Kinetics Parameters and Reactivity Measurement Experiments
Introduction To analyze reactor kinetics, we need to know the kinetics parameters of the reactor. Also, we build the reactor, operate it in the critical state, and determine the reactivity of each control rod. This chapter explains the primary steps of th
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Kinetics Parameters and Reactivity Measurement Experiments Yoshiaki Oka
Introduction To analyze reactor kinetics, we need to know the kinetics parameters of the reactor. Also, we build the reactor, operate it in the critical state, and determine the reactivity of each control rod. This chapter explains the primary steps of these operations.
4.1
Critical Approach Experiment (Inverse Multiplication Coefficient Method)
The operation to increase the reactivity of subcritical reactor and to set the reactor to the critical state is called the critical approach. The first critical approach of the reactor just constructed is called the “initial critical approach.” The critical approach has the following steps: Add the fuel gradually Reduce the soluble toxicant concentration of the moderator Increase the moderator or reflector If the subcritical reactor has no neutron source, its fission chain reaction is dropped quickly. If the reactor has the neutron source, the neutron flux is constantly formed according to the intensity of the source and the subcritical reactivity as shown by Eq. (2.35). The inverse number of subcritical reactivity 1 k is written by M as follows: M¼
1 1k
(4.1)
M1 is called the inverse multiplication factor. When it closes to the critical state, value k closes to 1 and, therefore, M1 closes to zero. In the critical approach procedure, we place multiple measurement systems in each of the reactor and add the fuel gradually around the neutron source. As the Y. Oka and K. Suzuki (eds.), Nuclear Reactor Kinetics and Plant Control, An Advanced Course in Nuclear Engineering, DOI 10.1007/978-4-431-54195-0_4, # Springer Japan 2013
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Y. Oka
Fig. 4.1 Concept of critical approach with inverse multiplication factor
ratio of counting rate Ci of each step to the initial counting rate C0 is an inverse multiplication coefficient, we plot the inverse multiplication factor in the figure by setting the fuel amount on the horizontal axis. We extrapolate it and determine a predictive value of fuel amount to have the zero C0/Ci ratio. In the next step, we assume the amount of loaded fuel is half of the difference from the predicted value and we repeat this operation. The concept of this procedure is shown in Fig. 4.1. Because we use multiple neutron detectors, we can obtain the predictive value from each plot. We use the minimum one. The extrapolation points match closely to the critical point. In the initial critical approach, we do not know the fuel amount that causes the reactor to enter the critical state. Therefore, we need to perform the above procedure carefully. If the subcritical reactor does not have a neutron source, the neutron flux is not formed and neutron counting by the fission chain reaction is not obtained. Although different curve-lines are plotted depending on the detector position, the extrapolation points match each other in the vicinity of critical point. If the reactor is in a subcritical state, we need to insert the neutron source into the core and make the critical approach. This
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