Patent Description:
A well-known standard technology for producing fuel pellets in the production of ceramic nuclear fuel from uranium dioxide (enriched up to <NUM>% uranium <NUM>) includes: obtaining and preparing uranium dioxide press-powder (mixed with a binder), pressing into pellets in a die, sintering pellets in a gaseous atmosphere, dry or wet grinding of pellets, drying, and checking pellets for compliance with specifications and drawing, packaging of satisfactory product, transferring them to filling of fuel elements of nuclear reactors (<NPL>).

A method for obtaining uranium-molybdenum alloy U-<NUM>%Mo by cold pressing followed by sintering is known [<NPL>]. In this method, uranium and molybdenum powders were mixed for <NUM> hours without adding a binder. After mixing the powders of uranium and molybdenum, they were pressed to obtain briquettes ~ <NUM> long and with <NUM>. <NUM> square cross-section, and cylinder-shaped briquettes. The sintering was performed for <NUM> hours in vacuum at <NUM>. Metallographic testing of the cold-pressed and sintered samples showed that the alloyed elements fully react with each other. The density of resulting pellets was <NUM>/cm<NUM>.

The disadvantage of this method is that there is an uneven distribution of molybdenum throughout the volume and the need to create high pressure during cold pressing of mixed powders of uranium and molybdenum in the samples obtained this way.

A method for producing ceramic fuel pellets for nuclear reactor fuel elements is also known comprising preparation of uranium dioxide powder mixed with a binder, pressing into pellets in two stages and sintering them (see <CIT>, IPC G21C <NUM>/<NUM>, BI No. <NUM> dated <NUM>.

The disadvantage of this method is a two-stage pressing of uranium dioxide pellets and the use of a plasticizer in the preparation of the press-powder.

Preparing binary uranium-molybdenum alloys is known (<NPL>). The alloys were produced from metal powders (e.g., uranium and molybdenum powders) to further investigate their fundamental mechanical properties. In one embodiment, hot pressing was used for their production, and in the other cold pressing followed by sintering.

During the hot pressing operation blended powders were compacted in vacuum at about <NUM> for <NUM>/<NUM> hour. A pressure of <NUM> tsi (about <NUM> MPa) was normally applied. The resultant compacts consisted essentially of discrete particles of uranium and the alloy element. These alloys could be homogenized by isothermal heat treatment at higher temperatures for two hours at <NUM>. The grain size attained was approximately that of the particles of powder used. In the case of -<NUM> mesh (<<NUM> microns) powder, hot pressing resulted in grain sizes of <NUM> to <NUM> microns.

In the case of alloys prepared by cold pressing followed by sintering, the elemental powders were weighed out and tumble-mixed in glass bottles for two hours. Compacts were pressed to a size of <NUM>-<NUM>/<NUM>" x <NUM>/<NUM>' x <NUM>/<NUM>" (about <NUM>. <NUM>) at <NUM> tsi (about 700MPa). The compacts were then sintered under vacuum for four hours at <NUM>-<NUM>. The grain size of <NUM>-<NUM> microns was thus obtained (the initial size of the uranium powder particles used was <NUM>-<NUM> microns).

The method of the present invention comprises producing pelletized fuel for fuel elements of a nuclear reactor from uranium-molybdenum powders. Such pellets can be further used in the existing types of the WWER and PWR nuclear reactors. The prior art described above does not teach preparing such pelletized fuel from the uranium-molybdenum alloys, it only teaches preparing alloys and articles therefrom having various shape (but not the fuel pellets), in order to further study their fundamental mechanical properties, namely, dimensional stability of products made from the alloys of the U-Mo (U-Mo-X) and U-Nb (U-Nb-X) systems under conditions of thermal cycling, stretching and irradiation. The other difference of the proposed method from the prior art described above is that the uranium-molybdenum powder with uranium <NUM> enrichment to <NUM>% is used as an initial powder to comply with the requirements of non-proliferation and nuclear and radiation safety restrictions of commercial reactors (WWER and PWR types), where the resulting fuel pellets can be used. Moreover, the proposed method comprises sintering the pellets in an inert gaseous atmosphere, rather than under vacuum, as proposed in the document described above for the fine metal uranium powders. The increased heat transfer of the inert gas environment as proposed by the present invention compared to a vacuum does not allow slow cooling of the pellets core, and ultimately does not lead to grain size nonhomogeneity of the material, which allows to obtain high-quality pelletized fuel with increased uranium capacity is obtained according to the claimed method.

