RU2165654C2 - Method for life tests of thermionic power-generating element with fuel-emitter unit - Google Patents

Abstract

FIELD: nuclear power engineering; spacecraft nuclear power units. SUBSTANCE: thermal power of power-generating element is measured in ground reactor or in that incorporated in converter reactor and emitter temperature is evaluated at instant of abrupt drop in gas activity; then maximal residual life is calculated using formula that includes following quantities: relative gas density in fuel- emitter unit of power-generating element; ratio of emitter-shell thickness to its diameter; thermal power transferred from fuel to emitter shell of power- generating element; temperature of power-generating element emitter; emitter diameter; yield index of emitter shell material. EFFECT: enhanced precision of determining maximal residual life according to short circuit of electrodes. 6 dwg

Description

 The invention relates to nuclear energy, to the creation and surface mining of fuel elements, in particular electric generating elements (EGE), 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 and energy characteristics of the electricity generating channels that form the active zone of the TRP. EGC can consist of one EGE or represent a series-connected assembly of EGE, in which a complete cycle of conversion of thermal energy into electrical energy takes place. Therefore, resource tests of thermionic electric generating elements in a reactor (as part of a loop channel in a research reactor or as part of a TRP) are a decisive step in the creation of EGCs and TRP as a whole [1].

 Most of the experimental thermionic EGEs tested in reactors had shell emitters when the load created by the pressure of gaseous fission products (GPA) is perceived by the emitter shell. In this regard, for the analysis of test results, the behavior of materials, as well as factors affecting the resource characteristics of the EGE, theoretical and theoretical studies of the behavior of the fuel-emitter assembly (TEU) are of interest. Moreover, we will take the time until the short circuit of the emitter with a collector characterizing the cessation of electricity generation as a resource for the work of the EGE.

 TEU is a cylindrical emitter shell (EO) with the fuel material inside it (TM) and the GPA output system. GPA output systems can represent, for example, a central channel in the fuel core with a separation system in the end cap of the EGE or as a special gas removal device (GOU) made in the form of an axisymmetric tube with a capillary tip (nozzle) [2]. In the process of operation of the reactor, the GPA (Xe, Kr [3]) exit the TM to the GPA output system and thereby unload the EO. Reliable operation of the GPA output system in high-temperature thermionic EGCs using high-volatile uranium dioxide as a TM is especially relevant.

In the process of EGE operation, TM re-condensation occurs, its redistribution in terms of TEU volume, compaction with restructuring of the initial structure. As a result, over time, a central gas cavity may form in the TEU. The volume of the gas cavity is equal to the sum of the volumes of the initial technological porosity of the TM and the volume of the initial gap between the TM and EO. Depending on the mode of operation of the EGE, its design and the ratio of the geometric dimensions of the individual elements, the characteristics of the materials used, a situation is possible when a high-volatile TM as a result of recondensation clogs the GPA output channels. During this time period, the TEU is a sealed cylindrical shell, perceiving the load created by the pressure of the GPA. Therefore, the service life (R) of a thermionic EGE with a ventilated TEU can be divided into two time periods: the first period (τ ₁ ) is characterized by a workable system for removing GPA from TEU that relieves the load from EEG EGE from GPA; the second period (τ ₂ ) determines the residual life of the EGE. Obviously, the residual life of the EGE will be maximum (τ ₂ = τ $\genfrac{}{}{0ex}{}{\text{m}}{\text{2}}$ ^ax ) when the magnitude of the interelectrode gap (MEZ) at the beginning of the second period corresponds to the initial value.

