SwissNuclear project: Investigation of fuel assembly integrity under wet and dry storage

After the Fukushima nuclear accident with the very long term station blackout, the safety of spent fuel pools came into the focus of interest. With severe accident codes and experimental programs for validation of their models under spent fuel pool conditions, the uncertainty of code predictions could be reduced. Using the newly released MELCOR 2.2 code (February 2017) the integrity of nuclear spent fuel elements will be investigated in wet storage (without data comparison) and dry storage (comparing with Sandia fuel project data). The new multi rod model in MELCOR 2.2 will be used to calculate the behavior of dry stored spent fuel assemblies under different decay heat conditions.

A comparison with calculations by the validated models used in MELCOR 1.8.6 will show advantages and limitations of the new model if compared also with experimental data. Using the expected decay heat data for BWR and PWR spent fuel elements, it is possible to determine the minimum storage times for wet storage before dry storage can be applied, also considering the planned shut-down of nuclear power plants when the safety procedures may differ from those under power operation.

The PSI air oxidation model can be compared with the Sandia model for air oxidation and breakaway of the oxide layer in case of severe accident conditions. The data from a recently performed project investigating the influence of nitrogen on the acceleration of the oxidation process of Zircaloy cladding materials can be used to validate the calculated results.