Fukushima redux – design basis Godzilla?

I am currently compiling an update for the Fukishima Nuclear Accident as of March 17. The situation is developing very quickly and commenters on this thread (March 16 updates) are doing an excellent job at posting regular updates.

Meanwhile, here is a guest post from Luke Weston.

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Looks like we need an update post on the situation with the Fukushima nuclear power reactors. The situation continues to change and develop, but good, sensible, detailed information is still hard to find.

In the wake of the Fukushima incident, it has really helped me to understand what happened in 1979 at Three Mile Island; with nonsense all throughout the media, and FUD, and panic spreading, with good information almost impossible to find, and with the over-abundance of bad information leading to hysteria.

But this is the first time it has happened to the Facebook and Twitter generation; I’ve yet to determine whether that’s a good thing or a bad thing. We need to keep working hard to make sure it’s a net benefit for the good information, not the dodgy information.

Design Basis Godzilla

Why wasn’t the earthquake design basis set high enough?, some people ask. What if the next earthquake is magnitude 10? Magnitude 12? Magnitude 20? But where does it stop? Where do you set the design basis? What if the reactor is attacked by Godzilla?

No matter where you set the design basis, you will always exceed it one day, eventually. And when you do, the anti-nuclearists will complain that the design basis is not set high enough.

There is always some really extreme, really catastrophic situation that you can imagine, but what is its probability in any given year?

It’s all about Probabilistic Risk Assessment.

You know what the average probability per year of a magnitude 8 earthquake in the area is, what the probability of a magnitude 9 earthquake is, what the probability of a large tsunami is, etc. I’m not sure what the probability of Godzilla attack is. Need to ask an expert. Somebody get Matthew Broderick in here; he’s the guy to ask.

And you decide what the acceptable probability of a severe core damage (“meltdown”) incident (which won’t hurt anybody but will probably write off the reactor, like TMI) for the nuclear reactor is in any given year – let’s suppose it is decided that one such failure per 50,000 reactor-years of operation is acceptable. Then, with that in mind, you design the degree of seismic hardening and safety engineering for the nuclear power plant so that you hit that target.

It is the safest way to do it which is actually realistic. You can’t say that it’s absolutely 100% resistant to any hypothetical scenario of destruction that you can imagine, because there is always something that you can imagine that is more destructive.

Seawater injection into the primary containment of Unit 1 – not into the reactor vessel itself

On 12 March, TEPCO announced that they planned to cool the Unit 1 reactor with seawater, adding boric acid to the water as a nuclear poison, to prevent any possibility of unintended criticality. The injection of fresh water and seawater into the primary containment vessel through a fire-extinguishing system line commenced on the 13th of March.

This is not an injection of seawater into any part of the nuclear reactor or the Nuclear Steam Supply System itself. It is an injection of seawater into the containment structure surrounding the reactor pressure vessel.

These reports confirm my earlier prediction that they were not talking about actually putting seawater into the nuclear steam supply system, despite the lack of any previous clear, sensible announcements in the press to this effect.

Given that the reactor pressure vessel is completely intact and the control rods were fully inserted, normally, by the Reactor Protection System at the first sign of the earthquake, and the reactor is in a completely subcritical configuration, adding additional nuclear poisons (i.e. boron) to the system is seriously overkill; there is no pressing reason why it’s really necessary, other than to be extra conservative.

ASIDE: “Going critical” isn’t some kind of catastrophe, it’s what a fission reactor is designed to do, which it normally does under normal operating conditions.

Status of Unit 2

Over March 14 and March 15, Unit 2 has been in a similar low-coolant condition to Unit 1, with coolant water being boiled off through the torus, and with the Emergency Core Cooling Systems, such as the Low Pressure Coolant Injection system, operating.

Some kind of explosion was detected in the Unit 2 reactor building on the 15th of March, in a different area to the hydrogen ignition in Unit 1 and Unit 3. This hydrogen explosion was possibly within or close to the torus (the toroidal overpressure-supression tank), which is below grade, near the bottom of the reactor building.

Although this explosion may have damaged the torus, it has not damaged any of the real containment structures, such as the primary containment structure, the drywell, the reactor pressure vessel, or the outer reactor building.

There are valves that can be closed to isolate the vents that connect the torus to the drywell. Therefore, even if the torus was damaged or breached, it can be isolated from the drywell, meaning that there’s no real pathway between the drywell (the primary containment vessel) and the reactor building, through the damaged torus.

That means that your key layers of containment are all still intact.

Status of Unit 3, and its MOX fuel

There has also apparently been a loss of some of the normal reactor coolant systems in Unit 3, and the precautionary injection of borated water into the primary containment vessel, as at Unit 1, also began on March 13 and continued through to March 14.

As with Unit 1, seawater has been being injected, in addition to fresh water, but it is into the drywell – not into the reactor vessel. There has also been a hydrogen explosion in the rooftop refueling crane space above the reactor building at Unit 3 – very similar to the explosion in the same area at Unit 1.

