Determination of Liquid Density for Nuclear Materials Accountability

Accountability tanks in nuclear facilities are typically equipped with bubbler probe systems for measuring pressure to determine liquid content. With such systems, accurate measurements of tank liquid density--at its measurement temperature--are required to determine the level (height) of liquid in the tank above some reference point with sufficient accuracy for accountability purposes. A method for making in-tank determinations of liquid density that applies to tanks equipped with two or more bubbler probes of differing lengths is discussed in this paper. This method initially requires a careful determination (calibration) of the vertical distance (separation) between the tips of two probes. The calibration must be done with a liquid (e.g., water) whose density is known with sufficiently high precision at all measurement temperatures. An algorithm is presented for computing the probe separation from simultaneous determinations of liquid height above the tips of the respective probes. A formula is given for determining the unknown density of some liquid. The formula is expressed in terms of the estimated probe separation and requires simultaneous measurements of the pressure exerted by the liquid at the tips of the two (calibrated) probes. A measure of uncertainty is provided for the resulting density determination. With state-of- the-art instrumentation and careful application of the proposed computational algorithms, it is possible to make density measurements whose accuracy and precision approach those of measurements made in the laboratory. The methods presented in this paper are based on one of several draft international standards that, taken in aggregate, provide a comprehensive and technically sound set of procedures for tank calibration and volume measurement.