Method for uncasing fuel assemblies and removing spent nuclear fuel

Abstract

FIELD: nuclear engineering.SUBSTANCE: invention relates to a method for uncasing fuel assemblies (FA) and removing spent nuclear fuel (SNF). The method includes cutting the spent fuel assembly shell and end parts, and disassembling the spent fuel assembly into individual fuel elements. Individual fuel elements are subjected to compression in the vertical diametrical direction due to mechanical action along the entire length of the shell with a force not exceeding the plasticity limit of the shell’s structural material, up to a size of 0.5 of the initial diameter of the fuel element. Then the fuel element is turned 90° and subjected to additional compression in the vertical diametrical direction to a value of 0.5 of the initial diameter of the fuel element. Then, one end of the fuel element is cut off and the spent nuclear fuel is removed by mechanical action. The fuel element cladding is crushed after the spent nuclear fuel is removed, and the crushed fuel element and spent nuclear fuel are forwarded for separate processing.EFFECT: increase in the completeness and efficiency of spent nuclear fuel removal from the fuel assembly cladding while ensuring technological simplicity and radiation safety of the process.1 cl, 1 ex

Description

[0001] The invention relates to nuclear technology, in particular to the reprocessing of spent nuclear fuel (SNF), and more specifically to the removal of SNF from the volume of fuel elements (FA).

[0002] A known method for cutting spent fuel assemblies (patent JP 2000180588A, IPC G21C 19 / 37, G21F 9 / 30, June 30, 2000) includes a cutting device for cutting the spent fuel assembly, a vise table for loading the spent fuel assembly, and a spent nuclear fuel collection device. The spent fuel assembly, installed in a vertically movable main body of a band saw, is loaded into the vise table, and after lowering it, the assembly is cut in a direction perpendicular to its axial direction. The band saw rotates endlessly, and the vise base is located such that the axial direction of the vise base is practically parallel to the vertical direction of the temporary base.

[0003] The disadvantages of the known method are the formation of a large number of small fractions of the assembly's structural material during cutting with a band saw and its mixing with spilled spent nuclear fuel, as well as large losses of spent nuclear fuel during cutting of the assembly, which falls into the cutting zone.

[0004] A method for cutting spent fuel assemblies is known (Machine-Building Encyclopedia, Vol. 2, Chapter 5.7, p. 313, Moscow, “Machine-Building”, 2005), which includes, after cutting off the end parts and preliminary flattening, cutting the assembly on an ARP unit in a horizontal position with two vertical knives with a cutting pitch of 10 to 30 mm.

[0005] The disadvantage of the known method is the large crumpling of the ends of the assembly sections during the impact of the knives, which prevents the complete removal of spent nuclear fuel from the fuel assembly cladding.

[0006] A known method for uncasing spent fuel rods (RU Patent No. 2192676, IPC G21C 19 / 37, 10.11.2002) involves cutting the fuel rod cladding with disks. The cladding is cut by disks rotating at the same angular velocity. The cutting edges of these disks are involute-shaped and oriented so that the protrusions of the involute of one disk coincide with the valleys of the involute of the other. As the disks rotate, each protrusion of one cutting disk, coming into contact with the fuel rod cladding, cuts into the cladding and deforms the fuel rod, forcing it into the valley of the other cutting disk. After cutting through the cladding, the disk wedge-shapedly penetrates the core, further crushing the core, pushing apart the edges of the cut cladding, and then exits the core, allowing the crushed core contents to freely spill out. Each projection of the two cutting disks, rotating in opposite directions, repeats the above operation, pulling through the entire fuel rod section.

[0007] The disadvantages of the known method include the destruction of the fuel rod structure during uncasing, as well as the mixing of crushed fuel rod materials with the spent nuclear fuel being crushed, contaminating the nuclear fuel and complicating its subsequent reprocessing. Furthermore, when the cutting edges of the discs penetrate the ceramic nuclear fuel and crush it, intense wear of the cutting discs and dust formation will occur. Replacing the discs will require decontamination of the device, and dust formation will degrade radiation safety, requiring not only effective air filtration but also subsequent replacement of contaminated filters.

