Nuclear Power


Are Radiation Levels at Fukushima High?

Dear Dr. Zoomie – I heard that radiation levels have gone sky-high at Fukushima. What gives? Do we need to worry that it’s still melting down or giving worse? Should I avoid the Pacific Ocean and the West Coast? Help!

Yeah…this was pretty interesting. And of course the question is why the radiation levels went up and what that means. But first, let’s talk a little bit about the radiation level that the story talks about.

No way around the fact that 530 Sieverts per hour (Sv/hr) is a whopping radiation dose rate. A dose rate of 10 Sv (1000 rem) is lethal 100% of the time, so this dose rate would give you a fatal dose of radiation in about a minute. So – yes – this is a serious dose rate. And let’s face it, if even a robot can only handle it for a few hours then we know it’s a high dose rate! For what it’s worth, I’ve been involved in radiation safety for 35 years and this is far and away the highest dose rate I’ve ever heard of. So there is no doubt that the dose rate reported is serious. The question we have to ask is whether this is a new dose rate (that is, did something change within the core to cause dose rates to go up) or was it like this all along and it was just found.

The Fukushima I Nuclear Power Plant after the 2011 Tōhoku earthquake and tsunami. Reactor 1 to 4 from right to left.

The Fukushima I Nuclear Power Plant after the 2011 Tōhoku earthquake and tsunami. Reactor 1 to 4 from right to left.

If this dose rate represents a change in core conditions then there might be cause for concern – it could indicate that something’s changed. For example, maybe some of the fuel dropped down to a new location. If it’s the latter then it could indicate that conditions might still be unstable. There are those who have speculated that perhaps the melted fuel has somehow achieved criticality again – this is highly unlikely. The reason is that it takes a great deal of engineering to achieve criticality in a nuclear reactor; the possibility that the melted fuel somehow rearranged itself to become critical is vanishingly unlikely.

It is possible that the components of the wrecked core have shifted and, in the rearrangement, a high-dose rate piece (or pieces) ended up near the instruments. This is more likely than a criticality, but given some other facts (I’ll get to these in a moment) this is also less likely than are other possibilities.

The key piece of information in this story was that TEPCO was pushing radiation instruments into a part of the plant that hadn’t been investigated before. As their instrument-bearing robot pushed into new territory it encountered new conditions – including parts of the ruined core. So it’s almost certain that the increased radiation levels are due to moving the instruments into new – and far more radioactive – territory. Think of moving your hand over a candle flame – as it passes over the flame you feel your hand heat up. This isn’t because someone just lit a candle – it’s because you moved your hand into the hot air above the candle that was already burning.

Interestingly, the robot took a number of photos as it was making its rounds, giving us an idea of what the core and reactor vessel now look like. Needless to say, it’s a mess – but that’s to be expected. There are lumps of what we can speculate are solidified fuel, a place that looks as though molten fuel melted its way through the floor grating, and so forth. It’s evident even to an untrained eye that there’s been a lot of damage – unfortunately, without knowing what the plant looked like before the accident (and being unfamiliar with this particular reactor plant design) it’s hard to know if anything non-obvious has changed.

The bottom line is that, while the newly reported radiation levels are dangerously high they probably don’t represent any changes in the conditions at the Fukushima reactors – much more likely is that they represent the first push into an area that has had extraordinarily high radiation levels every since the accident. So you don’t need to avoid the Pacific Ocean!

Is Nuclear Energy the Solution to Global Warming, and is it Safe?

Dear Dr. Zoomie – I’m worried about global warming because I live near the beach. The pro-nucks tell me that nuclear reactors don’t give off any greenhouse gases, but the anti-nukes tell me that you can’t make a safe reactor and we’ll just keep having more meltdowns will contaminate the whole world. What’s the real story?

