Fig. 7-6 Measured and calculated Doppler effect in a mockup core for a prototype oxide-fueled FBR

The Doppler effect is important as it introduces a negative reactivity insertion mechanism in the transient behavior of a Fast Breeder Reactor. To improve the prediction accuracy of the safety characteristics, including the Doppler effect during over-power events, new devices were developed at the Fast Critical Assembly (FCA) to measure the Doppler effect at temperatures up to 2000 degrees cent. Two methods were used: The first is a reactivity worth measurement of a sample heated up to 1500 degrees cent. and the other is a reaction rate measurement of a foil heated by a laser up to 2000 degrees cent. The construction of these experimental measurement devices required especially highly skilled techniques to satisfy severe limitations, the device's small size and its high surface temperature. For the analysis, a new cell calculation code with an ultra-fine group structure, PEACO-X, was developed to improve the calculation accuracy. The code can precisely analyze the resonance interaction effects between the 238U of the sample and the tungsten of the experimental measurement device. The PEACO-X code predicts the measured Doppler effect better than the conventional code. Fig. 7-6 shows the measured and the calculated results in a mock-up core for a prototype oxide-fueled FBR. The calculation for the reactivity worth measurement slightly underestimates the Doppler effect. The calculated foil Doppler effect was within the measurement error of the measured value. These measurements are to be continued using various cores at the FCA to obtain the neutron spectrum dependence of the high-temperature Doppler effect.

Reference S. Okajima et al., Doppler Effect Measurement up to 2000 degrees cent. at FCA, Proc. of Int. Conf. on Nuclear Data for Science and Technology at Galin- burg, USA (1994), 1009.

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