[en] A systematic approach is described for defining the accuracy required in measuring Pu-contaminated solid waste streams and for the selection of the optimal monitor. The proposed method is based on comparing the inaccuracy of waste measurements with the uncertainty in the material unaccounted for resulting from plutonium inventory-taking. Results from measurements of waste streams in plutonium reprocessing and fabrication plants are demonstrated and discussed. The measurements were performed with a variable dead-time counter which detects the spontaneous fission neutrons of 240 Pu