ed ▁Bo hr ▁to ▁explain ▁why ▁u ran ium ▁seemed ▁to ▁f ission ▁with ▁both ▁very ▁fast ▁and ▁very ▁slow ▁neut rons . ▁Bo hr ▁had ▁an ▁e pi ph any ▁that ▁the ▁f ission ▁at ▁low ▁energ ies ▁was ▁due ▁to ▁the ▁u ran ium - 2 3 5 ▁is ot ope , ▁while ▁at ▁high ▁energ ies ▁it ▁was ▁mainly ▁due ▁to ▁the ▁ ▁more ▁abund ant ▁u ran ium - 2 3 8 ▁is ot ope . ▁The ▁former ▁makes ▁up ▁just ▁ 0 . 7 % ▁of ▁natural ▁u ran ium ; ▁while ▁that ▁latter ▁accounts ▁for ▁ 9 9 . 3 %. ▁ ▁On ▁ 1 6 ▁April , ▁Bo hr , ▁P lac zek , ▁Whe eler , ▁Eug ene ▁W igner ▁and ▁Leon ▁Rosen feld ▁discussed ▁whether ▁it ▁would ▁be ▁possible ▁to ▁use ▁a ▁nuclear ▁chain ▁reaction ▁to ▁make ▁an ▁atomic ▁bomb , ▁and ▁concluded ▁that ▁it ▁was ▁not . ▁Bo hr ▁observed ▁that ▁ ▁" It ▁would ▁take ▁the ▁entire ▁efforts ▁of ▁a ▁country ▁to ▁make ▁a ▁bomb ." ▁ ▁British ▁response ▁ ▁In ▁Britain , ▁scient ists ▁also ▁considered ▁whether ▁an ▁atomic ▁bomb ▁was ▁practical . ▁At ▁the ▁University ▁of ▁Liverpool , ▁Ch ad wick ▁and ▁the ▁Polish ▁refuge e ▁scient ist ▁Joseph ▁Rot bl at ▁tack led ▁the ▁problem , ▁but ▁their ▁calculations ▁were ▁incon clus ive . ▁At ▁Cambridge , ▁Nobel ▁Prize ▁in ▁Physics ▁la ure ates ▁George ▁P aget ▁Thom son ▁and ▁William ▁Lawrence ▁B rag g ▁wanted ▁the ▁government ▁to