Saturday, March 12, 2011

Nuclear Reactor Safety, Cooling, and Failure Explained - Keeping the Japanese Event in Perspective

The Japanese disaster is significant and we cannot discount the current issues surrounding the nuclear reactors.  We should, however, keep the nuclear issue in perspective.  This was an enormous 8.9 magnitude earthquake followed by a serious of tsunamis and multiple aftershocks.  If the reactors are intact and there is no significant release of radiation then we have seen just how well they are designed and protected.  They may not be operable afterwords, but they will not have exploded or released radioactive materials in some apocalyptic event.  Contrast that with a LNG plant, the refineraries, fuel depots, chemical plants and other forms of electrical generation.  Would you want to be near them during this event?

Think of the environmental and human impact of all the chemicals, fuel, oil, building materials etc. spread across the country side. The worst outcome we are seeing now for a nuclear accident is a small release of radioactivity probably equivalent to Three Mile Island. That means no deaths and no significant radiation exposure.

Current status of the Japanese event as reported by NEI (Nuclear Energy Institute).

Let's step back and explore one of the fundamental concepts of reactor theory and the FACTS that make reactors in the US and most of the world inherently safe. I am talking about the temperature coefficient of reactivity.  I think if people better understood the science and engineering behind western reactor designs that it would alleviate some of the concern with reactors supposedly being able to "blow" up or melt down in some China Syndrome event.




The sad news for the nay Sayers is that reactors are safer than ever and US reactors are designed such that they shutdown when something goes wrong. Current reactor technology (these are the designs that are being licensed and built in the US and around the world) uses less equipment and less automation, focusing on passive systems. Passive is the key.  New reactors do not need electricty to keep the shutdown core cool. Unlike the Japanese and current operating reactors which require offsite or emergency power to pump coolant water into the shutdown reactor.

When something goes wrong in a nuclear reactor temperature is likely to rise in the reactor core. A negative temperature coefficient of reactivity means that as temperature goes up...reactivity goes down. When reactivity goes down the reactor is essentially turning itself off like pulling your foot off the gas of your car.

Reactivity is the engine of fission in a reactor. Reactivity equals more neutrons per unit time (neutron density) and therefore more fission, therefore more energy released, therefore an increase in temperature. That increase in temperature is harnessed as steam to drive a turbine and create 20% of the power in the US.


A negative temperature coefficient of reactivity makes a reactor inherently stable. Example: As power demand increases on the turbine, more steam is used, the coolant circulating through the steam generator and the reactor is cooled slightly. As the temperature goes down the reactivity....goes up! So we push on the gas pedal and get more neutrons and energy as we increase fission and compensate for the temperature drop by increasing reactivity and reactor power to match steam demand.

As you can see this stability allows for a mitigated emergency response for a major casualty leading to an increase in temperature. If I lose reactor coolant and cannot cool the core as effectively the reactor will shutdown.  Now a shutdown core still needs to be cooled.


When disaster struck Three Mile Island, containment was in place and there was very little release to the environment (maximum offsite radiation dose 0.1 rad and total population dose was approximately 10 person-rems see [ref1] and NRC analysis http://www.nrc.gov/reading-rm/doc-collections/fact-sheets/3mile-isle.html).  This is the likely scenario that is unfolding with the Japanese reactors. Fuel may be exposed due to a lack of ability to provide adequate cooling water.  At that point the fuel will overheat and melt.  The key is that the melted fuel is contained within the reactor pressure vessel.  Furthermore the overall reactor containment vessel provides another layer of defense.  Now pressure in the reactor will increase and will have to be released via controlled valve openings to the containment building.  Eventually pressure will increase in the containment building surrounding the reactor which will be released to the environment.  At that point radioactive gases are released to the environment.  This is not a steam explosion or massive release of radioactivity.  We will see the final result, but we must understand that we are not talking about massive radiation doses to the general population.  When we know the actual numbers we can equate the radiation exposure to a number of x-rays at the doctor or dentist or to the number of hours of time flying in a plane.

Contrast TMI and the Japanese reactors with Chernobyl. Russian designed reactors had a net overall positive temperature coefficient of reactivity (graphite moderator with water coolant thus positive steam void reactivity and positive reactivity of initial control rod motion [Ref1]). See where we are going here?!? Temperature goes up and reactivity goes up. Therefore power goes up and therefore temperature goes up.... leading to disaster. Chernobyl also did not have sealed containment. It also had an enormous reactor core which lead to fluctuating reactivity and flux..essentially three or four different reactors all within the same core behaving independently yet as a whole. All of this lead to a difficult to control reactor that was not inherently stable.

When the casualty hit, the reactor was unable to be controlled (There are multiple factors) and fission products and gases were released to atmosphere (no containment) NRC analysis http://www.nrc.gov/reading-rm/doc-collections/fact-sheets/chernobyl-bg.html.



Ref 1 Intro to Nuclear Engineering, John Lamarsh and Anthony Baratta

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