Thermal anemometry grid sensor for spatially-resolved measurement of gas phase temperature and velocity in fuel element subchannels

Since the Fukushima accident the spent fuel pool has gained some special focus in nuclear safety research, since it is potentially endangered by a longer persisting station black-out or a loss of the coolant due to a leakage in the pool liner. Behind this background the German national joint project SINABEL (SIcherheit NAsslager BrennElement Lagerbecken) has been launched, which targets an experimental investigation and an improved modelling of the thermal hydraulics in fuel element mock-up in the spent fuel pool under accident scenario conditions. The goal is to develop validated simulation tools for the prediction of temperature courses and the support of the emergency management.
Experimental investigations within the project are performed in a mock-up of a 10 x 10 boiling water reactor fuel element. Surface temperatures of the electrical heated rods are measured by means of 134 thermocouples at different locations. Another parameter of interest is the steam flow velocity in the sub-channels. For this purpose no commercial measurement technique is applicable due to limited accessibility and high temperatures. For this reasons a special thermal anemometry grid sensor has been developed. It consists of a grid of thermal resistors in the bundle cross-section with one resistor in each sub-channel. Via the resistors fluid temperature as well as flow velocity can be obtained. Applied measurement techniques are resistance thermometry and thermal anemometry respectively. The measurements are obtained simultaneously by sampling the data with a special multiplexed excitation scheme. Our contribution gives a brief description of the thermal anemometry grid sensor along with first results from studies on a potential natural circulation in the completely dried state.