The present invention relates to a process for sintering a compacted powder of at least one oxide of a metal selected from an actinide and a lanthanide, this process comprising the following successive steps, carried out in a furnace and under an atmosphere comprising an inert gas, dihydrogen and water: (a) a temperature increase from an initial temperature T.sub.I up to a hold temperature T.sub.P, (b) maintaining the temperature at the hold temperature T.sub.P, and (c) a temperature decrease from the hold temperature T.sub.P down to a final temperature T.sub.F, in which the P(H.sub.2)/P(H.sub.2O) ratio is such that: 500<P(H.sub.2)/P(H.sub.2O)≦50 000, during step (a), from T.sub.I until a first intermediate temperature T.sub.i1 between 1000° C. and T.sub.P is reached, and P(H.sub.2)/P(H.sub.2O)≦500, at least during step (c), from a second intermediate temperature T.sub.i2 between T.sub.P and 1000° C., until T.sub.F is reached.

A method of making a nuclear fuel pellet for a nuclear power reactor. The method includes providing a nuclear fuel material in powder form, providing an additive, forming a so-called green pellet, wherein said additive is added either to said nuclear fuel material in powder form or to the green pellet, sintering the green pellet, wherein said additive is such that it causes larger grains in the nuclear fuel pellet, and wherein said additive is made of or includes a substance which causes the larger grains and which substantially leaves at least an outer portion of the pellet before and/or during the sintering step.

A methodology is disclosed for compaction of a ceramic matrix of certain nuclear fuels incorporating neutron poisons, whereby those poisons aid in reactor control while aiding in fuel fabrication. Neutronic poisons are rare-earth oxides that readily form eutectics suppressing fuel fabrication temperature, of particular importance to the fully ceramic microencapsulated fuel form and fuel forms with volatile species.

This invention relates to a composition and method for manufacturing a large-grained uranium oxide nuclear fuel pellet containing an additive. The nuclear fuel pellet is configured such that a uranium oxide powder and an additive powder composed of an Mg compound and a Si compound or Ca compound and a Al compound are mixed together, thus increasing a grain size to thus suppress the release of fission products, thereby increasing the stability of nuclear fuel, preventing cladding tubes from breaking, and contributing to the stable operation of nuclear power plants, ultimately increasing the overall stability of nuclear power plants including nuclear fuel.

Proposed is a nuclear fuel pellet manufactured with UO.sub.2 powder and being in a cylindrical shape, the nuclear fuel pellet including: a dish (10) provided in a shape of a spherical groove having a predetermined curvature and a diameter of 4.8 to 5.2 mm at a center of each of top and bottom surfaces of the nuclear fuel pellet; a shoulder (20) provided in an annular plane along a rim of the dish (10); a first chamfer (310) provided along a rim of the shoulder (20) while being adjacent to the shoulder (20); and a second chamfer (320) provided along a rim of the first chamfer (310), wherein a width (SW) of the shoulder (20) is 0.4565 mm to 0.6565 mm, an angle between the first chamfer (310) and a horizontal plane is 2.0°, and an angle between the second chamfer (320) and the horizontal plane is 18.0°.

The present invention concerns a fuel assembly for a nuclear power boiling water reactor. The fuel assembly comprises fuel rods. At least 95% of the fuel rods comprise nuclear fuel material in the form of U enriched in 235U. At least 20% of the fuel rods belong to a first set of fuel rods. The fuel rods in this first set comprise both U enriched in 235U and Th. The first set comprises a first and a second subset of fuel rods. The ratio, with regard to weight, between Th and U, in each fuel rod of said first subset, is higher than the ratio, with regard to weight, between Th and U, in each fuel rod of said second subset. The invention also concerns a nuclear power boiling water reactor and a manner of operating such a reactor.

A method of making a nuclear fuel pellet for a nuclear power reactor. The method includes providing a nuclear fuel material in powder form, pressing the powder such that a so-called green pellet is obtained, providing a liquid that comprises an additive which is to be added to the green pellet, contacting the green pellet with the liquid such that the liquid, with the additive, penetrates into the pellet, and sintering the so treated green pellet, wherein the additive is such that larger grains in the nuclear fuel material are obtained with the additive.

A method for converting uranium hexafluoride to uranium dioxide includes steps of hydrolysis of UF.sub.6 to uranium oxyfluoride (UO.sub.2F.sub.2) in a hydrolysis reactor (4) by reaction between gaseous UF.sub.6 and dry water vapour injected into the reactor (4), and pyrohydrolysis of UO.sub.2F.sub.2 to UO.sub.2 in a pyrohydrolysis furnace (6) by reaction of UO.sub.2F.sub.2 with dry water vapour and hydrogen gas (H.sub.2) injected into the furnace (6). The hourly mass flowrate of gaseous UF.sub.6 supplied to the reactor (4) is between 75 and 130 kg/h, the hourly mass flowrate of dry water vapour supplied to the reactor (4) for hydrolysis is between 15 and 30 kg/h, and the temperature inside the reactor (4) is between 150 and 250° C.

A nuclear fuel powder production plant comprises a conversion installation (2) for the conversion of uranium hexafluoride (UF.sub.6) into uranium dioxide (UO.sub.2) having a hydrolysis reactor (4) for the conversion of UF.sub.6 into uranium oxyfluoride powder (UO.sub.2F.sub.2) and a pyrohydrolysis furnace (6) for converting the UO.sub.2F.sub.2 powder into UO.sub.2 powder. The nuclear fuel powder production plant also includes a packaging unit (20) for the UO.sub.2 powder comprising a filling station (22) having a chamber (26) for receiving a container (24) to be filled, a filling duct (28) supplied from the furnace (6) and a suction system (32) comprising a suction ring (34) disposed at the outlet (30) of the filling duct (28) for sucking an annular air flow (A) around a stream (P) of UO.sub.2 powder falling from the outlet (30) from the filling duct (28) into the container (24).

An installation for the conversion of uranium hexafluoride (UF.sub.6) to uranium dioxide (UO.sub.2) comprises a hydrolysis reactor (4) for the conversion of UF.sub.6 into uranium oxyfluoride powder (UO.sub.2F.sub.2), a pyrohydrolysis furnace (6) for converting the UO.sub.2F.sub.2 powder supplied by the reactor (4) into UO.sub.2 powder, a supply device (8) comprising reagent injection ducts (10) for the injection of UF.sub.6, water vapor or H.sub.2, and a control system (16) designed to control the supply device (8) so as to supply at least one of the reagent injection ducts (10) with a neutral gas during a shut-down or start-up phase of the conversion installation.