Quote:
Originally Posted by Grog
Must have figured they had enough ways to cool the core in an emergency. I agree with you, that for safety reasons, that natural circulation would be better. Maybe the newer designs have it, I don't know.
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Basic BWR design has numerous system to remove residual heat following a shutdown. The problem is that all of these require
power... When the tsunami took out the backup diesel generators, things got serious.
Nominally (speaking from my basic GE BWR background) the system used to remove heat initially is aptly called the Residual Heat Removal (RHR) system. There are two pumps and two heat exchangers per unit (and each are very large). This system circulates water through the core and then rejects the heat to a closed cooling water system (not seawater or cooling towers). If this fails, there is a system called the Core Spray system that is intended to keep cooling water in the core and above the fuel. Again, two very large pumps with their own heat exchangers.
If the pressures in the reactor are too high to pump water in (that can happen), then there is a system called the High Pressure Coolant Injection (HPCI) system that "injects" water into the reactor. It runs off of residual steam (not electricity). The problem with this system is that it is low volume - you're not going to pump large amounts of water into the reactor this way. There is also a system called the Reactor Core Isolation Cooling (RCIC), that functions to put water in the reactor.
All of the pumps for these systems are located well below the reactor level to maintain net positive suction head. During nominal operation, a BWR bleeds off some steam from the turbines to drive pumps that pump condenser water back into the operating reactor, but without steam, these are useless.
If all of these fail, you throw the hail mary pass and inject borated water into the reactor, which absorbs the neutrons. It ruins the reactor, but at this point that is the least of your worries.
One thing that I do not understand is how the Japanese are pumping seawater into the reactor. I don't understand an easy way to do that...