RU2224306C2 - Method for life tests of thermionic power- generating cell with cooling system of its fuel- emitter assembly - Google Patents

Abstract

FIELD: nuclear power engineering. SUBSTANCE: method includes installation of power-generating cell incorporated in power-generating channel into reactor. Immediately after reactor has gained steady-state thermal power level temperature T₂ of face can of fuel-emitter assembly is additionally measured. Maximum temperature T₀ in fuel-emitter assembly and serviceability of cooling system are determined. Cooling system serviceability is found from definite equation. EFFECT: reduced time of mastering thermionic power-generating cell in reactor. 1 cl, 3 dwg

Description

 The invention relates to nuclear energy, to the creation and ground-based mining of fuel elements, in particular electric generating elements (EGE), the thermionic assembly of which is called an electric generating channel (EGC).

 The most important stage in the development of a thermionic converter reactor (TRP) is the confirmation of the resource-energy characteristics of EGCs that form the active zone of the TRP. EGC can consist of one EGE or represent a series-connected electric generating assembly 5EGE, in which a complete cycle of conversion of thermal energy into electrical energy takes place. Therefore, the resource tests of the EGE in the reactor (as part of the loop channel (PC) in the research reactor, go as part of the TRP) are the determining stage in the creation of EGC and TRP as a whole [1]. The experimental development of fuel rods in surface reactors is associated with high economic costs and the longer the planned experiment, the higher the costs.

 Most of the experimental EGEs tested in reactors had clad thermionic fuel elements when the load created by the pressure of gaseous fission products (GPA). perceived by the emitter shell. Moreover, a quality solution to reduce the pressure on the cladding of a fuel rod from the GPA is the organized withdrawal of the GPA through the ventilation system, which can significantly increase the resource of the fuel rod. A particularly efficient ventilation system is important in the development of thermionic fuel elements in which the cladding of a fuel element operates at high temperatures (~ 2000 K and more [1]) and cannot withstand high pressures from the GPA emitted during fission, which leads to an emitter short circuit with the collector and the cessation of electricity generation. Therefore, for high-temperature thermionic fuel elements, the EGE service life will be largely determined by the reliable GPA output, i.e. operability of the ventilation system.

 For the operational analysis of reactor tests, during the resource development of thermionic EGE, it is of interest to use engineering methods that reflect theoretical and theoretical studies of the behavior of fuel in a fuel-emitter unit (TEU).

 We consider a high-temperature TEU (with an emitter temperature of ~ 2000 K and higher), consisting of a cylindrical emitter shell, with high-volatile nuclear fuel (such as uranium dioxide) inside it, and a GPA ventilation system. The ventilation system is made in the form of an axisymmetric central tube made of refractory metal (for example, alloys based on W or Mo) penetrating the fuel core [2]. In order to reduce the exit from the TEU of fuel vapors leaving together with the GPA, the end of the tube in the hot zone of the TEU is made in the form of a capillary tip. For the considered high-temperature thermionic fuel elements in the fuel core, intense processes of heat and mass transfer (tens of hours [3]) occur, leading to the rearrangement of the initial structure of nuclear fuel and the formation of a central gas cavity with an isothermal surface where gaseous fission products flow. Particularly intense processes of fuel mass transfer are observed for highly volatile fuel materials (HM), such as uranium dioxide. The operability of the ventilation system will be determined by the ability to freely remove the GPA beyond the TEU and depends on the relative geometric position of the surface of the central gas cavity and the tube with a capillary tip. Obviously, for the GPA to freely exit the central gas cavity, a part of the tube with a capillary tip should be in the gas cavity zone, which is a necessary condition for the operability of the TEU ventilation system.

 Known methods of life tests of thermionic EGE with ventilated TEU in the development of the reactor-thermionic converter of space purpose "TOPAZ-2" [4]. Single cell EGCs were tested with a ventilation channel in the fuel core and an optimized structure of the fuel material (TM). The main feature of the life tests of the EGC data was that the tests were carried out at a relatively "low" maximum temperature of the emitter shell (~ 1600 K and less), as a result of which there was a slight axial mass transfer of the TM in the ventilation duct and its very slow overgrowth with the TM condensate was not the main factor determining the planned life of the EGC data. A more significant contribution to the resource characteristics of these EGCs in this temperature range is provided by the deformation of the emitter shell from the swelling of the “cramped” fuel in the region of the distance controllers belts, and this factor will probably be the determining factor in the predicted resource.

