Steam drum process dynamics and level control of a pressure tube BWR

The Advanced Heavy Water Reactor (AHWR) is a pressure tube type light water cooled heavy water moderated Boiling Water Reactor (BWR) with natural circulation at all power levels. It has parallel inter-connected loops with 452 boiling channels in the Main Heat Transport (MHT) system configuration. These multiple (four) interconnected loops influence the Steam Drum (SD) level control adversely. Such a behavior has not been reported in the open literature. The MHT configuration has been chosen based on comprehensive overall design requirements and certain Postulated Initiated Events (PIEs). This does not allow the partitioning of the Common Reactor Inlet Header (CRIH). If partitioning of the CRIH into four segments is allowed then, it will make each loop independent. Then the SD level control problems subside as the unaccounted interaction among the loops is eliminated. It has also been observed that the open loop response is stable, non-oscillatory and non-diverging for a step change in the feed flow rates. A conventional individual 3-element SD level controller cannot account for the highly coupled and interacting behaviors, of the four loops and SD levels. To overcome these interactions it is proposed to interconnect all the four steam drums in the liquid and vapor regions respectively. The influence of the interconnect configuration and the level controller are studied in detail to find a robust solution. The response obtained for unsymmetrical core power disturbances shows that the SD levels do not diverge and quickly settle to the set points assigned with SD interconnect. The proposed scheme also works well for most of the PIEs considered. (orig.)