Thanks – it’s been awhile since I’ve been asked a question that deals with the fate of the world. I guess, for starters, I have to say that I don’t think that nuclear energy is the solution to our problems – but I think that it’s part of the solution. I should also say up front that, while I have never worked for a commercial nuclear power plant (or for any sort of commercial power plant), I did spend 8 years in Naval Nuclear Power, including two years as an instructor and four years on a nuclear submarine. I also spent nearly two weeks in Japan after the Fukushima accident, helping to train emergency responders and medical responders in how to care for patients coming from the contaminated areas. So I think I’ve seen both some of the good and bad that nuclear power can offer – I hope that this helps me to be objective.

The Fukushima I Nuclear Power Plant after the 2011 Tōhoku earthquake and tsunami. Reactor 1 to 4 from right to left.

The Fukushima I Nuclear Power Plant after the 2011 Tōhoku earthquake and tsunami. Reactor 1 to 4 from right to left.

So – this is one of those “Do you want the good news or the bad news first?” sort of things. I normally go for the bad news first, so let’s start there. The bottom line is that, in the 70 years since Fermi oversaw the world’s first reactor criticality, the world has had three serious nuclear accidents in the nuclear age as well as a number of near-misses or less serious accidents. The result of these accidents is that large swathes of land in Japan, Ukraine, and Belorussia are currently evacuated and might not be reoccupied for decades. Huge amounts of radioactivity were put into the environment – the fallout cloud from Chernobyl and Fukushima blanketed the Northern Hemisphere, and Fukushima also dumped large amounts of radioactivity into the oceans. The accidents themselves have cost hundreds of billions of dollars and have had a global impact on the way that we view nuclear energy. And – at least in the case of Chernobyl – people have died. I should add that other accidents have killed people as well.

Chernobyl Disaster Aftermath:very extensive damage to the main reactor hall (image center) and turbine building (image lower left)

Chernobyl Disaster Aftermath:very extensive damage to the main reactor hall (image center) and turbine building (image lower left)

On the other hand, I should also point out that the World Health Organization conducted an extensive study of the people around Chernobyl in 2006 – on the 20th anniversary of the accident – and concluded that fewer than 100 people have died of radiation sickness or from radiation-induced cancer from that accident, and they also concluded that nobody in Japan will die from radiation-induced disease (including cancer) from Fukushima. Nobody got sick or died from the Three Mile Island accident, and only a handful of people have died from any of the other nuclear reactor accidents that have taken place. When we compare the number of deaths per gigawatt-hour of energy produced, nuclear actually stacks up quite favorably to other forms of energy – coal mining, hydrocarbon extraction and processing, and transportation of these fossil fuels are dangerous activities that cause hundreds to thousands of deaths annually; far more than the deaths from these accidents. And that doesn’t even get into the health effects of smokestack emissions, acid rain, and so forth. I’m not trying to say that nuclear power is harmless – just that we have to remember that no form of energy is without risk.

The Diablo Canyon Power Plant in San Luis Obispo County, California. This electricity-generating nuclear power plant near Avila Beach has operated safely since 1985. Photo by marya from San Luis Obispo, USA: http:// flickr.com/photos/35237093637@N01

The Diablo Canyon Power Plant in San Luis Obispo County, California. This electricity-generating nuclear power plant near Avila Beach has operated safely since 1985. Photo by marya from San Luis Obispo, USA: http:// flickr.com/photos/35237093637@N01

Ah – I hear you say – but if we build more nuclear reactors it’s only a matter of time until the next accident, and the more reactors we build the more accidents there will be. Well – yes and no. If we assume that the risk of an accident is the same for every reactor built then yes, this logic makes sense. But reactors have been getting safer with time – reactors built now are much less likely to suffer catastrophic accidents than those built in the 1980s. And – in case you’re wondering – factors that make reactors less likely to melt down or to have catastrophic accidents include operating at lower pressures (we can’t avoid high temperatures), simplifying the engineering designs, and making use of basic laws of physics to provide cooling in an emergency rather than pumps and other reengineered safeguards that require power to operate properly. Reactors powered by thorium, for example, have a number of features that make them almost impossible to melt down (sorry – this is far too much to get into here, but maybe at a later time; in the interim, http://energyfromthorium.com/ is a site where you can find some information if you’re interested). So if today’s reactors are safer than those built in the past, doubling the number of reactors won’t necessarily double the number of accidents. The bottom line is that some of the newest designs are virtually melt-down proof – not completely so, but pretty close. The bottom line is that the risk of meltdowns might increase, but not as much as one might expect. In fact, if we replace older reactors with the newer designs (in addition to building new ones), the risk of a meltdown might actually drop somewhat.

