If a leak is found, attempt to isolate is by changing demineralizers and filters. If leak cannot be isolated prepare to shut down the plant and then attempt to make repairs.
If pressurizer level can be maintained, proceed to locate leak as in item 9, page 6 of 10.
Note: Annunciators h, i, j indicate a rupture into the containment.
They are: SS-21V and SS-22V Boiler Blow Down PP-57V PP System Isolation SA-45V 1st Sample Loop PD-20V Containment Drain Tank PD-21V Containment Vessel Drain PD-39V Effluent Cond. Tank SL-8V Return flow Buffer Seals WL-20V Containement Drain Tank Vent CW-201V CW PP-C1, PP-C1 Letdown Coolers Return CW-202V CW PP-C3 PP-C4 Letdown Coolers Return CW-203V CW Shield Tank Return CW-205V CW-CC-C1 Containment Cooling Return CW-206V CW-CC-C2 Containment Cooling Return CW-207V CW PS-P1 Return CW-208V CW PS-P2 Return CW-209V CW PS-P3 Return CW-210V CW PS-P4 Return
PS-4V, PS-6V, PS-3V, PS-5V.
A large rupture of any attendant part of the primary system can result in serious damage to the reactor core. This can result from a loss of cooling water from the primary system through the ruptured area. Decay heat following shutdown and a lack of cooling water can result in the eventual melting of the core, beginning approximately 5 minutes after the inception of the accident. It is, therefor, imperative that all possible measure be taken to prevent this occurrence. This mainly involves actions taken to isolate the leak, if possible, and to make up the lost coolant as quickly as possible.
As soon as it is obvious that a large leak or rupture has affected the primary system, the primary gate valves should be closed. This will isolate the boilers and a portion of the primary system from the reactor in case a leak has occurred on the boiler side of the isolation valves. The next important step is to start the emergency cooling system in order to supply the cooling water required by the core. This is necessary because primary water flow cannot continue once the primary gate valves have been closed.
Since it is important to maintain as much water in the primary system as possible, the normal flow of water to the purification system will be shut off.
All sources of spare water supplies should be utilized to makeup losses from the primary system. Normal makeup as a rate of 5-10gpm is available from the main condenser (5150 gals), DFT (4720 gals), distilled water tank (5700 gals.)*, boiler feed tank (16,325 gals.)* The waste transfer pump can supply water at a rate of 50 gpm from the waste storage tanks (4100 gals.)*. The filter backflushing system can provide 30 gpm from the main condensate system.
If the leak continues, valves on the lines leading from the pressurizer may be closed in order to isolate any leak which may have occurred there.
At this point all possible measures should have been taken to prevent a core melt-down. If remains now to monitor the radiation levels in and around the containment vessel and to determine if the leak is in an area which will permit repair or isolation so that startup may be accomplished. In order to insure complete containment until it has been determined that no radiation hazard exists, all air actuated containment isolation valves should remain closed. Since the salt water side of the DK system, when in operation, represents a potential leak directly to the sea through a single gland such as the packing of DK-P1, the system should be secured when it has finally been determined that the MCA has occurred. This should be done as soon as any indication is obtained of gross fuel melting or failure.
If the leak can be located and isolated, it may be possible to start up again. If not, the Emergency Nuclear Incident Plan should be placed into effect since it is probably than an MCA will occur.
The Maximum Credible Accident (MCA) is a nuclear incident caused by a gross leak in the primary system which exposes the core. Fuel element cladding melts allowing the fuel to drop to the bottom of the reactor vessel, and fission products escape into the containment vessel.
As soon as a gross leak is observed, every attempt should be made to isolate the leak as outlined above.
The following description is a short summary of the Maximum Credible Accident:
The Emergency Nuclear Incident Plan is to be used when it becomes obvious that maximum available makeup water cannot maintain normal level in the primary system after a gross leak occurs.
Approximately five minutes after the MCA occurs, cladding will begin to melt, and approximately eight minutes after the MCA occurred, the fuel pins in the high flux regions of the reactor will begin to melt. Theoretically, 50 percent of the pins will have failed approximately 40 minutes after the system rupture. Decay-heat will continue to be generated after the fuel-clad mixture has melted and dropped to the bottom of the reactor vessel.
The fuel temperature, however, will not be sufficient to melt through the vessel due to natural circulation of cooling water. A gap of approximately 3/4 inches is provided between the insulation of the lower head of the vessel to permit cooling by natural circulation of water after loss of coolant from the primary system. The cooling water that becomes available for this purpose after the incident results from the primary water collection to a depth of approximately 30 inches in the bottom of the containment vessel. Water enters at the bottom center of the insulation and flows upward in the gap between the insulation and vessel to outlet holes places 18 inches above the bottom of the insulation.
Pressure within the containment would rise to a maximum of 173 psig in approximately one minute and drop gradually to approximately 60 psig in approximately one hour. Temperature would rise to 360F in approximately 30 seconds and drop gradually to a value of 285 F in approximately 3 hours.