Monthly Archives: September 2015


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.

How Do Sodium Iodide (Scintillation) Detectors Work?

Dear Dr. Zoomie – your last posting discussed how gas-filled detectors work, but I’ve got a different type. I think it’s called a sodium iodide detector. Can you tell me how this works and when I should use it? Thanks!

You’ve got one of my favorite detectors (and yes, I know that this raises me way up on Geek sale)! There are two fundamental families of detectors – the gas-filled detectors I wrote about the last time, and the scintillation detectors, of which the sodium iodide (abbreviated NaI) is one. The vast majority of radiation detectors out there – and virtually every detector used in general situations – fall into one of these two families. So since I wrote about the gas-filled detectors last time, this is a good chance to write about the other major family. Here’s how the scintillation-type detectors work.

First, an important point. Most scintillation detectors are only sensitive to one type of radiation. So NaI detectors will pick up gamma radiation, but not alpha or beta, zinc sulfide (ZnS) will only pick up alpha radiation, and so forth. In actuality, there might be some sensitivity to other radiations – NaI, for example, will sometimes pick up high-energy betas – but you should only use a detector for the type of radiation it’s designed to pick up. Now, with that out of the way, on to how the things work!

radiation_Detection_scintillation_detectors

The basic principle is the same for every scintillation-type detector: when radiation strikes the scintillator it causes it to give off photons of visible light (that’s the scintillation part). These photons pass through the crystal and they strike a thin metal foil called a photocathode – when this happens the light enters the second part of the detector, called a photo-multiplier tube (PMT). When the photon hits the photocathode it causes an electron to be ejected from the photocathode. Just past the photocathode there is a set of metal cups, each with a voltage applied to it (typically several hundred to a thousand volts) – the electron is accelerated by this voltage to a high energy and it strikes the cup with enough energy that it knocks loose a number of other electrons. Each of them, in turn, is accelerated towards the next metal cup, where each of the “new” electrons knocks loose a number of additional ones – by the end of the PMT the initial signal has been multiplied by a factor of a million or so. From there, it’s up to the instrument manufacturer to figure out how they’re going to use the light that’s emitted.

As one example, every time a gamma hits the crystal it starts this whole process that culminates in a pulse of electrons arriving at the far end of the detector. The simplest way to deal with this is to simply count the pulses of electrons as they arrive at a counting circuit – this is a great way to measure contamination (which we normally record in terms of counts per minute or counts per second). We can also use this mode to measure radiation dose rates, but only by assuming that every count carries with it a specific amount of energy. If you remember the posting on gas-filled detectors, this is the same way that Geiger counters work and it’s one of the reasons that Geiger counters aren’t always accurate for measuring radiation dose rates. We’ll get back to that in a moment. Oh – one other thing to be careful about with an NaI detector is that the larger crystals (say, 2”x2”) can be sensitive to drastic temperature changes – they can thermally stress the crystal, eventually breaking it. So if you’re working outside on a blazing hot (or bitingly cold) day, you probably want to leave the crystal outside, rather than bring it into a much colder (or warmer) office a few times a day.

Something else that we can use NaI detectors for is identifying specific radionuclides by measuring the energy of each individual gamma that enters the crystal – this process is called gamma spectroscopy, or can also be called multi-channel analysis (and the instrument set up for this purpose is called a gamma spectroscopy device, or a multi-channel analyzer – abbreviated MCA). The basic principle behind gamma spectroscopy is that every gamma-emitting radionuclide emits a gamma ray (or a few gammas) with very specific energies – like a fingerprint – and if we can identify the gamma energies precisely enough then we can identify the radionuclide(s) present. For example, cesium-137 (Cs-137) gives off a gamma with an energy of 662 thousand electron volts (abbreviated keV) – if we analyze a gamma ray spectrum and find a peak with an energy of 662 keV then we know that Cs-137 is present. Along the same lines, seeing twin gamma peaks at about 1.1 and 1.3 million electron volts (MeV) tells us that we’ve found cobalt-60 (Co-60).

twin_gamma_peaks

The problem is in figuring out how much energy is in each gamma photon – luckily we can do this with a scintillation detector. When a gamma ray interacts with the NaI crystal it deposits energy – this energy is what causes the photons to be given off. Not only that, but a predictable number of photons are emitted depending on the energy deposited – in a sodium iodide crystal, depositing 1 MeV of energy will cause about 42,000 scintillation photons to be emitted. We know the number of photons that it takes to eject a single electron from the photocathode, and we know the amount of amplification (and the size of the electrical pulse) for each electron ejected. So if we can measure the size of the output pulse then we know how much energy was deposited in the crystal – and we can know what radionuclide emitted the gamma that we just detected.

So these are two uses of NaI detectors – measuring contamination levels (in counts per minute or counts per second) and identifying radionuclides by measuring gamma energies – now for a third, measuring radiation dose rate.

Radiation dose rate is a measure of the amount of energy deposited in an object. You’d think that this would be fairly simple with an NaI detector since the number of electrons coming out of the detector is proportional to the energy deposited in the crystal. But that’s not how most manufacturers handle things – most of the time they simply count the pulse rate and, as with a Geiger counter, assume that they are all coming from Cs-137. So, for example, the manufacturer might determine that 175 counts per second are equivalent to a dose rate of 1 mR/hr. This means that NaI detectors set up to measure dose rate this way have the same limitations as a Geiger counter – unless you are measuring exactly what they were calibrated with the dose rates you measure are going to be wrong. Thus, if you’re using a scintillation detector to measure radiation dose rate you need to make sure that either you’re measuring the same nuclide to which it was calibrated or you need to have a set of correction factors that will let you convert the meter reading to the correct dose rate (assuming that you know the nuclide that’s actually present). This graphic, which is for a sodium iodide scintillation detector, show that the meter will show only half the actual dose rate if the radiation is from a Co-60 source, but will over-respond by a factor of 6 or 7 if the radiation is coming from the Am-241 in, say, a box of smoke detectors.

energy_response_for_ludlum_model_19

OK – so that covers how the NaI detector works and what it can be used for (again, to measure gamma contamination, for nuclide identification, or – with caveats – to measure radiation dose rate). Now a little on other types of scintillators.

Zinc sulfide (ZnS) is used to measure alpha contamination. The ZnS crystals are razor-thin – only about as thick as a single human hair (give or take a little). But since alpha particles can’t penetrate very far into any materials the crystals don’t need to be any thicker than this. For a number of reasons we don’t worry about dose rate from alpha radiation, so the only thing we need to measure is count rate – a fairly simple matter of counting pulses. The biggest problem with ZnS detectors is that they can be fragile (remember how thin the crystals are). It can also take a long time to do a proper alpha contamination survey since alphas are so easily shielded and have such a short range in air. But for alpha counting, ZnS is about as good as it gets, in spite of its limitations.

Finally, there are also beta scintillators. Liquid scintillation counting is normally performed in the laboratory using fairly expensive (and large) machines – chances are that you won’t have to use one of these unless you work in a laboratory. There are also beta scintillation crystals that you use the same way you use an NaI detector – these tend to be made of plastic (called organic scintillators). While not as fragile as ZnS or NaI, the photo-multiplier tube is the same in all of these detectors and is not very sturdy – no matter what kind of scintillation detector you’re using, you need to treat it gently.

So to sum all of this up, scintillation detectors have their limitations, but they are essential pieces of equipment for just about anyone making radiation measurements. The big things are to treat them gently to keep from breaking them, and to use them properly and within their limitations.