RU2064692C1 - Ventilated fuel element - Google Patents

Abstract

FIELD: nuclear power engineering. SUBSTANCE: uranium dioxide core is arranged in radial space relation within cladding. Radial clearance depends on coefficient of linear thermal expansion of core and cladding, core and cladding diameter, and average working temperatures of core and cladding. EFFECT: improved design. 2 dwg

Description

 The invention relates to nuclear energy, and more specifically, to ventilated fuel elements of a thermionic nuclear converter reactor, where uranium dioxide is used as the main fuel material.

In the scientific, technical and patent literature, the design of fuel elements and the diversity of the approach to selecting materials for them are rather widely considered (B. Frost, “Fuel elements of nuclear reactors,” translated from English by M. Energoatomizdat, 1986; Zaimovsky A.S. et al. " Fuel elements of atomic reactors ", M. Atomizdat, 1986). The main resource-limiting factor in the design of a uranium dioxide fuel rod is shell deformation under the influence of a swelling core. UO ₂ -based industrial fuel rods are known in which the fuel core is placed with a radial clearance relative to the shell, and the latter is filled with a heat transfer medium, for example, liquid sodium, sodium alloys with potassium, metal melts or helium (R.S. Korsakov, V.F. Vygovsky and others. "Technology of reactor engineering", M. Atomizdat, 1977). The use of a heat transfer medium in fuel rods of industrial reactors with an uncoated core is a prerequisite, since the heat fluxes realized in them (≥100 W / cm ² ) otherwise lead to an unacceptable temperature increase reaching the melting temperature of uranium dioxide of the fuel core. A significant decrease in the temperature difference at the radial gap between the core and the shell achieved due to the heat transfer medium allows us to assign large initial gaps to compensate for the swelling of the core from Uo ₂ . Nevertheless, the initial radial clearance is mainly limited to 0.15 mm, since its further increase is accompanied by an increase in the swelling rate of Uo _{2 by the} formation of folds on the cladding and heterogeneous restructuring of the core structure during the operation of a fuel rod (RB Kotelnikov, S.N. Bashlykov et al. "High-temperature nuclear fuel", M. Atomizdat, 1978). A ventilated fuel element for a thermionic nuclear converter reactor is known, including a uranium dioxide fuel core placed with a radial clearance in the shell and a gas outlet device in the form of a tube with a capillary at the end located in the central cavity of the core for outputting gaseous fission products and trapping fuel vapor (Yu.G. Degaltsev, N.N. Ponomarev-Stepnoy, V.F. Kuznetsov, “Behavior of high-temperature nuclear fuel during irradiation,” M. Energoatomizdat, 1987, p. 110-120). Ventilation of the internal cavity of the fuel element provides the unloading of the shell from the pressure of the gaseous fission products leaving the fuel core (Uo ₂ ). However, the combination of vacuum conditions and high temperatures leads to a radial mass transfer of Uo ₂ to the shell and the initial radial clearance to become less dense with respect to the initial Uo ₂ condensate. The authors' task is to reduce the radial deformation of the cladding of the ventilated fuel element caused by the swelling of the fuel core from Uo ₂ .

где: δ_з исходный радиальный зазор, мм;
α_c, α_об коэффициенты линейного термического расширения сердечника и оболочки, соответственно, 1/град;
d_с, d_об диаметры сердечника и оболочки, соответственно, мм;
T_с, T_об средние рабочие температуры сердечника и оболочки, соответственно, град.To solve this problem, the authors propose the design of a ventilated fuel element that contains a core of uranium dioxide placed with a radial gap in the shell, while the radial gap between the shell and the core is selected from the ratio:

where: δ _s initial radial clearance, mm;
α _c , α _{about the} coefficients of linear thermal expansion of the core and the shell, respectively, 1 / deg;
d _c , d _{about the} diameters of the core and shell, respectively, mm;
T _with , T _{about the} average operating temperature of the core and the shell, respectively, deg.

The proposed value of the radial clearance between the shell and the core does not allow the contact of a less dense condensate layer with the evaporation surface Uo ₂ during mass transfer. The initial radial clearance in the proposed design of the fuel element is no less than 2-2.5 times greater than the used clearances in the well-known ventilated fuel elements of thermionic converter reactors, and the indicated quantitative change leads to a qualitatively new result: rearrangement of the equilibrium Uo ₂ structure to a columnar structure with a characteristic decrease grain boundaries and developed boundary porosity and, as a consequence, to reduce fuel swelling Uo ₂ . The choice of the required gap size and the achievement of decreasing the deformation of the cladding of a fuel rod by reducing the swelling of Uo ₂ with a rebuilt structure are justified by the results obtained by the authors in the process of post-reactor studies of fuel rods with different values of the initial radial clearance irradiated for 3000 hours in the composition of the nuclear zone of the thermionic emission reactor.

Figure 1 is a structural diagram; figure 2 the results of post-reactor studies of the fuel rod. Figure 1 shows: 1-cladding of a fuel rod; 2-fuel core placed in the shell; 3-radial clearance between the core and the sheath. Under the nominal operating mode of the fuel rod, fuel, uranium dioxide, evaporates from the surface of the core 2, condenses on the shell 1, thereby forming a columnar structure Uo ₂ with radially oriented grains, while the evaporation front freely passes through the entire depth of the core, completely rebuilding the outgoing structure Uo ₂ into fuel element.

This process becomes possible due to the selected value of the gap 3, when a less dense condensate does not lead to the closure of the surfaces and condensation Uo ₂ . To ensure complete recondensation of uranium dioxide, the ratio of the radial gap to the diameter of the core should exceed the difference between the thermal expansions of the core and shell by (0.01-0.02), which is confirmed by the reactor experiments shown in Fig.2. In FIG. Figure 2 presents the results of these studies: a change in the diameter of the cladding, the fraction of condensate in the cross section of the core and the macrostructure at extreme values of the radial clearance between the core and the cladding, which confirms the achievement of a reduction in deformation and justifies the selected range of radial clearance in the fuel element.

The proposed fuel rod design allows the formation of a Uo ₂ structure favorable from the point of view of swelling, using for this purpose tablets obtained by industrial technology and temperature conditions of the fuel rod operation as technological for restructuring the Uo ₂ structure, therefore, the proposed fuel rod design is accompanied not only by a decrease in the Uo ₂ swelling and a corresponding increase in the resource of the electric generating channel of the thermionic reactor, but also the economic effect.

Claims (1)

A vented fuel element mainly for a thermionic nuclear converter reactor containing a uranium dioxide core placed with a radial gap in the shell, characterized in that the radial gap between the shell and the core is selected from the ratio

where δ _s is the initial radial clearance, mm;
α _s , α _rev - linear thermal expansion coefficients of the core and shell, respectively, 1 / deg;
d _c , d _{about the} diameters of the core and shell, respectively, mm;
T _c , T _{about the} average operating temperature of the core and the shell, respectively, deg.

Images