 The main disadvantage of this method of life testing is that it does not cover the high temperature range (with an emitter shell temperature of more than 1600 K), where the process of restructuring the fuel structure as a result of mass transfer is sharply activated with increasing temperature. This is especially true for highly volatile fuel materials, for example, uranium dioxide, as a result of which the contribution to the resource component of such factors as the initial TM structure, the stresses on the emitter shell from the swelling TM will decrease and the axial mass transfer of TM, which leads to overgrowing of the ventilation duct, will increase sharply. it is out of order, and therefore a violation of the performance of the EGE.

Closest to the invention in technical essence is a method of life tests of thermionic EGE with a TEU ventilation system, including its installation as part of the EGC in the reactor, measuring the thermal power of the EGE at constant thermal power of the reactor and estimating the emitter temperature described in [5]. The essence of the method is that during the experiment, the thermal power of the EGE is measured and the temperature of the emitter is estimated at the time of the abrupt drop in the activity of the GPA, then the maximum residual life of the thermionic EGE is determined by the proposed expression, which includes, in addition to the parameters listed above, the geometric characteristics of the thermoelectric power and thermal characteristics emitter shell materials and TM. The life of a thermionic EGE with a ventilated TEU is divided into two time intervals: the first interval (τ ₁ ) is characterized by a functioning GPA ventilation system from TEU, which removes the load from the EGE emitter shell from the GPA; the second interval (τ ₂ ) determines the residual life of the EGE, when the cladding of the fuel rod receives pressure from the swelling TM and from the GPA accumulated in the central channel of the TEU. Moreover, in this method, the maximum residual life of the EGE is determined (τ ₂ = τ $\genfrac{}{}{0ex}{}{\text{max}}{\text{2}}$ ), when the value of the interelectrode gap (MEZ) at the beginning of the second interval corresponds to the initial value.

 The main disadvantage of this method of life tests is that at the beginning of the tests the necessary condition for the operability of the TEU ventilation system is not analyzed, thereby unreasonably prolonging the terms of expensive reactor tests of EGCs.

 The objective is to reduce the time required for experimental development of a thermionic EGE in a reactor with a TEU ventilation system.

где π₄ = (λ_э/λ_г)((T₀-T₂)/(T₀-T_E))(r $\genfrac{}{}{0ex}{}{\text{2}}{\text{T}}$ /(L_c•l_т));
ε - относительное содержание горючего;
λ_э- эквивалентная теплопроводность трубки, Вт/(м•град);
λ_г - теплопроводность горючего, Вт/(м•град);
r_т, l_т - радиус и длина трубки системы вентиляции, м;
L_c - длина топливного сердечника, м;
Т₀, [К]; Т₂, [К]; Т_Е, [К].The objective is achieved by the fact that in the method of life tests of a thermionic power generating element with a ventilation system of its fuel-emitter unit, including its installation as a part of the power generating channel in the reactor, measuring the thermal power of the electro-generating element at a constant thermal power of the reactor, estimating the temperature of the emitter, immediately after the output reactor at a constant level of thermal power additionally measure the temperature of the end shell of the fuel-emitter node T ₂ at the junction I have it with the ventilation system, determine the maximum temperature in the fuel-emitter bridle T ₀ , and the operability of the ventilation system is determined by the ratio

where π ₄ = (λ _e / λ _g ) ((T ₀ -T ₂ ) / (T ₀ -T _E )) (r $\genfrac{}{}{0ex}{}{\text{2}}{\text{T}}$ / (L _c • l _t ));
ε is the relative fuel content;
λ _e - equivalent thermal conductivity of the tube, W / (m • deg);
λ _g - thermal conductivity of the fuel, W / (m • deg);
r _t , l _t - radius and length of the tube of the ventilation system, m;
L _c is the length of the fuel core, m;
T ₀ , [K]; T ₂ , [K]; T _E , [K].

 In FIG. 1 shows a general view of a thermionic electric power generating element with a ventilation system of a fuel-emitter unit. Figure 2 presents the structural diagram of the reactor with a loop channel. Figure 3 presents graphs explaining the essence of the method.

