Formation and development of a surface layer in the BBER-440 fuel core

Abstract

For the thermophysical calculation of a fuel element, it is necessary to know the radial distribution of energy release in the fuel core, which influences the temperature field in the field core [1], and the consequent radial distribution of burnup in the fuel core. It is especially important to know the radial distribution of fuel burnup for average (over the transverse crosssectional area of the fuel element) burnup exceeding 45-50 MW.day/kg, when it starts to exceed 60-75 MW.day/kg in a thin peripheral layer. This level of burnup is the threshold from the standpoint of the formation of an easily distinguishable, altered, microstructural region at the boundary of the fuel tablet --the so-called rim layer [2]. This layer is characterized by the presence of many small gas bubbles, vanishing of the initial grainy structure, and formation of new, much smaller, grains (less than 1 ~tm).The surface layer strongly influences the integral characteristics of the fuel element as a whole. The fuel in this layer is characterized by a sharp decrease of the thermal conductivity and strength and a substantial intensification of the yield of gaseous fission products, including the athermal yield. The formation and development of such a layer results in the production of a thermal barrier in the path of the heat from the fuel and degradation of thermal conductivity of the fuel--casing gap accompanying the escape of gaseous fission products. As a result, the fuel temperature increases substantially, the total yield of gaseous fission products increases, and in consequence the pressure of the gas medium under the casing increases significantly; in the case of burnup along the fuel element exceeded 60 MW.day/kg, the pressure could become a determining factor influences the working capacity of the fuel element.In the present work, we calculated the radial distribution of energy release and burnup in the fuel core as a function of the burnup averaged over the transverse cross section. In the calculations, the transverse cross section of the fuel core v, as divided into 20 zones of equal area and, depending on the burnup, the change in the nuclide composition of the fuel and the relative energy release were determined in each zone.In the fuel elements of power reactors (with low-enriched fuel), the change in energy release at the initial moment of operation is small and is caused by the decrease in the thermal-neutron flux density as a result of absorption of the neutrons primarily in the outer layers of the fuel. With burnup, however, plutonium, which is formed as a result of resonance absorption of the moderated neutrons by 238U nuclei, accumulates. This accumulation is most rapid in the outer layer of the fuel. The redistribution of the fissioned isotopes along the radius of the fuel core increases the nonuniformity of energy release [1].The neutron-physical characteristics of two types homogeneous BBER-440 assemblies were calculated, using the fuel assembly program [3] in application to a core with 349 fuel assemblies with initi...

Prediction of the Material Composition of the VVER-type Reactor Burned Pellet with Use of Neutron-Physical Codes

“…For example, MIE-2017 area with modified microstructure, so-called rim-zone appears in the outer layer of the fuel rod of thermal-neutron reactors. Its formation is associated with high concentration of fission products [5][6][7]. Therefore, application of constant concentrations over the whole fuel rod can lead to distortion of the simulation results.…”

Consistent neutron-physical and thermal-physical calculations of fuel rods of VVER type reactors

“…For example, area with modified microstructure, so-called rim-zone appeares in the outer layer of the fuel rod of thermal-neutron reactors. Its formation is associated with high concentration of fission products [6][7][8]. Therefore, application of constant concentrations over the whole fuel rod can lead to distortion of the simulation results.…”

Simulation of neutron-physical processes in the surface layer of a fuel kernel

“…One such process gives rise to an edge zone with an easily distinguishable altered microstructure on the periphery of the fuel pellets. The structure of the edge zone is characterized by the presence of many fine gas bubbles, vanishing of the initial grain structure, and formation of new, much smaller subgrains (<1 µm) [1,2].It has been found experimentally that an edge zone starts to form in uranium dioxide fuel pellets in thermal reactors at burnup higher than 45-50 MW·days/kg. This layer is 100-200 µm thick and is characterized by much higher burnup than the main part of the fuel pellet.…”

“…One such process gives rise to an edge zone with an easily distinguishable altered microstructure on the periphery of the fuel pellets. The structure of the edge zone is characterized by the presence of many fine gas bubbles, vanishing of the initial grain structure, and formation of new, much smaller subgrains (<1 µm) [1,2].…”