  The ability to infer the reactor flux from spent fuel or seized fissile
material would enhance the tools of nuclear forensics and nuclear
nonproliferation significantly. We show that reactor flux can be inferred from
the ratios of xenon-136 to xenon-134 and cesium-135 to cesium-137. If the
average flux of a reactor is known, the flux inferred from measurements of
spent fuel could help determine whether that spent fuel was loaded as a blanket
or close to the mid-plane of the reactor. The cesium ratio also provides
information on reactor shutdowns during the irradiation of fuel, which could
prove valuable for identifying the reactor in question through comparisons with
satellite reactor heat monitoring data. We derive analytic expressions for
these correlations and compare them to experimental data and to detailed
reactor burn simulations. The enrichment of the original uranium fuel affects
the correlations by up to 3 percent, but only at high flux.
