Abstract In lakes and reservoirs, variability among sediment samples can mask temporal changes in radionuclide inventories. For Pond B, an abandoned reactor cooling reservoir in South Carolina, USA, we determined if a decline in the sediment 137 Cs inventory beyond radioactive decay could be detected over a 10-yr interval. Because the 95% confidence interval for the decay-corrected change in inventory (−7.7% to +0.8% yr −1) included 0, we could not conclusively determine that a change in inventory beyond radioactive decay had occurred. Given the sample size of 30 pairs of sediment cores, the minimum change that could be detected reliably would have been −5.8% yr −1. By contrast, we estimate that the average export of 137 Cs in surface water over the 10-yr interval was <0.6% yr −1. For lakes and reservoirs with low rates of radionuclide export, hundreds to thousands of sediment cores may be required to detect changes in radionuclide inventories beyond radioactive decay over time spans less than several decades.