 To allow insertion of the 3 @.@ 62 @-@ inch ( 92 mm ) diameter plutonium pit , containing the 0 @.@ 8 @-@ inch ( 20 mm ) diameter " Urchin " modulated neutron initiator , as late as possible in the device 's assembly , the spherical 8 @.@ 75 @-@ inch ( 222 mm ) diameter depleted uranium tamper surrounded by a 0 @.@ 125 @-@ inch ( 3 @.@ 2 mm ) thick shell of boron impregnated plastic had a 5 @-@ inch ( 130 mm ) diameter cylindrical hole running through it , like the hole in a cored apple . The missing tamper cylinder , containing the pit , could be slipped in through a hole in the surrounding 18 @.@ 5 @-@ inch ( 470 mm ) diameter aluminium pusher . The pit was warm to touch , emitting 2 @.@ 4 W / kg @-@ Pu , about 15 W for the 6 @.@ 19 kilograms ( 13 @.@ 6 lb ) core . 
