Comparison of Density Equations for Plutonium - Uranyl - Nitric Solution in a Tank and its Application to Find Unexpected Error Sources

Density equations proposed by Maimoni, Sakurai, Cauchetier and Kumar were compared based on the process data and the analytical results taken in the JNC's Plutonium Conversion Development Facility (PCDF). In the plant specific range of plutonium concentration, uranyl concentration, acid concentration and temperature, the density calculated by the Kumar's equation is most consistent with the measured density by the vibrating tube density meter (VTDM) at laboratory or the digital-quartz electromanometer (ELTM) at tank. The difference between calculated value and measured value was 0.2±0.5 % for plutonium nitric solution, and 0.3±0.7 % for plutonium-uranyl nitric solution, whereas the density difference between VTDM and ELTM was negligible (< 0.1 %) after adjusting the temperature difference between tank and laboratory. To adjust the temperature difference, the equation was partially differentiated by temperature, and analytically determined or estimated solute concentrations were put into the equation. The result of partial differentiation is not so sensitive and almost linear to the solute concentration. Therefore, a typical value (0.06 % per degree) is used in PCDF to evaluate quickly the temperature difference between tank and laboratory. Instruments/equipment used to determine accurately the large plutonium inventory in a tank are carefully calibrated and/or checked periodically under the domestic rule and/or the agreements between the government and the IAEA. At inspection, the sample is usually divided and analysed individually at three laboratories and compared each other. However, a small error source may unexpectedly occurs beyond the instruments and leads to a hidden but crucial bias. For example, thin film of water in a sample bottle, precipitated small particles in a sampling line, some residuals of solution in a sampling line, pressure drop due to precipitation in a dip-tube, and so on. It is difficult to find such error sources by means of conventional method, however it is possible to do so by means of comparing VDTM, ELTM and density calculation by Kumar's equation. This comparison could be completed within a day. JNC had started this comparison in PCDF since 2001 at the most important flow KMP