Pressure drop measurements at the KNK II annular fuel element with replaceable test bundles TOAST

An annular fuel element with replaceable 19-pin test bundles for the Tolerance Enlargement Study TOAST is planned to be operated in the third core of KNK II. Due to different reasons, the up to now used coolant mixing devices in the element head and the element foot had to be revised. Therefore the pressure drops had to be measured again to support the thermal-hydraulic design of this special subassembly and its individual components. Additionally the hydraulic suppression of the test bundle in the annular element had to be demonstrated. In parallel to that, measurements with models of original size were performed in the water circuit of the IRB. The following results can be pointed out: 1. The pressure drop of 0.2 bar of the modified mixing head at nominal flow is comparable to that of the previous design. 2. The measured total pressure drop of the annular element with test bundle is 1.37 bar at nominal flow. It agrees within 3 % with the precalculated value. The sufficiently exact precalculation of the orifices in the test bundle and the coolant partition have been confirmed also by the experiments. 3. The hydraulic suppression of the test bundle in the annular element and consequently of the total subassembly in the core assembly is guaranteed