Chemical interaction of D9 alloy clad with B4C in liquid sodium: Studies employing XPS, XRD, and SEM

A cell representing typical control rod subassembly of the prototype fast breeder reactor is designed to probe the liquid sodium-mediated chemical interaction of boron carbide (B4C) control rod with the D9 alloy clad. The cell was equilibrated at 973 K for 5000 hours with liquid sodium in the annular gap. XRD shows the formation of Cr3C2 and Fe2B along with oxides of Ni, Cr, and Fe. XPS studies reveal the diffusion of boron and carbon up to a depth of about 160 and 120 mu m, respectively. A boron-rich region is observed up to a depth of about 40 mu m which consists of B3+ arising from oxides and a nearly constant elemental boron region, extending up to a depth of 160 mu m. The highly reacted zone extends up to a depth of about 40 mu m consisting of oxides of most of the elements of D9 clad exhibiting a higher valence states. As the cell is helium leak tested, oxygen out gassed from B4C at 973 K played a major role in chemical oxidation of the constituents of the D9 alloy which can be minimized by using high-density B4C.