The method of determining the speed of removal of the fuel material from the vented fuel

 

(57) Abstract:

The invention relates to the development of vented fuel elements, their experimental development in nuclear reactors, in particular thermionic fuel elements, while creating electricity generating channels thermionic reactor Converter. In the process reactor testing of vented fuel element at a point in time measure heat capacity, record the pressure of the gaseous fission products in the ventilation system, the temperature of the shell of a fuel rod and estimate the speed of the removal of the fuel material on the proposed terms. The technical result is greater accuracy in the determination of the speed of ejection of fuel and simplification of the experiment. 4 Il.

The invention relates to the development of vented fuel elements, their experimental development in nuclear reactors, in particular to thermal emission method of converting thermal energy into electrical energy and reactor physics and can be used in the program of creation of thermionic fuel elements energonaprjazhenie electricity generating channels (EGK), forming the core thermionic reactor Converter (TRD).

One of the factors that determine the resource ventilated is followed by condensation of TM on the "cold" parts of the exhaust tract. Uncontrolled removal of the TM of the fuel rods can lead to overlapping channels output GPA, their blockage, which in turn leads to intense deformation of the membrane of a fuel rod. A particularly important control removal TM high-temperature thermo-emission fuel elements in the composition of TFE. Condensation TM on structural elements martelinho space in TFE leads to leakage current, reducing power and efficiency, i.e. the occurrence of the failure type "degradation characteristics". Therefore, the definition of the speed of removal of the TM through the ventilation system, and therefore the resource of this process is a major task when creating energonaprjazhenie rods.

There is a method of determining the rate of removal of TM from vented fuel element using the Poiseuille flow equation for the flow rate Q of gas flow through the channel in the form of a pipe of circular cross section, describing viscous for [1].

Q = a4Pa(P2-P1)/(8l), (1)

where a is the radius of the pipe; l - length; is the viscosity of the gas; P2and P1the pressure measured at the entrance to the channel and the exit channel, respectively; Pa- the arithmetic average of the P1and R2.

The disadvantage of this method is the need to measure what about the fuel. To carry out the measurement of these pressures in the reactor conditions, taking into account the sharp dependence of the vapour pressure TM temperature, very difficult.

Closest to the invention to the technical essence is a way of determining the speed of removal of the TM vented from the fuel rod, including the process reactor test fuel rod measurement of heat capacity and the rate of removal of TM (oxide fuel) through the ventilation system of the fuel and the emitter node thermal emission EGK, given in [2]. The method consists in measuring the time-dependent heat dissipation (Q) in the fuel and the emitter node, and heat dissipation (q) condensate fuel released through the ventilation system and condensed outside the fuel-emitter node, these dependencies estimate timeithe rate of change of heat in the CHP plant dQ/d and condensate fuel dq/d, and the speed of ejection of fuel mithrough the ventilation system at the momentievaluate the expression

< / BR>
where Mo- initial mass of the fuel in the fuel-smitheram node; Qi- heat in the CHP plant at time =i; qithe heat of condensation of the fuel that came out of the ate through the ventilation system at the time BP is riment, measurement of heat q in the condensate TM that it is difficult for the following reasons:

1) in addition to the main sensor heat capacity for measuring Q heat generation in the fuel rod, to measure the heat in the condensate TM, out of TVEL, q requires additional system is very sensitive sensors, because the number of released TM may be very small, i.e. below the threshold of sensitivity of these sensors;

2) the error in the measurement of q is also due to the fact that TM, released from the fuel condenses in the ventilation system is not compact, and is distributed according to the structures inside the ventilation system, which can lead to mutual interference of the sensors for measuring q and Q;

3) to avoid interference with the sensors q and Q in the design of the ventilation system shall be provided special cold trap for condensate recovery TM, which leads to the complexity of the design of the ventilation system;

4) the process of measuring q and Q must be continuous in time, which requires the introduction of additional registration systems, complicating the experiment.

In addition, the drawback of this method can be considered that the speed of removal of the TM from the fuel rod p is produce according to Q() and q().

The technical result that is achievable with the use of the invention is to improve the accuracy in the determination of the speed of removal of the fuel, the simplification of the experiment.