Closest to the invention in technical essence is a method of life tests of thermionic EGE with TEU, including its installation as part of the EHC in the reactor, measuring the thermal power of the EHE at a constant thermal power of the reactor, estimating the temperature of the emitter, monitoring the activity of gases at the outlet of the EHC, described in [ 4]. In the loop channel ES-6-3, the EGE resource was tested with sealed TEUs, the resource of which is determined by the second period corresponding to the maximum residual resource for ventilated EGE (τ ₁ = 0; R = τ ₂ = τ $\genfrac{}{}{0ex}{}{\text{m}}{\text{2}}$ ^ax ). The heat power released in the TEU was measured in two independent ways: the method of autoradiography of the detectors, which gives a spatial picture of the distribution of heat release over the core volume, and the calorimetric method using the radiation-thermal divergent heat flux. Using the data obtained on the distribution of heat, the temperature distributions of the emitters of the elements of the tested assembly were calculated [5]. During the testing of EGCs in ES-6-3, the resource — time to short circuit of the EEG electrodes — when the electric power was reduced, the EGC was recorded by changing the current-voltage characteristics at constant thermal power. The short circuit of the EGE electrodes was fixed by an almost equidistant shift of the current – voltage characteristic. However, along with the short circuits of the EGE, the main reason for the decrease in the electric power of the EHC with an equidistant shift of the current – voltage characteristics should be considered an increase in the collector temperature much higher than the optimal value [4], which is associated with changes in the thermal resistance of the multilayer collector package. As a result, the reliability of this method for determining the EGE resource by the short circuit factor of the EGE electrodes by recording changes in the current-voltage characteristics at constant thermal power is significantly reduced.

 The technical result obtained by using the invention is to increase the accuracy of determining the maximum residual life of a thermionic EGE with a vented TEU by the short circuit factor of the electrodes in the EGE.

где τ $\genfrac{}{}{0ex}{}{\text{m}}{\text{2}}$ ^ax - максимальный остаточный ресурс [с];
ε_п - относительный объем газовой полости в топливно-эмиттерном узле электрогенерирующего элемента;
δ_э - отношение толщины эмиттерной оболочки к его диаметру;
q_F- тепловая мощность, поступающая из топлива на эмиттерную оболочку электрогенерирующего элемента [Вт/м²];
T_э - температура эмиттера электрогенерирующего элемента [K];
D - диаметр эмиттера [м];
m - показатель ползучести материала эмиттерной оболочки;
B₁ - коэффициент ползучести материала эмиттерной оболочки [(Па)^-m· c^-1];
d - величина межэлектродного зазора в электрогенерирующем элементе [м];
ΔT - перепад температуры по топливу в топливно-эмиттерном узле [K];
размерность коэффициента 4,14 · 10^-13 [1/град].The specified technical result is achieved by the method of life tests of a thermionic power generating element with a fuel-emitter unit, including its installation as a part of the power generating channel in the reactor, measuring the heat power of the power generating element at a constant thermal power of the reactor, estimating the temperature of the emitter, monitoring the gas activity at the outlet of the power generating channel, As an electric generating element, an electric generating element with a ventilated fuel itternym node, and measurement of the thermal power electricity generating element and the emitter temperature estimate produced at the time of abrupt fall gas activity is then determined maximum residual resource thermionic power generating element according to the expression

where τ $\genfrac{}{}{0ex}{}{\text{m}}{\text{2}}$ ^ax - maximum residual life [s];
ε _p - the relative volume of the gas cavity in the fuel-emitter node of the power generating element;
δ _e - the ratio of the thickness of the emitter shell to its diameter;
q _F is the heat power coming from the fuel to the emitter shell of the power generating element [W / m ² ];
T _e - the emitter temperature of the power generating element [K];
D is the diameter of the emitter [m];
m is the creep rate of the material of the emitter shell;
B ₁ - creep coefficient of the material of the emitter shell [(Pa) ^-m · s ^-1 ];
d is the magnitude of the interelectrode gap in the power generating element [m];
ΔT is the temperature difference in fuel in the fuel-emitter unit [K];
coefficient dimension 4.14 · 10 ^-13 [1 / deg].

 The measurement of the thermal power of the EGE and the estimation of the temperature of the emitter can be carried out according to the methods described in [6], in particular, the control of thermal power using heat flux sensors.

 In FIG. 1 shows a structural diagram of a reactor. In FIG. 2 shows a General view of the power generating element (EGE). In FIG. 3,4,5 presents typical structural variants of ventilated fuel-emitter units (TEU), and in FIG. 6 is a graph explaining a method.

 In FIG. 1 is indicated: 1 - reactor; 2 - active zone; 3 - electricity generating channel (EGC); 4 - power generating element (EGE); 5 - side reflector; 6 - organs of the control and protection system (CPS); 7 - end reflector; 8 - end reflector; 9 - a tube for the output of gaseous fission products (GPA); 10 - activity sensor; 11 - sedimentation tank; 12 - activity sensor.

