Reactor overheating and malfunction of relief valve Given that the steam generators were no longer receiving feedwater, heat transfer from the reactor coolant system (RCS) was greatly reduced, and RCS temperature rose. The rapidly heating coolant expanded and surged into the pressurizer, compressing the steam bubble at the top. When RCS pressure rose to , the pilot-operated relief valve (PORV) opened, relieving steam through piping to the reactor coolant drain tank in the containment building basement. RCS pressure continued to rise, reaching the reactor protection system high-pressure trip setpoint of eight seconds after the turbine trip. The reactor automatically tripped, its control rods falling into the core under gravity, halting the nuclear chain reaction and stopping the heat generated by fission. However, the reactor continued to generate decay heat, initially equivalent to approximately 6% of the pre-trip power level. Because steam was no longer being used by the turbine and feed was not being supplied to the steam generators, heat removal from the reactor's primary water loop was limited to steaming the small amount of water remaining in the secondary side of the steam generators to the condenser using turbine bypass valves.
REactOR oveRhEATing anD malfUnctIon Of relIeF vAlve givEN thAt tHe STeAm GeNEratOrs were nO lonGEr RECeIving fEEDwaTer, heAt TraNSfer frOM The reactoR CoOLant SYsteM (rcs) Was greatly ReDuCed, ANd rcS temperaTure Rose. the rapIDlY HEatiNg coolanT ExPaNdeD and surged into tHe PreSsuRiZer, cOmpressInG the stEAm bubble aT thE TOP. wHeN RCs prEssure Rose tO , thE pilot-operatED rElief valve (porv) OpeneD, relieVInG stEam ThroUgh pIping to thE rEactOr cOoLant dRAIn taNk in tHe ConTAINMent building bASEMent. rcs pressUre ConTinUeD To rise, rEaching the ReactoR pRotecTioN system HiGH-pREssure trIP setpOINt Of eigHt sECoNds aFtEr The tURBINe tRIP. thE reactor AUtOmATiCaLly Tripped, its cOntrOl roDS faLLing into the CorE undeR gravitY, halting tHe Nuclear cHain rEacTiON And stoPping ThE hEat Generated By fissIon. HOWevEr, tHe Reactor cONTinUed to geNerAte deCay heaT, inItIAllY equIValENt to AppRoXimately 6% oF the PrE-tRiP pOwER lEVel. beCausE steam WAs no loNGer beinG used By the tuRbine and Feed waS Not bEing suppliEd tO THe STEam geNeratoRS, heat rEmOvaL froM the reaCtor's primary WaTer loop wAs limiTEd tO steaminG tHE smaLL amOuNt of watER remaining In the SecondAry side of The SteaM generAtOrs to tHe cOndenSer usInG turbinE byPASs vALvES.