Investigation of the neutron fields in the nuclear power plants and of the influence of their diversity on the determination of radiological protection quantities

A multisphere neutron spectrometer has been developed and used in order to characterize the neutron fields inside the Swiss nuclear power plants. Prior to the use of the Bonner spheres system, its response matrix has been determined. This has been evaluated by computation using the ANISN particles transport code and a recent condensed cross section library. The matrix has been then extended up to 400 MeV. The adjustment of the calculated functions to the experimental calibration points, established in a previous work, has lead to coherent results. To determine the neutron spectrum from the spheres indications three unfolding codes, based on different mathematical algorithms, have been used in order to validate the obtained results. These codes have been tested by means of a variety of synthetised spectra. Both the spectrometric and dosimetric results of this test have been satisfactory. The sensitivity of the unfolding procedure to the measurement statistics and to the number of spheres used has been investigated. It has been established that with reasonable counting rates ( a few thousands pulses) the spectrometric and dosimetric results are reliable. Moreover it is possible, in the case of the neutron fields encountered in the nuclear power plants, to reduce, when necessary, the number of spheres used without losing too much spectrometric information. The unfolding of the measurements performed in reference beams lead to neutron spectra in good agreement with ISO standard. The dosimetric values obtained match well with the reference values and this results validates both the unfolding procedure and the response matrix. The measurements performed in the nuclear power plants allowed the characterization of the radiation fields inside the containments. (author) figs., tabs., refs