 In FIG. 2-5 are indicated: 4 - EGE; 13 - fuel-emitter unit (TEU); 14 - collector EGE; 15 - collector insulation; 16-case pipe EGK; 17 - thermal power sensor allocated to the TEU; 18 - remote control; 19 - isolation; 20 - fuel material (TM); 21 - emitter; 22 - emitter shell (EO) TEU; 23 - the central channel in the TM; 24 - getter filter material or labyrinth screens; 25 - heat transfer gaskets made of refractory metal; 26 - tube gas outlet device (GOU); 27 - jet.

In FIG. 6 denotes: N _A —characteristic change in the activity of gaseous fission products (GPA) emerging from a ventilated TEU EGE; q _F - thermal power coming from the TM to the EE EGE; R is the resource of work of the EGE; τ ₁ - the first period of the EGE with a workable system of valves GPA unloading EA; τ $\genfrac{}{}{0ex}{}{\text{m}}{\text{2}}$ ^ax is the maximum residual resource corresponding to the second period of the EGE operation with the failed GPA ventilation system (as in the case of a sealed TEU).

 Expression (1) is obtained by calculating the deformation of the EE of the EGE in the model of the cylindrical shell of the GPD deformable by internal pressure that accumulate during the operation of the EGC. EO operates in the regime of steady creep in the region of elastic stresses. It is assumed that the creep properties of the EO material are constant over the entire service life. Cylindrical EOs are assumed to be long (cross sections remote from the ends are considered).

 It is assumed that the TM operates under compression conditions, and the stresses practically do not change along the radius, since the TM does not carry a load due to the relatively low strength at operating temperatures, but transfers it like a liquid. The load is perceived by EO, which is under conditions of tensile stresses.

To describe the creep strain of EO, a power-law dependence of the form is adopted [7]
ζ _II = B ₁ σ ^m , (2)
where ζ _II = dε / dτ is the creep rate of the EA material in the second period;
m is the creep rate of the EA material;
B ₁ - creep coefficient of the material EO;
σ is the voltage in the EA;
ε is the relative elongation of EE EGE in the circumferential direction.

In accordance with the assumptions made above, from the relation known as the Dallas equation [8], we determine the voltage in the EO
σ = P · (D-δ) / (2 · δ) = P · (1-δ _e ) / (2 · δ _e ), (3)
where δ is the thickness of the EA;
δ _e = δ / D;
P is the GPA pressure exerted on the EEG EGE from the GPA accumulated in the TEU after the gas outlet channels are blocked by the TM condensate and determined from the Mendeleev-Clapeyron equation.

P = n (τ) · k · T ₀ , (4)
where n (τ) is the concentration of the GPA accumulating in the gas cavities of the TEU;
T ₀ = T _e + ΔT is the absolute gas temperature corresponding to the maximum temperature in the TEU EGE;
ΔT is the temperature difference in TM;
k is the Boltzmann constant.

Knowing that the number of Xe and Kr atoms per one separated U ²³⁵ atom is 0.25 [3] and 3 · 10 ¹⁰ divisions / s approximately corresponds to 1 W of thermal power released in a TM [9], we can obtain expression (4) in form
P = 4.14 · 10 ^-13 · q _F · (T _e + ΔT) · τ / (ε _p · D · (1-2δ _e )). (5)
Expressing the relative elongation of the EE EGE in the circumferential direction (ε) through an increase in the emitter radius Δr (τ), we obtain
ε = 2 · Δr / (D · (1-δ _e )). (6)
Substituting expression (3) in (2) taking into account (5) and (6), after integrating (2) over τ, taking into account that the maximum increase in the emitter radius before stuttering with the collector Δr (τ) corresponds to the interelectrode gap (d) EGE, we obtain the expression (1).