 In FIG. 1 is indicated: 1 - thermionic electric power generating element (EGE); 2 - EGE emitter; 3 - collector EGE; 4 - fuel-emitter unit (TEU); 5 - nuclear fuel; 6 - ventilation system TEU; 7 - EGE remote control; 8 - end shell of TEU; 9 - switching jumper; 10 - temperature sensor; 11 - a tube; 12 - the surface of the Central gas cavity; 13 - capillary tip; 14 - sensor thermal power allocated to the TEU; 15 - collector insulation; 16 - cover tube EGC; 17 - isolation.

 In figure 2, it is indicated: 18 — GPA sedimentation tank; 19 - a nuclear reactor; 20 - reflector with control and protection system bodies; 21 - active zone; 22 - loop channel (PC); 23 - electricity generating channel (EGC).

In FIG. Figure 3 shows the dependence of the coefficient of relative coverage of the outer surface of the tube with a nuclear fuel condensate on the coefficient π ₄ .
The method is implemented as follows. Thermionic EGE 1 with a ventilation system 6 of the fuel-emitter unit 4 as part of the electricity generating channel 23 is placed in the loop channel 22, equipped with the necessary recording devices (a thermal power sensor 14 allocated in the TEU 4, a temperature sensor 10 located at the junction of the shell 8 with the tube 11 ventilation systems 6). A PC 22 with an EGC 23 is placed in the cell of the active zone 21 of the nuclear reactor 19. The reactor 19 is brought to the planned thermal power, fixing the thermal power of the EGE 1 using the sensor 14 according to the method described in [6], in particular using heat flux sensors. During operation of the reactor 19, when it is brought to the planned thermal power, nuclear fuel 5 is divided in TEU 4 with the formation of gaseous fission products leaving the ventilation system 6 outside the TEU 4 and then reactor 19 into the settling tank 18. Heat released during the fission reaction of fuel 5, the emitter 2 heats up, thus causing the emission of electrons from the emitter 2 and their condensation to the collector 3. At the same time, with the increase in the thermal power of the reactor 19, the temperature of the fuel core increases with a maximum m in the central zone of TEU 14. This circumstance leads to intensive re-condensation of nuclear fuel 5 (especially for fuel materials with high vapor elasticity and low thermal conductivity, such as uranium dioxide) with the restructuring of its structure and the formation of a central gas cavity with an isothermal surface 12. The presence of the system ventilation 6 in the TEU 4 leads to the fact that part of the heat from the central part of the TEU 4 is removed to the periphery of the shell 8 through a pipe 11 made of a material with a higher heat coefficient rovodnosti fuel than 5, causing the isothermal transformation process gas surface 12 of the central cavity. Immediately after the reactor 19 is brought to the planned power level, we measure the temperature of the end shell 8 of the TEU 4 at the junction of it with the tube 11 of the ventilation system 6 using the sensor 10 (these measurements can be performed, for example, according to the methods described in [6]). We estimate the temperature of emitter 2, for example, using one of the methods described in [6]. Knowing the heat generation in TEU 4 and the temperature of emitter 2, we determine the maximum temperature in TEU 4, for example, using a special case of solving the heat equation for a hollow cylinder with heat sources cooled from the outer surface [7]. Knowing the geometric characteristics of the ventilation system 6 and TEU 4, the physical characteristics of the fuel 5 used, as well as the relative content of fuel 5 in the TEU 4, we determine the necessary condition for the operability of the ventilation system of the thermionic EGE 1 by the relation (1).

We present the derivation of relation (1). We take the parameter K _s = l _s / l _t , the relative coverage of the outer surface of the tube with nuclear fuel condensate, where l _s is the length of the condensate on the outer surface of the tube, and l _t is the length of the tube. Obviously, a necessary condition for the operability of the TEU ventilation system is the ratio K _s <1. We establish a system of criteria relations expressing the main quantitative laws of the change in the parameter K _s , after processing the results of numerical calculations of the temperature field of the heterogeneous fuel core of the TEU taking into account the condensation of fuel in the form of generalized dimensionless parameters.

We consider a mixed boundary value problem for the equation
div (λ gradT) = - q, (2)
where the temperature distribution function over the emitter shell of the power generating element is known at the outer boundary, the isothermal condition is used on the surface of the central gas cavity in the final state (in the equation λ, Т, q is the heat conductivity coefficient, temperature, and heat density, depending on the coordinates, respectively).

