Correction factors to apply to fission rates measured by miniature fission chambers in various neutron spectra
CEA develops and makes use of miniature fission chambers (MFCs, with radius down to 1.5 mm) for reactor physics conducted in experimental reactors such as EOLE and MINERVE zero power reactors (CEA Cadarache). When measuring fission rate, it is known that the neutron spectrum in the irradiation channel can be modified by the detector and the detector fixture. So the result of the measurement does not give a direct access to the desired quantity (fission rate, neutron flux,etc.) To overcome this problem, it is possible to make use of Monte Carlo calculations based on a detailed modeling of the detector. It could then be included in the 3D reactor model but this leads to large and time consuming calculations. In this case, measurement results can be combined directly with calculated values to produce the desired quantity. Another possibility is to calculate correction factors to apply to the biased measurement, i.e. to perform two-step calculations. Those factors depend on the detector geometry, the neutron spectrum and the fissile isotope at stake. A method to determine those factors is presented in this paper. The previously calculated neutron spectrum is fed to a simplified calculation route that includes only the detector and its close environment. Correction factors are obtained from two calculations results (with and without the detector fixture). In this case, the measured fission rates are corrected before being further processed. This paper details a parameter study on the impact of MFC parts and its environment (cable, connector) on the observed fission rate. Precise models of CEA-made MFCs have been developed for that purpose and used to produce correction factors for various fissile isotopes and neutron spectra. It is shown that fission rates can be greatly underestimated because of neutron radiative capture in MFC parts close to the fissile coating (9% in the worst case). The impact on standard reactor physics measurements is then discussed.