A Continuous Sampling Plan for theTaking of Physical Inventories

Whenever it is necessary to take a physical inventory for the purpose of establishing the amount of special nuclear material contained in a particular material bal- ance area two kinds of measurements are required; a bulk measurement (weight or volume) of the items containing the material and analyses to determine chemical and/or isoto- pic composition. One method used to establish the amount of SNM on hand at inventory time involves determining that all of the items appearing in the records are physically present, weighing (or measuring the volume) a subset con- taining a given composition of material and sampling this subset on a random basis for analysis. Although the weigh- ing is accomplished quickly and inexpensively the analysis is relatively slow and expensive and so the number of sam- ples drawn is generally small compared to the total number of items. Even so, the time required to do chemical and isotopic analyses results in a considerable delay before a conclusion can be reached regarding the total amount of SNM in inventory. In addition, if all inventory strata are established simultaneously, a heavy load is placed on the analytical laboratory at the time of inventory taking, thus further increasing the delay. In this paper a method is presented that would eliminate this time lag by doing the sampling on a continuous basis, with minimal effect on production, while the items in inventory are being shipped out and replenished. Although this weakens the confidence one has in the knowledge of the amount of SNM at the end of the accounting period, it strengthens the confidence in the intervening interval during which such samples are ordinarily not taken. The smoothing out of the intermittent load on the analytical laboratory that results from the con- tinuous sampling procedure is in itself a desirable bonus.