What Happened at the Japanese Nuclear Reactors?

UPDATE: Thanks to Eli the Wacky Wabbit, here is a link with more complete information. It has a few minor corrections to what I have here. Some of the very short live isotopes I did not mention are included. He listed cesium and iodine and what a minor worry they are in a neat way. Also the portable generators appeared to arrive in time, they just didn't have the right plug to hook up the power! I need to double check that because it should only take minutes to splice in directly.

The earthquake and tsunami that struck Japan is a tragic combination of natural disasters. I feel deeply for the Japanese people. On top of the huge destruction by natural forces is the nuclear incident.

As far as worst case scenarios go, the Fukushima reactor is about as close as you get. While accounts are still sketchy, it appears that the plant went through its standard "scram" or shutdown for the situation. The control rods were positioned to stop the reaction. While the reaction was stopped, the reactor core is still hot, so cooling water has to be pumped through the reactor core until the temperature drops, ideally to below 100 degrees C, but below the temperature where the core is damaged will do. Since the Tsunami appears to have flooded the back up diesel generators, the plant only had 4 hours of battery operation for emergency cooling. Because of all the other emergencies caused by the combination earthquake and tsunami, portable cooling equipment was not brought to the reactor in time. Without cooling, the reactor core experienced fuel damage which is a partial meltdown.

Engineers are not really sick individuals, but incidents like this are learning experiences. To be a truly walk away, passively safe design, there would have been no damage to the fuel in this case. Terms have different meanings, so passively safe can mean no significant loss of ionizing radiation, meaning no public health disaster. Whether I disagree with the meaning of the term or not is immaterial, the plant designers have to explain their meaning to the buyers which have to explain it to the public. The circumstances of the incident do qualify it as and extremely unlikely event or combination of events. As such, the final chain of safeties, containment, are designed to prevent significant release of radiation.

The pressure vessel containing the reactor core is the first level of containment. Pressure is the first job of the vessel. It is designed to hold pressure much greater than the operating pressure of the system. Three times operating pressure is a good number with an average burst pressure about twice that. This is just my recollection of design criteria, not the actual dsign of the plant. Pressure relief valves have different settings, normally starting a ten percent over operating pressure. So this plant with an operating pressure of 1000 psi would have 1100 psi first relief valve with 3000 psi pressure vessel rating and an average burst rating of 6000 psi. Since there was an apparent steam explosion, that probably happened outside the main containment building. This is often a design feature so that an explosion in a worst case scenario causes the least possible damage, which in this case is major release of radioactive material.

The containment building is the second level of containment. It is the reinforced concrete building that houses the reactor. Three feet or more of reinforce concrete is a nominal design for a containment building. While a containment building may be designed to contain some pressure, its main design is to contain heat and radiation. The floor of the containment building is the thickest construction to contain molten radioactive fuel in the unlikely event that it burns through the pressure vessel.

With this design, the radiation most likely released is tritium, a radioactive form of hydrogen. Tritium forms during operation when water, the moderator of the reaction, captures two neutrons. thanks to fairly complex statistics, the amount of tritium formed in the reactor cooling water is a small percentage of the normal water and heavy water (also formed in operation which is a stable isotope)in the cooling system. So the total amount of harmful tritium that can be released is small. Tritium occurs in nature and we are exposed to it all the time. The concentration of tritium is the factor of concern. maximum levels of allowed tritium release are normally a fraction, roughly a quarter, of tritium that may be in drinking water in some locations. It is naturally occurring so zero exposure is impossible even without a nuclear reactor. The released tritium as a gas, is spread over a wide area. This appears to be a problem but the wide area decreases the tritium concentration to safe levels, often unmeasurable levels, rendering it harmless. It is a popular misconception that wide spread release of radioactivity is a bad thing. It is actually an important natural safety factor.

The absolute worst case scenario is a complete burn through which should not be confused with a melt down. A burn through is when the nuclear material melts down, then burns through the pressure vessel and the containment building. Should this happen, the molten radioactive material can cause a steam explosion with ground water releasing the more toxic heavier ionizing radioactive elements. In order for this to happen, an extremely unlikely sequence of events would have to happen. First, enough of the fuel would have to melt, all of it pretty much, then it would have to burn through the bottom of the pressure vessel. Again, nearly all of the fuel would have to drop through the bottom of the pressure vessel, then melt through the concrete floor of the containment building. The thermal mass of the containment building is figured into the design to cool several times the amount of "possible" molten fuel that may fall out of the pressure vessel. Then enough molten fuel would have to strike enough ground water to cause an explosion of enough force to blow a path out of the containment building to allow the spread of the heavier radioactive materials. Without going through all the math, the probability of this happening is about as likely as being struck by lightning every day for a couple weeks in a row.

Finally, should the nearly impossible happen, the spread of the heavy radioactive elements is greatly limited by their weight. So the vast majority of the radiation does not go very far. That is bad for the area of the site but a good thing for the local population. The area around the reactor has several rings outlining levels of safety (on a site map not painted on the floor). I will try to dig up a drawing showing regions of risk. Chernobyl, BTW, did not have a containment building, so I will try to compare the two designs.

So what should you expect the actual damages to be in terms of human life lost due to the event? Emergency workers in the vicinity of the reactor have a large risk of being caught in a steam explosion. God love them, firefighting is a dangerous job. In an event of this type it is often best to stay away and let the design its job. Hopefully, emergency operations managers will recognize this in the future. If you exclude emergency workers, by which I am in no way implying their lives are less valuable, the loss of live would be very near zero.

The biggest lesson that should be learned from this incident is trust the design and walk away. The overall design provides for more than enough time to actually walk away to a safe distance.

One reason I promote small modular light water reactors is that the smaller amount of fuel in the reactor allows the system to be "truly" walk away. While mega large scale reactors are most common because of various reasons, the scale of the project increases the probability of reactor core damage. Reactor core damage turns the former containment building into a few billion dollar monument to poor choice. Even though that possibility is small it is significant enough to be concern as this case illustrates.

When this situation plays out, try to keep an open mind about the real damages, instead of the sci-fi hype potential catastrophe damages.