Safety Instrumented Systems

More on Fukushima

Harry Forbes Disagrees with Bela Liptak's Automation Could Have Prevented Fukushima Article. Find Out Why

By Harry Forbes

I have a GE certificate in boiling water reactor (BWR) technology. Béla's analysis (May, 2013, "Automation Could Have Prevented Fukushima, 2") is incorrect in several aspects. He is confusing "passive" emergency cooling with the systems that were designed in. BWRs have emergency cooling systems designed to work both at high and low pressures, but none are fully passive.

This statement of Béla's is incorrect: "The plant was neither provided with elevated water storage tanks (to take advantage of gravity to flood the reactors), nor with backup cooling water pumps driven by steam turbines, as steam energy was available."

In fact, BWR IVs have two different emergency cooling systems, which are designed to keep the reactor cool by using steam. They're called RCIC and EDS. RCIC has a steam turbine-driven pump. These systems do require some electric power (to run their I&C!). This is part of the reason why emergency diesel generators and associated busses/switchgear are designed into the plant. No emergency power was available at Fukushima except station batteries, which only lasted a short time as expected. 

The meltdowns occurred because emergency electric power could not be supplied.

Electric power could not be supplied because the critical switchgear was under water. The critical switchgear was under water because the tsunami had flooded it. The tsunami flooded critical switchgear because the tsunami was higher than the plant's design basis.

The design basis tsunami estimate for Fukushima proved to be inadequate.  Tepco realized this possibility during the plant life, after the plant was built, and made some adjustments, but did not change the formal design basis. Had they done so, the inadequate elevation of the critical switchgear would likely have been noted and changed. There are very serious financial, regulatory and organizational barriers to changing a plant's design basis, especially after it is built.