The method for calculating the temperature fields of a heterogeneous fuel core of thermionic EGE, used to derive relation (1), was considered in [2]. Based on the results of a numerical calculation, the dependences of the parameter Ks on ε and the dimensionless complex π ₄ are constructed, which consist of parameters determining the relative length of the condensate of the fuel material on the tube:
π ₄ = (λ _e / λ _g ) ((T ₀ -T ₂ ) / (T ₀ -T _E )) (r $\genfrac{}{}{0ex}{}{\text{2}}{\text{T}}$ / (L _c • l _T )), (3)
where λ _e = λ _T (1-r $\genfrac{}{}{0ex}{}{\text{2}}{\text{2}}$ / r $\genfrac{}{}{0ex}{}{\text{2}}{\text{T}}$ ); λ _T is the thermal conductivity of the tube material; r ₂ , r _t - inner and outer radii of the tube, respectively. The dimensionless complex π ₄ has the physical meaning of heat leakage through the TEU ventilation system relative to the heat released in the fuel core of the EGE and going to the cylindrical shell of the EGE.

откуда и подучено соотношение (1).The calculation results performed by the method [2] for uranium dioxide and given in [8] with the determination of the final configuration of the surface of the central gas cavity were processed in the coordinates of dimensionless parameters and presented graphically in the form of the dependence K _s = f ((π ₄ , ε) on figure 3. The obtained calculated dependences in figure 3 in the interval ε = 0.5-0.8, typical for thermoelectric emission thermoelectric EGE, are well described by the empirical dependence of the form

whence relation (1) was obtained.

Having obtained the dependence K _S = f (π ₄ , ε), it is possible, based on the known boundary conditions (T ₂ , T _E ), thermophysical characteristics (λ _e , λ _g , λ _t ), geometric characteristics (r _t , l _t , r ₂ , L _c , R _c ), the relative fuel content in the TEU (ε) and the known heat dissipation in the TEU, to predict the degree of coverage of the fuel pipe with condensate and, thus, determine the feasibility of the necessary working condition for the ventilation system of the TEU of the thermionic EGE.

As a specific example, we consider the use of the method of life tests of thermionic EGE with the ventilation system of its TEU for the constructional version of the TEU shown in figure 1, where uranium dioxide is used as fuel. Thermionic EGE with a vented TEU as part of the EGC is placed in a PC equipped with the necessary recording devices (thermal power sensor emitted in the TEU. Temperature sensor for the end shell of the TEU). PCs with EGCs are placed in the core of a nuclear reactor. The reactor is brought to the planned thermal power and maintained unchanged during these life tests. In the calculations, the thermal conductivity coefficient of uranium dioxide is taken equal to λ _g = 2.5 W / (m • deg), and the thermal conductivity coefficient of a tube made of a tungsten-based alloy is taken equal to λ _T = 100 W / (m • deg). Let us take the following geometrical parameters of a TEU with a ventilation system, characteristic of a typical EGE: R _c = 10 ^-2 m; L _c = 0.1 m; r _t = 2 • 10 ^-3 m; r ₂ = 1.5 • 10 ^-3 m; l _t = 6 • 10 ^-2 m. The relative fuel content in the TEU is taken ε = 0.7.

где r_в - внутренний радиус полого топливного цилиндра, который можно определить в первом приближении, зная ε, из выражения r_в=R_c(1-ε)¹/₂=5,5•10^-3 м.During the experiment, we measure the thermal power released in the TEU using a sensor, which can be used as a partitioned calorimeter of the integral heat flux [9]. Suppose that, as a result of the measurement, we obtained the heat release in the TEU Q = 2.2 • 10 ³ W, whence the density of the volume heat release in the nuclear fuel-based TEU q = Q / (π • R _c ² • L _c • ε) = 10 ⁸ W / m ³ . We estimate the temperature of the emitter T _E , for example, using the heat balance method [10], as a result we obtain T _E = 2010 K. Using a temperature sensor, for example, a thermocouple, we fix the temperature of the end face of the TEU at the junction with the tube of the TEU ventilation system T ₂ = 1900 K Assuming the contact of the fuel core with the emitter shell, knowing the heat density in TEU q and the temperature of the emitter shell T _E , we determine the maximum temperature in TEU T ₀ equal to the temperature on the surface of the central gas cavity, for example, using in a special case of solving the heat equation for a hollow cylinder with heat sources cooled from the outer surface, from [7] according to the expression

where r _in - inner radius of the hollow cylinder of the fuel, which can be determined in the first approximation, knowing ε, expression of r _in = R _c (1-ε) _^1/2 = 5,5 • 10 ^-3 m.