где t(r) - распределение температуры по радиусу полого цилиндра;
R_в - внутренний радиус полого цилиндра;
R_н - наружный радиус полого цилиндра;
Т_н - температура на наружной поверхности полого цилиндра;
q_v - плотность объемного тепловыделения;
λ - теплопроводность материала полого цилиндра.To determine ΔT - temperature difference by TM in TEU - you can use a special case of solving the heat equation for a hollow cylinder with heat sources cooled from the outer surface [10]

where t (r) is the temperature distribution along the radius of the hollow cylinder;
R _in - the inner radius of the hollow cylinder;
R _n - the outer radius of the hollow cylinder;
T _n - temperature on the outer surface of the hollow cylinder;
q _v is the density of volumetric heat release;
λ is the thermal conductivity of the material of the hollow cylinder.

В нашем случае:

q_v = 4·q_F/(D·(1-ε_п)·(1-2δ_э)). (10)
Температура наружной поверхности Т_н соответствует температуре эмиттера Т_э ЭГЭ и может быть оценена одним из методов, описанных в [11].Where does the temperature difference in TM in TEU ΔT come from (7) for r = R _in

In our case:

q _v = 4 · q _F / (D · (1-ε _p ) · (1-2δ _e )). (10)
The temperature of the outer surface T _n corresponds to the temperature of the emitter T _e EGE and can be estimated by one of the methods described in [11].

 The method is implemented as follows.

EGC 3, consisting of one EGE 4 with a vented TEU 13 or representing a series-connected assembly of EGE 4 with a vented TEU 13, with a thermal power registration system 17 allocated to the TEU 13, is placed in the active zone 2 of reactor 1. During operation of reactor 1, rated power, a stationary mode of operation of thermionic EGC 3 is established. Using activity sensors 12 and 10, the activity values (N _A ) of gases (GPA) are recorded at the outlet of the EGC 3 and at the outlet of the settling tank 11, respectively. Gas (GPA) leaves the ventilated TEU 13 EGE 4 from the EGC 3 through the tube 9. The heat power (q _F ) released in the TEU 13 is recorded by the sensor 17. As a result of the condensation of TM 20 over a period of time τ ₁ , determined by the intensity of the heat mass transfer in TEU 13, clogging of GPA output channels (or central channel 23, or gas-absorbing filtering material, or labyrinth screens 24, or GOU tube 26, or nozzle 27) with TM 20 condensate occurs, and TEU 13 is sealed, which leads to an abrupt decrease in N activity _A gas (GAP) at the outlet of the EG 3, a detectable activity sensor 12. At the time of the fall of activity is fixed by the sensor 17, the thermal power q _F, coming from the TM 20 to 22 EO EGE 4 and the emitter 21 appreciate T _e temperature in TEU 13, for example, by heat balance [11] . Then, by the expression (1), the maximum residual EGE resource is determined.

As a specific example, we consider the use of the method of life tests of thermionic EGE with TEU for the constructional version of EGE with ventilated TEU shown in FIG. 3, where uranium dioxide is used as a TM, and a single-crystal Mo-3XNb alloy is used as the material of EO EGE. The coefficient of thermal conductivity of uranium dioxide in the calculations is taken equal to 2.6 W / (m. grad). The creep index (m) and creep coefficient (B ₁ ) of the EO material are determined from the experimental dependences of the steady-state creep rate of the Mo-3% Nb single-crystal alloy on stresses according to [12]. We take the following geometric parameters characteristic of a typical EGE: ε _p = 0.3; δ _e = 0.1; D = 1.5 · 10 ^-2 m; d = 3 · 10 ^-4 m. Suppose that at the time of a sudden drop in the activity of the GPA removed from the ventilated TEU, the thermal power q _F released in the TEU EGE was fixed at 9 · 10 ⁵ W / m ² and the emitter temperature T _{e was} estimated according to one of the methods [10] equal to 2173 K. Moreover, from [12] for the single-crystal alloy Mo-3% Nb, at T _e = 2173 K, we have m = 3.52 and B ₁ = 1.58 · 10 ^-27 (Pa) ^-m · s ^-1 . To estimate ΔT, the temperature difference in TM in TEU, by expression (8), taking into account (9) and (10), we find ΔT ≈ 500 K. Substituting the obtained values in expression (1), we determine the maximum residual EGE resource t $\genfrac{}{}{0ex}{}{\text{m}}{\text{2}}$ ^ax ≈ 5 · 10 ⁵ s.
Thus, a method for life tests of a thermionic power generating element with a fuel-emitter assembly is proposed, which allows:
1. Calculate the final engineering effect: the output of gaseous fission products from the fuel material and the deformation of the emitter shell of the fuel-emitter assembly of the power generating element, depending on the operating parameters. Such a model has simplifications in detailing physical representations and includes parameters that allow normalizing the model to an experimental result.

