ide ▁( U O 2 ) ▁or ▁equivalent ▁pressed ▁into ▁pel lets ▁and ▁assemble d ▁into ▁fuel ▁ro ds . ▁ ▁React ivity ▁is ▁controlled ▁by ▁control ▁ro ds ▁that ▁can ▁be ▁inserted ▁into ▁the ▁re actor ▁from ▁above . ▁These ▁ro ds ▁are ▁made ▁from ▁a ▁neut ron ▁absor bing ▁material ▁and , ▁depending ▁on ▁depth ▁of ▁insert ion , ▁h inder ▁the ▁chain ▁reaction . ▁If ▁there ▁is ▁an ▁emer gency , ▁a ▁re actor ▁shut down ▁can ▁be ▁performed ▁by ▁full ▁insert ion ▁of ▁the ▁control ▁ro ds ▁into ▁the ▁core . ▁ ▁Primary ▁cool ing ▁circ uits ▁▁ ▁As ▁stated ▁above , ▁the ▁water ▁in ▁the ▁primary ▁circ uits ▁is ▁kept ▁under ▁a ▁constant ▁elev ated ▁pressure ▁to ▁avoid ▁its ▁bo iling . ▁Since ▁the ▁water ▁trans fers ▁all ▁the ▁heat ▁from ▁the ▁core ▁and ▁is ▁ir rad i ated , ▁the ▁integrity ▁of ▁this ▁circuit ▁is ▁cru cial . ▁Four ▁main ▁components ▁can ▁be ▁distinguished : ▁▁ ▁Re actor ▁vessel : ▁water ▁flows ▁through ▁the ▁fuel ▁rod ▁assemb lies ▁which ▁are ▁he ated ▁by ▁the ▁nuclear ▁chain ▁reaction . ▁ ▁Volume ▁compens ator ▁( press ur izer ): ▁to ▁keep ▁the ▁water ▁under ▁constant ▁but ▁controlled ▁pressure , ▁the ▁volume ▁compens ator ▁regul ates ▁the ▁pressure ▁by ▁cont rolling ▁the ▁equilibrium ▁between ▁s atur ated ▁steam ▁and ▁water ▁using ▁elect rical ▁he ating ▁and ▁relief ▁val ves . ▁ ▁Ste am ▁generator : ▁in ▁the ▁steam ▁generator , ▁the ▁heat ▁from ▁the ▁primary ▁cool ant ▁water ▁is ▁used