On the Effect of Systematic Errors in Near Real Time Accountancy

Systematic measurement errors have a decisive impact on nuclear materials accountancy. This has been demonstrated at various occasions for a fixed number of inventory periods, i.e. for situations where the overall probability of detection is taken as the measure of effectiveness. In the framework of Near Real Time Accountancy (NRTA), however, such analyses have not yet been performed. In this paper sequential test procedures are considered which are based on the so-called MUF-Residuals. It is shown that, if the decision maker does not know the systematic error variance, the average run lengths tend towards infinity if this variance is equal or longer than that of the random error. Furthermore, if the decision maker knows this variance, the average run length for constant loss or diversion is not shorter than that without loss or diversion. These results cast some doubt on the present practice of data evaluation where systematic errors are tacitly assumed to persist for an infinite time. In fact, information about the time dependence of the variances of these errors has to be gathered in order that the efficiency of NRTA evaluation methods can be estimatet realistically.