▁energy ▁re actor ' ▁( i . e . ▁water - co o led ▁water - mod er ated ▁energy ▁re actor ). ▁The ▁design ▁is ▁a ▁type ▁of ▁press ur ised ▁water ▁re actor ▁( P WR ). ▁The ▁main ▁distingu ishing ▁features ▁of ▁the ▁V VER ▁compared ▁to ▁other ▁P WR s ▁are : ▁▁ ▁Hor izontal ▁steam ▁gener ators ▁ ▁H ex ag onal ▁fuel ▁assemb lies ▁ ▁No ▁bottom ▁pen etr ations ▁in ▁the ▁pressure ▁vessel ▁ ▁High - cap acity ▁press ur izers ▁providing ▁a ▁large ▁re actor ▁cool ant ▁invent ory ▁ ▁Re actor ▁fuel ▁ro ds ▁are ▁fully ▁imm ers ed ▁in ▁water ▁kept ▁at ▁ 1 5 ▁M Pa ▁pressure ▁so ▁that ▁it ▁does ▁not ▁bo il ▁at ▁the ▁normal ▁( 2 2 0 ▁to ▁over ▁ 3 0 0   ° C ) ▁operating ▁temper atures . ▁Water ▁in ▁the ▁re actor ▁serves ▁both ▁as ▁a ▁cool ant ▁and ▁a ▁moder ator ▁which ▁is ▁an ▁important ▁safety ▁feature . ▁Should ▁cool ant ▁circul ation ▁fail , ▁the ▁neut ron ▁moder ation ▁effect ▁of ▁the ▁water ▁dimin ishes , ▁reducing ▁reaction ▁intensity ▁and ▁compens ating ▁for ▁loss ▁of ▁cool ing , ▁a ▁condition ▁known ▁as ▁negative ▁void ▁coefficient . ▁Later ▁versions ▁of ▁the ▁react ors ▁are ▁enc ased ▁in ▁massive ▁steel ▁pressure ▁shell s . ▁F uel ▁is ▁low ▁en rich ed ▁( ca . ▁ 2 . 4 – 4 . 4 % ▁ 2 3 5 U ) ▁u ran ium ▁dio x