Furthermore, Eiss A. et al is silent about the order of operations as proposed in the present invention for producing pelletized fuel, such as preparation of powder, pressing into pellets in a die, sintering them in a gaseous atmosphere, grinding, drying, rejection of the pellets, wherein the pellets are sintered in an inert atmosphere, and uranium-molybdenum powder with uranium <NUM> enrichment to <NUM>% and molybdenum content of <NUM> to <NUM> wt % is used as an initial powder. The other embodiment of the proposed invention, characterized by the order of operations for producing pelletized fuel from uranium-molybdenum powders for nuclear reactor fuel elements, comprising preparation of powder, stage-by-stage mixing with a binder, pressing into pellets in a die, thermal removal of the binder, sintering the pellets in a gaseous atmosphere, grinding, drying, rejection of the pellets, characterized in that the pellets are sintered in an inert atmosphere, and uranium-molybdenum powder with uranium <NUM> enrichment to <NUM>% and with molybdenum content of <NUM> to <NUM> wt% is used as an initial powder, is also not disclosed in the state of the art.

Thus, the purpose of the method disclosed by Eiss A. et al is to prepare alloys and products therefrom having various shape (but not the pellets) to investigate their fundamental mechanical properties, namely, dimensional stability of products made from the alloys of the U-Mo (U-Mo-X) and U-Nb (U-Nb-X) systems under conditions of thermal cycling, stretching and irradiation. Such purpose differs from that of the claimed invention, which is intended for preparing uranium-molybdenum fuel pellets with uranium <NUM> enrichment to <NUM>% for the use in the WWER and PWR nuclear reactors. The use of uranium-molybdenum powder with uranium <NUM> enrichment to <NUM>% and molybdenum content of <NUM> to <NUM> wt % as an initial powder followed by sintering the pellets in an inert atmosphere is not known for producing pelletized fuel for nuclear reactors fuel elements. Technological operations of the processes are also different since they are developed for preparing different products as described above.

The closest is a method for producing ceramic fuel pellets for fuel elements of a nuclear reactor (<CIT> IPC G21C <NUM>/<NUM>, BI No. <NUM> dated <NUM>. <NUM>) comprising preparation of uranium dioxide press-powder enriched to <NUM>-<NUM>% uranium <NUM>, stage-by-stage mixing with a dry binder (metal-free) and uranium oxide powder, pressing into pellets in a die, thermal removal of the binder, sintering of pellets in a gaseous reducing atmosphere, wet grinding of pellets with a diamond disk, drying, and rejection of pellets.

The disadvantage of the method is that the method requires significant power consumption during pressing, and the resulting fuel has smaller uranium intensity.

The objective of the invention is to develop a method for producing pelletized fuel from uranium-molybdenum alloys enriched to <NUM>% uranium <NUM> for nuclear reactor fuel elements, which increases the safety of nuclear reactor operating conditions and performance.

The technical result of the proposed invention according to the first and second embodiment is aimed to obtain uranium-molybdenum pellets enriched to <NUM>% uranium <NUM> for fuel elements of a nuclear reactor, which increases the uranium intensity of the fuel, reduces the amount of heat buildup in a reactor core, and lowers the amount of energy released in the event of abnormalities in the operation of a nuclear reactor, thus providing increased reactor safety and resilience to accidents.

The technical result according to the first embodiment is achieved in a method for producing pelletized fuel from uranium-molybdenum powders for fuel elements of a nuclear reactor, comprising powder preparation, pressing into pellets in a die, sintering in a gaseous atmosphere, grinding, drying, and rejection of pellets. The pellets are sintered in an inert atmosphere, and uranium-molybdenum powder with uranium <NUM> enrichment to <NUM>% and molybdenum content of <NUM> to <NUM> wt % is used as an initial powder.

Uranium-molybdenum powder fraction size is no more than <NUM>. Before pressing into the pellets in the die, the uranium-molybdenum powder is heated in argon at <NUM> for <NUM>-<NUM> hours. The pellets are pressed in the die with a force of up to <NUM> MPa. The pellets are sintered in argon at <NUM> - <NUM> for <NUM>-<NUM> hours.

The technical result according to the second embodiment is achieved in a method for producing pelletized fuel from uranium-molybdenum powders for nuclear reactor fuel elements, comprising powder preparation, stage-by-stage mixing with a binder, pressing into pellets in a die, thermal removal of the binder, sintering the pellets in a gaseous atmosphere, grinding, drying, rejection of pellets. The pellets are sintered in an inert atmosphere, and uranium-molybdenum powder with uranium <NUM> enrichment to <NUM>% and with molybdenum content of <NUM> to <NUM> wt% is used as an initial powder. Uranium-molybdenum powder fraction size is no more than <NUM>. The pellets are pressed in the die with a force of up to <NUM> MPa. The thermal removal of the binder is done by heating the pellets in argon at <NUM> to <NUM> for <NUM>-<NUM> hours.

