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

Andrianov, Anatolij N.; Баранов, В. Г.; Tikhomirov, G. V.; Khlunov, A. V.

doi:10.1007/s10512-008-9056-3

Cited by 7 publications

(1 citation statement)

References 3 publications

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“…Recentemente tem havido um grande interesse em medidas de distribuição das taxas de reação nuclear no interior de varetas combustíveis [1,2,3]. Esse interesse surgiu pela necessidade de compreender de forma efetiva o comportamento da auto-blindagem gerada pelas ressonâncias dos nuclídeos 235 U, 238 U, 239 Pu e 240 Pu nas próprias pastilhas do combustível nuclear [4,5].…”

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Determinação experimental de taxas de reação no 238U e 235U ao longo do raio da pastilha de UO2 do reator IPEN/MB-01

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This research presents and consolidates an alternative methodology for determining nuclear reaction rates along the radial direction of the fuel pellets which does not require high neutron flux. This technique is based on irradiating a thin UO 2 disk inserted into a removable fuel rod at the IPEN/MB-01 reactor core.Several gamma spectrometries are performed after irradiation using a HPGe detector. Six lead collimators with different diameters are sequentially alternated during this process, thus, the nuclear radioactive capture which occurs in 238 U and the fissions which occur in both 235 U and 238 U are measured according to six different radial regions of the fuel disk. Geometric efficiency corrections due to the introduction of collimators in HPGe detection system are determined by MCNP-5 code. The fission rate measurements are performed using the 99 Mo. This radionuclide was studied and proved ideal for these measurements because it is formed in linear behavior in the reactor core, have a high yield fission and emits low-energy photons.Measurements were performed irradiating UO 2 disks (with 4.3%enrichment) in the central position of the IPEN/MB-01 core at 100 watts power level during one hour. Some measurements were performed using a cadmium glove wrapped in the fuel rod to determine the nuclear reaction rates in the epithermal energy range. The experimental results obtained are compared with nuclear reaction rate calculations by means of MCNP-5 with ENDF/B-VII.0 data library showing discrepancies of up to 9% in 238 U capture rates and 14% for U fission rates for epithermal energies. Uncertainties regarding the nuclear capture rates have maximum values of 4.5% and the fission rates has maximum values of 11.3%.

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Determinação experimental de taxas de reação no 238U e 235U ao longo do raio da pastilha de UO2 do reator IPEN/MB-01

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Simulation of nuclear-physical processes in the surface layer of a fuel kernel with a consumable absorber

et al. 2008

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The structure, density, and chemical and phase state of nuclear fuel change considerably with deep burnup. In the present work, the neutron-physical processes in the peripheral layer of a fuel kernel with a consumable absorber are simulated. It is shown that plutonium will actively accumulate in a 50-200 µm thick surface layer. The burnup depth in this layer will be much greater than the average value over the fuel assemblies. A combined fuel element with an absorber where the central part has the usual enrichment (4-5% 235 U) and does not contain consumable absorbers and the peripheral layer consists completely of the consumable element is proposed to decrease the influence of consumable absorbers on fuel properties. Particularities of the Burnup of Nuclear Fuel with a Neutron Absorber.The profitability of power reactors depends on how efficiently the nuclear fuel is utilized, which is determined by burnup. The burnup of off-loaded fuel is increased mainly by increasing the initial enrichment and by using absorbers. In VVER-1000, fuel to which Gd 2 O 3 has been added is used for these purposes [1]. As a rule, gadolinium is present in only a small fraction of the fuel elements (6 or 12 of 312) in a fuel assembly. To calculate accurately the change of the isotopic composition of fuel containing the consumable absorber, it is necessary to calculate the burnup of each fuel element in the fuel assemblies in the reactor. Such calculations require complex computer programs and powerful computers [2]. However, simpler models can be used to obtain estimates. For this, the following must be taken into account:• the isotopic composition of the natural gadolinium consists of seven stable isotopes (mass %): 152,154,155,156,157,158,160 Gd 0.2, 2.2, 14.8, 20.5, 15.7, 24.8, and 21.8, respectively; • only 155 Gd and 157 Gd burn up; they have a large radiative capture cross section; as they burn up, 155 Gd transforms into the stable isotope 156 Gd and 157 Gd into the stable isotope 158 Gd. 156 Gd and 158 Gd have a relatively small radiative capture cross section; thus, the total amount of gadolinium in nuclear fuel does not change much; the largest differences between the cross sections for 155 Gd, 157 Gd and 156 Gd, 158 Gd are found in the thermal range; their absorption cross sections are several thousand times larger; • gadolinium burnup, which affects the multiplication factor of a fuel assemble, is completed at 10-15 MW·days/kg. Simulation of the Burnup of Nuclear Fuel with a ConsumableAbsorber. Different mathematical models are used to simulate the change of the neutron-physical characteristics and isotopic composition of spent nuclear fuel during the operation of a reactor. The choice of the model depends on the problem under study. All models can be divided into three classes: at the level of a fuel element (cell) or fuel assembly and full-scale models of the core. As a rule, the sim-

