In pyroprocessing it is important to determine the amount of Pu in the various streams of materials involved. This paper presents two approaches to determine the Pu mass of spent fuel assemblies using nondestructive assay and burnup simulation code. Cm balance is adopted and the concept of "Cm ratio," the mass ratio of Pu to Cm, is used for the nuclear material accountancy for the model pyroprocessing facility. The biggest error of the nuclear material accountancy is expected to arise from the determination of Pu mass and Cm ratio in input homogeneously mixed uranium oxide powder, which is assayed nondestructively. One approach to determine the Pu mass and Cm ratio is to apply the average burnup of spent fuel and determine the Pu mass and Cm ratio by using the ORIGEN code. The estimated error in Pu mass determined by this method ranges from 0.94% to 2.33% for a total of 225 spent fuel assemblies of various burnup, initial enrichment, and cooling time. The other approach is to use the functional relationship between the neutron emission rate and Pu mass of spent fuel. The error in Pu mass calculated using this method ranges from -1.68% to 3.86%.