One last thing that’s touted as being a risk from nuclear reactors is the radioactive waste that they produce – more specifically, where it can be stored safely. At the moment the US has no long-term radioactive waste solution, but this is more a function of politics than of science. Again, there’s not enough space here to go into a full-blown discussion about radioactive waste disposal, but here’s something to consider: about two billion years ago a particularly rich uranium deposit achieved criticality and operated as a reactor for at least 100,000 years in what’s now West Africa. In the time since then, all of the fission products (the nuclear waste) have stayed put, in spite of the fact that the reactor zones are in porous and fractured sandstone and below the water table for most of that time. This bodes well for our ability to safely contain radioactive waste in an engineered facility set in much less-porous rock well above the water table.

The bottom line is that nuclear energy has its drawbacks, as does every form of energy. But these drawbacks are understood and can largely be managed – they certainly don’t call for abandoning nuclear energy altogether. Incidentally, you might also be interested to know that fossil fuels (coal, oil, natural gas) are associated with natural radioactivity due to the geochemistry of uranium. The radiation dose from burning any of these fuels is actually higher than the radiation dose from nuclear energy for every GW-hour of energy produced.

Now let’s look at the other side – the part of your question regarding nuclear energy and global warming. Here the argument is pretty simple – nuclear energy produces a lot of power and virtually no greenhouse gas at all; pretty attractive in a world increasingly worried about global warming. And – yes – some greenhouse gases are produced by the mining, processing, and transportation of uranium. But this pales in comparison to the greenhouse gas produced by fossil fuel plants.

There are other forms of low-carbon energy – solar, wind, tidal, and geothermal power come to mind. But each of these has its limitations – solar power doesn’t work well at night for example, the strongest winds are usually distant from the cities that use the most power, geothermal is only useful in a few location, and tidal power isn’t very useful in the continent’s interior. As of today, nuclear energy is the only form of non-fossil fuel power that can be used anywhere at any time. That’s not to say that nuclear energy is the answer to all of our problems (it isn’t) or that there’s no place for alternative energy sources (there is) – just that for reliable baseline power that can be put virtually anywhere on Earth and that doesn’t emit any greenhouse gases we just can’t get any better than nuclear at the moment. One of these days we might have fusion reactors, solar power satellites, or other exotic forms of energy – but not today and not in the plannable future.

What does it mean when a nuclear reactor goes critical?

Dear Dr. Zoomie – I was watching a movie the other day and everyone was upset that the reactor was going critical. What’s this mean? How does a reactor go critical, and how dangerous is it really?

The word “critical” certainly sounds bad, but it’s not as dangerous as you might think. Physicists use the term “critical” all the time – as one example, if you shine a light at a piece of glass it will pass through it up to a point – once you reach a certain angle the light will reflect off the glass. This angle is called the critical angle and, among other things, it helps light to reflect endlessly along fiber optic lines. But I digress…in the case of nuclear reactors, “criticality” simply means that the reactor is in a configuration that will let it operate at a steady power level. It’s a term developed by physicists during the Second World War and it just stuck around as reactors went from being scientific demonstrations to a mainstream source of power. Here’s how it works.