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Numerical modeling of the temperature distribution in a VVER fuel element

et al. 2010

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The temperature distribution in a VVER fuel element with deep burnup of nuclear fuel is studied. Numerical and analytical methods are used. It is shown that a stationary temperature distribution is established in no longer than 1 min. Analytical methods are used to obtain the temperature dependence of the radius of a fuel element in a stationary regime.One way to realize down trending fuel-cycle costs in nuclear power is to increase the burnup of heavy nuclei. Higher burnup is desirable from the standpoint of a positive solution to economic questions as well as to draw high-enrichment uranium and plutonium into large-scale nuclear power, since the transition to deep burnup and long fuel run is impossible without increasing the enrichment of or adding plutonium to uranium fuel [1].With deep burnup of oxide nuclear fuel, a large change occurs in the structural-phase state, chemical composition, and density of the fuel kernel of a fuel element [2]. As fission products accumulate, the oxygen potential changes and new localized phases precipitate [3,4] and the thickness of the most highly damaged surface layer of a fuel pellet increases [5,6]. In the peripheral layer, the initial grains with typical size ~15 μm disperse under irradiation into 0.2-0.3 μm subgrains and low-density dislocations. The result is that a fine-grain structure with elevated porosity of quite large size (micron) forms in the surface layer of the pellets as burnup increases and fission products accumulate. For example, with average uranium burnup 105 GW·days/tons the local burnup in the surface layer of a pellet reaches 300 GW·days/tons. In the process, a so-called ultra-high burnup structure [7-9], where the pore size reaches 15 μm as a result of coalescence or migration of bubbles, forms, increasing the local porosity to 22% [10].The thermophysical processes in the fuel composition and in a fuel element as a whole are of scientific and practical interest. For oxide nuclear fuel, as burnup increases, the thermal conductivity decreases noticeably as a result of the accumulation of soluble fission products and the formation of radiation-induced defects due to the action of fission products. This decrease of the thermal conductivity is especially strongly manifested at temperatures below 1000 K.The strongest changes can occur in the surface zone. It is important to know the particulars of and regularities in the variations of the state and properties of the fuel composition to refine the fuel element design, choose the operating regimes of the reactor, and predict the behavior of fuel intended for deep burnup.For a fuel element as a source of nuclear energy, the thermal conductivity of the materials and heat transfer in the radial direction are important but difficult to determine experimentally. For this reason, it is of interest to perform a theoretical analysis and modeling of the heat transfer taking account of the surface layer, appearance and growth of an oxide layer on the zirconium cladding, and decrease of the gap between the fuel composit...

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

Cited by 7 publications

References 3 publications

Determinação experimental de taxas de reação no 238U e 235U ao longo do raio da pastilha de UO2 do reator IPEN/MB-01

Determinação experimental de taxas de reação no 238U e 235U ao longo do raio da pastilha de UO2 do reator IPEN/MB-01

Simulation of nuclear-physical processes in the surface layer of a fuel kernel with a consumable absorber

Numerical modeling of the temperature distribution in a VVER fuel element

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