leads to the swelling of the fuel rod, the violation cooled geometry, and further to the rupture of the membranes /3/. In the end, radioactive gases and particles of fuel and fission products, washed out at the contact of the fluid with the fuel composition are ejected in the first cons radioactive products is uncontrolled - all that is under the shell, freely come out.

The technical problem to be solved in this invention is the prevention of swelling of the fuel rod rupture of a fuel rod during a severe accident, preventing contact of the fuel composition with the fluid and uncontrolled release of radioactive products in the first circuit, reducing emissions of most toxic radionuclides.

The invention consists in that the heat-generating element with a fuel core, enclosed in a sealed envelope, and having a free fuel composition gas-filled volume, intended for collection of gaseous fission products, is supplied fusible safety valve to vent gas from the internal volume of the fuel rod to the coolant circuit placed at the top or bottom plug of a fuel rod and is made of metal, the melting point of which is below the melting temperature of the sheath of the fuel element and selected from the operating conditions and analysis of emergency situations of the reactor installation.

The technical problem is solved due to the fact that in an emergency situation with loss-of-coolant temperature of a fuel rod, including the temperature of the shell and sahlene inside the fuel elements and the primary circuit is aligned. When this is achieved a number of significant advantages in comparison with the flow of such accidents with standard sealed fuel rods, including with prototype:

a) saved cooled geometry of fuel assemblies, which greatly facilitates emergency core cooling and prevent defragmentation membrane in re Gulf;

b) prevents or at least significantly reduces the contact of the fuel composition with the carrier and its leaching into the first circuit, which is inevitable when rupture of the membranes;

in) is organized by the dumping of radioactive gases in the first circuit, allowing you to use the built-in TVEL special filters, the same way as is done in the "vented" TVEL, which can significantly reduce emissions of the most dangerous radioisotopes of iodine, cesium, tellurium, etc.

Description of drawings

In Fig. 1 shows a fuel element reactor VVER-440 with fusible safety valve: 1 - top cover 2 - sleeve, 3 - trap gaseous products, 4 - gadobenic, 5 - lock, 6 - shell, 7 - tablet UO2, 8 - lower bracket, 9 - fusible safety valve.

In the upper cover plate or rivets (soldering, fill or otherwise) that seals the fuel under normal operating conditions. The melting temperature of the filler metal must be below the melting temperature of the sheath of the fuel element and is selected from the operating conditions and analysis of emergency situations specific reactor plant.

In Fig. 2 shows the dependence of the temperature change shells by accident type LOCA (Loss of Coolant Acident) from time to time after the occurrence of a large leak in the primary circuit, obtained at different (conservative and more realistic) assumptions /2/. Here: 10-model with a conservative approach, 11 - model with a more realistic approach, 12 - a sharp decrease in the coolant flow rate, 13 - start of Gulf zone 14 zone filled with water).

In normal operation, when the temperature of the shell and gases gazosbornik close to the temperature of the coolant, the proposed construction works as a standard sealed fuel elements, without creating problems with radioactive contamination of the primary circuit.

In an emergency situation with loss-of-coolant temperature of a fuel rod, including the temperature of the shell and the insert is growing rapidly. When reaching a certain temperature, selected on the basis of the analysis and the when, where intense deformation of the shell, fuse is melted and through the hole the gas comes out, the pressure inside the fuel elements and the primary circuit is aligned. When you do this:

a) saved cooled geometry of fuel assemblies, which greatly facilitates emergency core cooling and prevent defragmentation membrane in re Gulf;

b) prevents or at least significantly reduces the contact of the fuel composition with the carrier and its leaching into the first circuit, which is inevitable when rupture of the membranes;

in) is organized by the dumping of radioactive gases in the first circuit, allowing you to use the built-in TVEL special filters, the same way as is done in the "vented" TVEL, which can significantly reduce emissions of the most dangerous radioisotopes of iodine, cesium, tellurium, etc.

All these advantages are manifested in severe accident conditions, when the mass depressurization of the fuel rods is almost inevitable and care must be taken to minimise the radiological consequences of such an accident.

An example of a possible use of the invention may be the introduction of the proposed fuel rod with fusible safety valve in reacto is shown on Fig. 2. Fuel temperature also increases, in this case there are additional fragmentation gases. As a result of internal pressure in the fuel rod is increased up until the shell begins to deform. This deformation occurs in the temperature range 900-1300oC. Given that the growth temperature of the stub is delayed relative to the rising temperature of the shell in the most hot place (temperature stub in normal conditions when the reactor rated power is ~ 350o(C) as a material for the fusible safety valve (9, Fig. 1) you can choose, for example, aluminum with a melting point of 660oC. In the melting of the safety valve, the excess pressure inside the fuel rod will be reset, which will prevent bloating and defragmentation of the shell. Gaseous fission products through the trap (3, Fig. 1), will be largely captured by the filter, which will reduce the radiological consequences of an accident.

List of used sources

1. B. Frost. The fuel rods of nuclear reactors. M , Energoatomizdat, 1986.

2. Walter A. , Reynolds, A. breeder Reactors on fast neutrons. - M. : Energoatomizdat, 1986.

3. O. B. Samoil">

Heat-generating element with a fuel core, enclosed in a sealed envelope, with free fuel composition gas-filled volume, characterized in that the upper and/or lower cover provided with a fusible safety valve, made of metal, the melting point of which is below the melting temperature of the sheath of the fuel element and selected from the operating conditions and analysis of emergency situations of the reactor installation.

 

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