Nuclear fuel pellet having enhanced thermal conductivity, and preparation method thereof

Abstract

The invention relates to nuclear physics, and specifically to reactor fuel elements and units thereof, and particularly to the composition of solid ceramic fuel elements based on uranium dioxide, intended for and exhibiting characteristics for being used in variously-purposed nuclear reactors. The result consists in a more reliable, special structure and a simple composition of uranium dioxide without heterogeneous fuel pellet additives, approaching the characteristics of a monocrystal having enhanced, and specifically exceeding reference data, thermal conductivity as temperature increases, and a simple production method thereof. The result is achieved in that pores of between 1 and 5 microns in size are distributed along the perimeters of grains in the micro-structure of each metal cluster in a nuclear fuel pellet, and in that located within the grains are pores which are predominantly nano-sized. In addition, the metal clusters comprise between 0.01 and 1.0 percent by mass. The invention provides for a method of preparing a nuclear fuel pellet, including precipitating metal hydroxides, in two stages, having different pH levels. Uranium metal is melted at a temperature exceeding 1150 DEG C., sintering is carried out in an insignificant amount of liquid phase at a temperature ranging between 1600 and 2200 DEG C. in a hydrogen medium until forming uranium dioxide, the structure of which includes metal clusters dispersed therein. An X-ray photon spectroscope is used for identifying the new structure of the UO2 pellet and the additional UU chemical bond.

Claims

1. A nuclear fuel pellet, comprising: a pellet structure of a pressed and sintered uranium dioxide powder; wherein the pellet structure is made up of evenly distributed pores among grains of the uranium dioxide powder; wherein nanopores and metal clusters of chemically bonded uranium cations are located inside the grains; wherein the nanopores are between 1 and 200 nm in size and comprise at least 50% of a total porosity of the pellet structure; wherein the metal clusters are surrounded by the uranium dioxide powder; wherein a total content of the metal clusters is between 0.01 and 2 wt %; wherein the nuclear fuel pellet has a thermal conductivity in a range of 7.6 to 8.7 W/m.Math.degrees over a temperature range of 600 to 900 C.

Description

(1) This invention is illustrated by detailed description, examples of implementation and illustrations where:

(2) FIG. 1 shows the microstructure of the proposed nuclear fuel pellet according to the invention;

(3) FIG. 2 shows the microstructure of the proposed nuclear fuel pellet of uranium dioxide with pore sizes between 1 and 200 nm making up 50% of the total pellet porosity;

(4) FIG. 3 shows the microstructure of the standard uranium dioxide nuclear fuel pellet;

(5) FIG. 4 shows the temperature-dependence plot of uranium dioxide nuclear fuel pellet thermal conductivity;

(6) FIG. 5 shows the temperature-dependence table of thermal conductivity of different uranium dioxide pellets.

(7) The nuclear fuel pellet having enhanced thermal conductivity (hereinafter referred to as the pellet) has a structure of pressed and sintered uranium dioxide powder (FIG. 1). The pellet structure is made up of pores of 1 to 5 m in size evenly distributed along the grain boundaries, and nanopores between measured between 1 and 200 nm in size located inside the grains (FIG. 2). The latter make up at least 50% of the total porosity. Metal clusters of uranium chemical compounds with a valency of 0 and 2.sup.+ are surrounded by UO.sub.2. The total content of metal clusters (the clusters) in the form of a mixture of uranium chemical compounds with a valency of 0 and 2.sup.+ is between 0.01 and 2 wt % and represent chemically bonded uranium cations (chemical bond UU). Microhardness of such metal clusters is at least 1.5 time lower than the reference data. Due to metal clusters, the O/U ratio is reduced to 1.996-1.999 inside the grains, and O/U ratio is between 2.000 and 2.002 along the grain boundaries due to oxidation during storage in open air. This improves the pellet thermal conductivity. FIG. 3 shows the structure of the standard uranium dioxide nuclear fuel pellet without metal clusters for comparison.

