Search

EP-4736189-A1 - NUCLEAR FUEL PELLET EMBEDDED WITH A THERMALLY CONDUCTIVE INSERT IN THE FORM OF BRANCHING ARMS DISTRIBUTED FROM A HOLLOWED-OUT INNER CYLINDER TOWARDS THE OUTSIDE OF THE PELLET

EP4736189A1EP 4736189 A1EP4736189 A1EP 4736189A1EP-4736189-A1

Abstract

The invention relates to a nuclear fuel pellet (6) embedded with a thermally conductive insert (7) in the form of branching arms (7.1 to 7.6) distributed from a hollowed-out inner cylinder (61) towards the outside of the pellet. The invention essentially consists of a nuclear fuel pellet (6) embedded with, within a straight cylindrical ring (60) of fissile material, a thermal conductor (7), in particular a metal conductor, in the form of main branching arms (7.1 to 7.6) that extend from the inside of the ring (60) to the outside and are advantageously distributed homogeneously within the ring. The insert made up of its macrostructure arms and branches makes it possible to reduce the maximum temperature of the fuel by several hundreds of degrees Celsius according to its dimensions, its constituent material and the distribution of its arms in the straight cylindrical fuel ring (60).

Inventors

  • CHAUVIN, Nathalie
  • JOSIEN, Marc

Assignees

  • Commissariat à l'Energie Atomique et aux Energies Alternatives

Dates

Publication Date
20260506
Application Date
20240628

Claims (14)

  1. 1. Nuclear fuel pellet (6), comprising: - a straight cylindrical ring (60) of fissile material with a central axis (X) delimited by an inner straight cylinder (61) and an outer straight cylinder (62) whose length defines the length (H) of the pellet, the inner straight cylinder being hollowed out and defining the inner diameter (0int), the outer straight cylinder defining the outer diameter (0 ex t) of the pellet; - an insert (7) made of thermally conductive material, in the form of at least three branched main branches (7.1 to 7.6) which extend over all or part of the length of the pellet and radially from the periphery of the hollowed-out inner straight cylinder towards the outside of the pellet, each of the main branches being branched symmetrically or not at least p times, p being greater than or equal to 1; the number n of symmetrical or not branches (70, 71, 72, 73) per main branch and per branch being equal to n, n being greater than or equal to 2; the main branches with their branches being regularly distributed in the volume of the ring and spaced apart from each other.
  2. 2. Pastille (6) according to claim 1, the number p being between 1 and 7, the number n being equal to 2.
  3. 3. Pastille (6) according to one of claims 1 or 2, the main branches with their ramifications being straight or curved.
  4. 4. Pastille (6) according to one of the preceding claims, the main branches with their ramifications being of square, rectangular, circular or elliptical cross section.
  5. 5. Pellet (6) according to one of the preceding claims, each of the main branches and its ramifications extending radially to the outer diameter of the pellet.
  6. 6. Pastille (6) according to one of the preceding claims, the ratio between the section of each branch and that of each main branch being less than 1.
  7. 7. Pellet (6) according to claim 6, the section of each main branch being less than 0.5 mm 2 , preferably less than 0.1 mm 2 .
  8. 8. Pastille (6) according to one of the preceding claims, the ratio between section and length of each of the main branches and its ramifications being less than 0.5 mm, preferably less than 0.1 mm.
  9. 9. Pellet (6) according to one of the preceding claims, the material of the main branches and its ramifications being chosen from Molybdenum (Mo), niobium (Nb) or their alloys, preferably an NbZr alloy or an NbZrC alloy.
  10. 10. Pellet (6) according to one of the preceding claims, the fissile material of the right cylinder being chosen from uranium (IV) oxide (UO2), mixed oxide (U, Pu)O2 or a mixed mixture based on uranium oxide and reprocessed plutonium oxides (MOx).
  11. 11. Pastille (6) according to one of the preceding claims, the volume percentage of the main branches with their ramifications being between 1 and 20%.
  12. 12. Nuclear fuel needle (1) extending in a longitudinal direction (XX') comprising: a plurality of nuclear pellets (6) according to one of the preceding claims, stacked on top of each other; a sheath (2) made of material transparent to neutrons surrounding the stack of pellets.
  13. 13. Needle (1) according to claim 12, the sheath being made of zirconium alloy, in particular Zircaloy-4 (Z), or M5® alloy (ZrNbO).
  14. 14. Use of a nuclear fuel pellet (6) according to one of claims 1 to 11 or of a nuclear fuel needle (1) according to claim 12 or 13 in a fast neutron reactor (RNR), in particular cooled by liquid metal, such as liquid sodium (RNR-Na), a pressurized water reactor (PWR), a boiling water reactor (BWR).

