Research on Isotope Dilution Gamma Spectroscopy for Determining Plutonium Concentration

Abstract

Preliminary validation of isotope dilution gamma spectroscopy (IDGS) for plutonium concentration measurement was investigated in this study. A high 239Pu abundance plutonium solution was used as an isotope dilution agent. By studying the changes in isotopic abundance of 241Pu and 239Pu before and after adding the dilution agent to plutonium working sample, and combining it with high-purity germanium gamma spectrometry, a method for measuring plutonium concentration and isotopic composition using IDGS technology was established. In this experiment, the concentration and isotopic composition of the plutonium diluent working standard and the plutonium working sample were accurately calibrated by the controlled potential coulometry and mass spectrometry separately. Different gamma ray pairs with similar energy from appropriate energy regions were selected to calculate the isotopic abundance ratio, and the efficiency calibration was performed on this gamma ray pairs. The optimal volume and concentration of the plutonium gamma measurement standard solution were explored and determined. Using IDGS technology, the test samples with plutonium concentration of 2 g/L in the nitric acid plutonium solution and the abundance range of 239Pu from 57% to 70% were measured. The results show that within 1.5 h, the RSD of IDGS technology for plutonium concentration measurement is ≤0.68%, and the measured values are in good agreement with those measured by the Coulomb method, with deviations from 0.3% to 2.3%. When measuring the isotopic composition, the RSD of 241Pu/239Pu is ≤0.46%, and the RSD of 239Pu abundance is ≤0.66%. The RSD of the measurement results for two parallel samples of the same plutonium sample is ≤0.98%. With the optima dilution ratio range of 1∶4 to 4∶1, a small amount of dilution agent could achieve good consistency with the Coulomb method measurement results, but beyond this dilution ratio range, the deviation of the measurement results from the Coulomb method is larger. These results indicate that using domestically high-quality plutonium samples to prepare diluent working standards, adopting portable medium-high energy gamma spectrometry to directly measure liquid plutonium sources, and selecting the 241Pu/239Pu atomic ratio to determine plutonium concentration by the IDGS technology is feasible. It simplifies the sample preparation process, shortens the measurement time, and reduces the analysis costs. Compared with the isotope dilution mass spectrometry, IDGS technology is easier to operate and has a shorter analysis cycle. Compared with the hybrid K-edge method, it can simultaneously measure both plutonium concentration and isotope composition. Therefore, this technology can be used for rapid measurement of plutonium concentration in spent fuel solutions, providing technical support for the management of nuclear materials in our spent nuclear fuel reprocessing plants in the future.

Keywords

• plutonium
• isotope dilution method
• gamma spectroscopy