[0008] The closest in technical essence to the claimed method is a method for emptying nuclear fuel cladding and a machine for grinding nuclear fuel by deformation of the cladding (patent RU 2506658, IPC G21C 19 / 36, 10.02. 2014), which includes passing the cladding through a machine that produces deformation of the cladding by ovalization, repeated over a plurality of cycles, leading to plastic deformation of the cladding, wherein the ovalization in the cycles increases from one cycle to the next and then decreases, so as to grind the fuel and then return the cladding to its original round shape.

[0009] A disadvantage of the known method is that the uneven mechanical action of the rollers on the cladding can cause the spent nuclear fuel to be pushed not toward the already deformed portion of the fuel assembly, but in the opposite direction—toward the portion of the fuel assembly that is just being ovalized, causing significant deformation and an increase in its cross-section. This can lead to a failure and halt in the spent nuclear fuel removal process from the cladding, necessitating reconfiguration of the equipment or an emergency response under conditions of high radiation exposure to personnel. Furthermore, the process of removing spent nuclear fuel from the fuel assembly requires complex equipment that requires time-consuming and precise adjustments for different types of fuel assemblies. This significantly complicates and increases the time required to remove spent nuclear fuel from the cladding.

[0010] The purpose of the present invention is to increase the completeness and efficiency of removing spent nuclear fuel from the fuel assembly cladding while ensuring technological simplicity and radiation safety of the process.

[0011] The method is based on the different plasticity properties of the fuel rod cladding structural material and the nuclear fuel pellets (NF). Since NF, like a ceramic material, lacks plasticity and fractures under mechanical stress (impact or pressing pressure). A network of interconnected cracks forms within the NF pellets, compromising their integrity. This process is particularly effective in the confined conditions of the NF cladding. Furthermore, in VVER fuel rods, the central hole in the NF pellets acts as a stress concentrator and the primary direction for crack propagation, as it is a weakened zone within the pellet material.

[0012] Unlike nuclear fuel, the fuel element material, being highly plastic, will only change its shape under mechanical impact, but will not be destroyed, provided that the plasticity limit of the fuel element cladding material is not exceeded.

[0013] At least two impacts on the fuel rod cladding will result in intensive fragmentation of the spent nuclear fuel. Leaving the fuel rod cladding cross-sectional area after trimming one end will allow for the complete removal of the spent nuclear fuel from the fuel rod without difficulty.

[0014] To achieve the set goal, after cutting the spent fuel assembly cladding and end parts, disassembling the spent fuel assembly into individual fuel elements, the following operations are performed:

[0015] - an individual fuel element is compressed due to mechanical action in the vertical direction along the entire length of the shell;

[0016] - an individual fuel element is subjected to mechanical action in the vertical direction along the entire length of the shell with a force not exceeding the plasticity limit of the shell structural material;

[0017] - an individual fuel element is compressed by mechanical action in the vertical direction along the entire length of the shell to a size of 0.5 of the initial diameter of the fuel element;

[0018] - after which the TVEL turns 90;

[0019] - then an individual fuel element is subjected to mechanical action in the vertical direction along the entire length of the shell with a force not exceeding the plasticity limit of the shell structural material up to a size of 0.5 of the initial diameter of the fuel element;

[0020] - one of the ends of the fuel element is cut off;

[0021] - removal of spent nuclear fuel from the fuel element cladding is carried out by mechanical action;

[0022] - the fuel element cladding freed from spent nuclear fuel is crushed;

[0023] - Crushed fuel assemblies and spent nuclear fuel are sent for separate reprocessing. The advantages of the proposed method for uncasing fuel assemblies and removing spent nuclear fuel are:

[0024] - low material costs for uncasing fuel assemblies and removing spent nuclear fuel due to the use of standard equipment;

[0025] - increasing the completeness of spent nuclear fuel extraction from fuel elements by preventing the spent nuclear fuel from being left in the cladding when the end ends of individual parts of the fuel elements are crushed;

[0026] - technical and technological simplicity of the method application, ensured by the use of standardized equipment;

[0027] - increasing the radiation safety of spent nuclear fuel removal operations by crushing it in the volume of a sealed fuel element and the possibility of almost complete automation of the entire process.

