Summary Of Recent Investigations Related To Predictions Of The Early Die-away Time Tau From The DDSI Instrument

The Differential Die-Away Interrogation (DDSI) instrument detects neutrons in coincidence and is sensitive to the content of fissile material and neutron absorbing material in a fuel assembly. Its response to spent nuclear fuel (SNF) with varying properties was investigated under the Next Generation Safeguards Initiative (NGSI) and the efforts included simulations as well as the manufacturing of a prototype instrument that was successfully tested in the field. Due to the interest of applying machine-learning techniques to support safeguards verification of SNF, we have investigated how to speed up the predictions of the early die-away time tau from the DDSI instrument for a large SNF inventory. One way to do this has been to develop a parametrization function for tau as a function of the fuel parameters initial enrichment, burnup and cooling time. However, in order to assess the validity of the function it has been necessary to investigate the sensitivity of the DDSI response to the various parameters in the depletion calculations of the underlying SNF such as e.g. boron concentration, fuel pin geometry and operational history. In this work, we summarize the recent efforts made to resolve these questions.