Description

Description Title: Nuclear fuel pellet incorporating a thermally conductive insert in the form of ramified branches distributed from a hollowed-out inner cylinder towards the outside of the pellet. Technical field The present invention relates to the field of fuel elements for nuclear reactors, in particular Fast Neutron Reactors (FNRs), including those cooled by a liquid metal, in particular by liquid sodium (FNR-Na). More specifically, it is located in the field of ceramic fuels made from uranium or uranium and plutonium oxide (U, PujCh or MOX (acronym for “mixed oxide”). The invention essentially aims to improve the thermal properties of these fuels as well as the reduction of gaseous swelling, both in nominal operation and in incidental or accidental conditions (for example in situations of increase in power or loss of refrigerant). By "nuclear reactors", throughout the application, is understood the usual meaning of the term to date, namely power plants for producing energy from nuclear fission reactions using fuel elements in which fissions occur which release the heat power, the latter being extracted from the elements by heat exchange with a heat transfer fluid which ensures their cooling. By "nuclear fuel rod", throughout the application, we understand the official meaning defined for example, in the dictionary of Nuclear Sciences and Technology, namely a narrow tube of small diameter, closed at both ends, constituting the core of a nuclear reactor and containing fissile material. Thus, a "nuclear fuel needle" is a nuclear fuel rod but whose terminology is used for fast neutron reactors. Although described with reference to the application to Fast Neutron Reactors (RNR), including those cooled by a liquid metal, in particular by liquid sodium (RNR-Na) but also lead, lead-bismuth, etc., the invention relates to fuel elements which can be dedicated to all types of reactors for electrogenic, calogenic or experimental, such as Boiling Water Reactors (BWR), Pressurized Water Reactors (PWR) and all advanced 3rd and 4th generation reactors. Previous technique Nuclear reactors that use fission energy to produce heat can be classified into several different categories based on their characteristics: the form of final energy produced (electricity, heat, etc.), the type of neutron flux (fast neutrons or thermalized neutrons), the coolant used (liquid metal, water, etc.), the physical state of the coolant (liquid or gaseous), the pressure level of the coolant, etc. Fuel assemblies intended for use in liquid sodium-cooled fast neutron reactors (RNR-Na) have a specific mechanical structure, in particular to allow liquid sodium to pass through them. Figures 1 and 1A show a fuel assembly 1 conventionally used in an RNR-Na nuclear reactor. Such an assembly 1 of elongated shape along a longitudinal axis X firstly comprises a tube or casing 10 of hexagonal section, closed and sealed around the perimeter, the upper portion 11 of which forms the gripping head of the assembly and houses an upper neutron protection device (PNS), and the central portion 12 of which envelops fuel needles 100. The portions 11, 12 form the same tubular envelope 10 or housing of identical hexagonal section over its entire height. The head 11 of the assembly has a central opening 110 opening into it and used for its handling. The central portion 12 of an assembly comprises a plurality of nuclear fuel needles 100. Each needle 100 is in the form of a sealed cylindrical sheath tube made of steel and closed at both ends by a welded cap inside which is stacked a column 14 of fissile fuel pellets within which the nuclear reactions that release heat occur. All the columns 14 define what is usually called the fissile zone which is approximately located halfway up an assembly 1. The sheath of the needles 100 thus constitutes the first barrier of containment whose integrity it is very important to preserve by protecting it from external aggressions such as mechanical shocks/constraints or excessive temperatures. This power is evacuated towards the cold source of the primary circuit (for example the liquid cooling sodium) by encountering a certain number of thermal resistances which can be synthesized as follows: - the low thermal conductivity of the fissile material consisting of an oxide ceramic which leads to a strong thermal gradient between the center and the periphery of the pellet; - the thermal resistance of the clearance between the pellet and the needle sheath; - radial thermal conduction through the sheath; - the resistance due to the radial convective exchange between the external face of the sheath and the heat transfer fluid. Figure 2, from publication [6] gives the order of magnitude of the temperatures in a fuel pellet in nominal operating mode. There are different ways to improve the power evacuation function of a (U,Pu)O2 fuel pellet or pellet stack. Since some characteristics can hardly be modified, such as the primary fluid or the c