2022:09 Assessment of modelling approaches for axial gas flow inside LWR fuel rods

SSM perspective


The fuel rod analysis program SCANAIR has been developed by IRSN (Institut de Radioprotection et de Sûreté Nucléaire) for analysis of reactivity initiated accidents (RIA) in light water reactors. The Swedish Radiation Safety Authority (SSM) has access to SCANAIR in exchange for annual contributions for its further development. The development and administration of the program is done by Quantum Technologies AB on assignment from SSM. The production and release of gaseous fission products in fuel rods are important phenomena that can be analysed by most computer codes for fuel rod thermal-mechanical behaviour. Bulk flow of gas caused by axial pressure gradients is a phenomenon acting in transient scenarios, and it is still an area of development in these computer codes.
The 2021 contribution to the development of SCANAIR is a review of experimental and analytical work for pressure-driven axial gas flow.


This report is an in-depth investigation into models for bulk flow that could be used in computer programs. The report discusses theoretical approaches that are suitable for development of such a model and gives recommendations to further development and implementation. The work describes the complexity of modelling and observes that parameters for gas flow is necessary to compute in connection with other state parameters. For example, the state of the pellet-cladding gap strongly affects the accuracy of the computations and the gap state depends on non-trivial parameters like pellet and cladding deformations and extent of pellet cracking. Besides being a theoretical foundation for further development, the report also discusses the experimental basis for development and validation. A conclusion is that there is little such information available in open literature and that more tests would be useful.


Knowledge of what is happening in a fuel rod during an event and how it is implemented in analytical tools is essential to SSM for the supervision of nuclear power plants and their safety analysis. The participation in the development of SCANAIR also enables SSM to actively be a part of the large efforts that are made internationally with testing, understanding and improving the tools for analysis of reactivity initiated accidents.

Need for further research

Continued work on developing SCANAIR’s analysis capabilities is planned in cooperation with IRSN. The current work is a foundation for development of a computational model and a continuation in this direction is planned. There is also a need for tests of fuel behaviour to include gas transport in order to expand the basis for models development.