(8) Pellet thermal conductivity increases as temperature increases above 500-600 C. and exceeds the reference and design data by 1.5 to 3 times at 1000 C. (FIG. 4, 5). It is attributable to the following. The nature of temperature dependence of thermal conductivity measured using the conventional axial thermal flux method for the proposed UO.sub.2 pellet is very similar to the nature of temperature dependence of thermal conductivity for monocrystalline UO.sub.2. For a monocrystal, thermal conductivity does not depend on its size or orientation. At 700 C., monocrystal thermal conductivity is 60% higher than the average thermal conductivity of the sintered polycrystalline UO.sub.2. At 1000 C., monocrystal thermal conductivity is 5.9 W/m.deg., which is 2.4 times higher than the thermal conductivity of the sintered polycrystalline uranium dioxide.

(9) To produce a nuclear fuel pellet with enhanced thermal conductivity, a method is applied that includes deposition of metal hydroxides in two stages with pH, incinerating, sintering of the uranium dioxide mixture powder, pressing, and application of an X-ray photon spectrometer. For the method implementation, deposition is performed by simultaneous draining of uranyl nitrate solutions and ammonia to the buffer at 55-602 C. in two stages. At the first stage, pH is maintained between 6.5 and 6.7, at the second stage, final deposition of polyuranate ammonia (PUA) is performed at pH level between 9.0 and 10.5. The incinerating is performed at temperatures between 600 and 680 C. until UO.sub.2 reduction. Uranium metal is melted at a temperature exceeding 1150 C., and sintering is carried out in an insignificant amount of liquid phase at temperatures between 1600 and 2200 C. in a hydrogen-nitrogen medium until metal clusters are formed. Sintering in a liquid phase results in the required porosity and pellet structure. Pores with the size of 1 to 5 m are formed along the grain boundaries, and nanopores with the size of 1 to 200 nm are formed inside the grains making up at least 50% of total porosity. The O/U ratio reduces to 1.996-1.999 in the UO.sub.2-U system. Uranium dioxide is formed with dispersed metal clusters of uranium chemical compounds with a valency of 0-2.sup.+ surrounded by UO.sub.2. The new structure of the UO.sub.2 pellet and an additional UU chemical bond are identified by means of an X-ray photon spectrometer showing that such metal clusters amount to from 0.01 to 2 wt % in the pellet.

(10) In an embodiment with an extended range of method application and preparation of catalysts, the deposition is performed by simultaneous draining of the nitric-acid solution with uranium and added metal and ammonia to the buffer at 55-602 C. in two stages as well: At the first stage, pH is maintained between 7.0 and 7.2, at the second stage, final deposition of polyuranate ammonia (PUA) is performed at pH level between 8.0 and 8.5. Chromium, tin, titanium, aluminum, etc. are used as metal additives. Additives are catalysts contributing to partial, in the areas near the additives, reduction of uranium dioxide nanoparticles to uranium metal during pellet sintering.

(11) When applying the standard technology, an ammonia-containing additive in the amount of 0.01 to 0.5% is stirred in mechanically to the UO.sub.2 powder, wherein the following is used as such ammonia-containing additive: ammonia carbonate or bicarbonate, paraphenylenediamine, triazole, etc.

EXAMPLE 1

(12) Nuclear fuel fillet having enhanced thermal conductivity was prepared as follows.

(13) Deposition was performed by simultaneous draining of uranyl nitrate solutions and ammonia to the buffer at 55-602 C. in two stages. The ammonium solution was supplied to the ammonium polyuranate sediment bowl. At the first stage, pH was maintained between 6.5 and 6.7, at the second stage, final deposition of polyuranate ammonia (PUA) was performed at pH level between 9.0 and 10.5. The incinerating was performed at temperatures between 600 and 680 C. until UO.sub.2 reduction. Uranium metal was melted at a temperature exceeding 1150 C., and sintering was carried out in an insignificant amount of liquid phase at 1750 C. in a hydrogen-nitrogen medium until metal clusters were formed. Sintering in a liquid phase resulted in the required porosity and pellet structure. The new structure of UO.sub.2 pellet and an additional UU chemical bond were identified using an X-ray photon spectroscope. The pellet structure has pores evenly distributed along the grain boundaries and inside the grains. Pores with the size of 1 to 5 m were identified along the grain boundaries, and nanopores with from 1 to 200 nm were identified inside the grains making up at least 50% of total porosity. In addition, it was noted that the size of nanopores is even smaller than the microscope resolution, i. e. less than 1 nm. A the same time, sintered pellets in the UO.sub.2U system had a UO.sub.2 phase composition and O/U ratio of 2.002 at grain boundaries and 1.998 inside grains. Dispersed metal clusters of uranium chemical compounds with a valency of 0-2.sup.+ surrounded by UO.sub.2 were identified in the uranium dioxide structure. Such metal clusters of a mixture of uranium chemical compounds with a valency of 0 and 2.sup.+ amounted to 0.01-2 wt % of the pellet.

