Determination of Kinetics Parameters Using Stochastic Methods in a ²⁵²Cf Driven System

Abstract

Safety analysis and control system design of nuclear systems require the knowledge of neutron kinetics related parameters like effective delayed neutron fraction, neutron lifetime, time between neutron generations and subcriticality margins. Many methods, deterministic and stochastic, are being used, some since the beginning of nuclear power, to measure these important parameters. The method based on the use of the ²⁵²Cf neutron source has been under intense study at the Oak Ridge National Laboratory, both experimentally¹ and theoretically,² during the last years. The increasing demand for this isotope in industrial and medical applications and new designs of advanced high flux reactors to produce it make the isotope available as neutron source (only few micrograms are necessary). A thin layer of ²⁵²Cf is deposited in one of the electrodes of a fission chamber which produces pulses each time the ²⁵²Cf disintegrates via α or spontaneous fission decay; the smaller pulses associated with the α decay can be easily discriminated with the important result that we know the time when v _c neutrons are injected into the system (number of neutrons per fission of ²⁵²Cf). Thus, a small (few cm³) and nonintrusive device can be used as a random pulsed neutron source with known natural properties that do not depend on biases associated with more complex interrogating devices like accelerators. This paper presents a general formalism that relates the kinetics parameters with stochastic descriptors that naturally appear because of the random nature of the production and transport of neutrons.