Neutron Transport Theory
Neutron transport theory is concerned with the transport of neutrons through various media. As was discussed neutrons are neutral particles, therefore they travel in straight lines, deviating from their path only when they actually collide with a nucleus to be scattered into a new direction or absorbed.
Transport theory is relatively simple in principle and an exact transport equation governing this phenomenon can easily be derived. This equation is called the Boltzmann transport equation and entire study of transport theory focuses on the study of this equation. In general, the neutron balance can be expressed graphically as:
The Boltzman transport equation is a balance statement that conserves neutrons. Each term represents a gain or a loss of a neutron, and the balance, in essence, claims that neutrons gained equals neutrons lost. It must be added it is much easier to derive the Boltzmann transport equation than it is to solve it. Deterministic methods solve the Boltzmann transport equation in a numerically approximated manner everywhere throughout a modeled system. This task demands enormous computational resources because the problem has many dimensions. But the Boltzmann transport equation can be treated in a rather straightforward way. This simplified version of the Boltzmann transport equation is just the neutron diffusion equation. Naturally, there are many assumptions that have to be fulfilled when using the diffusion equation, but the use of the diffusion equation usually provides sufficient accurate approximation to the exact transport equation.
Nowadays reactor core analyses and design can be performed using nodal two-group diffusion methods. These methods are based on pre-computed assembly homogenized cross-sections and assembly discontinuity factors (pin factors) obtained by single assembly calculation with reflective boundary conditions (infinite lattice). Two methods exist for calculation of the pre-computed assembly cross-sections and pin factors.
- Deterministic methods that solve the Boltzmann transport equation.
- Stochastic methods that are known as Monte Carlo methods that model the problem almost exactly.