a sequence of physical events - Three Mile Island

shut. Closing this valve, with pump MU-P1A still running,caused a rapid increase in pressurizer level.March 28,1979-7:35 a.m.At 3 hours 35 minutes, it was noted that the auxiliarybuilding basement was flooded. It will be recalledthat the rupture disk on the auxiliary buildingsump tank had previously broken, so that much ofthe water pumped from the reactor building hadwound up in the auxiliary building basement. Highradiation readings were found in many areas of theauxiliary building.The PORV block valve was reopened at 3 hours41 minutes. Thirty seconds earlier, there was a suddenjump in the source range neutron detectors.[The jump may have been due either to water in thedowncomer flashing into steam or to a disturbanceof the core geometry. The change in the sourcerange is believed to be due to an event internal tothe core representing a change of geometryunrelatedto external events.]At 3 hours 56 minutes, there was an ES actuationbecause of high reactor building pressure (thesetpoint for actuation is 4 psig). When the ES actuated,the reactor building was automatically isolated.Isolation means that valves in alI systems notabsolutely essential for cooling the core are closedand the systems are shut down. Makeup pumpMU-PlC started, and the intermediate closed coolingpumps were tripped automatically. The intermediateclosed cooling pumps are needed for letdown andseal cooling, so the building isolation and ES weredefeated 4 minutes after actuation and the pumpswere restarted. [The delay in building isolation isdiscussed in Section II.C.1.c.]March 28,1979-8:00 a.m.About 4 to 4 1 /2 hours into the accident. incorethermocouple temperature readings were taken offthe computer; many registered question marks.Shortly after, at the request of the station superintendent,an instrumentation control engineer hadseveral foremen and instrument technicians go to aroom below the control room and take readings witha millivoltmeter on the wires from the thermocouples.The first few readings ranged from about200'F to 2300°F. These were the only readings reportedby the instrumentation control engineer tothe station superintendent. Both have testified thatthey discounted or did not believe the accuracy ofthe high readings because they firmly believed thelow readings to be inaccurate. In the meantime, thetechnicians read the rest of the thermocouples-anumber of which were above 2000°F-and enteredthese readings in a computer book which was laterplaced on a control room console. The techniciansthen left the area when nonessential personnel wereevacuated. [We have not developed evidence thattheir superiors were conscious of these additionalreadings on March 28.]An attempt was made to start reactor coolantpump RC-P1A at 4 hours 18 minutes. Current andflow were monitored to see if the pump could beoperated. The starting current was normal, butcurrent quickly dropped to a low value and flowdropped to zero. This indicates that a slug of watermay have been forced through, but the pump wasnot working continuously. It was stopped a minutelater.March 28,1979-8:18 a.m.Both makeup pumps (MU-P1A and 1C) werestopped at 4 hours 18 minutes. Two unsuccessfulattempts were made to restart pump 1A. The controlswitch was then put in the "pull-to-lock" position.This completely defeated automatic starts ofthe pump. [The reasons for doing this were apparentlythe difficulties experienced in attempting torestart the pump, and a desire to avoid the possibilityof having the pump come on if ES actuated. Thepressurizer indicated full, and the operators wereconcerned about full high pressure injection flowcoming on with an apparently "solid" system.][Actually, a very large part of the RCS was filledwith steam and gas, and the system was far frombeing solid. This condition could have been recognizedfrom the fact that the RCS hot legs were superheated.There was no danger of overpressurizingthe RCS by high makeup flow.]There was, in fact, another ES actuation at 4hours 19 minutes. Decay heat pump DH-P1A started,and the intermediate closed cooling pumptripped, but makeup pump MU-P1A did not start.The ES actuation was immediately defeated and theintermediate closed cooling pump was restarted.Only one channel had been actuated, but the factthat one channel was defeated satisfied the "twoout of three" logic which is required for ES actuation.Makeup pump MU-P1B was started by the operatorat 4 hours 22 minutes, and MU-PlC at 4 hours27 minutes.Problems in the condensate system were continuing.The condensers had been steadily losingvacuum. It was also necessary to maintain steam tothe main turbine seals in order to operate the condenserat a vacuum. When main steam is not avail-327