University of Wisconsin-Madison
NE 428 Nuclear Reactor Laboratory
April 30, 2018
In operating power reactors, the neutron population generates a large amount of heat. This causes the temperature of the system to change in addition to its material densities, due to thermal expansion. Due to macroscopic cross sections being proportional to densities and temperatures, neutron flux spectrum depends also on the density of moderator, these changes in turn will produce some changes in reactivity. An important parameter in determining the operational characteristics and safety of nuclear reactors is the temperature coefficient of reactivity (). This parameter is important in reactor design, because the reactivity feedbacks influence the stability of the reactor. It will in addition assist in assuring appropriate reactor power boundaries so that the reactor does not reach supercritical (>1).
The temperature effects on reactivity in the University of Wisconsin-Madison Nuclear Reactor (UWNR) are studied while at a supercritical state. The UWNR was pulsed with multiple reactivity insertions, this defines conditions which establish stable heat generation. The delayed neutron fraction (), and the prompt neutron lifetime (lp) assist in determining the performance of a pulsating reactor.
Pulsating the reactor requires a large amount of energy which needs to be produced in a short period of time. The Fuchs-Nordheim model is used to quantitatively predict this reactor power during transient. Using this model, it is assumed that the delayed neutron effect can be neglected, and the reactor is adiabatic. Under these assumptions the period, pulse width, asymptotic period, peak power, and total energy released were measurements obtained from the power trace data. The Fuchs-Nordheim model of reactivity feedback of the prompt neutron lifetime and the negative temperature coefficient of reactivity was computed. Two methods of the Fuchs-Nordheim model were used to calculate the negative temperature coefficient. These models produced values which were relatively close 118.9 C and 119.26C. The computed values for the delayed neutron fraction and the prompt neutron lifetime were fairly close in comparison to the theoretical values of 0.007 and second presenting an error of less than 8%. The Fuchs-Nordheim model proved to be valid, as it qualitatively provided information about reactor pulses.
The focus of this experiment is to determine two parameters, the prompt neutron lifetime (lp) and energy coefficient of feedback reactivity (). The Fuchs-Nordheim model of reactivity is used to determine these parameters, in addition to the negative temperature coefficient ().
The assumptions used to complete this model are accomplished due to the reactor self-limiting power excursion, which occurs during a short period of time.
It is assumed that all heat loss is negligible (adiabatic approximation), the resulting change in...