From where, by (5), we determine T ₀ = 2341 K, and from (3) π ₄ = 1.55 • 10 ^-2 . From expression (4) we find K _s = 0.38, i.e. K _s <1, which corresponds to relation (1), the necessary condition for the operability of the TEU ventilation system is fulfilled.

Thus, a method of life tests of a thermionic emitting electro-generating element with a ventilation system of its fuel-emitter unit is proposed, which allows:
1. To solve the problems of predicting the service life of a high-temperature thermionic power generating element, to determine, in the coordinates of the generalized variables, the necessary condition for the operability of the ventilation system of the fuel-emitter unit, which are key in the design of the thermionic converter reactor.

 2. Predict the timing of the experimental development of thermionic power generating elements and the power-generating channel as a whole, while reducing the cost of expensive reactor tests.

LITERATURE
1. Kornilov V.A., Sinyavsky V.V., Yuditsky V.D. On the effectiveness of introducing into the program for creating energy-intensive and long-life thermionic EGCs the stage of loop reactor tests of ampoule devices with mock-ups of fuel-emitter units with the neutron diffraction method of non-destructive testing. Fifth International Conference "Nuclear Energy in Space". Sat reports under the general ed. prof. I.I. Fedika. Part 2. Podolsk, Moscow. reg., 1999, p. 300-309.

 2. Kornilov V.A., Sukhov Yu.I., Yuditsky V.D. Method for calculating the temperature fields of a heterogeneous fuel core of a thermionic fuel element. Atomic Energy, 1980, Volume 49, Issue 6, pp. 393-394.

 3. Kornilov V. A., Yuditsky V. D. Modeling of heat and mass transfer in the core of a thermionic fuel element. Atomic Energy, 1982, Volume 53, Issue 2, p. 74-76.

 4. Degaltsev Yu.G., Weak V.D., Gontar A.S. A generalization of the results of post-reactor studies of single-element EGCs that underwent nuclear power research in experimental facilities I-82, 81, and resource prediction. Fifth International Conference "Nuclear Energy in Space". Sat reports under the general ed. prof. I.I. Fedika. Part 2. Podolsk, Moscow. reg., 1999, p. 272-279.

 5. Pat. RU 2165654 C2, MKI G 21 D 7/04, H 01 J 45/00. The method of life tests of a thermionic electric generating element with a fuel-emitter unit / V.A. Kornilov // Inventions. 02/20/2001, bull. N5.

 6. Sinyavsky V.V. Methods for determining the characteristics of thermionic fuel elements. M .: Energoatomizdat, 1990, p. 39-95.

 7. Zaimovsky A.S., Kalashnikov V.V., Golovnin I.S. Fuel elements of nuclear reactors. M .: Atomizdat, 1966, p. 504.

 8. Kornilov V.A. Processes of heat and mass transfer in high-temperature fuel rods of thermionic emission electro-generating channels / Sat .: PKT. Cep. XII. Issue 2-3 // RSC Energia, Korolev, 1996. Space thermionic nuclear power plants and high-power electric rocket engines. Part 2, pp. 99-112.

 9. [6], p. 48.

 10. [6], p.73.

Claims (1)

The method of life tests of a thermionic electric generating element with a ventilation system of its fuel-emitter unit, including its installation as a part of the electric generating channel in the reactor, measuring the thermal power of the electric generating element at a constant thermal power of the reactor, estimating the emitter temperature T _E , characterized in that immediately after the output of the reactor the constant level of heat output additionally measured end shell temperature of the fuel and the emitter node T ₂ at connection with a Stem ventilation, determine the maximum temperature in the fuel-emitter node T _0, and the performance of the ventilation system is determined by the ratio

ε is the relative fuel content; λ _e - equivalent thermal conductivity of the tube, W / (m · deg); λ _g - thermal conductivity of the fuel, W / (m · deg); r _T , l _T is the radius and length of the tube of the ventilation system, m; L _C is the length of the fuel core, m; T ₀ , [K]; T ₂ , [K]; T _E [K].

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