 2. To solve the problems of predicting the service life of the electric generating element, which are key in the design of the thermionic converter reactor.

 3. To reduce the time for experimental testing of thermionic power generating elements and the power generating channel as a whole in case of expensive reactor tests.

LITERATURE
1. Tests of multi-element thermionic experimental assemblies / V. I. Berzhaty, N. A. Griboedov, V. P. Gritsaenko and others. -Atomic energy, vol. 31, no. 6, 1971, p. 585-588.

 2. Neutron diffraction studies of thermionic EGCs during loop reactor tests / E.S. Bekmukhambetov, A.S. Karnaukhov, V. A. Kornilov et al. / Sat: RKT. Ser. XII // RSC Energia, Korolev, 1996. Issue. 2-3: Cosmic thermionic emission nuclear power plants and high-power propulsion engines. Part 2, p.116.

 3. Degaltsev Yu.G., Ponomarev-Stepnoy N.N., Kuznetsov V.F. The behavior of high-temperature nuclear fuel during irradiation. Moscow. Energoatomizdat. 1987, p. fifteen.

 4. Resource tests of the thermionic converter / E.S. Bekmukhambetov, V. I. Berzhaty, V. P. Gritsenko, etc. - Atomic energy, vol. 35, no. 6., 1973, p. 387-390.

 5. Some results of post-reactor studies of a six-element thermionic assembly, which worked for 2670 hours / G.A. Batyrbekov, E.S. Bekmukhambetov, V.I. Berzhaty et al. - Atomic energy, vol. 40, no. 5, 1976, p. 382-384.

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

 7. L.M. Kachanov. Creep theory. Fizmatgiz. Moscow, 1960, p. 20.

 8. V.I. Feodosiev. Strength of materials. Fizmatgiz. Moscow, 1962, p. 294.

 9. Zaimovsky A.S., Kalashnikov V.V., Golovnin I.S. Fuel elements of nuclear reactors. Moscow. Atomizdat. 1966, p. 9.

 10. [9], p. 504.

 11. [6], p. 73.

 12. Zubarev P.V., Tachkova N.G. Increasing the heat resistance of single-crystal refractory metals. Questions of atomic science and technology. Series: Atomic Materials Science, Issue 5 (16). State Committee for the Use of Atomic Energy of the USSR. 1982, p. 28.

Claims (1)

The method of life tests of a thermionic power generating element with a fuel-emitter assembly, including installing it as a part of the power generating channel in the reactor, measuring the heat power of the power generating element at constant thermal power of the reactor, estimating the temperature of the emitter, monitoring the activity of gases at the outlet of the power generating channel, characterized in that as an electro-generating element, an electro-generating element with a vented fuel-emitter unit was used, and measured e thermal power electricity generating element and the emitter temperature estimate produced at the time of abrupt fall gas activity is then determined maximum residual resource thermionic power generating element according to the expression

where τ $\genfrac{}{}{0ex}{}{\text{m}}{\text{2}}$ ^ax - maximum residual life, s;
ε _p - the relative volume of the gas cavity in the fuel-emitter node of the power generating element;
δ _e - the ratio of the thickness of the emitter shell to its diameter;
q _F is the heat power coming from the fuel to the emitter shell of the power generating element, W / m ² ;
T _e - emitter temperature of the power generating element, K;
D is the diameter of the emitter, m;
m is the creep rate of the material of the emitter shell;
B ₁ - creep coefficient of the material of the emitter shell, (Pa) ^-m · s ^-1 ;
d is the magnitude of the interelectrode gap in the power generating element, m;
ΔT is the temperature difference in fuel in the fuel-emitter unit, K;
coefficient dimension 4.14 · 10 ^-13 [1 / deg].

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