This technical result is achieved by the proposed method of determining the speed of removal of the fuel material from vented fuel element, including a process reactor test fuel rod measurement of heat capacity and the rate of removal of the fuel material initially measure the time precondensation fuel materialtothen at time +tomeasured heat capacity, record the pressure (P) of gaseous fission products in the ventilation system, the temperature (Tabout) shell fuel and appreciate the speed of removal (J) of the fuel material in expression

< / BR>
where qf- the density of thermal power supplied to the membrane of a fuel rod from a fuel source at time +to, W/m2;

r - radius of the inner membrane of a fuel rod, m;

- coefficient of thermal conductivity of the fuel material, W/(mgrad);

the relative volume fraction of porosity of the fuel material, Rel.ed. ;

R is the total resistance of the ventilation system, 1/m;

A and b are coefficients, hung P [PA]; Tabout[To].

According to the invention, to determine J in the expression (3) it is necessary to measure the time precondensation TMto. In the manufacture of fuel rods of the nuclear fuel is put in the form of pellets, forming a fuel cell unit with a gap between TM and the inner surface of the shell of a fuel rod that creates a large thermal resistance to heat flow coming from the TM to obolochke of a fuel rod, and thus leads to a significant jump in temperature TM. Thereby activates the process of precondensation TM on the shell of a fuel rod and seal it with the formation of gas Central cavity. Time precondensation TM can be fixed with the help of the testimony of any devices (e.g., thermocouples on the shell of a fuel rod or on the carrier pipe EGK). Moreover, it is important not quantity, say thermocouples and temporal dynamics in the reading device. Let us explain this point. When testing fuel elements, in particular thermionic fuel elements comprising TFE, in research reactors was marked by the fact of change of the heat flow from the fuel rod at a constant power research reactor), in particular it noted the readings of thermocouples on the carrier pipe TFE, which is associated with the process Percodan azionario state with a constant value of qf. The time interval to the exit of the curve at a constant value toassociated with the completion of the process of precondensation TM, and for high-temperature fuel elements this time interval may be very small [3]. With the completion of the process of precondensation TM redistributed on the inner surface of the shell of a fuel rod with the formation of isothermal gas Central cavity [4]. The fact of the contact TM with the inner surface of the shell of a fuel rod suggests the equality of the cladding of a fuel rod and TM at the point of contact that used in deriving (3).

In Fig.1-3 schematically shows the main structural variants common types of vented fuel rods, which can be implemented this way. In Fig.4 schematically shows a nuclear reactor, which is being vented fuel element.

In Fig. 1-3 is displayed: 1 - Fe, 2 - shell, 3 - fuel material (TM), 4 - ventilation system, 5 - temperature sensor 6 sensor heat capacity, 7 - tube, 8 - capillary tip. In Fig.1 the ventilation system 4 is made in the form of a Central channel penetrating TM for the entire length of a fuel rod. In Fig.2 and 3, the ventilation system 4 consists of a Central axisymmetric tube 7 with capillary is, 13 - tank-tank GPA.

The method is implemented as follows.

TVEL 1 with recording devices (sensor thermal power 6 and the temperature sensor 5 of the shell 2 of a fuel rod 1) is placed in the cell 10 of the active zone 11 of a nuclear reactor 12. During operation of the reactor 12 in a vented fuel element 1 is the division of nuclear fuel in TM 3 with the formation of gaseous fission products escaping from the ventilation system 4 outside of the fuel rod 1 and the reactor 12 in the reservoir tank 13 GPA. In the process of selection of thermal power in TM 3 is heated and reconcretion TM 3 on the cooler inner surface of the shell 2 of a fuel rod 1. The fact of the termination of the process of precondensation TM 3 is determined by the dynamics of the testimony of the registration device, for example using thermocouples 5, thereby fixing the timetoprecondensation TM 3. Simultaneously with the GPA through the ventilation system 4 or through the Central channel (Fig.1) or through the capillary tip 8 and axisymmetric tube 7 (Fig.2) go and molecules TM 3, diffusing into the gas-vapor medium consisting of GPA and TM. We are interested in the time +toregister sensor thermal power of 5, which can befcoming on the shell 2 of a fuel rod 1 from TM 3. Using the pressure sensor 9 mounted on the output gap of the ventilation system, record the pressure P. the temperature Sensor 5, for example a thermocouple, record the temperature Taboutshell 2 of a fuel rod 1. Knowing the geometrical characteristics of vented fuel element 1, the relative volume fraction of porosity TM 3 and physical characteristics of the used fuel material 3, by using the expression (3) estimate the rate of removal of J TM 3.

