Designing for stability in a natural circulation based pressure tube type boiling water reactor

Full text of publication follows: Stability design of NC BWRs, where several different instability mechanisms can be simultaneously active is somewhat involved and is not well documented. Strictly speaking, natural circulation systems shall be analyzed for all known instabilities. However, well-established analysis procedures do not exist for all instabilities. Fortunately not all instabilities are commonly observed in a natural circulation system. Design is usually carried out for the commonly observed instabilities like the Ledinegg and density wave instability (DWI). Even so, DWI can be in-phase instability or out-of-phase instability and both can be neutronically coupled. For design, the first issue is which of them is controlling? Probing calculations are necessary to identify this and generate the corresponding stability map which is the locus of all neutrally stable points. Generally the stability map, which delineates the stable and unstable zones, is established by a linear stability analysis. Characteristic of the neutrally stable point is that it oscillates with the same amplitude and hence the reactor cannot be operated at these points. Now the question is how far away from the neutrally stable point can we operate the system so as to avoid instability? A low decay ratio was considered as an adequate and sufficient indicator of stability. However, recent instability events have raised questions regarding the adequacy of the decay ratio as an indicator of stability. This is particularly true in case of dual oscillations where the in-phase and out-of-phase modes are simultaneously active. As a result, there is a discontinuity in the decay ratio. Reported studies also suggest that an operating point with a small decay ratio can be closer to instability than an operating point with a larger decay ratio. Instability can also be obtained on the stable side of the linear stability boundary. Besides, there is evidence from simple loop facilities that the instability threshold is not a unique value, but depends on the operating procedure due to the hysteresis phenomenon. Also, the same neutrally stable condition when approached with different operating procedures can have different amplitude of oscillations. With certain operating procedures even the neutrally stable condition can be reached harmlessly since the amplitude of oscillations is small. Due to these, the operating line concept is followed in AHWR design to avoid instability. In this concept first the stable and unstable zones are identified using the linear stability method. Then an operating line through the stable zone is chosen such that it is sufficiently away from the instability thresholds. Subsequently, nonlinear analysis covering the entire operating line including the start up, power raising and step back is carried out to establish its stability and operational margins. After partial validation by tests in simulated integral test facilities nonlinear analysis is used to predict the neutronics effect and to revise the operational margins if required. The present paper describes the operating line concept adopted in AHWR for the stability design. (authors)