Explain the concept of adjoint flux in reactor theory.

• A. The adjoint flux is the neutron flux itself.
• B. The adjoint flux is the solution to the adjoint transport equation that weights importance of neutrons to a given response; used in perturbation and sensitivity analyses.
• C. Adjoint flux measures neutron speed.
• D. Adjoint flux is used to compute reactor power only.

Answer: (B)

The adjoint flux represents how important neutrons are to a specific response you care about in the reactor. It is the solution of the adjoint transport equation, which is constructed so that it weights neutrons by their contribution to that chosen response. In practice, you think of it as an importance field: for neutrons at a given point, direction, and energy, the adjoint flux tells you how much that neutron would affect your target outcome (detector reading, reaction rate in a region, etc.).

This usefulness becomes clear in perturbation and sensitivity analyses. When a small change occurs in a cross section or material property, the resulting fractional change in the response can be expressed by integrating the product of the forward neutron flux, the adjoint flux, and the perturbation over space, angle, and energy. So the adjoint flux is not the actual neutron population, nor a measure of speed, and it’s not limited to calculating reactor power; it’s a general weighting function tied to the particular response you’re studying.