The pellets are sintered in argon at <NUM> - <NUM> for <NUM>-<NUM> hours.

This technology for producing pelletized fuel from uranium-molybdenum metal powders for nuclear reactor fuel elements helps obtain uranium-molybdenum pellets with uranium <NUM> enrichment up to <NUM>%, molybdenum content from <NUM> to <NUM> wt%, and the pellet density at least <NUM>/cm<NUM> (over <NUM>% of the theoretical density). The performance of a nuclear reactor fueled with uranium-molybdenum pellets increases due to the increased thermal conductivity of uranium-molybdenum fuel compared to the existing uranium dioxide fuel and the use of lower uranium <NUM> enrichment, since the density of uranium-molybdenum pellets is <NUM> times higher than that of uranium dioxide pellets. Higher nuclear reactor fueling in terms of uranium mass due to the higher density of uranium-molybdenum alloy (<NUM> to <NUM> wt%) compared to the density of uranium dioxide expends the fuel cycle time in the reactor without the need to increase fuel enrichment. The density of uranium dioxide by uranium is <NUM>/cm<NUM>, and the density of uranium-molybdenum alloy (<NUM> to <NUM> wt%) by uranium is ~<NUM>/cm<NUM>. Thus, with the same charge of uranium-molybdenum fuel in the reactor, the amount of fissile component increases and is ~<NUM>%.

It is known [<NPL>] that during tempering of U-Mo γ-alloy within the temperature range of <NUM>-<NUM>, γ-phase converts into a eutectoidal mixture of α-uranium and U<NUM>Mo intermetallide (γ'-phase). However, this process runs slowly. When the initial powder is held in vacuum for <NUM>-<NUM> hours at <NUM>, the phases get partially separated to produce γ-phase, a eutectoid mixture of α-phase and γ'-phase. The use of powder, which is a eutectoidal mixture of phases, helps obtain strong "raw" pellets at lower pressing forces (up to <NUM> MPa). Further sintering of such pressings (pellets) at temperatures above <NUM> is accompanied by a complete reverse transition of the eutectoid to the γ-state.

The optimum result is achieved when the initial metal powder of uranium-molybdenum with molybdenum content of <NUM> to <NUM> wt% is held in an inert gas at a temperature of <NUM> for <NUM> to <NUM> hours, due to the fact that the initial metal powder of uranium-molybdenum in the form of γ-phase is subjected to partial separation of phases to γ-phase, eutectoidal mixture of α-phase and γ'-phase; all this helps obtain strong "raw" pellets at pressing forces of up to <NUM> MPa.

Lower molybdenum content in the initial uranium-molybdenum powder less than <NUM> wt% leads to an increased density of sintered uranium-molybdenum pellets (up to <NUM>/cm<NUM> with zero molybdenum content), and higher molybdenum content in the initial uranium-molybdenum powder over <NUM> wt% leads to a decreased density of pellets, which is not allowed by technical requirements for the production of pellets for nuclear reactors.

Increasing the sintering temperature above <NUM> leads to melting of the uranium-molybdenum pellet, and temperatures below <NUM> make it impossible to obtain a pellet without internal pores. However, the pellet density remains low and is only <NUM>-<NUM>/cm<NUM>.

The best results were achieved with uranium-molybdenum pellets with molybdenum content of <NUM> to <NUM> wt% at a sintering temperature (in an inert gas) of <NUM> to <NUM> for <NUM> to <NUM> hours.

The developed method helps obtain γ-phase from uranium-molybdenum powders for nuclear reactor fuel elements, with molybdenum being the basic alloying element that contributes to the preservation of uranium γ-phase throughout the operating temperature range of a fuel element. Molybdenum not only modifies the kinetics of phase transformations to produce a randomly oriented fine-grained structure, but also stabilizes uranium γ-phase, thereby increasing fuel element performance.

Due to the fact that the thermal conductivity of uranium-molybdenum fuel is higher than that of uranium dioxide fuel, the amount of heat buildup in a reactor core can be reduced, and the amount of energy released in the event of abnormalities in the operation of a nuclear reactor can be reduced, thus providing increased reactor safety and resilience to accidents.

Provided below are examples of embodiment of the proposed uranium-molybdenum pellet production method.

