Experiments in LEENA facility with modified wire type leak detector layout in large sodium pipelines

Sodium cooled Fast Breeder Reactors (SFRs) are envisaged in the second phase of Indian nuclear power programme. Liquid sodium is used as the coolant in the SFRs due to its favourable nuclear properties and excellent heat transfer properties. Leaks in sodium systems have the potential of being exceptionally hazardous due to the reaction of liquid sodium with oxygen and water vapour in the air. When a sodium leak occurs, the sodium leak rate, the total quantity of sodium leaked and leak detector layout governs the detection time. Other factors to be considered are insulation material packing condition, distance between the leak point and detector, heater layout, pipe geometry, temperature etc. Potential regions of leakage in Fast Breeder Reactor (FBR) sodium circuits are near welds, high stress areas and regions subjected to thermal striping. Early detection of leak is needed for minimizing the quantity of sodium leaked to outside and consequent damages. Three wire type leak detectors (WLDs positioned at 90°, 180° and 270°) working on conductivity principle are used for detecting sodium leak in the large horizontal secondary sodium pipelines of Prototype Fast Breeder Reactor (PFBR). It was found from the upper boundary curve based on LEENA (LEak Experiments in NAtrium) facility experimental results with present PFBR leak detector layout, that a leak rate of 200 g/h and above can be detected within 6 h. To improve the detection capability, modified leak detector layout was proposed for FBR 800 NB size horizontal secondary circuit pipelines of FBR with seven WLDs positioned at 45°, 90°, 135°, 180°, 225°, 270° and 315° by taking top of pipeline as 0°. It is required to assess the performance of wire type leak detectors along with its layout on large horizontal sodium pipelines and confirm that they are meeting the requirements. LEENA facility was re-commissioned and five sodium leak experiments with modified leak detector layout on large horizontal pipeline were conducted with different sodium leak rates. From the experimental data it was observed that there was considerable reduction in detection time with modified WLD layout. This is due to the reduction in distance between leak point and leak detector (Reduced from 0.63 m to 0.31 m). From the experiments it was found that a leak rate of 200 g/h can be detected in one hour.