Limit design studies for LMFBR subassemblies after blockage and fuel meltdown

In the safety assessment of fast breeder reactors, the so-called subassembly accident is supposed to consist in a meltdown of the fuel pins inside a single subassembly following a hypothetical flow blockage at the entrance. The blocked hexagonal subassembly tube is still cooled from the outside, by the sodium flowing through the intersubassembly gaps. The neighbour subassemblies are intact and in nominal operating conditions. This situation is studied in the experimental inpile program SCARABEE. It has been shown that the molten materials create by refreezing at the outlet of the fissile zone of the pin bundle another blockage, and inside the region between the in and outlet blockages a boiling pool develops which pressurized the hexcan walls while submitting them to high heat fluxes. The speed and conditions of propagation of this melt to the neighbouring subassembly depend on the thermal or mechanical failure mechanisms of the hexcan walls. In this paper the limit design method, which is classical for calculating the collapse load in many structures with a homogeneous material, has been extended to a pressurized hexagonal tube submitted to a very high temperature gradient due to high heat fluxes. When the temperature at the hot face of the wall exceeds the temperature at which the limit stress of the material becomes zero, one can represent the collapse pressure by a very easy formula as a function of the cold face temperature and the heat flux. Deformations beyond the bending collapse load are as easy to calculate. Some applications to SCARABEE tests are given. (author)