Reactimeter

The ''reaetimeter'' is an apparatus by which the reactivity in a reactor, particularly during reactivity transients, can be measured directly and instantaneously. The apparatus has a measuring range from ± 50 x 10-5 to ± 5000 x 10-5 . TJie accuracy of measurement is ± 1% for a power range of about 2.5 decades. From the behaviour of the reactor, given by Nordheim equations, the Pagels circuit enables one to obtain a current i = Kpn, where p is the reactivity and n the neutron flux. The Pagels circuit consists of resistor and capacitor subcircuits representing neutron families. An electronic divider enables one to obtain the reactivity term p ∼ (kpn/n) directly. The apparatus is housed in a 3U2 drawer subchassis and consists of a measuring amplifier, Pagels circuit, electronic divider, and LT and HT supply for the compensated ionization chamber. Comparative studies were carried out on the basis of conventional measurements and an accuracy of the order of ± 1% was obtained on the following reactors: Alizé (CEN Saclay), Azur (CEN Cadarache) and Siloëtte (CEN Grenoble). When the period of the reactor is positive no precautions need to be taken with regard to the positioning of the chamber. When the period is strongly negative (shut-down) it is sufficient to arrange the chamber in such a way that part of the core is between the rod being studied and the chamber. In this way rod anti-reactivity can be measured with an accuracy of ± 1%. The ''reaetimeter'' has several applications: reactor safety, measurement of reactor poisoning; and evaluation of rod anti-reactivity with reactor power rising or falling. (author)