Thermal-hydraulics/thermal-mechanics temporal coupling for unprotected loss of flow accidents simulations on a SFR

In the frame of ASTRID designing, unprotected loss of flow (ULOF) accidents during which primary pumps are lost but not the secondary ones, are considered. As the reactor is not scrammed, power evolution is driven by neutronic feedbacks, among which Doppler effect, linked to fuel temperature, is prominent. Fuel temperature is calculated using thermal properties of fuel pins (we will focus on heat transfer coefficient between fuel pellet and cladding, H(gap), and on fuel thermal conductivity, λ(fuel)) which vary with irradiation conditions (neutron flux, mass flow and history for instance) and during transient (mainly because of dilatation of materials with temperature). In this paper, we propose an analysis of the impact of spatial variation and temporal evolution of thermal properties of fuel pins on a low void coefficient (CFV-like) core behavior during an ULOF accident. These effects are usually neglected under some a priori conservative assumptions. The vocation of our work is not to provide a best-estimate calculation of ULOF transient, but to discuss some of its physical aspects. To achieve this goal, we used TETAR, a thermal-hydraulics system code developed by our team to calculate ULOF transients, GERMINAL V1.5, a CEA code dedicated to SFR pin thermal-mechanics calculations and APOLLO3, a neutron transport code in development at CEA. It is shown that the impact of the spatial variations of fuel pins was found to be about +30 C. degrees on sodium temperature during ULOF transient. It is mainly due to H(gap), and simple zones averages seem to be enough for λ(fuel). The combined effect of local thermal properties and local Doppler coefficients leads to an impact of about +35 C. degrees. On the other hand, the temporal coupling, because of the opening of the gap, improves the reactor behavior during ULOF and leads to a decrease of about 45 C. degrees of the sodium temperature. This improvement of the core behavior is very strong and could help greatly to demonstrate the safety of large SFRs