Example <NUM> (according to the first embodiment). A uranium-molybdenum powder with molybdenum content in the alloy of <NUM> wt% obtained by centrifugal atomization from an ingot of the same alloy, with uranium <NUM> enrichment to <NUM>%, is used as an initial powder. Centrifugal atomization of an uranium-molybdenum ingot with molybdenum content in the alloy of <NUM> wt% helps obtain a uniform molybdenum content in the initial powder. The powder is screened through a <NUM> sieve. The screened uranium-molybdenum powder with a molybdenum content of <NUM> wt% is heated at <NUM> for <NUM> hours in a top-loader vacuum furnace SShVE (in argon). The resulting powder is pressed in a cylindrical die at a molding pressure of <NUM> MPa without adding a binder (plasticizer). The pellets are sintered in argon (with water content not exceeding <NUM> ppm) at (<NUM>+<NUM>/-<NUM>) °C at an isothermal exposure time of <NUM> hours in an SShVE furnace (or XERION XVAC-<NUM>). Heating to isothermal exposure is carried out in a stream of argon <NUM>/min at a heating rate not exceeding <NUM>/min, followed by cooling in static argon at a cooling rate of (<NUM>-<NUM>) °C/min. After that, pellets are ground, dried, and rejected for compliance with technical requirements.

Example <NUM> (according to the first embodiment). A uranium-molybdenum powder with molybdenum content in the alloy of <NUM> wt% obtained by centrifugal atomization from an ingot of the same alloy, with uranium <NUM> enrichment to <NUM>%, is used as an initial powder. Centrifugal atomization of an uranium-molybdenum ingot with molybdenum content in the alloy of <NUM> wt% helps obtain a uniform molybdenum content in the initial powder. The powder is screened through a <NUM> sieve. The screened uranium-molybdenum powder with a molybdenum content of <NUM> wt% is heated at <NUM> for <NUM> hours in a top-loader vacuum furnace SShVE (in argon). The resulting powder is pressed in a cylindrical die at a molding pressure of <NUM> MPa without adding a binder (plasticizer). The pellets are sintered in argon (with water content not exceeding <NUM> ppm) at (<NUM>+<NUM>/-<NUM>) °C at an isothermal exposure time of <NUM> hours in an SShVE furnace (or XERION XVAC-<NUM>). Heating to isothermal exposure is carried out in a stream of argon <NUM>/min at a heating rate not exceeding <NUM>/min, followed by cooling in static argon at a cooling rate of (<NUM>-<NUM>) °C/min. After that, pellets are ground, dried, and rejected for compliance with technical requirements.

Example <NUM> (according to the second embodiment). A uranium-molybdenum powder with molybdenum content in the alloy of <NUM> wt% obtained by centrifugal atomization from an ingot of the same alloy, with uranium <NUM> enrichment to <NUM>% and screened through a <NUM> sieve, is used as an initial powder. Centrifugal atomization of an uranium-molybdenum ingot with molybdenum content in the alloy of <NUM> wt% helps obtain a uniform molybdenum content in the initial powder. <NUM>% aqueous solution of polyvinyl alcohol with <NUM>% glycerol (<NUM>% by weight of the uranium-molybdenum alloy) is used as a plasticizer (binder). Mixing includes three stages. At the first stage, the entire amount of the binder and uranium-molybdenum alloy powder in an amount up to <NUM> wt% are mixed to get a homogeneous mixture. At the second stage, the resulting mixture is mixed with up to <NUM> wt% uranium-molybdenum alloy powder to get a homogeneous mixture. At the third stage, the remaining amount of uranium-molybdenum alloy powder is added into the mixture obtained at the second stage and mixed to get a homogeneous mixture. The powder is mixed in a Turbula mixer for <NUM>-<NUM> minutes. The prepared powder is pressed in a cylindrical die at a molding pressure of <NUM> MPa. Before sintering, the pellets are heated in argon at <NUM> to <NUM> for <NUM> hours to remove the binder. The pellets are sintered in argon (with water content not exceeding <NUM> ppm) at (<NUM>+<NUM>/-<NUM>) °C at an isothermal exposure time of about <NUM> hours in a top-loader furnace SShVE. Heating to isothermal exposure is carried out in a stream of argon <NUM>/min at a heating rate not exceeding <NUM>/min, followed by cooling in static argon at a cooling rate of (<NUM>-<NUM>) °C/min. After that, pellets are ground, dried, and rejected for compliance with technical requirements.

<FIG> shows uranium-molybdenum pellets after sintering and machining.

Claim 1:
A method for producing pelletized fuel from uranium-molybdenum powders for fuel elements of a nuclear reactor, comprising preparation of powder, pressing into pellets in a die, sintering them in a gaseous atmosphere, grinding, drying, rejection of the pellets, characterized in that the pellets are sintered in an inert atmosphere, and uranium-molybdenum powder with uranium <NUM> enrichment to <NUM>% and molybdenum content of <NUM> to <NUM> wt % is used as an initial powder.