RU2526856C1 - Imitator of fuel element of nuclear reactor - Google Patents

Abstract

FIELD: power engineering.

SUBSTANCE: invention may be used in investigation of behaviour of fuel elements (FE) of nuclear reactors by experimental modelling of thermal and hydrodynamic processes under various modes of reactor operation, including emergency ones. A fuel element imitator comprises a shell, in which there is a column of pellets of natural fuel with a central hole and an electric heater arranged with a gap in holes of pellets and equipped with upper and lower shunts. Between the pellets there are circular aligning spacers from high-temperature electric insulation material with the same outer diameter as in fuel pellets. The diameter of the central hole of spacers and the distance between them is determined on the basis of two ratios that take into account diameters of the heater, holes of the pellets; coefficients of linear expansion of heater, pellets and spacers materials; heater temperature; a coefficient that characterises the method of heater end closing.

EFFECT: imitator makes it possible to provide for complete modelling of processes in fuel elements of reactors on imitators with the same dimensions, as the natural fuel elements, using natural fuel materials and the same temperature as in actual conditions of fuel elements testing.

4 cl, 1 dwg

Description

The invention relates to the field of thermophysical research and can be used to study the behavior of fuel elements (fuel elements) of nuclear reactors by experimental modeling of thermal and hydrodynamic processes under various operating conditions of the reactor, including emergency ones.

A known design of the TVEL simulator, consisting of a tubular body (shell) with a diameter of 9.1 mm from an alloy of zirconium E110, end elements (plugs) and uranium dioxide tablets placed inside the case. The simulator is placed vertically inside a tube heater coaxial with it [Collection of reports of the VIII Russian Conference on Reactor Materials Science; Dimitrovgrad, May 21-25, 2007, from 21-40]. The placement of the heater external to the simulator allows testing the elements of the fuel element design with a geometry that exactly matches the geometry of full-scale products, and with full-scale tablets of nuclear fuel.

A disadvantage of the known design is the impossibility of carrying out the correct simulation of thermal processes and the stress state of the fuel and shells, since the direction of heat fluxes and temperature gradients with an external heater are opposite to full-scale ones.

Known simulator of a fuel element [A.c. SU 1 264 754, IPC G21C 17/06, publ. 09/27/1999 // S.M. Balashov et al. //], including a shell and a tubular heater placed inside it. Between the shell and the heater there is an electrically insulating material - periclase powder (MgO).

A disadvantage of the known simulator is the impossibility of a complete simulation of thermal and hydrodynamic states, chemical interaction, since the design does not provide for placement of tablets made of natural fuel in the simulator case. The thermophysical properties and elemental composition of the cavity filling simulator of the electrical insulating powder are significantly different from the properties of the material of the tablets.

Also known is a simulator of a fuel element of a nuclear reactor containing a shell, a fuel column in the form of natural fuel pellets made of uranium dioxide, an internal rod heater located with a gap in the channel formed by the central openings of the tablets, and current-carrying nodes [Gontar A.C., Gridnev A.A. and others. Uranium dioxide with controlled restructuring of the structure of the fuel element of the thermionic converter reactor. International Conference "Nuclear Energy in Space", Moscow - Podolsk, March 1-3, 2005, collection of reports, vol.2, p.267-270]. In terms of the set of essential features, this simulator is closest to the claimed one and is selected as a prototype.

This design provides the preliminary heating of natural fuel tablets with the aim of further heating them by directly passing electric current through the tablets.

A disadvantage of the known simulator is the inability to fully simulate the thermal and hydrodynamic state of the fuel column and shell. For a complete simulation of the processes, the simulator must provide geometric proportions and a shape that determine the degree of similarity of thermal and hydrodynamic conditions on the surface of the cladding, as well as the thermal properties and energy release in materials of natural fuel elements. In the known simulator, the ratio between length and diameter, which is characteristic for full-time fuel elements, is not maintained. The length of the fuel column of natural diameter tablets in the device adopted for the prototype is 60 mm, while the full-length fuel rods have a fuel column length of more than a meter. The number of uranium dioxide tablets in the fuel column of the simulator of the same length as the fuel rod is more than 100. In this case, it is impossible to ensure uniform heating of the fuel column by directly passing current through it due to the large number of contacts between the tablets, where there is a high probability of overheating. Modeling the temperature profile along the length of a fuel rod with direct current transmission is also impossible.