Let us derive the expression (3), using the phenomenon of diffusion of TM in the one-dimensional case in the two-component system (GPA and a pair TM), described by the first law Fika [6]. It is assumed that the ventilation system of the fuel rod is designed in a way that does not allow condensation of molecules TM inside it or this condensation is negligible and does not affect the performance of the ventilation system.

In this case, the first law Fika can be written in the form:

J = -D(no-n0)/R, (4)

where J is the rate of removal of TM released from vented fuel element; D is the diffusion coefficient of molecules TM in the gas mixture GPA and molecules TM; - molecular weight TM; nothe concentration of TM at the outlet of the ventilation system of a fuel rod; n0- maximumpoolsize ventilation system in the form of an axisymmetric channel in TM, as shown in Fig.1, in the first approximation we can assume

R = Lc/(2r2in). (5)

In case of performance of the ventilation system in the form of Central axisymmetric tube with a capillary tip (Fig.2, 3)

R = l1/(r21)+l2/(r22). (6)

In a first approximation, the diffusion coefficient D of molecules TM to nonequilibrium stationary gas mixture of molecules TM and GPA (mostly molecules Heh [7]) is calculated by the formula [8]

D = u*/3, (7)

where u is the average velocity of thermal motion of molecules TM;

*- the average length of the free path of the molecules TM.

The velocity u, we define the expression given in [9], and *from the expressions given in [10], considering that the GPA consist mainly of Heh, as follows from [7]

u = (8kT/())1/2, (8)

*= kT/(((d+dXe)/2)2(1+/Xe)1/2P), (9)

where k is Boltzmann's constant; T - temperature; d, dHehthe diameters of the molecules TM and Eh, respectively; ,Xe/- molecular weight molecules TM and Eh, respectively; P is the pressure in GPA.

Knowing the density of the TM , to determine d from the relation d = 1,122(/)1/3[15], and dHehfrom [16].

Given the exponential dependence of the pressure BR>
where a*and are coefficients depending on the type TM.

Where the expression for the maximum concentration of TM in Fe, taking into account the ratio P=nkT from [11], can be written in the form

n0=AND*exp (- /T)/(CT). (11)

Given that the temperature structure in which condensation occurs TM, out of TVEL, much less than the maximum temperature TM in Fe, and taking into account (11) - exponential dependence of the concentration of molecules TM temperature

n0> no. (12)

Given the above, substituted in (4) expressions (7) and (11), taking into account(8), (9), (12)

J = AT1/2exp(-B/T)/(PR), (13)

where the factor a depends on the type TM and is determined from the expression

< / BR>
The temperature T for the expression (13) corresponding to the maximum temperature TM in Fe, determine based on that given in [14] for a hollow fuel cylinder with heat, cooled from the outer surface.

< / BR>
where qv- density volumetric heat generation in the TM of a fuel rod; r and rinrespectively the radii of the outer and inner surfaces of the hollow fuel cylinder; - thermal conductivity of TM; Taboutthe temperature on the outer surface of the fuel cylinder is equal to the temperature of the tee, we can assume that the relative volume fraction of porosity of the fuel material (free volume not occupied TM in Fe)

= (rin/r)2(16)

Express

Convert the expression (15), taking into account (16) and (17), to mind

T = qfr/(2)(1+ln/(1-))+Tabout. (18)

Substituting (18) into (13), we obtain the expression for the rate of removal of the fuel material from the vented fuel

< / BR>
As an example, consider the use of the method for determining J, where the TM will take uranium dioxide, and the ventilation system made in the form of Central axisymmetric tube with a capillary tip, as shown in Fig.2 and 3.

