Active zone of lead-cooled fast reactor

Abstract

An active zone includes a homogeneous uranium-plutonium nitride fuel, the mass fraction of which is a minimum 0.305, and consists of central, intermediate and peripheral parts which form fuel assemblies comprising fuel elements with geometrically identical shells but differing heights. The radial distribution of the fuel across the volume of the active zone has a stepped shape. The radius of the central part is from 0.4 to 0.5 of the effective active zone radius, while the height of the fuel column in the fuel elements in the central part is from 0.5 to 0.8 of the height of the fuel column in the peripheral part. The heights of the fuel columns forming a stepped intermediate part for diameters ranging from 0.5 to 0.85 of the effective active zone diameter are within the range from 0.55 to 0.9 of the height of the fuel column in the peripheral part.

Claims

1. An active zone of a lead-cooled fast reactor, comprising a homogeneous uranium-plutonium nitride fuel which is contained in geometrically identical shells of cylindrical fuel elements, wherein the fuel elements are arranged in fuel assemblies so that a mass fraction of the fuel in the active zone is a minimum of 0.305, said fuel assemblies create a central part, an intermediate part and a peripheral part of the active zone, wherein a diameter (d.sub.1) of the central part of the active zone ranges from 0.4 to 0.5 of an effective diameter (D) of the active zone, while a height (h.sub.1) of the fuel column in the fuel elements of the fuel assemblies in the central part of the active zone is from 0.5 to 0.8 of a height (H) of the fuel column in the fuel elements arranged in the fuel assemblies in the peripheral part of the active zone, and a diameter (d.sub.2) of the stepped intermediate part of the active zone ranges from 0.5 to 0.85 of the effective diameter (D) of the active zone, while heights (h.sub.2) of the fuel columns in the fuel elements in the fuel assemblies forming the stepped intermediate part are from 0.55 to 0.9 of the height of the fuel column in the fuel elements arranged in the fuel assemblies in the peripheral part of the active zone.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is longitudinal cross-sections of fuel elements which define fuel assemblies in the peripheral, intermediate and central parts of the reactor active zone in accordance with the present invention.

(2) FIG. 2 is a diagram showing nuclear uranium-plutonium fuel distribution in the reactor active zone in accordance with the present invention, which distribution has a stepped shape in a longitudinal section.

EMBODIMENTS OF THE INVENTION

(3) A fuel element of a fuel assembly defining a peripheral part of an active zone (FIG. 1a) consists of a tubular shell 1 with end elements 2 and 3, wherein inside the shell 1 there is uranium-plutonium fuel 4 in the form of a column of height H. An upper part of the fuel element comprises a cavity 5 filled with inert gas. An upper part of the gaseous cavity 5 comprises a neutron absorber, for example, in the form of a tungsten-carbide rod 6 of 5 cm in height, and a structural component for fuel fixation made, for example, in the form of a spring 7.

(4) A fuel element of a fuel assembly defining a central part and an intermediate part of an active zone having in a longitudinal section a stepped fuel distribution (FIG. 1b) consists of a tubular shell 1 with end elements 2 and 3, wherein inside the shell 1 there is uranium-plutonium fuel 4 in the form of a column of fuel pellets of height h. The height h is selected from the range from 0.5 to 0.8 of H for the central part and from 0.55 to 0.9 of H for the intermediate part. An upper part of the fuel element comprises a cavity 5 filled with inert gas. An upper part of the gaseous cavity 5 comprises a neutron absorber, for example, in the form of a tungsten-carbide rod 6 of 5 cm in height, and a structural component for fuel fixation made, for example, in the form of a spring 7.

