ier ls ▁were ▁ex cluded ▁from ▁the ▁most ▁important – and ▁secret – war ▁work ▁being ▁done ▁by ▁O li ph ant ' s ▁team ▁at ▁B irmingham , ▁that ▁on ▁rad ar . ▁However , ▁O li ph ant ▁would ▁ask ▁Pe ier ls ▁a ▁theoretical ▁question ▁about , ▁say , ▁the ▁solution ▁to ▁Max well ' s ▁equations ▁in ▁a ▁hem is pher ic ▁cav ity . ▁Pe ier ls ▁knew ▁that ▁questions ▁of ▁this ▁nature ▁related ▁to ▁the ▁work ▁on ▁mic row ave ▁rad ar , ▁and ▁O li ph ant ▁was ▁doubt less ▁aware ▁of ▁this ▁too , ▁but ▁the ▁fa ç ade ▁of ▁secre cy ▁was ▁maintained . ▁The ▁nuclear ▁investigation ▁was ▁not ▁yet ▁secret , ▁so ▁Fr isch ▁was ▁available ▁to ▁work ▁on ▁it . ▁He ▁began ▁experiment ing ▁with ▁u ran ium ▁en rich ment ▁through ▁thermal ▁diffusion , ▁a ▁process ▁first ▁demonstrated ▁in ▁Germany ▁by ▁Klaus ▁Cl us ius . ▁Progress ▁was ▁slow ; ▁the ▁required ▁equipment ▁was ▁not ▁available , ▁and ▁the ▁rad ar ▁project ▁had ▁first ▁call ▁on ▁the ▁available ▁resources . ▁ ▁Francis ▁Per rin ▁had ▁defined ▁a ▁critical ▁mass ▁of ▁u ran ium ▁to ▁be ▁the ▁smallest ▁amount ▁that ▁could ▁sust ain ▁a ▁chain ▁reaction , ▁and ▁had ▁calculated ▁the ▁critical ▁mass ▁of ▁u ran ium ▁ox ide ▁( not ▁metal ) ▁to ▁be ▁about ▁. ▁He ▁reck oned ▁that ▁if ▁a ▁neut ron ▁reflect or ▁were ▁placed ▁around ▁it ▁of ▁materials ▁such ▁as ▁iron ▁or ▁lead ▁which ▁did ▁not ▁greatly