End point determination for spent nuclear fuel drying operations

When spent nuclear fuel is being dried ahead of either interim storage or long term disposal it is necessary to be able to confirm that the required level of dryness has been achieved. This has typically involved a vacuum rebound test however this method has certain limitations in terms of both reliability of the result and also by introducing an additional step that is time consuming and depending on the drying system used requires additional equipment at high system cost. It would be preferable if the end point and requisite dryness could be confirmed from online data readings recorded during the drying process. This paper presents the results of a number of vacuum drying tests using a benchtop drying rig in which online dew point, temperature, pressure and mass flow rate readings were compared to the results of vacuum rebound tests. Mass flow rate, pressure and dew point readings all showed cliff edge behaviour as the drying process progressed. Flow rate provided a good indicator of progress however it was clear from the behaviour that the resolution of the instrument was not sufficient to confirm dryness. Neither pressure nor dew point readings alone were capable of indicating whether a test would be passed successfully however it was found that in combination an envelope existed in which a vacuum rebound test was always passed. Testing of the same techniques for flowed gas drying methods was limited due to the lack of a suitable way of confirming whether dryness was achieved however there was an indication that dew point measurements would be capable of confirming that a set level of dryness had been achieved.