Friday, March 11, 2011

How Shutdown and Core Cooling of the Japanese Reactors Likely Functions

Information from a licensed senior reactor operator/control room supervisor on a boiling water reactor (BWR) similar to the Fukushima plant.

Good info from a CNN article comment post by someone named jjj4591. The following was taken directly from the CNN article comment.  I did not verify every detail, but it appears to be a solid description of BWR shutdown and core cooling methods.  I typically do not like to point to and reproduce someone's work, but the info is key to get out to people to avoid the hysteria and false statements regarding the safety of nuclear power plants.  Let the record show that this information is the product of jjj4591.

 I’ve worked in the US nuclear industry for 30+ years and for 18 years I was a licensed senior reactor operator/control room supervisor on a boiling water reactor (BWR) similar to the Fukushima plant. I helped write the emergency procedure guidelines that are used at US BWRs. There is a great deal of information flying around that just does not make sense. There just seems to be no detailed technical information getting out to the public on this. At the risk of over simplifying the system, a BWR is like a giant pot of boiling water. Regular light water, not heavy water, goes through the reactor, is heated by the splitting of uranium atoms, turns to steam and spins a turbine-generator to make electricity. The steam is condensed back to water and pumped back into the reactor to continue the cycle. There are 3 basic barriers to the release of radiation, the metal clad that encases the uranium fuel, the reactor pressure vessel, and the containment. If 2 of these 3 are compromised and the third is in jeopardy, US plants will advise shelter or evacuation of nearby residents. The reactor operates at a normal pressure of about 1000 psig. During an earthquake of this magnitude, the reactor would be expected to automatically shut down (called a reactor scram). Control rods are hydraulically driven into the core in less than 7 seconds. I do not know if this took place but if it did not, we’d probably hear about it because it would be such a big deal. Even with rods inserted, the reactor continues to produce heat equivalent to about 3% of its full power level. This is not the same as taking a pot off the stove and letting it cool. There are still some atoms splitting and fission products decaying that produce heat. This drops off slowly and is why there needs to be layers of redundant cooling with backup power. During such an earthquake, power from outside the plant would not be expected to be available. The plants have several back up diesel generators (locomotive style engines) that supply power to motor driven cooling systems that will supply high flow of water up to about 300 psig.. There are also steam driven systems to supply cooling water up to 1100 psig. There are also pressure relief systems that active at about 1100 psig. If reactor pressure gets too high, relief valves open and discharge steam to a water filled pool inside the containment.
Here are some things that do not make sense, Reports that the pressure is 1.5 times normal. There are at least 10 relief valves and any one can handle the energy after a plant shut down. CNN reports the US military has flown coolant to the site. The coolant they use is regular water; I can’t imagine why the US would need to fly in coolant.
Right now I’d want to know a few things.
Are all rods fully inserted? What is the water level in the reactor? It’s normally about 12 feet above the top of the fuel. What injection systems are available? What is the reactor pressure? What is the status of containment?
Based on limited information, this is what I think might happen.
Earthquake hits, high vibration on the main turbine automatically trips the turbine by rapidly closing stop valves. Reactor automatically shuts down (scrams) all rods go in. Earthquake disrupts off site power to the plant and back up diesel generators should have started, maybe they did not. Main sources of water to the reactor are not available. If there is no pipe break off of the reactor, the pressure will slowly increase. After about an hour, a relief valve(1 of about 10) will open at about 1100 psig and drop pressure to about 1080. The steam is sent to a pool of water called a suppression pool in the containment that condenses the steam. This valve will cycle open and close every 5-10 minutes. Operators would use a small steam driven turbine (RCIC) to supply water at high pressure to the reactor under these circumstances for several hours. You can sit like this a long time, hot and at 1000 psig it’s no big deal as long as water covers the fuel in the reactor pressure vessel. If that turbine is not available, there is a larger steam driven turbine (HPCI) that supplies more water meant to provide make up if there was a pipe break. If neither of these systems is available, the relief valve will continue to cycle and reactor water level will slowly drop. At some point before the water level lowers to the point of uncovering the fuel, all the relief valves would be open (ADS) and pressure would be reduced to below 300 psig to allow the low pressure but high flow systems (CS & LPCI) to restore water level and cooling. These pumps however, need electricity, like from the diesel generators, to run. If things get this far but there is no injection, in US plants there are things like diesel fire pumps that can be tied in to provide alternate sources of water. I’m not sure if they are set up to do this in Japan. Without cooling, eventually the fuel temperature will exceed 2200 deg F and the clad will melt. Fission products that are highly radioactive will get dispersed into the reactor vessel. If there is a pipe break or relief valve open, those fission fragments get dispersed through containment.
The USNRC has some technical info on this link for those of you that wish to know more.


  1. Thanks, I have no experience on a BWR plant so this really helps. I am concerned about the spent fuel pool on this 40 year old plant, what are your feelings on that?

  2. Spent fuel is removed from the reactor and stored in a pool of water or in dry casks if the spent fuel pool is full. Spent fuel generates some heat and must be cooled, however the amount of cooling is low. The biggest concern right now is getting the just shutdown reactor cooled off. The spent fuel is only a radiation concern which can be dealt with at a later time. I am assuming that the pool or casks are intact as the likelihood of failure is very low.

  3. Nathan, please comment on the explosion. Could this have occured if cooling was proceeding normally? Does it imply rods were exposed? WTF is going on?

  4. No this would not have happened if proper cooling was in place. The uncovered fuel began to melt and lead to the initiation of various exothermic chemical reactions between the molten material and the water steam mixture, some of which produce hydrogen. That hydrogen along with other gases were released to the reactor building which somehow ignited. Keep in mind the reactor building is just a weather structure. The hardened containment building was not damaged and is intact. It looks bad, but containment is still in place. I wish they would properly report what is going on!!

    Excellent explanation here:

  5. Since the Fukushima I Nuclear Power Plant is a BWR, the control rods would have been inserted from below using motors or hydraulics. On a marine nuclear power reactor, the control rods fall from above using only gravity and springs. The clamps that grab the control rods, pull them from the pile and hold them up have to have electricity to stay closed. If power is interupted, the clamps open and the rods fall and the reactor SCRAMS in about 4 seconds.
    Were the control rods at Fukushima successfully inserted?

  6. Yes rods were successfully inserted within seconds of the earthquake. The turbine senses the excessive vibration and trips (shuts down steam supply to the turbine) then the reactor scrams or shuts down. The problem was a station black out due to the emer diesels being taken out by the tsunami. The plant cooled on battery power for eight hours after that. Power was not restored until Tuesday 22Mar. During that period temporary cooling to spent fuel pool and seawater supply to the reactors were used to provide cooling. We will know all the facts soon, but it appears the reactors are now under control and being actively cooled. That assumes the electrical power is effectively hooked up and that the piping and systems are functional or can be made functional in short order.