A new system for in-core wide range neutron monitoring

For in-core reactor application, we have developed a new system in order to measure the thermal neutron flux from 107 n.cm^-2.s^-1 up to 1014 n.cm^-2.s^-1 at a temperature of 310degC. For this purpose, a new fission chamber with an external diameter of 3 mm has been developed. To obtain the wide range, the single detector operates continuously in the three modes, pulse counting, Campbelling or mean-square-voltage measurement and current. The new electronics system is built to work with these 3 modes simultaneously. Its design is based on massive use of digital technology and advanced digital signal processing. To transmit the signal from the fission chamber to the electronic cabinet, we have used a mineral coaxial cable with typical characteristics specially designed in order to operate in pulse mode and an external diameter of 1.4 mm to be introduced into the core of the reactor. Experiments were carried out in the ISIS and OSIRIS reactors, both experimental facilities available on the CEA Saclay site. These experiments have shown the capability of the system to obtain a linear measurement of the neutron flux from 106 n.cm^-2.s^-1 up to 2.1014 n.cm^-2.s^-1 at a temperature of 37degC. The next step, planned at the end of the year 2004, is to operate at the temperature of 310degC during 1000 hours.