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The temperature measurements of mixed oxide (MOX) and UO 2 fuels during irradiation suggested that the thermal conductivity degradation rate of the MOX fuel with burnup should be slower than that of the UO 2 fuel. In order to explain the difference of the degradation rates, the quasi-two phase material model is proposed to assess the thermal conductivity degradation of the MIMAS MOX fuel, which takes into account the Pu agglomerate distributions in the MOX fuel matrix as fabricated. As a result, the quasi-two phase model calculation shows the gradual increase of the difference with burnup and may expect more than 10% higher thermal conductivity values around 75 GWd/t. While these results are not fully suitable for thermal conductivity degradation models implemented by some industrial fuel manufacturers, they are consistent with the results from the irradiation tests and indicate that the inhomogeneity of Pu content in the MOX fuel can be one of the major reasons for the moderation of the thermal conductivity degradation of the MOX fuel.

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This paper considers the generation of almost‐periodic oscillation and chaos which occur in a parametric excitation circuit having an external force, and the region in which they occur. In a resonance circuit, if one of the reactances is excited with a frequency which is almost twice the resonance, a subharmonic parametric oscillation of one‐half order, i.e., the parametric oscillation for short, is sustained. If an external force with frequency near the resonance is applied to the circuit, the parametric oscillation disappears. The characteristics of this oscillation disappearance are analyzed by using the nonlinearized Mathieu equation with a forcing term which describes this system. For this we assume a solution containing only two harmonic components and apply the method of harmonic balance. In this paper, using a more detailed investigation of the characteristics of the oscillation disappearance, it is clarified that chaotic phenomena occur in a part of the response curves. Advancing the forementioned method, we obtain a set of four autonomous differential equations. Plotting the solution curve of these equations in the phase plane, we may investigate the behavior of this system. Consequently, it is clarified that under proper condition of circuit parameters, the solution curve tends to a limit cycle or chaos, by only several percent variation of the frequency of the external force. A limit cycle is correlated with an almost‐periodic oscillation. Therefore, the behavior of this system is revealed in more detail.