On the average chord length in reactor physics

By elaborating on the meaning of the average chord length in reactor physics, it is shown that the average chord length for a convex body in an isotropic flux is given by the bodyʹs volume divided by its average projection area. The relation is known in literature (Weinberg, A.M., Wigner, E.P., 1958. The Physical Theory of Neutron Chain Reactors./The University of Chicago Press, Chicago), but in view of a recent technical note on the average chord length by Sjöstrand [Ann Nucl Eng 29 (2002) 1607] it seems useful to explain the background of the simplicity of the average chord length for a convex body in an isotropic flux. For another angular flux distribution the average chord length cannot be expressed in such an elegant way, and has to be calculated for each body (and orientation) separately.