Determination of the local void fraction in BWRs from in-core neutron noise measurements requires the knowledge of the axial velocity of the void. The purpose of this paper is to revisit the problem of determining the axial void velocity profile from the transit times of the void between axially placed detectors, determined from in-core neutron noise measurements. In order to determine a realistic velocity profile which shows an inflection point and hence has to be at least a third order polynomial, one needs four transit times and hence five in-core detectors at various axial elevations, whereas the standard instrumentation usually consists only of four in-core detectors. Attempts to determine a fourth transit time by adding a TIP detector to the existing four LPRMs and cross-correlate it with any of the LPRMs have been unsuccessful so far. In this paper we thus propose another approach, where the TIP detector is only used for the determination of the axial position of the onset of boiling. By this approach it is sufficient to use only three transit times. Moreover, with another parametrisation of the velocity profile, it is possible to reconstruct the velocity profile even without knowing the onset point of boiling, in which case the TIP is not needed, although at the expense of a less flexible modelling of the velocity profile. In the paper the principles are presented, and the strategy is demonstrated by concrete examples, with a comparison of the performance of the two different ways of modelling the velocity profile. The method is tested also on velocity profiles supplied by system codes, as well as on transit times from neutron noise measurements.

从堆芯中子噪声测量中确定 BWR 中的局部空隙率需要了解空隙的轴向速度。本文的目的是重新审视根据轴向放置的探测器之间空隙的传播时间确定轴向空隙速度分布的问题，该时间由堆芯中子噪声测量确定。为了确定显示拐点并因此必须至少为三阶多项式的真实速度剖面，需要四个渡越时间，因此需要在不同轴向高度的五个堆芯探测器，而标准仪器通常仅包括四个核内探测器。迄今为止，通过将 TIP 检测器添加到现有的四个 LPRM 并将其与任何一个 LPRM 进行互相关来确定第四渡越时间的尝试均未成功。因此，在本文中，我们提出了另一种方法，其中 TIP 检测器仅用于确定沸腾开始的轴向位置。通过这种方法，仅使用三个传输时间就足够了。此外，通过速度剖面的另一种参数化，即使不知道沸腾的起始点，也可以重建速度剖面，在这种情况下，不需要 TIP，但代价是速度剖面的建模灵活性较低。本文介绍了原理，并通过具体示例演示了该策略，并比较了两种不同的速度剖面建模方法的性能。该方法还在系统代码提供的速度剖面以及中子噪声测量的传输时间上进行了测试。