[0028] These advantages are ensured by the fact that the process of uncasing fuel assemblies and removing spent nuclear fuel:

[0029] - is performed on a separate fuel element by mechanical action on it through pressing, impact action, etc., along the entire length of the cladding, which avoids the uncontrolled release of crushed spent nuclear fuel into the production environment;

[0030] - mechanical action on the fuel element is performed along its entire length in the vertical diametrical direction with a force that does not exceed the plasticity limit of the structural material of the shell, which helps to avoid destruction and depressurization of the fuel element;

[0031] - mechanical action is performed on the fuel element until it is compressed in the vertical diametrical direction to a size of 0.5 of the initial diameter of the fuel element, which allows for the intensive destruction of the spent nuclear fuel, but does not reduce the cross-sectional area of ​​the cladding;

[0032] - the fuel element is rotated by 90° and its additional compression in the vertical diametrical direction to a value of 0.5 of the initial diameter of the fuel element allows for the intensive destruction of the spent nuclear fuel, but does not reduce the cross-sectional area of ​​the shell;

[0033] - one of the ends of the fuel element is cut off and the spent nuclear fuel is removed from it by mechanical action - impact, vibration, etc.;

[0034] - the fuel element freed from spent nuclear fuel is crushed and sent for reprocessing to remove the unremoved spent nuclear fuel.

[0035] An example of the method implementation is given below.

[0036] Example.

[0037] A VVER-1000 reactor spent fuel assembly was received for reprocessing. After trimming the end pieces and uncasing the spent fuel assembly, the fuel elements were individually removed from the assembly using a manipulator.

[0038] Individual fuel rods were placed on a press table. The ends of the fuel rods were secured in rotating clamping bushings.

[0039] After this, the main part of the fuel rods was subjected to mechanical stress using a superpress with a pressure of approximately 220-250 MPa (which is significantly less than the plastic limit of Zircaloy - at least 300 MPa). The fuel rod claddings were compressed vertically to a size of 4.5 mm (≈ 0.5D). ТВЭЛ). After which, by rotating the clamping bushings, the fuel rods were rotated by 90°. And further compressed using a superpress with a pressure of approximately 250 MPa to a size of 4.5 mm (≈ 0.5D). ТВЭЛ ) in the vertical direction.

[0040] Then one end of the fuel rods was cut with a circular saw. The crushed spent nuclear fuel was removed from the fuel rods by raising the end of the press table opposite the cut end of the cladding and vibrating it. The SNF recovery rate from the fuel rods was 95-98%.

[0041] The released fuel element claddings were crushed using the GIT-1000 unit to pieces measuring 40 mm and sent for reprocessing by leaching the spent nuclear fuel with nitrogen solutions under heating.

[0042] The extracted spent nuclear fuel was further crushed in a crusher to a fraction of -0.5 mm and sent for processing by dissolution in nitrogen solutions with heating.

Claims

A method for uncasing a fuel assembly and removing spent nuclear fuel, including cutting the spent fuel assembly cladding and end pieces, disassembling the spent fuel assembly into individual fuel elements, characterized in that the individual fuel elements are subjected to compression in the vertical diametrical direction due to mechanical action along the entire length of the cladding with a force not exceeding the plasticity limit of the cladding structural material, to a size of 0.5 of the initial fuel element diameter, after which the fuel element is turned by 90° and subjected to additional compression in the vertical diametrical direction to a size of 0.5 of the initial fuel element diameter, then one of the ends of the fuel element is cut off and the spent nuclear fuel is removed due to mechanical action, the fuel element cladding is crushed after the spent nuclear fuel is removed, the crushed fuel element elements and the spent nuclear fuel are sent for separate processing.