The collapse of spark-induced cavitation bubbles in a flowing system was studied by means of high speed photography. The migration of cavitation bubbles toward a nearby solid boundary during collapse and rebound was observed. Near its minimum volume the bubble typically formed a high speed microjet, which struck the nearby surface causing individual damage craters on soft aluminum.

Rocket sled data have been used to evaluate statistically best values for threshold velocity and velocity exponent as well as the coefficient n in the relation: MDPR=K(Vsinθ-V0)α/sinnθ It is found that a reasonable assumption for most materials is that threshold velocity is zero, n = 1, and that there is a rough correlation between K and α. The correlations with the data are relatively poor. However, within the basic precision attained, a single figure of merit for the material in terms of either K or α could be derived easily. A more basic relation for the evaluation of MDPR has been assumed in terms of the energy flux model. In this relation the material properties are described in terms of an energy per volume term describing material failure. It has been found that the material energy term is best described as proportional to ultimate resilience. No correlation with any efficiency term relating liquid and material properties alone has been found.

Westinghouse is currently conducting the pre-application licensing of the International Reactor Innovative and Secure (IRIS) on behalf of the IRIS Consortium. One of the key aspects of the IRIS design is the concept of safety-by-design. The PRA is being used as an integral part of the design process. As part of this effort, a PRA of the initial design was generated to address two key areas. First, the IRIS PRA supported the evaluation of IRIS design issues by providing a solid risk basis for design and analyses required for the pre-licensing evaluation of the IRIS design. The PRA provides the tool for quantifying the benefit of the safety-by-design approach. Second, the current PRA task is beginning the preparation of the more complete PRA analyses and documentation eventually required for Design Certification. One of the key risk-related goals for IRIS is to reduce the EPZ to within the exclusion area by demonstrating that the off-site doses are consistent with the US Protective Action Guidelines (PAGs) for initiation of emergency response so that the required protective actions would be limited to the exclusion area. The results of the preliminary PRA indicated a core damage frequency of 1.2E−08 for internal initiators. This is a very good result but much work is needed to meet the ambitious goal of no emergency response. The next phase of the PRA analyses will involve a two-fold expansion of the PRA. First, as the design and analyses approach a greater level of detail, the assumptions used for the initial PRA will be reviewed and the models will be revised as needed to reflect the improved knowledge of the system design and performance. Furthermore, as the full plant design advances, the PRA will be expanded to incorporate risk associated with external challenges such as seismic and fire, and to address low power and shutdowns modes of operation. As with the initial work, the PRA will serve as a tool to help improve, not just to evaluate the design.

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