Safety at the hands of Physics

I have to say that I was really impressed at the response I got to my video.  The idea of passively safe nuclear power seems to strike a chord with a lot of people, especially given the current situation in Japan.  Knowing that if the Fukushima Diiachi nuclear power plant had been built with modern Generation III technology what we have seen happen over there would not be possible is a very intriguing notion.  I have been claiming for the last few weeks that the Fukushima incident couldn't have happened if it was a modern reactor, and I bet by now you are sick of me saying this without giving you proof.  So here we go...a little proof.

As it turns out, passive safety systems are both numerous and complex in design.  It took me quite a while to pour through texts talking about passive safety systems and decipher whatever code they were using.  It seems that nuclear engineers are as keen on using acronyms as the military is, which will be interesting being that I will be a nuclear engineer in the Navy.  Anyway, I am going to give you a warning that passive safety systems are extremely complex, often depending on multiple valve systems and thermal properties of materials that can be hard to follow.  My eyes are still crossed from reading about them, but I think I have boiled it down to several key ideas.  This, and I am blaring the Mumford and Sons right now to get my mind in the right place, which I highly recommend to everyone.  Maybe they can help us weave through this just a little to understand the premise, to help us gain the proof we are looking for.

As I mentioned above, there are many types of passive safety.  The one I talked about in the video was something known as a negative feedback mechanism.  Such mechanisms are designed to stop the reactor from continuing to heat up as it gets hotter.  Doppler broadening is one such mechanism.  The reactor I talked about in my video used the high heat capacitance of liquid metals to pull heat away from the reactor.  These systems are designed to keep the reactor from running away,  so that they operate at a safe temperature even when all cooling is lost.  They really interesting in themselves, but I will defer taking about these as I want to talk about the type of passive safety currently at hand; the type that would have prevented the disaster at Fukushima.

You probably heard people referring to something called decay heat when they were talking about Fukushima.  You see, the decay heat was the cause of the problem when it came to keeping the reactors under control.  What is decay heat?  Well, after fission takes place, daughter products from the fission reaction are left over.  These are just the split halves of the nucleus that underwent fission.  The daughter products are often extremely unstable themselves causing them to decay further.  Unlike the fissile material in the reactor (the nuclear fuel), the daughter products don't need neutrons around to decay.  So even when the reactor is shut down, the daughter products are decaying and releasing heat.  This is why the reactor must be cooled even after the reactor has been shutdown.  It is this cooling that failed in Fukushima.  There were many redundant systems in place to ensure that cooling would always be working, but apparently engineering redundant systems is not enough to insure against the improbable.

This is where passive safety steps into play.  So what exactly do I mean when I call something passively safe?  I mean that the system does not rely upon outside sources to operate it.  It does not need a back up generator or batteries, or even power at all.  They rely simply on the never failing laws of physics.  And if those fail, we have bigger problems to face than a nuclear power plant if you catch my drift. 

There are many types of passive safety systems that are designed to ensure that decay heat is pulled away from the reactor once it has been shutdown.  They all work off the same basic ideas though, so I will avoid the metric ton of technical information it would take to fully explain these and talk just about the basic design that they all share (in general).  If you really want to read about them though, I say more power to you and point you to this article.  For the rest of us, lets just stay here on Earth while discussing this.

The first source that powers passive safety systems is good old gravity itself.  Modern light water reactor (most commercial reactors around the world are light water reactors) make use of gravity to deliver water to the reactor if the cooling system fails.  Fukushima relies upon pumps to deliver water to the reactor, but once the pumps no longer work, they are just kind of left up a creek without a paddle.  When modern systems loose cooling abilities, water can still be delivered by water storage tanks that are placed above the reactors.  Modern reactors have to have a water source located above the reactor so that water can gravity feed to keep the core cool.  It is kind of like using a water tower to store water.  Even when the power goes out, the town will still be supplied water thanks to gravity!

Some reactors used tanks that are pressurized.  When a loss of coolant is detected, water automatically is injected into the reactor through valves which open based on pressure differences.  When a reactor looses coolant, there is a pressure drop in the reactor vessel which causes the valve to open and water to be delivered to the core.  Note that this does not take any "intelligent" input.  It relies on physics alone.

So, this is great...we have water delivered to the core, but last time I checked, water can't pull that much heat away from the core if it is just sitting around the core.  The water must have the heat removed from it as well.  This is done by circulating the water through a device known as a heat exchanger, where the heat in stored in the core water is transferred to a medium that can cool to the atmosphere.  This is necessary as we only want limited amounts of water to contact the core, and we want to keep this water contained in a closed loop.  We can't just dump the water that has been through core into the cooling ponds.  Besides the safety reasons, I get the feeling the NRC just wouldn't like this.  Anyway, the problem is that we need to circulate the cooling water through the heat exchanger without the use of outside power.  Sound impossible?  It's not.

I grew up in a house that had a ranch behind it.  There were no houses for miles behind where I grew up.  In fact, you couldn't see any houses behind my house when I was young.  You could only see Pikes Peak.  I freaked me out when I got up early one summer morning and there were all the sudden houses back there in the distance.  Did somebody build them overnight?  Not exactly...  You see, there was a hill somewhere between where I lived and the houses that were back there.  Thanks to the hill, you couldn't see the houses most of the time, but on that morning when the air was cool and the sun was strong, the heat from the houses actually appeared to make them rise from out of nowhere.  My point:  heat rises!

Ok, that was a long winded and perhaps pointless story to just get that across, but I had to do something to break up the monotony.  Anyway, heat rises!  Passive safety takes advantage of this.  It turns out that we can use this fact as well as gravity to circulate the water through the reactor.  The hot water or steam rises up through a heat exchanger which cools the water.  After cooling, the water uses gravity to return back to the reactor.  This is done using several different techniques, but I will spare you explanation of the methods.  The cool part is that natural convection and gravity can circulate the water without having to use powered pumps!  Nature has allowed us to build a pump that needs no electricity!  The only downside to this part of the passive safety system is that a person must manually initiate this process.  Physics does not automatically turn this one on for us, but at least we will know that it will always work.  It doesn't need those backup generators that have been washed away by the tsunami!

There has been a lot of discussion lately about whether we can safely harness the power of the nucleus.  Some tend to view it as we are simply trying to play with the power of God by taking on such endeavors.  I think that the safety part of the industry is just an engineering challenge though.  We can do it, though we might make a few mistakes along the way.  In general though, I think we can build nuclear reactors to be as safe as anything else in this world.