EXAMPLE 2

(14) Nuclear fuel fillet having enhanced thermal conductivity was prepared as follows.

(15) Deposition is performed by simultaneous draining of the nitric-acid solution with uranium and added metal and ammonia to the buffer at 55-602 C. in two stages as well. At the first stage, pH was maintained between 7.0 and 7.2, at the second stage, final deposition of polyuranate ammonia (PUA) was performed at pH level between 8.0 and 8.5. Chrome was used as an additive to metal. Additives contributed to partial, in the areas near the additives, reduction of uranium dioxide nanoparticles to uranium metal during pellet sintering. Then uranium metal was melted at a temperature exceeding 1150 C., and sintering was carried out in an insignificant amount of liquid phase at 1750 C. in a hydrogen-nitrogen medium until metal clusters were formed. Sintering in a liquid phase resulted in the required porosity and pellet structure. The new structure of UO.sub.2 pellet and an additional UU chemical bond were identified using an X-ray photon spectroscope. The pellet structure has pores evenly distributed along the grain boundaries and inside the grains. Pores with the size of 1 to 5 m were identified along the grain boundaries, and nanopores with from 1 to 200 nm were identified inside the grains making up at least 50% of total porosity. In addition, it was noted that the size of nanopores is even smaller than the microscope resolution, i. e. less than 1 nm. At the same time, sintered pellets in the UO.sub.2U system had a UO.sub.2 phase composition and O/U ratio of 2.002 at grain boundaries and 1.998 inside grains. Dispersed metal clusters of uranium chemical compounds with a valency of 0-2.sup.+ surrounded by UO.sub.2 were identified in the uranium dioxide structure. Such metal clusters of a mixture of uranium chemical compounds with a valency of 0 and 2.sup.+ amounted to 0.01-2 wt % of the pellet.

EXAMPLE 3

(16) In a uranium dioxide powder prepared by the standard method, 0.5 wt % of 4-amino-1,2,4-triazole powder (the triazole) was added by mechanical stirring. Pellets were pressed and sintered in a hydrogen medium at 1750 C. During sintering, the ammonium-containing triazole radical ion decomposed emitting hydrogen that contributed to the reduction of adjacent areas of uranium dioxide within the pellet volume. As a result, metal clusters and substoichiometric composition were formed in the internal part of pellets.

(17) Then uranium metal was melted at a temperature exceeding 1150 C., and sintering was carried out in an insignificant amount of liquid phase at 1750 C. in a hydrogen-nitrogen medium until metal clusters were formed. Sintering in a liquid phase resulted in the required porosity and pellet structure. The new structure of UO.sub.2 pellet and an additional UU chemical bond were identified using an X-ray photon spectroscope. The pellet structure has pores evenly distributed along the grain boundaries and inside the grains. Pores with the size of 1 to 5 m were identified along the grain boundaries, and nanopores with from 1 to 200 nm were identified inside the grains making up at least 50% of total porosity. In addition, it was noted that the size of nanopores is even smaller than the microscope resolution, i.e. less than 1 nm. A the same time, sintered pellets in the UO.sub.2U system had a UO.sub.2 phase composition and O/U ratio of 2.001 at grain boundaries and 1.999 inside grains. Dispersed metal clusters of uranium chemical compounds with a valency of 0-2.sup.+ surrounded by UO.sub.2 were identified in the uranium dioxide structure. Such metal clusters of a mixture of uranium chemical compounds with a valency of 0 and 2+ amounted to 0.01-2 wt % of the pellet.