Materials considerations in accelerator targets

Abstract

Future nuclear materials production and/or the bum-up of long lived radioisotopes may be accomplished through the capture of spaUationproduced neutrons in accelerators. Aluminum clad-lead and/or lead alloys has been proposed as a spallation target. Aluminum was the cladding choice because of the low neutronabsorptioncross section, fast radioactivitydecay, high thermalconductivity, andexcellent fabricability. Metallic lead andlead oxide powders were considered for the targetcore with the fabrication options being casting or powder metallurgy (PM). Scoping tests to evaluate gravity casting, squeeze casting, and casting and swaging processes showed that, based on fabrieability and heat transfer considerations, squeeze casting was the preferred option for manufacture of targets with initial core cladding contact. Thou_.,ands ofaluminumclad aluminum-lithium alloy core targets andcontrol rodsfortritium production havebeenfabricated by coextrusion processes andsuccessfully irradiated intheSRS reactors. Tritium retention in, andrelease from, thecocxtruded product wasmodeled fromexperimental andoperational data. Themodelassumed that tritium atoms, formed bythe6Li(n,a)3He reaction, wereproduced insolid solution inthe AI-Li alloy. Because ofthelowsolubility ofhydrogen isotopes inaluminum alloys, theirradiated Al-Lirapidly becamesupersaturated intritium. Newlyproduced tritium atomsweretrapped bylithium atoms toforma lithium tritide. The effective tritium pressure required fortrap or tritide stability was the equilibrium decomposition pressure oftritium overalithium tritide-aluminum mixture. The temperature dependence oftritium release wasdetermined by thepermeability ofthecladding totritium andthelocal equilibrium atthetrap sites. The modelcanbe usedtocalculate tritium release fromaluminumclad, aluminum-lithium alloy targets during postulated accelerator operational andaccident conditions. This paper describes themanufacturing technologies evaluated andpresents themodelfortritium retention in aluminum clad, aluminum-lithium alloy tritium production targets.