▁bar riers ▁▁ ▁A ▁typical ▁design ▁feature ▁of ▁nuclear ▁react ors ▁is ▁layer ed ▁safety ▁bar riers ▁prevent ing ▁escape ▁of ▁radio active ▁material . ▁V VER ▁react ors ▁have ▁four ▁layers : ▁▁ ▁F uel ▁pel lets : ▁radio active ▁elements ▁are ▁retained ▁within ▁the ▁cry stal ▁structure ▁of ▁the ▁fuel ▁pel lets . ▁ ▁F uel ▁ro ds : ▁the ▁z ir cal oy ▁tub es ▁provide ▁a ▁further ▁bar rier ▁resist ant ▁to ▁heat ▁and ▁high ▁pressure . ▁ ▁Re actor ▁shell : ▁a ▁massive ▁steel ▁shell ▁enc ases ▁the ▁whole ▁fuel ▁assembly ▁herm et ically . ▁ ▁Re actor ▁building : ▁a ▁concrete ▁contain ment ▁building ▁that ▁enc ases ▁the ▁whole ▁first ▁circuit ▁is ▁strong ▁enough ▁to ▁resist ▁the ▁pressure ▁sur ge ▁a ▁bre ach ▁in ▁the ▁first ▁circuit ▁would ▁cause . ▁ ▁Comp ared ▁to ▁the ▁R B M K ▁react ors ▁– ▁the ▁type ▁involved ▁in ▁the ▁Ch ern ob yl ▁dis aster ▁– ▁the ▁V VER ▁uses ▁an ▁inher ently ▁sa fer ▁design . ▁It ▁does ▁not ▁have ▁the ▁graph ite - mod er ated ▁R B M K ' s ▁risk ▁of ▁a ▁power ▁sur ge ▁trans ient ▁or ▁critical ity ▁accident . ▁Also ▁the ▁R B M K ▁power ▁stations ▁were ▁constructed ▁without ▁contain ment ▁structures ▁on ▁grounds ▁of ▁cost ▁as ▁well ▁as ▁the ▁relative ▁ease ▁of ▁re - f uel ing . ▁( F uel ▁elements ▁in ▁an ▁R B M K ▁can ▁be ▁replaced ▁while ▁the ▁re actor ▁is ▁running ▁at ▁its