Lifetime Analysis of Beryllium Pebbles in a Hybrid Fusion Blanket

Thermo-mechanical and neutron lifetime modelling and design of Be pebbles in the neutron multiplier for the LIFE engine

“…The estimated temperature range in the neutron multiplier is 600 to 650 • C, with fast (neutron energy >1 MeV) fluences of up to 9.1 × 10 23 n cm −2 [4]. Thus, Be-pebble lifetime estimates obtained from previous experimental campaigns at temperatures of typically 450 • C and fast fluences of ≈1.2 × 10 23 n cm −2 , corresponding to fluxes with energies well below 14 MeV [5], are not applicable for LIFE. Figures 2(a) and (b) show, respectively, the neutron spectrum at the beginning and end of life (BOL, EOL), and the total fluence as a function of lifetime, averaged over the Be blanket in the DU and WG-Pu versions of the LIFE reactor.…”

“…Several authors have suggested that swelling can be tolerated up to a maximum value of 30% (volumetric), at which either crumbling occurs [5] or saturation is observed [25], although values of up to 40% have been measured at fast fluences of ∼10 22 n cm −2 and T > 1100 • C [16]. In this work, we assume that this limit results in failure regardless of the stress state inside the pebble.…”

“…In the same fashion as Miller et al [5], we estimate the contact stresses by resorting to Hertzian contact theory, which states that the maximum tensile stress at the contact point between two spheres is…”

“…For the former, we assume that Be pebbles flow gravitationally in the (stationary) coolant and, hence, the most severe value of f c is given by f c = ρ flibe V g, where ρ flibe is the density of the coolant, V = 4πR 3 /3 is the pebble volume and g is the acceleration of gravity. With ρ flibe = 2.0 g cm −3 [2] and R = 15 mm, we have f c = 0.3 N, which results in values of a = 0.02 mm at BOL and 0.03 mm at EOL 5 . This corresponds to σ max = 82 MPa at BOL and 44 MPa at EOL.…”

“…Depending on the temperature, creep may relax this stress buildup before the tensile limit is reached. Several authors have suggested that, for temperatures between 400 and 500 • C, thermal creep is not sufficient to relax the stresses caused by swelling, and that, approximately at 30% volumetric swelling, crumbling occurs and the pebble fails [5].…”