Fukushima I Unit 3 is currently fueled with MOX fuel, containing recycled plutonium dioxide in its initial fuel load, in addition to low-enriched uranium dioxide.

You might see some slightly different characteristics in this MOX fuel with regards to things like the void reactivity coefficient compared to ordinary LEU fuel, because the fission cross section, plotted as a function of the incident neutron energy, will look slightly different for ²³⁹Pu compared to ²³⁵U. So, if the cross-section doesn’t drop away as rapidly as you increase the incident neutron energy, for ²³⁹Pu as compared to ²³⁵U, then you might see a negative void reactivity coefficient that isn’t quite as strong. Stuff like that.

But in practice there’s no evidence that we’re seeing any neutronic behavior or thermal behavior in the fuel that is any different, in practice, as far as the emergency situation is concerned, at Unit 3 as compared to Unit 1.

There is no significant difference in the composition of the radioactive fission products, or in their potential to be released to the environment, in the MOX fuel compared to the LEU fuel in the other reactors.

What we will see, however, is a lot of “plutonium phobia” in the media with regards to this MOX fuel, which is completely independent of any rational, fact-based discussion of the science and engineering. There’s this idea in the popular consciousness that plutonium is some mysterious, almost mythical, horrifying and terrible thing, and you’re going to turn inside out and die if you ever even look at it the wrong way.

But plutonium isn’t something that’s been spawned out of the arse of a demon or something, it’s actually just another element. It’s radioactive, but less radioactive than most of the fission products formed in a nuclear reactor.

The potentially harmful radioactive materials that might be released into the environment from a LOCA scenario such as the Fukushima I-1 situation, or the Three Mile Island accident, are the radionuclides of the high-volatility fission products, such as radioisotopes of xenon, krypton, and to a lesser extent, radioisotopes of iodine. Tritium is also on the list of key radionuclides that are present within the coolant water and are volatile enougn to potentially be relased, but its relatively low specific activity compared to the hort-lived fission products and its very low beta decay energy means that tritium is not significant in terms of its potential for harm.

Uranium and plutonium, for example, are heavy metals, they’re not soluble in the coolant water, and they are nowhere near as volatile as these gaseous and volatile fission products. This means that there’s no way that these elements can escape out into the primary coolant and eventually find their way out into the reactor building atmosphere, as these more volatile elements can. This means that there’s no way that they will be released out into the atmosphere in the same way that very small quantities of tritium and gaseous fission products may be be released. Even if they could be, uranium has a negligible specific activity, and it is a negligible source of radiation dose – and plutonium is not extremely radioactive either; being less radioactive than the majority of the fission products.

The Used Nuclear Fuel

There is a small fuel transfer pool in the reactor building at each of these GE BWRs, near the top of the reactor pressure vessel, that is used for the temporary transfer of used nuclear fuel during refueling. However, the longer-term storage of the used nuclear fuel is done in a pool elsewhere on the site. Those storage pools, outside the reactor buildings, are seismically hardened and defended-in-depth, just like the reactors themselves, and there are no indications of any problems with them. Since there was no refueling going on at the damaged reactors at the time of the earthquake, there is little or no fuel in the fuel transfer pools.

That’s my understanding of the situation, anyway.

Is there actually some used nuclear fuel stored in the Unit 4 fuel transfer pool at the time of the earthquake? I don’t believe there is much, if any.

Some media reports suggest there might be; but I’m extremely distrusting of the incomplete, garbled information being filtered out through the mainstream media on this whole issue – which is why I’m trying to patiently take information in from a diverse range of sources, and to patiently, carefully and skeptically piece it together into a self-consistent picture of what we actually know about the situation, based on my knowledge of the physics and my limited but partial knowledge of nuclear engineering.

If there is some fuel stored in the transfer pool, we need to know a few things about it.

How much is stored there? How long has it been there for? When was the reactor that it was taken from powered down, prior to fuel unloading? The decay heat of used nuclear fuel drops off exponentially following reactor shutdown, as the short-lived fission products that account for most of the radioactivity rapidly and exponentially decay. After a short period of cooling, the used fuel would not be significantly heating the pool water, nor would it require active cooling.

Immediately after fission is shut down in an operating nuclear power reactor, the power output from nuclear fission ceases. However, the nuclear fuel is still extremely radioactive, at least initially, due to the presence of very short lived fission products with very high specific activities. But that decay heat output falls off exponentially over the coming hours, as the very short lived fission products decay.

Thanks to Kirk over at Energy From Thorium for preparing this excellent chart, which I’ve borrowed from the EFT blog – all credit to him. Go and check out Energy From Thorium – it’s an excellent site.

Initially, the decay heat power level – the heat emitted by the radioactive decay of the radionuclides present in the used nuclear fuel, in a subcritical configuration – is approximately 5% of the fission reactor’s normal thermal power output.

The Fukushima I Unit 1 reactor has a nameplate capacity of 460 MW_e. At a thermodynamic efficiency of approximately 35%, the reactor has a thermal power output of approximately 1.31 gigawatts.

