What is the purpose of perturbation theory in reactor physics?

• A. To exactly re-solve the full transport problem for each change.
• B. To estimate large, nonlinear changes in the system.
• C. To estimate the effect of small changes in system parameters on k_eff or reaction rates without full re-solution.
• D. To measure the physical dimensions of the reactor.

Answer: (C)

Perturbation theory in reactor physics is about predicting how small changes in system parameters affect quantities like k_eff or reaction rates without re-solving the full transport problem each time. You start from a known reference state with a calculated eigenvalue and flux, then linearize the response to small parameter changes. The result is a first-order estimate of how sensitive k_eff is to things like cross-section tweaks, temperature shifts, or density changes. This approach is valuable because it avoids the heavy computation of re-solving the full problem for every variation. It works best for small changes where the linear approximation holds; for large or strongly nonlinear changes, the estimate can be off. It’s not about measuring physical dimensions, and it doesn’t require redoing the full solution for every change.