Accept: = 0,3; =2.5 W/(grad.); r=0.01 m; l1=410-3m; r1=510-5m; l2=1,610-2m; r2=10-3m from which (6) R5,141051/m

Find the coefficients a and b for uranium dioxide. Convert the equation of equilibrium between the steam and the adsorbed phase stoichiometric uranium dioxide, is given in [13],

LgP [mm RT.article] =-32258/T + 12,183

to the form (10), taking into account the International system of units,

P[PA]=2,0271014exp(-74277/T).

Where AND*= 2,0271014PA;=74277 deg. From the expression (14) find the value of coefficient A= 2,9,195>103after the exit of the reactor at full power on the dynamics of changes in the sensor readings of the cladding of a fuel rod identified the time precondensation TMto=1800 S.

Then at time +tothe reading of the sensor heat capacity measure, suppose that qf=5105W/m2. Assume that the recorded pressure GPA recorded using, for example, a pressure sensor, P=103PA and the temperature of the shell of a fuel rod Tabout= K and estimate the speed of removal of the fuel material according to the expression (3) J=9,410-12kg/s

Thus, the proposed method allows you to:

1) to provide control over the rate of removal of fuel vented from the fuel rod in the reactor conditions;

2) to simulate the working modes vented fuel elements in a real design and manufacturing technology;

3) thereby to increase the accuracy of determining the speed of ejection of fuel through the ventilation system of the fuel element.

In turn, increased the accuracy of determining the speed of removal allows us to estimate the limiting resource is vented fuel elements, in particular thermionic fuel elements in the composition of TFE, the factor of the removal of fuel or to give recommendations for improving the design of the ventilation system of the fuel elements and modes of operation with the aim of increasing its resource.

Sources of information

1. C. Desman. Scientific foundations of vacuum Tagus fuel through the ventilation system of the fuel and the emitter node thermionic power generation channel/ Century A. Kornilov, V. C. Sinyavsky. - N94023472/07; claimed 21.06.94; publ. 27.07.97, bull. N21.

3. Kornilov C. D., yuditsky C. D. Modeling of heat and mass transfer in the core thermionic fuel elements: Atomic energy, 1982, T. 53, vol.2, S. 74-76.

4. Legalizes Y. G., Ponomarev-Stepnoi N. N., Kuznetsov, C. F. the high-temperature Behavior of nuclear fuel during irradiation. M.: Energoatomizdat, 1987, S. 116.

5. Sinyavsky Century Century Methods of determining the characteristics of thermionic fuel elements. M.: Energoatomizdat, 1990, S. 48.

6. Jaworski C. M., Detlef A. N. The Handbook of physics. By "Nauka", M., 1971, S. 211.

7. [4], S. 15.

8. [6], S. 213.

9. [6], S. 207.

10. [1], S. 68.

11. [1], S. 12.

12. Kotelnikov R. B. and other high-Temperature nuclear fuel. Ed. 2-E. M., Atomizdat, 1978, S. 40.

13. Gorban Y. A. and other Study evaporation dioxide and carbides of uranium. Atomic energy, 1967, I. 22, vol.6, S. 465-467.

14. Simovski A. S. and other Fuel elements of nuclear reactors, M. , gosatomizdat, 1962, S. 355.

15. [1], S. 42.

16. Physical quantities, the Handbook edited by I. S. Grigoriev, E. H. Malikova. M.: Energoatomizdat, 1991, (PL. Mendeleev).

The method of determining the speed cynoscephalae power and the rate of removal of the fuel material, characterized in that initially measure the time precondensation fuel materialtothen at time +tomeasured heat capacity, fixed pressure P of the gaseous fission products in the ventilation system, the temperature Taboutshell fuel and estimate the speed of the takeaway J fuel material in expression

< / BR>
where qf- the density of thermal power supplied to the membrane of a fuel rod from a fuel source at time +to, W/m2;

r - radius of the inner membrane of a fuel rod, m;

- coefficient of thermal conductivity of the fuel material, W/(mgrad);

the relative volume fraction of porosity of the fuel material, Rel.ed. ;

R is the total resistance of the ventilation system, 1/m;

A and b are coefficients depending on the type of the fuel material; And [kg2/(m2with3deg1/2)]; [Deg];to[]; []; J [kg/s]; R [PA]; Tabout[To].

 

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