Contact Author:
Gerhard Sauer
gerhard.sauer@tuev-sued.de
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Gerhard Sauer
TÜV SÜD Industrie Service GmbH
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München, 80997
Germany
Estimation of the influence of plutonium agglomerates in MOX fuel on the pellet temperature
G. Sauer, W. Besenböck (TÜV SÜD Industrie Service GmbH)
Estimation of the influence of plutonium agglomerates in MOX fuel on the pellet temperature
G. Sauer, W. Besenböck
TÜV SÜD Industrie Service GmbH, Westendstraße 199, D-80686 München, Germany
Abstract
In MOX fuel the plutonium is generally not homogenously distributed. Many investigations using α-autoradiography and electron probe microanalysis have shown that the plutonium is concentrated in agglomerates. The size of these agglomerates ranges from 1 µm to 100 µm with an average size of about 30 µm. Due to the concentration the plutonium content in the agglomerates may be several times higher than the average plutonium content specified for the fuel. Consequently, the plutonium content outside the agglomerates is very small; in this region only few and very small Pu particles are found. One can therefore conclude that the fuel pellet consists of two regions of different composition: The areas belonging to the first region are filled with (U,Pu)O2 particles and the areas of the second region by UO2 particles.
The heterogeneous distribution of the plutonium has consequences for the heat generation in the fuel. When such fuel is freshly inserted in a reactor the heat is almost exclusively pro-duced in the Pu agglomerates. The heat developed in the UO2 region is rather small com-pared to that of the Pu agglomerates since depleted uranium is used as matrix for the MOX fuel. In the course of irradiation the heat production in the UO2 particles increases due to the formation of Pu from neutron capturing in 238U and partly fissioning of the newly formed Pu isotopes. At the same time the fission rate in the Pu agglomerates decreases because of the consumption of fissile Pu isotopes. It is easy to conceive that the uneven heat production influences the fuel temperature. The impact of the uneven heat production is further amplified by the reduced heat conductivity of PuO2.
In usual computer codes for the thermal and mechanical design of fuel rods the fuel is treated as homogenous substance. The power generation in the fuel can be described as function of the fuel radius in these codes but it is impossible to simulate hot spots that may develop in the Pu rich areas. The assumption of fuel homogeneity may lead to an underesti-mation of the fuel temperature. This underestimation can be corrected by adjusting the fuel heat conductivity coefficient provided the impact of the hot spots on the fuel temperature is known. This paper is concerned with the evaluation of possible correcting factors.
The temperatures in the MOX fuel with heterogeneously distributed Pu are computed with a planar finite element model. The elements that represent the Pu agglomerates are randomly selected. The amount of Pu heterogeneity in the fuel is varied between a Pu agglomeration in 10% of the fuel up to a homogeneous Pu distribution. By decreasing the heterogeneity the Pu is distributed in more agglomerates and thus the number of agglomerates increases. The increase of agglomerates is accompanied by a reduction of the Pu content in the agglomer-ates. The reduction of the Pu content leads to changes of the heat conductivity of the ag-glomerates. Simultaneously, the local power generation in the fuel alters. Further changes result from the consumption of Pu in the agglomerates and the formation and fissioning of Pu in the UO2 parts of the fuel during irradiation. All these changes are accounted for by special models. The variation of heat conductivity in dependence on the initial Pu content is, for in-stance, described by a flexible heat conductivity relation. This relation allows the prescription of individual heat conductivity coefficients to agglomerates with different PuO2 and UO2 con-tent. The local generation of power in the fuel is computed on the basis of the actual concen-tration of fissile isotopes. How the Pu concentration and the heat generation are affected by burn-up is graphically illustrated.
Since it is aimed to exclusively quantify the influence of Pu heterogeneity on the fuel tem-peratures, other factors influencing the fuel temperature are switched off in the computations. Especially, the heat transfer from fluid to surrounding is held constant, fuel swelling and cracking are suppressed and any burn-up dependent degradation of the fuel conductivity is omitted. These factors would impair the computed temperature and thus the derived adjust-ing factor. A fixed heat generation rate is prescribed in the computations.
The searched adjusting factor is developed by comparing the temperatures obtained for the fuel with heterogeneously distributed Pu to those computed for homogeneous MOX fuel. It is shown that the phonon term of the heat conductivity coefficient for homogenous fuel should be modified by this adjusting factor to capture the effect of the heterogeneous distribution of Pu in MOX fuel when performing design computations with a code in which the fuel is mod-elled as homogenous material. The impact of the temperature increase in such MOX fuel on design parameters is exemplarily shown for the gas release from the fuel centre. This exam-ple stresses the need to either consider the Pu clustering in MOX fuel in the design process or to avoid such agglomeration by using a fuel production process that ensures a homoge-nous distribution of Pu in the fuel.