Demonstration of the effect of impuritie

This paper reports and discusses the oxygen sensors tests achieved on the STELLA loop in 2006. Sensors are foreseen to be used in liquid lead alloys for use either as a nuclear coolant or as liquid spallation target in ADS system for nuclear waste transmutation. These tests validated the sensor housing design over efficient operation for 3000 hours. The issue of the time drift is now understood: the fouling of the zirconia ceramic by impurities led to the creation of an isolated liquid volume between the ceramic and the sheath, which presents a different chemical behaviour, in particular with the release of the corrosion products from the sheath. Fouling is caused by the presence of impurities in excess, which was the initial conditions of the tests, combined to the ceramic sheath. The technique for in-situ calibration used for assessing the effect of impurities gave preliminary results that are promising for checking the good behaviour of the sensor during operation, which would increase the reliability. Finally, the understanding of the liquid metal chemistry, notably during the degraded conditions, which happen at one moment or the other (maintenance, repair, incident) increased significantly. The coolant chemistry control of the lead alloys remains complex and critical for the operation of a nuclear system, but prospects for control were identified, and the oxygen monitoring system is now more reliable, although studies must be continued.