The temperature levels necessary for modeling the thermal and hydrodynamic state, temperature profiles along the length, and energy release can be provided using a central rod heater. However, with large lengths of the heater and small gaps between the heater and the tablets (0.5-1.5 mm), between the tablets and the metal sheath (0.065 mm for a WWER fuel element), due to the possible distortion of the heater with increasing temperature, it is likely that the conductive rod touches with tablets and coated tablets. With increasing temperature, UO ₂ loses its insulating properties. For example, to provide a temperature of 1200 ° C on the shell, which is required by the test conditions during a design basis accident, the temperature of the heater should be ~ 2000 ° C, and that of uranium dioxide tablets 1400-1600 ° C. The electrical resistivity of UO ₂ with such heating is reduced by three orders of magnitude - from 1 Ohm • m to 10 ^-3 Ohm • m [Krzhizhanovsky PE, Stern Z.Yu. Thermophysical properties of non-metallic materials, L., "Energy", 1973], while the resistance of the heater increases. Since the thickness of the fuel pellet is small (~ 2 mm in VVER fuel elements), the electrical resistance of the heater-shell gap when the rod touches the pellet becomes comparable to the heater resistance, the length of which can exceed several meters.

Touching the current-carrying rod of the tablet leads to electrical breakdown and destruction of the simulator.

Thus, the main drawback of the known design is the temperature limitation when modeling processes with full-scale material (uranium dioxide) and full-scale dimensions of the simulator when studying the behavior of fuel rods in various reactor operating modes, including emergency ones.

Even greater temperature restrictions should be established during model tests of fuel elements with promising fuel compositions, in which the electrical conductivity is higher than that of uranium dioxide.

The problem to which the invention is directed, and the technical result provided by it, is the completeness of the modeling of processes in the fuel elements of reactors on simulators with the same dimensions as full-time fuel rods, when using full-scale fuel materials and the same as in real test conditions for fuel rods, temperatures.

The solution of the problem and the technical result provided by the invention are achieved by the fact that in a fuel element simulator containing a casing with a fuel column made of natural fuel pellets placed inside a coaxial casing, in the central openings of which there is a rod heater equipped with current leads, according to the invention, ring-type centering spacers from high-temperature insulating material with the same as that of fuel pellets with an outer diameter and with a diameter ENTRAL openings defined by the relation:

, $$\frac{\left(\mathrm{one}+{\alpha }_{T}T\right)}{\mathrm{one}+{\alpha }_{D}T}{d}_T>d_D>d_N\frac{\left(\mathrm{one}+{\alpha }_{N}T\right)}{\left(\mathrm{one}+{\alpha }_{D}T\right)}\left(\mathrm{one}\right)$$

,

the distance between adjacent centering spacers does not exceed a value equal to:

, где $$0.25\frac{\mathrm{one}}{\sqrt{K}}\frac{\pi d_H}{\sqrt{{\alpha }_{H}T}}\left(2\right)$$

where

d _N , d _T , d _D - respectively, the diameters of the heater, the holes of the tablets, the holes of the centering spacers;

α _H , α _T , α _D - respectively, the coefficients of linear expansion of the materials of the heater, tablets and spacers;

T is the temperature of the heater;

K is a coefficient characterizing the method of sealing the ends of the heater.

Observance of the conditions represented by expressions (1) and (2) allows modeling on simulators with tablets not only from uranium dioxide, but also from conductive fuel compositions, such as uranium nitride.

As the material of the ring centering spacers, beryllium oxide can be used. This material has high electrical insulation properties and with high reliability ensures a constant gap between the heater and the fuel column.

An increase in the distance between the remote controllers (a decrease in the K coefficient) can be achieved by installing a flexible element on one of the current leads.

Expression (1), which sets the boundaries for the diameter of the holes of the centering spacers, allows one to take into account the difference in the diameters of the heater and the holes in the tablets and dispensers with increasing temperature and to prevent the diameter of the holes in the dispensers from exceeding the diameter of the holes in the tablets when it is possible to touch the heater with the material of the tablets .

The reason for touching the current-carrying rod (heater) of the tablets, leading to electrical breakdown and destruction of the simulator, is the bending of the rod. Bending can occur as a result of the sagging of the rod when the simulator is not strictly vertical (for heater lengths greater than 1 m, the deviation from the vertical should not exceed a fraction of a degree), and as a result of the appearance of axial load P due to the restriction of free elongation of the rod during thermal expansion.

The purpose of centering spacers is to prevent bending leading to contact with the fuel pellets.

Expression (2) for determining the required distance between adjacent spacers is obtained from the solution to the problem of stability of a compressed rod (Euler problem). The first critical force at which the rod begins to bend is written as:

, $$P_K=\frac{{\pi }^{2}EJ}{L^2}\left(3\right)$$

,

.where E is the temperature-dependent elastic modulus of the material, L is the length of the bar, J is the moment of inertia of the bar during bending, which is equal to a bar with a cross section in the form of a circle $$\raisebox{1ex}{$\pi d_H^{\mathrm{four}}$}\!\left/ \!\raisebox{-1ex}{$64$}\right.$$

.

