Time Delay of Thermal Neutron Detection during Power Burst in TRACY

Abstract

Time delay of thermal neutron detection at a micro fission chamber has been experimentally confirmed during an initial power burst caused by reactivity addition in the Transient Experiment Critical Facility (TRACY). In the experiment, the power was measured by using a fast response γ-ray ionization chamber, which was newly designed exclusively for the TRACY experiment, in addition to the fission chamber. As results of the experiment, the transient power measured by the fission chamber was observed about 4 ms later than that by the γ-ray chamber during the power increasing, and then the delay time expanded after the power peak. The time delay must be taken into account to evaluate accurately the transient power. To comprehend the observed time delay in detail, time-dependent analyses were performed using the MCNP4B code. In the analyses, time response of neutron detection at the fission chamber was calculated and used for the simulation of the transient power measured by the fission chamber. From results of the analyses, it was confirmed that the simulated power well agreed with the measured power and the observed time delay was caused from the thermalization of neutrons emitted at the TRACY core and the following flight to the fission chamber.

KEYWORDS:

• time delay
• micro fission chamber
• initial power burst
• reactivity addition
• TRACY
• transient power
• fast response gamma-ray ionization chamber
• MCNP4B
• time response
• thermal neutrons
• radiation detection