Abstract: During the production and operation of some bulk handling facilities, nuclear material deposits remaining after cleaning in and about process equipment, interconnecting piping, filters and adjacent work area or equipment, which is called holdup. In order to meet the requirements of nuclear material accounting management, the physical inventory of nuclear materials shall be carried out according to the specified frequency, and the amount of nuclear materials shall be the measured value. Quantitative measurement of holdup in equipment is one of the technical difficulties in nuclear material measurement for nuclear facilities. Holdup measurement is important for radiation protection, critical safety and nuclear material management. The valve is a key component to control the material transfer in the process of nuclear material production. The valve is welded to pipe, which is not conducive to sampling. In this paper, mass of uranium in the valve was measured by γ-ray spectroscopy. It is necessary to study absolute detection efficiencies of the detector for known source distribution. Firstly, the passive efficiency calibration was selected to simulate the absolute detection efficiencies. Analysis models consist of detector model and valve models, they were placed under the same observation system by the transforming coordinate method. Analysis models of detector and valves were built based on information of the detector and design drawing of the valves. Two point sources, ⁵⁷Co and ¹³⁷Cs, were used to verify the model. Under the combination of different crystal radius and length parameters, the absolute detection efficiencies for full-energy γ-rays peak were simulated. The crystal parameters of the LaBr₃ detector were adjusted. The root mean square of the relative deviation between the simulation results and the experimental results was calculated. The crystal parameters with the smallest root mean square are the best parameters. The relative deviation between the experimental results and Monte Carlo simulation results for point sources is within 5%. The standard uranium sample was used to carry out experiments. The count rate of characteristic γ-ray was measured by the LaBr₃ detector. The absolute detection efficiencies were simulated by the analysis model. The holdup was calculated according to the relationship between count rate of characteristic γ-ray and the mass of uranium. The results show that the relative deviation between the measured results and the reference values of uranium mass is within 15%. The feasibility of the method for measuring the uranium holdup in the valves is preliminarily confirmed by the experiments. The transforming coordinate, a simple and easy solution, by which the analysis model can be quickly and accurately built.