( ( The process ) ( ( is ( ( ( called ( nuclear fission ) ) , ) ( because ( it ( is ( ( the ( atomic nucleus ) ) ( that ( is split ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (DT The) (NN process)) (VP (VBZ is) (VP (VBN called) (NP (JJ nuclear) (NN fission)) (, ,) (SBAR (IN because) (S (NP (PRP it)) (VP (VBZ is) (NP (NP (DT the) (JJ atomic) (NN nucleus)) (SBAR (WHNP (WDT that)) (S (VP (VBZ is) (VP (VBN split))))))))))) (. .)))
The process is called nuclear fission, because it is the atomic nucleus that is split.
(ROOT (S (NP (JJ Nuclear) (NN energy)) (VP (VBZ is) (VP (VBN released) (SBAR (WHADVP (WRB when)) (S (NP (NP (DT the) (NN nucleus)) (PP (IN of) (NP (DT an) (NN atom)))) (VP (VBZ is) (VP (VBN split))))))) (. .)))