A flow sensor for determining the velocity and direction of a fluid flow including a substrate, a heat source located on the substrate, and a first and a second heat sensor located on the substrate to detect at least a portion of heat generated by the heat source. The plurality of IO lines may have different lengths and the shortest IO line from among the plurality of the IO lines may be adjacent to a longest IO line from among the plurality of the IO lines. A material is dispersed within pores of the porous silicon matrix, and the material has two stable states. may be used to determine what spacer types at which locations result in the best operating conditions and margins for example fuel bundles.