Therefore, immediately following reactor shutdown, the decay heat power output from the reactor’s nuclear fuel will be about 5% of that, or about 66 megawatts.

Over the last 5 days since the shutdown of the reactor, however, that power output has been dropping off exponentially, and it is now somewhere down around 6 megawatts of thermal power.

So, how much fuel is presently stored in the Unit 4 fuel transfer pool, if any? I might take an educated guess and say that it’s about one third of one full core loading, since about one third of the core fuel is replaced during one refueling. Let’s take a conservative guess, and say that it has been out of the Unit 4 reactor (which was already offline for maintenance prior to the earthquake) for about a week, at the least. It’s probably longer than that.

If those assumptions are valid, then the radiothermal power output from the used fuel in the fuel transfer pool will be only about one or two megawatts at the present time, and still decaying of course.

We know what the thermal power input from the fuel is, what the volume of water in the pool is, what the dimensions of the pool are, what the nominal temperature of the pool is, and what the specific heat capacity of water and the latent heat of evaporation of water are.

Therefore, we know exactly what the temperature of the water of the pool is going to do, and exactly what the volume of water in the pool is going to do. There is no need for guessing or speculation.

The nuclear fuel is made up of pellets of uranium oxide clad in tubes of Zircaloy. (Zircaloy is the trade name of the alloy in question, which is almost pure zirconium.) UO₂ is a refractory metal oxide with a very high melting point – it is not flammable or combustible in any way at all. It does not burn. In theory, the zirconium metal cladding could burn, if it was removed from all cooling water, exposed to an oxygen-containing atmosphere, and heated to an extremely high temperature. However, it is almost impossible to burn a piece of zirconium in this fashion in practice.

The rate at which the water in the fuel transfer pool would evaporate, in the absence of active cooling system functionality and in the presence of the fuel’s remaining thermal power output, is not more than a few percent of the water volume per day.

Given that there is approximately 16 feet or more of water above the surface of the fuel assemblies in the pool under normal conditions, it will take many days to weeks for this water to evaporate to the point where the fuel is potentially exposed above the surface, assuming that no additional water was added by any other mechanism.

The outermost layer of the multiple layers of containment surrounding the reactors – the cuboid-shaped reactor buildings – have walls and a roof made of solid concrete. On top of the concrete reactor buildings, however, there is an additional part of the structure – it is not made of concrete, but it is made of steel, with steel sheets over a steel frame. This steel building on top of the reactor building houses the fuel transfer crane, and it is built on top of the concrete roof of the reactor building.

That is, the part of the structure above the concrete shield plug and the refueling platform at the top of the concrete reactor building, as shown on the diagrams below.

It is this relatively weak steel structure on top of the reactor building, which is not really part of the reactor building proper, which has been blown out by a hydrogen explosion at Fukushima I Unit 1, as I described in my previous post, and apparently at one of the other Fukushima I reactor buildings as well.

The concrete roof of the reactor building proper – which is not where the hydrogen explosion occurred – is built over the top of the temporary fuel transfer pool; the pool is protected within the concrete reactor building.

Unit 4 Explosion and Fire

On 14 March, an explosion of hydrogen gas in the steel-framed structure atop the Unit 4 reactor building occurred, similar to the earlier explosion at Unit 1. As was the case at Unit 1, there was no real damage to the reactor building proper or any of the other layers of containment within.

On March 14, a fire was reported in the Unit 4 reactor building. The fire was caused by a leak of lubricating oil in the mechanical drivetrain that drives the reactor’s recirculation water pumps. Contrary to inaccurate information which was spread widely in the media and the social-media grapevine, this small fire had absolutely nothing to do with the used-fuel transfer pool or any fuel which may have been in it. The fire was extinguished shortly after.

Detection of radioactivity off the coast

It has been reported that the crew of the Nimitz-class carrier USS Ronald Reagan, off the Japanese coast, has detected the presence of radioactivity emitted from the Fukushima nuclear power plant. Since she’s nuclear powered, the vessel is obviously carrying sensitive instruments and sensors for detecting the presence of radioactivity around the ship, and obviously they’ve detected the trace levels of radioactivity that have escaped into the atmosphere.

It’s possible to measure the presence of small traces of radioactivity fairly easily with extremely high sensitivity. Just because we’re able to accurately measure it and quantify it at very low levels doesn’t mean we’re dealing with levels of radioactivity that are anywhere close to harmful levels.

Has anyone actually reported the actual quantitative presence of radioactivity that was detected aboard the ship?

The status of the Fukushima II plant

At the time of the March 11 earthquake, all four operating units at the Fukushima II station were automatically SCRAMmed by the Reactor Protection Systems.

Although there were some reports of disruptions to the Emergency Core Cooling Systems at Fukushima II, all the Emergency Core Cooling Systems at all the reactors were restored to operational status.

Between March 12 and March 14, every one of these reactors has been bought to a safe state of cold shutdown, where the decay heat from the nuclear fuel is being completely dissipated and the fuel is being kept cool, with no rise in fuel temperature.