▁and ▁a ▁nuclear ▁chain ▁reaction ▁was ▁briefly ▁sust ained . ▁ ▁God iva ' s ▁design ▁was ▁inspired ▁by ▁a ▁self ▁termin ating ▁property ▁ ▁discovered ▁when ▁incorrectly ▁experiment ing ▁with ▁the ▁J em ima ▁device ▁in ▁ 1 9 5 2 . ▁J em ima ▁operated ▁by ▁rem ot ely ▁lif ting ▁one ▁stack ▁of ▁en rich ed ▁u ran ium - 2 3 5 ▁dis ks ▁up ▁towards ▁another , ▁fixed , ▁stack . ▁On ▁ 1 8 ▁April ▁ 1 9 5 2 , ▁due ▁to ▁a ▁mis calcul ation , ▁J em ima ▁was ▁assemble d ▁with ▁too ▁many ▁dis ks ; ▁this ▁caused ▁an ▁exc urs ion ▁of ▁ 1 . 5 ▁x ▁ 1 0 1 6 ▁f issions — an ▁automatic ▁sc ram — but ▁no ▁damage . ▁ ▁On ▁ 3 ▁February ▁ 1 9 5 4 ▁and ▁ 1 2 ▁February ▁ 1 9 5 7 , ▁accident al ▁critical ity ▁exc urs ions ▁occurred ▁causing ▁damage ▁to ▁the ▁device , ▁but ▁only ▁ins ign ific ant ▁expos ures ▁to ▁personnel . ▁This ▁original ▁God iva ▁device , ▁known ▁as ▁Lady ▁God iva , ▁was ▁irre par able ▁after ▁the ▁second ▁accident ▁and ▁was ▁replaced ▁by ▁the ▁God iva ▁II . ▁ ▁God iva ▁II ▁ ▁God iva ▁II ▁was ▁constructed ▁inside ▁a ▁concrete ▁building ▁with ▁ ▁walls ▁and ▁ ▁roof ▁in ▁a ▁can y on ▁a ▁quarter - mile ▁( 4 0 0 ▁m ) ▁away ▁from ▁the ▁control ▁room . ▁ ▁In