(5) FIG. 2 is a diagram showing nuclear uranium-plutonium fuel distribution in the active zone, which distribution has a stepped shape in a longitudinal section. When arranging fuel assemblies according to this principle, an intermediate part of the active zone having a diameter from d1 to d2 creates a step and has fuel elements shown in FIG. 1b. The diameter d1 of the central part of the active zone is selected from 0.4 to 0.5 of its effective diameter D. Fuel assemblies of the intermediate part are arranged within the diameter d2, which is selected in the range from 0.5 to 0.85 of the effective diameter D of the active zone and comprises the fuel elements having the fuel column of height h.

(6) According to the present invention, the fuel assemblies and the fuel elements of the active zone together create a stepped shape distribution. The applicant hasn't found any technical solutions that would comprise the features related to establishing the inventive fuel distribution in an active zone which has in its longitudinal section a stepped shape. This solution, in terms of its simplicity and used structures, dramatically differs from the solution of the fuel zonal distribution by changing diameters of fuel elements and a pitch of their arrangement along the active zone radius. Reduction of fuel height in the central part of the active zone results in neutron flux space-and-energy redistribution, increase of neutron escape from the central part of the active zone and, consequently, reduction of a positive constituent of the void reactivity effect. This effect together with an impact of lateral and end reflectors and a neutron absorber which is mounted at the upper part of a gaseous cavity of the fuel element provides for achievement of a negative value of the void effect and for the entire reactor.

(7) Similar to the described above three-step fuel arrangement, an active zone having four and more steps created with fuel assemblies of different fuel height in fuel elements can be embodied. Selection of height of the fuel in the fuel elements of the fuel assemblies in the central part of the active zone impacts the power distribution along its radius. Calculation results show that the stepped fuel distribution in the active zone provides more uniform power distribution along the active zone radius. Different heights of fuel in the rod-type fuel elements in the fuel assemblies which increase stepwise from the active zone centre to its periphery allow reducing the non-uniformity of power distribution along the active zone radius, thus increasing average power density and optimizing fuel load in the active zone.

(8) The example of an embodiment of the active zone of the reactor BP-1200 based on the inventive solution with fuel radial distribution across the active zone volume which has a stepped shape in a longitudinal section. The active zone of the lead-cooled reactor BP-1200 with heat output of 2800 MW and an effective diameter of 576 cm consists of 692 jacket fuel assemblies, each of which comprises 169 fuel elements with uranium-plutonium nitride fuel (having Pu around 14.3%), so that a mass fraction of the fuel in the active zone (.sub.m) is minimum 0.305. Coolant heating in the active zone is performed at 120 C., and the maximum coolant rate is about 2 m/s. The first step of the central part of the active zone comprises 127 fuel assemblies, each of which is defined by the fuel elements the fuel column height of which is 68 cm. The second step of the central part of the active zone comprises 270 fuel assemblies, each of which is defined by the fuel elements the fuel column height of which is 78 cm. The peripheral part of the active zone comprises 295 fuel assemblies, each of which is defined by the fuel elements the fuel column height of which is 88 cm. All fuel elements of the fuel assemblies in the central, intermediate and peripheral parts of the active zone have the shell external diameter of 10.0 mm and are arranged in a triangular array at a pitch of 13 mm. The ration between the diameter of the central part of the active zone and its effective diameter is 0.404, while the ratio between the fuel heights in the fuel elements of the first and the second steps of the central part of the active zone and the fuel height in its peripheral part is 0.77 and 0.89, respectively.

(9) According to this embodiment of the active zone of the lead-cooled fast reactor with power of 2800 MW, the active zone comprises four jacket fuel assemblies and uses a stepped configuration of fuel load along the radius of said active zone equal to the height of a fuel column for fuel elements having geometrically identical shells; provides power density flattening with a non-uniformity factor along the radius which is no more than 1.27 and the negative void effect for the entire reactor. In this way, the advantages of the inventive configuration of the active zone with stepped fuel distribution along its radius, and the fuel assembly and fuel element configurations intended for creating said active zone allow increasing the safety of the high-power lead-cooled reactor system and providing a base for improvement the performance and economic properties.