Improved nuclear power plant operations and safety through performance-based safety regulation.

This paper illustrates some of the promise and needed future work for risk-informed, performance-based regulation (RIPBR). RIPBR is an evolving alternative to the current prescriptive method of nuclear safety regulation. Prescriptive regulation effectively constitutes a long, fragmented checklist of requirements that safety-related systems in a plant must satisfy. RIPBR, instead, concentrates upon satisfying negotiated performance goals and incentives for judging and rewarding licensee behavior to improve safety and reduce costs. In a project reported here, a case study was conducted concerning a pressurized water reactor (PWR) emergency diesel generator (EDG). Overall, this work has shown that the methods of RIPBR are feasible to use, and capable of justifying simultaneous safety and economic nuclear power improvements. However, it also reveals several areas where the framework of RIPBR should be strengthened. First, researchers need better data and understanding regarding individual component-failure modes that may cause components to fail. Not only are more data needed on failure rates, but more data and understanding are needed to enable analysts to evaluate whether these failures become more likely as the interval between tests is increased. This is because the current state of failure data is not sufficiently finely detailed to define the failure rates of individual component failure modes; such knowledge is needed when changing component-specific regulatory requirements. Second, the role of component testing, given that a component has failed, needs to be strengthened within the context of RIPBR. This includes formulating requirements for updating the prior probability distribution of a component failure rate and conducting additional or more frequent testing. Finally, as a means of compensating for unavoidable uncertainty as an obstacle to regulatory decision-making, limits to knowledge must be treated explicitly and formally. This treatment includes the formulation of probabilities through expert solicitation and the review of risk-informed, performance-based and engineering analyses used to evaluate proposed changes to existing technical specifications.

Droplet impacts upon liquid surfaces.

The absorption and rebounding of single droplets and streams of droplets (of diameter less than 1200 micrometers) impacting upon the surface of a deep liquid have been examined experimentally. Conservation of mechanical energy and momentum have been used to explain rebounding droplet interactions, and impaction criteria have been established regarding the absorption of droplet streams. Surface tension is the dominant mechanism governing the observed behavior. Single droplets were never observed to rebound.

Light scattering device for sizing and velocimetry of large droplets utilizing a ring-shaped laser beam.

A newly developed light scattering device for sizing and velocimetry of large droplets is described. A novel beam shaping technique is employed in conjunction with a laser operating in the circular TEM(01) (doughnut) mode as a light source to achieve a ring-shaped source beam. This particular beam geometry allows definition of a sheetlike scattering volume with an approximately constant power density across it resulting in improved sizing and sampling accuracy. The size resolution of the instrument is better than 7%, and the overall sizing accuracy for an input size range of 30 dB (droplets of ~45-1500 microm in radius) is ~10%. While the instrument reported in this paper was designed to analyze water droplets entrained in air, it should also be useful in other sizing applications if appropriate modifications are made.