Study On Concentration And Age Measurement Of Uranium Parts

Plutonium and high enriched uranium are the most important fission materials of nuclear weapon. Therefor, these two materials are the emphases of attention in the frame of arms control inspections after warhead disassembly. The detection method of plutonium has been researched roundly and deeply. And now it is hotspot and difficulty question of inspection technology to measure uranium assembly. The enrichment is a basic attribute of uranium material, which is always got attention in the inspections. Age of enriched uranium can offer some history information, which can help to verify whether the inspected nuclear material originates from the appointed nuclear weapon. So uranium age has been concerned increasingly.It is a common nondestructive method by analyzing character gamma rays spectra from nuclear materials. In this method the intensities of relevant nuclides can be obtained. The enrichment of uranium can be measured by activity ratioes among234U,235U and238U. And the uranium age can be determined by the activity ratio betweent daughter and parent as a function of decay time. It is obvious that the activity ratio between different nuclides is the measurement key of enrichement and age. However, the detection efficiency ratio is the most important factor in measuring activity ratio. In order to obtain the detection efficiency ratio, absolute efficiency method and relative efficiency method have been adopted in this paper.In the first, preliminary simulation about uranium assembly has been carried out in the paper and the absolute detection efficiencies of relevant characteristic rays have been offered by Geant4simulation. The uranium enrichment is calculated by absolute efficiencies and measured gamma spectra and the result accords with the reference value.The relative efficiency method is major research in this paper."Uhienr.pst" and "Uhienr.pst" are two parameters for analyzing uranium enrichment providing by PC/FRAM software. Detailed research about these parameters have been carried out in this paple and optimized parameter has been provided. This parameter is applicable to analyse the enrichment of weapons-grade uranium. When the parameter is used, the uranium enrichment can be obtained quickly because the background will be ignored. And the analysis results using the parameter are consistent with the reference value.The investigation of uranium age has also been carried out in the paper. In the first, U-Bi method has been carried out. The advantage and disadvantage of this method have been analysed. The analysis method with close efficiency is brought forward to resolve the shortcoming of U-Bi method. A set of characteristic rays with close energy have been chosed from uranium series and the235U-227Th method has been established. Several gamma spectra of a4kg HEU assembly have been measured and the HEU age obtained from235U-227Th method is50.5years, which is close with the known age of this material.The major production of this paper is to put forward a new method for HEU age determination by measuring the activity ratio227Th/235U. This method only use two characteristic gamma rays, which are236keV of227Th and233.5keV of235U. Because the energy of these rays are very close, the detection efficiencies will be equal approximately. Then the uncertainty induced by relative efficiency fitting will be minished greatly. In addition, this method can’t be influenced by the background. So the method have quick and exact advantage comparing with U-Bi method.