• nuclear reactor safety
• mixing
• in-vessel flow
• CFD
• boron dilution
• assessment
• validation

The present research aims at contributing to the CFD code assessment process for
nuclear reactor applications, and particularly for the predictive analysis of the fluid dynamic
phenomena occurring inside the reactor pressure vessel of a pressurized
water reactor. The importance of such phenomena relies, for instance, on the
influence that they can have on the spatial and temporal distribution of coolant
properties (such as temperature or boron concentration) at the core inlet during
certain accident transients involving perturbations of such properties with respect to
nominal conditions; furthermore, in-vessel mixing phenomena can also affect the
thermal interaction between coolant and pressure vessel during pressurized
thermal shock scenarios.
The contribution provided by this work consists in the proposal of a general and
systematic methodology to be applied in the CFD code assessment for in-vessel
flow investigations. Within the proposed approach, the relevant modelling issues
are identified and discussed, so as to point out the main capabilities and limitations
in the present state-of-the-art tools and methods. Then, the main steps of the code
application procedure are described and discussed analytically, thus providing
guidance for a quality-oriented use of the codes, and complementing the existing
best practice guidelines for this specific problem.
Furthermore, the research addresses the problem of the quantification of the
accuracy for numerical predictions (both from CFD and integral codes) about
coolant properties perturbations at the core inlet. As a result, a methodology is
proposed based on a set of accuracy indicators, which can represent a means for
judging whether the code results are sufficiently close to experimental data, once
acceptance thresholds have been defined and the method has been completely
assessed.
The work is supported by extensive CFD code validation and application results
obtained in the frame of several international research projects and co-operations,
and by a continuous interaction with the involved International scientific community.