The RCIC (Reactor Core Isolation Cooling System) is not technically part of the emergency core cooling system but provides and important safety role. This system provides cooling to the reactor when certain failures have happened, any one of these could cause the RCIC to be used:
1. Loss of access to the “ultimate heat sink” (the river, sea or lake used for cooling).
2. Loss of all electrical power.
The RCIC has certain capabilities, It can provide make up water to the reactor when the reactor is shut off from the condenser in the turbine building that normally pulls heat out of the reactor cooling system. The RCIC can inject water at the rate of 2,000 Liters/min (600 gpm) even when the reactor is at a high pressure level. It can pump water into the RPV from full operating pressure down to ~1 MPa(150 psia). The RCIC can not lower reactor pressure. Pressure is relieved through the normal process of steam relief valves that route excess steam to the suppression chamber (torus).
The RCIC can be deployed faster than the high pressure coolant injection system, taking only 30 seconds to take off after receiving an “on” signal. This system is capable of replacing water boiled off by the reactor and can compensate for small leaks.
The RCIC operates by using a small steam turbine that is powered by the steam still being created by the heat in the reactor. The valves are used to maintain the correct water level in the reactor by turning the unit off and on. If the RCIC were to stay open and running it could potentially flood the reactor and send excess water back through the steam line that feeds the RCIC turbine.
The RCIC can operate without electricity but the valves do require DC power from the plant battery system. This allows it to be started even in a total loss of AC power at the plant.
The RCIC sends the steam it uses to turn the small turbine on to the suppression chamber (torus). The RCIC system pulls water first from the condensate storage tank located outside the reactor building and can alternatively pull water from the suppression chamber (torus). The condensate storage tank has between 200,000 and 375,000 gal capacity.
Example of an RCIC turbine, more information about these pumps can be found on Peter Melzer’s blog.
Interrelated Systems
The turbine and pump of the RCIC shut down if any of the following happen:
- The RCIC turbine runs over the set speed or the exhaust pressure from the turbine is too high
- The high water level is exceeded inside the reactor
- Suction pressure for the pump goes too low
- It receives an automatic shut down signal
The steam supply to the RCIC turbine can be shut off if any of these occur:
- High temperature in the system area
- Low pressure in the reactor
- A high pressure differential across two pipe elbows in the steam supply line
The RCIC is independent of these systems:
- AC electrical systems
- Plant service air systems (these provide air under pressure for control purposes)
- External cooling water such as the ultimate heat sink (river, sea or lake)
These things could cause the RCIC system to fail:
- DC power loss or failure causing the DC operated valves to close
- Rising temperatures in the suppression chamber (torus) causing it to fill with steam rather than water resulting in loss of suction for the RCIC pump
- Inadequate cooling of lubricating oil for the system
- Overly high pressure in the reactor vessel causing the RCIC system to trip
Unit 2 Timeline (RCIC events highlighted)
3-11-11
14:46 Auto SCRAM – Seismic
14:47 Loss of offsite power
14:47 Turbine generators trip
14:47 emergency diesel gen’s start
14:47 MSIV’S close
14:52 SRV’S controlling pressure in AUTO
15:02 Operators start RCIC
15:27 Series of tsunamis begin flooding in turbine building and reactor building
21:30 Workers begin running temporary cable to power SLC pumps
3-12-11
02:55 RCIC verified in service on unit 2
04:00 Operators switch RCIC suction to torus
3-13-11
02:42 RCIC maintaining water level
3-14-11
11:01 Blowout panel in reactor building dislodged by explosion in unit 3
11:01 Secondary containment lost
13:25 RCIC trips resulting in loss of injection into the reactor
At time of trip, indicated reactor water level was approx 95 inches (2400 mm) above the top of active fuel
and drywell pressure was 67 psi (465 kPa)
17:17 Indicated RPV level below TAF
18:00 Operators successful in opening an SRV and start to depressurize the reactor
18:22 Reactor water level lowered below the bottom of the indicating range
19:20 While touring to check status of fire engines, workers discovered that the engine had run out of fuel and no seawater was being injected into the reactor
19:54 After refueling and starting a fire engine, seawater injection commenced into the ractor via the fire protection system
23:00 Based on increasing reactor pressure, operators suspected that there was not enough air left to open the selected SRV. The operators started to open other SRV switches in an attempt to depressurize the reactor
3-15-11
00:02 Operators worked to align the containment vent system however, containment pressure remained stable at approx 102 psi
00:22 Operators continued cycling SRV control switches in an attempt to depressuze the reactor. Reactor pressure however, remained above 160 psig
06:14 A loud noise was heard in the area around the TORUS. Operators in the unit 1-2 MCR felt a shock – different than what they felt when unit 1 reactor building exploded. While suppression chamber pressure dropped to 0 psia indicating a potential instrument failure, drywell pressure remained high, indicating 105.9 psia, and reactor water level was 106 inches below TAF
Unit 3 Timeline (RCIC events highlighted)
3-11-11
14:46 Auto SCRAM – seismic
14:47 Loss of offsite power
14:47 Turbine generators trip
14:47 Emergency diesel gen’s start
14:47 MSIV’s close
14:52 SRV’S controlling pressure in AUTO
15:06 Operators start RCIC BUT
15:26 RCIC trips due to high reactor water level
15:27 Series of TSUNAMIS begin flooding in turbine building and reactor building
NOTE: Post tsunami unit 3 had 125 VDC power on main bus panel A and B
3-12-11
02:30 RCIC in service maintaining reactor water level
11:36 RCIC malfunctions , no injection into the reactor vessel
12:35 Operators start HPCI
3-13-11
02:42 Operators secure HPCI in preparations for opening a relief valve and injecting using a diesel driven fire pump. The relief valve does not open, and reactor pressure is too high to inject water, resulting in loss of injection into the reactor
05:08 Operators attempted to restart HPCI , the steam stop valve would not remain open and the system would not start
09:08 Operators open a SRV (steam relief valve) to depressurize the reactor
3-14-11
01:10 Injection into the reactor stopped because of a lack of water in the seawater pit
03:20 Workers moved the fire engine around allowing the hose to drop deeper into the seawater valve pit and seawater injection into the reactor was restored using a fire engine
06:00 Workers began injecting boric acid into the unit 3 back wash valve pit
11:01 Hydrogen explosion destroying secondary containment
11:01 11 workers injured
11:01 Debris damages portable generators and temporary power cabling
References:
NRC – Boiling Water Reactor Systems
http://www.nrc.gov/reading-rm/basic-ref/teachers/03.pdf
ANS – Safety System Descriptions for Station Blackout Mitigation: Isolation Condenser, Reactor Core Isolation Cooling, and High-Pressure Coolant Injection
http://fukushima.ans.org/inc/Fukushima_Appendix_F.pdf
INPO – Fukushima Daiichi Accident
http://nas-sites.org/fukushima/files/2012/08/INPO-Meng-August-2012-NAS-FINAL.pdf
Fukushima Failure By Design
http://brainmindinst.blogspot.com/2011/07/fukushima-failure-by-design.html
Peach Bottom Reactor Statistics
http://library.thinkquest.org/25916/database/pennsylvania5.htm
Browns Ferry Reactor Statistics
http://library.thinkquest.org/25916/database/alabama3.htm