The axial load P acting on the rod, to maintain its rectilinear shape should not exceed P _cr . When rigidly terminating the ends of the heater, the axial load caused by the heating of the rod is determined by the expression:

, $$P=\epsilon EF=\frac{\Delta L}{L}EF\left(\mathrm{four}\right)$$

,

, тепловое удлинение стержня ΔL=Lα_HТ. Тогда P=α_HTEF.where ε is the axial deformation of the rod, the cross-sectional area of the rod $$F=\frac{\pi d_H^2}{\mathrm{four}}$$

, thermal elongation of the rod ΔL = Lα _H T. Then P = α _H TEF.

From the condition P _cr ≥P you can get:

$$\frac{{\pi }^{2}EJ}{{\alpha }_{H}TEF}\ge L^2\mathrm{and}l\mathrm{and}\frac{d_H^2}{16}\frac{{\pi }^2}{{\alpha }_{H}T}\ge L^2\left(5\right)$$

Expression (5) makes it possible to determine the permissible length of the rod (the distance between the attachment points) for the most difficult case, when the rigid termination of the ends does not allow the rod to elongate freely when heated. The methods used - the installation of bellows junctions or flexible current leads - reduce the axial load, but do not completely eliminate it.

The influence of the method of sealing the ends of the heater is taken into account by introducing the coefficient K. The value of K is equal to unity for the case of rigid sealing of the ends, for other variants of sealing K is defined as the ratio of the sum of the axial forces acting on the rod caused by the resistance of the elastic elements (flexible current supply or bellows) , thermal expansion of the current leads and the housing to the force calculated by the formula P = α _H TEF arising in the rod during hard termination of the ends:

. $$\frac{P_{\mathrm{At}}+R_D-R_{\mathrm{TO}}}{R}\left(6\right)$$

.

The resistance force Р _У is determined by calculation or experimentally: the axial force on the rod with an elastic element is measured, which is necessary for the same elongation as when heating. The additional force R _D on the rod associated with the thermal expansion of the composite current leads lying on the same axis as the heater is calculated as the sum of the forces from each part of the current lead taking into account the material and geometry of this part. Thermal expansion of the simulator housing (shell) in which the heater is located reduces the load on the heater. The force P _K caused by the elongation of the body in the expression (6) has a negative sign, it is calculated by temperature, coefficient of thermal expansion and the elastic modulus of the shell material.

Finally, the expression for determining the maximum permissible length of the rod between the elements of its fastening is written as:

. $$L=0.25\frac{\mathrm{one}}{\sqrt{K}}\frac{\pi d_H}{\sqrt{{\alpha }_{H}T}}\left(7\right)$$

.

Electrical insulating spacers with a hole, the diameter of which is less than the diameter of the holes in the tablets, act as intermediate supports - fasteners that prevent bending of the rod.

The essence of the claimed technical solution is illustrated by the drawing, which shows the design of the simulator of a fuel rod.

The simulator consists of a shell 1, a rod heater 2, fuel pellets 3, ring centering spacers 4, an upper current lead 5 and a lower composite current lead 6, equipped with a flexible element in the form of a lead wire 7 in insulating beads 8, pressure leads 9, 10.

The heater 2, which is a rod of circular cross section, is installed inside the shell 1 coaxially with the latter. Between the shell 1 and the heater 2 there is a column of fuel pellets 3 - cylinders with an axial hole for placing the heater. At intervals calculated using the expression given in the claims, centering spacers 4 are installed. The heater 1, made of tungsten, is connected by welding to molybdenum parts and then threaded to copper parts of the upper current supply 5 and the lower composite current supply 6. In series with the lower a flexible current-conducting bundle 7 made of copper wire placed in insulating ceramic beads 8 is installed with the current lead. The current leads are connected to the power source through pressure seal 9, 10.

The simulator works as follows. One or more simulators simulating a fuel assembly are mounted vertically in an airtight channel. From the power source through current leads 5, 6 and heater 2, current is passed. The material and diameters of the current leads are selected so that the main heat generation falls on the heater, where fuel pellets are placed coaxially with the heater. The heat flux from the heater enters the elements under study - fuel pellets, the shell, providing the levels and radial temperature gradients specified by the test conditions.

Installed between the tablets at calculated distances, centering spacers made of high-temperature electrical insulation material limit the heater's movements across its axis associated with possible loss of stability and bending, and do not allow the current-carrying rod to come into contact with fuel pellets, leading to electrical breakdown.

To conduct research on the deformation and corrosion behavior of fuel elements under such conditions, a simulator is needed that reproduces the geometric dimensions of full-size fuel elements and uses the same materials as in full-time fuel elements.

Example 1

As a first example of a specific implementation, a WWER reactor fuel element simulator can be considered, designed to simulate design emergency situations associated with a rupture of a coolant pipeline. Test conditions: shell temperature 1200, aggressive environment from the outside of the shell (superheated water vapor), pressure inside the shell up to 60 atm.

