Neutron Flux Measurements, Comparison of Experimental and Theoretical Results

Neutron Flux Measurements, Comparison of Experimental and Theoretical Results  

One part of the FDR zero power experiments consisted of measuring the neutron flux distribution in three dimensions. For this task aluminum tubes were inserted in the experimental quarter of the core parallel to the fuel rods (Fig. 12) inside these tubes semiconductor detectors with Li converter foils could be moved up and down. The a-particles emerging from the reactions Li (n,a) H induce pulses in the biased circuits of the semiconductors, these pulses being counted in a counting device 34). The measurements all over one quarter of the core resulted in an experimental Li absorption rate distribution (20, 34). 

To establish a comparison of experimental and theoretical results, a theoretical absorption rate distribution in Li was calculated. The calculations for this theoretical absorption rate distribution followed as closely as possible the methods used in the nuclear and thermal design of the FDR reactor (see Section IV). Thus it was possible to verify the codes and analytical methods, which had been used for the calculation of the power distribution in the FDR core, by an experimental absorption rate distribution. In this verification of analytical methods by experimental tests the following are the principal ideas both distributions— the power distribution, which is of main interest in the hot reactor, and the absorption rate distribution, which had been measured in the cold reactor—are determined by a reaction rate, the fission rate of in the first case, the absorption rate of Li in the second case. If the analytical methods give an absorption rate distribution for Li in good agreement with the experiment, the same methods may be assumed to give a correct fission rate distribution, i.e., power distribution. 

This argument takes into account that the two distributions are very similar and that nearly the same considerations are essential in the calculation of the two distributions. 

The determination of a power distribution for the hot reactor from the detector counting rates measured in the experiment for the cold 

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