( ( By ( 60 years ) ) ( , ( ( ( ( ( the ( initial ( short-lived isotopes ) ) ) , ) ( including cobalt-60 ) ) , ) ( ( will ( have ( decayed ( to ( background levels ) ) ) ) ) . ) ) ) )
(ROOT (S (PP (IN By) (NP (CD 60) (NNS years))) (, ,) (NP (NP (DT the) (JJ initial) (JJ short-lived) (NNS isotopes)) (, ,) (PP (VBG including) (NP (CD cobalt-60))) (, ,)) (VP (MD will) (VP (VB have) (VP (VBN decayed) (PP (TO to) (NP (NN background) (NNS levels)))))) (. .)))
By 60 years, the initial short-lived isotopes, including cobalt-60, will have decayed to background levels.
(ROOT (S (NP (DT The) (JJ long-lived) (NNS isotopes)) (VP (VBP require) (NP (NP (NNS thousands)) (PP (IN of) (NP (NP (NNS years)) (PP (TO to) (NP (NN decay)))))) (PP (TO to) (NP (NP (DT a) (JJ safe) (NN level)) (PP (IN in) (NP (DT a) (JJ nuclear) (NN reactor)))))) (. .)))