Nuclear Criticality Safety Assessment Calculations: Part 1. Calculating Power Histories in Nuclear Excursions.

摘要

We will develop the lumped-parameter model for nuclear kinetics and exploit certain useful approximations of that model. The lumped-parameter model, which eliminates detailed properties of the system by means of same averaging process, is usually called the point-reactor model. As we will see, the equations that describe the point-reactor model depend only on the time variable. Effects produced by geometry or energy of the neutron are contained in the parameters of the point-reactor model. The point-reactor model is satisfactory only when the chain-reacting system is close to critical. This restriction allows us to treat most cases of practical interest. For example, we want to describe how energy is generated during an excursion. Below critical, the power of a chain-reacting system decreases; above critical, it increases. It is difficult to increase the reactivity of a chain-reacting system very much above critical because the energy deposited in the system by the fission process shuts down the excursion. Hence, we only need a dynamic model that is valid near critical. The point-reactor model, of course, cannot describe spatially dependent dynamic effect. Our purpose in describing the point-reactor model is twofold. First, we want to give the reader an understanding of the phenomena involved in an accidental nuclear excursion. How much energy is likely to be produced during an accident. What is the behavior in time of the energy released. What properties of the system determine the shutdown mechanism of the nuclear excursion. Second, we want to give the criticality safety specialist some computational tools for estimating the behavior of an accidental nuclear excursion when the parameters describing the chain-reacting system are not precisely known. 22 figs.