Reduction of fluctuation and small bias observed in continuous volume monitoring taken in an annular tank for plutonium nitrate

Accurate measurement of density, level and volume by dip-tubes in an annular tank for plutonium nitrate is indispensable for material accountancy and safeguards in a reprocessing and conversion plant. The principle of this method is to measure true hydrostatic pressure at the tip of the tube, which postulates that solution in a tank is resting (motionless) during measurement. Such condition is satisfied by short time interruption of agitation at the measurement to determine plutonium mass. However, it is very difficult to continue such condition, because solution is being agitated for safety reasons. Therefore, the solution is circulated and causes some fluctuation and possibly some small bias on hydrostatic pressure measurement. The width of fluctuation and small bias could reach around one-third percent that depends on configuration of dip-tubes, airflow rate of agitation, solution level, and various supports inside the tank. As a result, volume monitoring data does not reflect correctly the amount of solution in a tank. Continuous monitoring data is used for operator to find a sign of unexpected event such as clogging of dip-tube or pipe leakage, and for inspector to check undeclared operation. So, it is strongly desired to improve accuracy and resolution of tank monitoring. Fig. 1a is the example of volume monitoring data. The effect of agitation stop appears coincidentally and symmetrically in density and volume. This is the result of volume calculation procedure. Fig. 1b is the example of mass monitoring simply calculated by volume multiplied by density. During the period shown in Fig. 1, plutonium nitrate solution was stored and IIV was carried out. The day before IIV, operator interrupted agitation very limited time and measured density, volume and temperature necessary for declaration. Inspector verified these data at IIV in the same manner followed by sampling. During sampling, solution was circulated between tank and sampling line using airlift and vacuum system, thus the amount of solution in the tank decreased. After sampling, solution temperature decreased a little, so density increases and volume decreases. However, the mass did not change because the volume of sample was very little (∼ 0.03L). The mass decreased only due to evaporation (∼ 0.1L/day). So, it was confirmed in Fig. 2b that mass monitoring is free from agitation and also free from temperature change. Mass monitoring reflects correctly the amount of solution in a tank. The reason of such bias and fluctuation is that the effect of solution flow on hydrostatic pressure measurement around the density tube (minor tube) is larger than the effect around the level tube (major tube) which is set close to the deepest point where solution flow is nearly stagnated shown in Fig. 2. The effect of solution flow is recognized as the change in excess pressure due to bubbling at the tip of the dip-tube. The excess pressure varies, for example, from 10 Pa to 60 Pa and integrated average is around 50 Pa in motionless water. The solution flow can shift easily the average in the order of ten Pa. The effect of solution flow on pressure measurement is not compensating between the major and the minor, so measured density has a small bias and fluctuation. As a result of calculation procedure, measured volume also changes. However, the mass calculated by volume multiplied by density is roughly proportional to the major pressure measured at the stagnated point, so the small bias and fluctuation is reduced. The uncertainty of density pressure measurement has a decisive influence not only on conventional volume measurement but also on continuous volume monitoring