The simulator shell is made of alloy E110, the outer and inner diameters are 9.13 and 7.73 mm, respectively. The fuel simulator tablets are from unirradiated uranium dioxide. The diameter of the tablets is 7.6 mm, the height is 11 mm, the diameter of the central hole is 4.2 mm. The height of the rod heater made of tungsten is 1275 mm, the column of fuel pellets is 1200 mm. The length of the shell in which the heater is located with current leads, a flexible conductive harness in aluminum oxide beads is 2000 mm. The total length of the simulator is 2100 mm.

For a heater with a diameter of 3 mm, the geometry of the hole in the centering spacers of beryllium oxide at an assumed heater temperature of 2000 ° C is determined by the condition:

, $$\frac{\left(\mathrm{one}+{\alpha }_{T}T\right)}{\mathrm{one}+{\alpha }_{D}T}{d}_T>d_D>d_N\frac{\left(\mathrm{one}+{\alpha }_{N}T\right)}{\left(\mathrm{one}+{\alpha }_{D}T\right)}$$

,

or calculated according to the values of the linear expansion coefficients and the maximum temperature, which is assumed to be the same for spacers, tablets and a heater (2000 ° C), condition 4.1> d _Д > 3.

The height of the spacer is selected from the condition of sufficient compressive strength equal to the weight of the fuel column with centering spacers (~ 0.4 kg per 1 meter of column), since the design of the simulator allows the fuel column to extend freely when heated. For the case under consideration, the height of the spacer is 3 mm, the diameter of the hole is 3.2 mm.

The distance between the remote controllers l and the number of remote controllers n placed in the simulator is calculated by the formula:

$$l=\frac{L}{n}=0.25\frac{\mathrm{one}}{\sqrt{\mathrm{TO}}}\frac{\pi d_N}{\sqrt{{\alpha }_{N}T}}$$

For the example in question $$l=\frac{1.275}{n}=0.25\frac{\mathrm{one}}{\sqrt{\mathrm{TO}}}\frac{3.14\cdot 3\cdot {10}^{-3}}{\sqrt{7\cdot {10}^{-6}\cdot 2\cdot {10}^3}}~\frac{0.02}{\sqrt{\mathrm{TO}}}$$

With a rigid termination of the heater ends (K = 1), the distance between adjacent centering spacers is ~ 20 mm; the number of centering remote controllers - 65.

Installation of a flexible element allows you to reduce the number of remote controllers, increase the distance between them. The experimentally determined value of the coefficient K for current leads with a flexible harness with beads is 0.12; in this case, l = 57.5 mm, n = 22. The value of K when installing an additional flexible element that provides relatively free elongation of the simulator body during heating is 0.036; and in this case, the distance between the centering spacers is ~ 105 mm, their number is 12.

Example 2

The simulator shell is made of special steel EP823, the outer and inner diameters are 9.1 mm and 8.1 mm. Fuel simulator tablets - from a fuel composition of uranium nitride. The diameter of the tablets is 8.0 mm, the height is 11 mm, the diameter of the central hole is 4.2 mm. The height of the tungsten rod heater is 600 mm, the column of fuel pellets is 580 mm. The length of the shell in which the heater is located with current leads, a flexible conductive bundle in aluminum oxide beads is 1100 mm. The total length of the simulator is 1200 mm.

The calculations determining the number of spacers along the length of the heater for example 2 are similar to the calculations given above for example 1.

The installation in the simulator of centering spacers made of high-temperature electrical insulation material with reasonable dimensions and the distance between them allows to increase the reliability of the test results of the fuel elements due to the use of full-scale fuel materials and to prevent the destruction of the simulators at high temperatures.

Claims (4)

1. A simulator of a fuel element of a nuclear reactor containing a shell in which is a pillar of natural fuel pellets with a central hole and an electric heater located with a gap in the holes of the tablets, equipped with upper and lower current leads, characterized in that ring-type centering spacers made of high-temperature electrical insulation are installed between the tablets material with the same as that of fuel pellets with an outer diameter and with a diameter of the central hole, determined from the ratio Nia:

,
the distance between adjacent centering spacers does not exceed a value equal to:

where
d _N , d _T , d _D - respectively, the diameters of the heater, the holes of the tablets, the holes of the centering spacers;
α _H , α _T , α _D - respectively, the coefficients of linear expansion of the materials of the heater, tablets and spacers;
T is the temperature of the heater;
K is a coefficient characterizing the method of sealing the ends of the heater. 2. The simulator according to claim 1, characterized in that the tablets are made of a conductive fuel composition. 3. The simulator according to claim 1, characterized in that the centering spacers are made of beryllium oxide. 4. The simulator according to claim 1, characterized in that the lower current supply is equipped with a flexible element in the form of a current-carrying harness placed in insulating ceramic beads.