Tuesday, March 29, 2011

Patience and Monitoring the Japanese Reactor Situation

The news is sparse over the past week.  Every little bit sounds more worrisome, but we must have patience as the final analysis of the facts are often different than that which was first reported.

Tune into reliable sources such as NEI.  Be careful and look for assumptions and keep that questioning attitude.  The end game here is a set of actionable lessons learned for all industry.  I would also like to see how the reactors faired compared to the refineries and gas facilities for total human and environmental impact based on breach of their containments and overall releases to the environment.  Will non-nuclear facilities be analyzing and applying lessons learned to mitigate future events?  Are we asking or expecting them to or do we have disproportionate expectations for nuclear?

Perfect example:  Pu found on reactor site indicates core and containment breach which leads us to assume that the MOX fuel is the source of the problem.  This leads to a discussion of the dangers of MOX fuel use and a discussion of more widespread contamination beyond the reactor site.

It is more likely that the Pu  background as described by NEI: 
"On Monday, TEPCO discovered minute levels of plutonium in the soil at five locations at the site. The plutonium measured is as little as was in the environment in Japan following nuclear weapons testing during the Cold War and poses no health risk to humans."

Or is it really from the core?: To determine whether these readings are the result of weapons testing the samples must now be compared with samples from outside the site.

Plutonium is a by-product of the nuclear power generation process. At unit 3 at the Fukushima-Daiichi plant it is an ingredient in mixed oxide, or MOX, fuel, but only 6% of the total fuel loadout is MOX. Plutonium is a health risk mainly when it is inhaled because it can remain in the lungs and other organs, causing long-term damage including cancer.

At the same time NEI also reports the following:
"On Monday, TEPCO reported radiation levels of more than 100 rem per hour on the surface of puddles in the reactor 2 turbine building and in a trench outside the building. TEPCO is using sandbags to keep the water confined to the trench, a concrete channel that does not connect to the ocean. The trenches at reactors 1 and 3 are also at risk of overflowing and measures are being taken to contain the water."

Now we have some data that substantiates highly radioactive water at the site with a concern on how to manage it so that it does not spread beyond the site.  We need more information to understand the true nature and source of this radiation. The amount and composition of isotopes in water that has leaked from the unit 2 primary loop and reactor pressure vessel indicates that fuel damage in this unit is most serious and that fuel might have begun to melt, according to calculations by French Institut de Radioprotection et de Sûreté Nucléaire (IRSN).
Containment has been an assumption from the start, but we still do not know the extent of damage to the reactor from the earthquake.  Time will tell.  I am hopeful that a rational fact based response will continue.  Despite some early shock reporting and the standard greenpeace response, there has not been a
serious anti-nuclear uprising.

More to follow as the facts come in.

Sunday, March 13, 2011

Japanese Reactor Building Explosion Explained

Here is what likely happened as described in my trusty nuclear engineering text book Intro to Nuclear Engineering 3rd edition Lamarsh:

The explosion 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 during controlled venting by the operators to control containment building pressure which somehow ignited. Keep in mind the reactor building is just a weather structure. It is not intended as a barrier.  The hardened containment building within the reactor building was not damaged and is intact. It looks bad, but containment is still in place. I wish they would properly explain what is going on!!

Here is an excerpt for hydrogen production explanation provided by Dr Josef Oehmen (see the link below and here)  Note: his explanation does not include core melt.
The problem is that at the high temperatures that the core had reached at this stage, water molecules can “disassociate” into oxygen and hydrogen – an explosive mixture. And it did explode, outside the third containment, damaging the reactor building around.

The Japanese are doing all the right things...evacuations, monitoring, etc.  We should applaud and recognize how well the reactors and procedures are working despite such a huge event.  Keep in mind that the refinery has been burning for three days with heavy black smoke plumes up to 3000ft into the air and thousands of gallons of spilled oil.  Which one has a more significant environmental and human effect?   

Excellent explanation of the overall scenario here

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

Latest Media updates on nuclear power stations in Japan

Please see ANS Cafe for up to date links to credible news on the Japanese situation.

Information below from ANS Cafe:

An 8.9 magnitude earthquake is affecting nuclear power stations in Japan.  ANS Nuclear Cafe begins at | 0800 | 2011 03 11 | a media clip service on breaking news about the status of nuclear energy facilities in Japan. The news reports will be in descending order based on time/date stamps where available or when posted.

Fixed Links

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.