Improving safety in a MOX plant by qualification of neutron measurements and criticality evaluation

Two of the major aspects of safety in a MOX fuel fabrication plant are radiation doses and criticality prevention. This paper presents two qualification processes which lead to improvements of the safety when handling and storing fissile materials. 1. Radiation protection of workers depends very much on the reliability of personal dosimeters. As a result of the introduction of the ICRP 60 recommendations and the increasing contribution of the neutron dose to the total dose of the Belgonucleaire personnel of the MOX fuel plant, the bubble detector became in use as a reliable personal neutron dosimeter. To evaluate the impact of the neutron energy on the detector response, real neutron spectra in the plant were determined by a two-sided approach. Measurements were carried out in rooms where representative neutron spectra were present, by means of a reference calibrated neutron spectrometer, in parallel with some types of personal bubble detectors. On the other hand, detailed calculations of the measured configurations were performed by means of the Monte Carlo MCNP 4 computer code with a 3D model. Comparison between measurements and calculations reveals good agreement. Additional calculations clarify the influence of some parameters as the presence of structure materials. The calibrated calculations allow also to separate the contribution of the low energy neutrons in the total dose rate in order to qualify the method of correcting the calibration factors of the bubble detectors on the basis of the typical spectra, in relation with the last ICRP fluence-to-dose conversion coefficients. 2. Criticality safety is a permanent issue for a MOX fuel fabrication plant. In order to reduce calculations uncertainties and improve consequently the safety while trying to reduce margins, BELGONUCLEAIRE is promoting the new KEOPS international R and D Program to be performed in collaboration with the Belgian Nuclear Research Centre (SCK·CEN), located close to the MOX fuel fabrication plant of BELGONUCLEAIRE. The Safeguard department of SCK·CEN has developed for many years an original neutron-counting device, based on double and triple neutron coincidence counting. The K-effective of the measured configuration (which can be as large as a 2001 waste drum) can be derived from neutron multiplication measurements with high accuracy. The KEOPS programme will consist on subcritical measurement of effective multiplication factors (Keff) of several materials available in a MOX fuel fabrication facility: pure PuO2 powders, primary blends containing 30% PuO2 and final product (fuel pellets). Simple and clean geometrical configurations, such as box or cylinder will be considered. (J.P.N.)