Nuclear fuel technology - Tank calibration and volume determination for nuclear materials accountancy - Part 4: Accurate determination of liquid height in accountancy tanks equipped with dip tubes, slow bubbling rate

ISO 18213 deals with the acquisition, standardization, analysis, and use of calibration to determine liquid volumes in process tanks for the accountancy of nuclear materials. This part of ISO 18213 is complementary to the other parts, ISO 18213-1 (procedural overview), ISO 18213-2 (data standardization), ISO 18213-3 (statistical methods), ISO 18213-5 (fast bubbling rate) and ISO 18213-6 (in-tank determination of liquid density). The procedure presented herein for determining liquid height from measurements of induced pressure applies specifically when a very slow bubbling rate is employed. A similar procedure that is appropriate for a fast bubbling rate is given in ISO 18213-5. Measurements of the volume and height of liquid in a process accountancy tank are often made in order to estimate or verify the tank's calibration or volume measurement equation. The calibration equation relates the response of the tank's measurement system to some independent measure of tank volume. Beginning with an empty tank, calibration data are typically acquired by introducing a series of carefully measured quantities of some calibration liquid into the tank. The quantity of liquid added, the response of the tank's measurement system, and relevant ambient conditions such as temperature are measured for each incremental addition. Several calibration runs are made to obtain data for estimating or verifying a tank's calibration or measurement equation. A procedural overview of the tank calibration and volume measurement process is given in ISO 18213-1. An algorithm for standardizing tank calibration and volume measurement data to minimize the effects of variability in ambient conditions that prevail during the measurement period is given in ISO 18213-2. The procedure presented in this part of ISO 18213 for determining the height of calibration liquid in the tank from a measurement of the pressure it induces in the tank's measurement system is a vital component of that algorithm. In some reprocessing plants, the volume of liquid transferred into or out of a tank is determined by the levels of two siphons. The high level corresponds to the nominal volume, and the low level to the heel volume. If the transfer volume cannot be measured directly, then it is necessary to calibrate this volume because the difference between the actual volume and that used for inventory calculations will appear as a systematic error. The ultimate purpose of the calibration exercise is to estimate the tank's volume measurement equation, which relates tank volume to measurement system response. Steps for using the measurement equation to determine the volume of process liquid in the tank are presented in ISO 18213-1. The procedure presented in this part of ISO 18213 for determining the height of process liquid in a tank from a measurement of the pressure it induces in the tank's measurement system is also a key step in the procedure for determining process liquid volumes