The Core behaviour analysis embraces the nuclear fuel aspects related with its in-core history. The nuclear fuel may experience a wide spectrum of working conditions, ranging from normal to accident conditions. To cover such a large range of parameter variations and to address the different multi-physics and multi-scale phenomena pertaining to the core behaviour, NINE has developed competences and expertise in the following closely interrelated fields:
•    Fuel behaviour, which addresses the thermal-mechanical performance of the nuclear fuel;
•    Reactor Physics, which deals with the power generation and distribution within the whole core as well as in each fuel rod, taking the fuel depletion into account;
•    Subchannel Thermal-Hydraulics, which investigates local phenomena connected with the clad-coolant heat transfer.
Such multi-disciplinary coupled approach allows realistic full-core behaviour analyses. This type of analyses have been carried out by NINE’s engineers in licensing and, more generally, safety assessment frameworks to estimate the number of failed fuel rods during DBA accidents (like reactivity initiated or loss-of-coolant accidents). Various water-cooled reactor designs were investigated, adopting the most advanced computational tools and developing appropriate evaluation models to meet the strictest qualification requirements.
NINE’s engineers are actively participating in several International research programmes organised in the framework of IAEA and OECD/NEA (particularly NSC, WGFS, WGAMA) experts groups, both performing analytical exercises and contributing to technical reports that often become key references for the international community.

NPP thermal hydraulic analysis addresses the prediction of the behaviour and evolution of the single- and two-phase fluid systems encountered in a nuclear power plant (and, more generally, in nuclear installations), in normal operation as well as during incident or accident scenarios, in steady-state as well as in transient conditions. The scope of the analysis generally covers the primary and secondary systems, emergency and auxiliary systems, containment system and balance of plant. The complexity of the system, with the many phenomena and processes involved, requires the use of advanced and complex tools, experienced code-users and robust and systematic procedures to manage a large amount of data. NINE’s engineers have competences and expertise in the following interrelated fields:
•    System Thermal-Hydraulic Analysis, to simulate the TH behaviour of the plant during postulated transients or accidents, taking all the key thermal-hydraulic phenomena into account;
•    Containment Analysis, by which the evolution of the Containment system is investigated and important parameters such as pressure, temperature, distribution of hydrogen and non-condensable gases etc. are predicted;
•    Regulatory Compliance, to prepare and review safety evaluation reports, Technical Specifications and Final Safety Analysis Report submittals;
•    Uncertainty Quantification for Best-Estimate Plus Uncertainty Licensing Applications, by which the uncertainties connected with system code calculations are quantified and associated to the code results.
NINE’s engineers have applied such skills to the preparation and review of safety analyses (particularly within licensing frameworks) of PHWR, PWR, WWER and small reactors, providing services to utilities and regulatory authorities.
The validation of the adopted computational tools is an essential step of a qualified safety analysis process. In this respect, NINE is participating in several international activities (mainly within OECD/NEA and IAEA umbrellas) involving simulation of experiments and code assessment.

Leading-edge approaches to safety analysis and design take advantage of advanced three-dimensional computer simulation tools for problems of both fluid and solid mechanics.
Namely, Computational Fluid Dynamics (CFD) relates to the numerical simulation of flow problems. There are a number of problems relevant to Nuclear Reactor Safety that one can tackle with the help of CFD, with different degrees of maturity; typical examples are: Pressurized Thermal Shock and Boron Dilution scenarios and, in general, problems involving the prediction of flow distribution and mixing in complex domains; estimation of pressure losses; problems involving fluid-solid heat transfer (Conjugate Heat Transfer) and many others. NINE specialists have a long-standing experience with the use of both commercial and research CFD codes and have actively participated in many International projects related to the development and validation of such tools. This area involves also the 3D thermal-hydraulic analysis of Containment systems, by means of CFD-like specialized codes, as well as the development and application of Uncertainty Evaluation methods.
On the Structural Mechanics side, NINE takes advantage of leading-edge Finite Element Analysis tools to support design tasks as well as safety investigation, including problems involving Fluid-Structure Interaction (coupling to CFD).
CFD and structural mechanics analyses can be integrated with System Thermal-Hydraulics, Sub-Channel Thermal-Hydraulics, Fuel Performance and Reactor Physics within multi-physics and multiscale analysis frameworks.

The Severe Accident (SA) analysis starts from the core damage and covers molten core interaction with vessel and containment. It includes the calculation of fission products release and transport in the primary side and in the containment, in the spent fuel pool, calculation of the source term to the environment and the dose evaluation.
SA analyses have been performed for VVER, PWR and RBMK plants and spent fuel pool experimental tests. Radioprotection analysis has been performed for the Radwaste Management Facility in Saluggia (Italy) for evaluation dose to the workers and population in normal operation and incidental conditions.
A renewed international interest exists in SAs because they are included in the accident conditions to be considered in the design of new plants, particularly to demonstrate the effectiveness of the designed mitigation measures.
MELCOR, RELAP5-SCDAP, GOTHIC, SMART, MCNP5 and MACCS2 are among the tools used for the analyses of SA, dose evaluation and radioprotection.

Nuclear safety analysis and design require dedicated thermal fluid dynamic experiments to investigate both basic phenomena and integral effects, in order to achieve a deep understanding of the relevant processes as well as to support the validation of predictive analysis tools.
NINE’s specialists have the competences to provide the design of, to supervise the construction of, and to operate experimental facilities using state-of-the-art technology and advanced instrumentation and measurement techniques.
As a result of past cooperation with Utilities, Industries, Universities and Research Centres for the design and conduction of experiments in the framework of both International Projects (within EC and OECD-NEA frameworks) and third-party contracts, NINE’s specialists gained experience over a wide range of technological aspects related to thermal fluid dynamic experiments, such as: design of customized measurement probes; development of Instrumentation & Control systems; design and construction of customized components (e.g. pressure vessels, electrical heaters, heat exchangers); design and construction of test facilities, as wells as definition and execution of commissioning tests, conduction of experiments, data management and analysis.
The implementation of the above activities benefits from a combined use of state-of-the-art Structural Mechanic and multi-scale thermal fluid dynamic analysis, the latter involving lumped-parameter (System Thermal-Hydraulics), component-specific (Sub-Channel) or detailed 3D (Computational Fluid Dynamics) analysis codes.