• sodium-cooled fast reactor
• SIMMER-III
• hypothetical core disruptive accident
• fluid-structure interaction
• fluid-dynamics
• structure code

This work concerns the assessment of the consequences deriving from a Hypothetical Core Disruptive Accident (HCDA) in a Sodium-cooled Fast Reactor. The analysis focuses on a late phase of the accident, where the whole fissile part of the core is liquefied due to nuclear energy deposited in proceeding power excursions during the so-called Transition Phase (TP). The high temperatures of liquid core materials induce high vapor pressures which can result in a rapid ejection of the melt from the core region into the upper sodium plenum. There, a powerful thermal fuel-coolant interaction (FCI) may happen, in which the high-tensed melt material expands and large amounts of sodium are liquefied. This is where the thermal-to-mechanical energy conversion takes place. The large vapor bubble displaces surrounding sodium and accelerates it towards the only escape path, the cover gas region. A mechanical thread arises from a possible impact of the sodium slug on the lid of the vessel and its walls potentially challenging the integrity of the pressure vessel.
In order to properly assess the consequences of such an accident, it is needed to take account of several aspects: thermal energy of melt materials, melt discharge rate, mechanical energy acting on walls, and structural response. With this purpose, the newly developed extended version of SIMMER-III was used. Such extension embeds a structural code for describing the vessel behavior, which is coupled to the original SIMMER-III safety code.
The first part of this work concentrated on a preliminary study aimed at extending the database of standalone SIMMER-III simulations carried out by Karlsruhe Institute of Technology (KIT). Five initial liquid fuel temperatures, ranging from 4000 K to 6000 K, have been evaluated. Several assumptions have been made to speed up simulations and be more conservative at the same time. It was observed a great influence of the initial fuel temperature on the severity of the accident and the efficiency of energy conversion.
The second part of the study focused on the assessment of the structural response with the use of the coupled code. Only two representative temperatures chosen from the preliminary study, namely 5000 K and 6000 K, have been considered for the coupled analyses. Here, for both cases the infinitely rigid wall behavior has been compared with the realistic, flexible wall behavior.
Only elastic deformations on the wall were observed in the 5000 K case, unlike the more energetic 6000 K case where plastic deformations appeared. In both cases, the mechanical energy and the pressure field along the vessel wall were affected by the structural response of the vessel wall.