When a uranium atom is hit by a neutron (a slow-moving neutron) it can split apart – this is the fission process. It splits into two smaller atoms (called fission products) and it also emits a few neutrons. A neutron might escape the reactor entirely, it might be absorbed by metal in the reactor machinery (or by anything that’s not another uranium atom), it can be absorbed by a control rod (more on these later) or it might be absorbed by another uranium atom, causing another fission. To keep reactor power steady we need to have one of these neutrons to cause another fission. When that happens – when the reactor is in a configuration such that one neutron from each atom fissioned causes another fission – the reactor is said to be “critical.” So to a reactor operator, “criticality” is what they’re shooting for and it’s certainly nothing to be feared. A military reactor I was assigned to was once critical for low-power physics testing and was making about enough energy to heat up a cup of coffee.

Atomic Chain Reaction

The image above is a diagram of an atomic chain reaction. In an atomic chain reaction a neutron (in blue) hits a uranium atom and causes it to split in two parts, and also release two neutrons.  The neutrons cause other atoms to split (fission), releasing even more neutrons; which create more fissions.  Energy is released with each fission.  In a nuclear bomb, this chain reaction is very fast and uncontrolled, causing a huge explosion.  In a nuclear reactor, the chain reaction is slow and controlled and is used to produce energy.

I mentioned earlier that slow neutrons are most effective at causing fissions. So say (to pick a number) a neutron travels 5 inches before it can cause another fission – the fuel has to be at least 10 inches in diameter to be large enough that most of the neutrons are likely to run into another uranium atom before they escape from the fuel (it’s actually a little larger, but you get the idea). With a density that’s nearly 20 times that of water, a ball of uranium that’s 10 inches in diameter weighs about 380 pounds – this is the mass of uranium required to achieve criticality – the critical mass. Incidentally, this is NOT the actual number – just made up to illustrate the point. The real number will vary depending on the enrichment of the uranium (e.g. natural uranium, reactor-grade, or weapons-grade) and a number of other factors.

But it’s not enough to have a critical mass of uranium – it has to be in the right configuration as well. Picture a thin sheet of uranium and one of the atoms fissions. The only way that a neutron will be able to find another uranium atom to cause a fission is if it’s emitted in the plane of the sheet of paper – this means that the overwhelming majority of neutrons are probably going to escape the sheet of uranium without encountering a uranium atom at all. So you can have a critical mass of uranium that can’t achieve criticality because it’s in the wrong configuration – it doesn’t have a critical geometry. Now, if you crumple up the sheet of uranium it will eventually be in a shape that where neutrons are likely to cause fissions – this is the critical geometry. Unless you have a critical mass of uranium AND it’s in a critical geometry you will not be able to make a reactor operate.

There’s another factor – neutrons from fission move very quickly and they have to be slowed down; they slow down by bouncing off of hydrogen atoms in the water that keeps the reactor cool. So this means that our reactor core has to space out the uranium with water in between the rods of uranium to give the neutrons water to pass through (this process is called “moderation”). This adds even more to the size of the reactor core, as well as calling for even more uranium since the core has grown.

Diagram of How Nuclear Power is Generated

Simple diagram of how nuclear power is generated. Click the image to see an enlarged view.

Yet another thing to consider is that we want our reactor to be able to operate for awhile before we have to add more uranium to make up for what has fissioned – any reactor that has exactly the minimum critical mass will be able to operate only once (just as we build a fire with more than just a single match). So it’s necessary to have still more uranium to keep the reactor running for months or years.

OK – so now we have a reactor core that has, say, 2 years’ worth of uranium in it. This is great from the standpoint of longevity, but it will have too much uranium to be stable unless we have a way to control it – this is where the control rods come in. Control rods are made of material that are great at absorbing neutrons – when control rods are inserted between the fuel rods they absorb neutrons and keep them from causing new fissions; when control rods are pulled out the fuel is exposed to neutrons and fission can proceed. So when the control rods are fully inserted into the core the reactor is shut down; to start up a reactor we pull the control rods out – at some point there will be enough fuel exposed to the neutrons that there will be a chain reaction; at this point the reactor will be critical.