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Nuclear Power

Appetiser: our energy mix

Here are a few facts about energy of which you may not be aware.

Canada is in the top ten for electricity generation and in the top three for hydroelectricity generation in the world.
Canadian average per capita power consumption is among the highest in the world at around 16.5 megawatt-hours per year.
Renewable energy, if properly implemented, can provide baseload power. The technology already exists but the political will is not there yet.
In Ontario, Canadian-designed CANDU nuclear reactors supply more than a third of the province's electricity demand (see figure below).

[graphic depicting the mix of power generation types in Ontario.]

In the remaining three seminars of this course, we will discuss (a) how nuclear power works, (b) what happened in the Fukushima and Chernobyl nuclear accidents, and (c) energy sustainability and anthropogenic climate change. By the end you should be well enough educated to make up your own mind about the direction energy policymaking should be taking. I hope you will conclude that advocating for environmental sustainability hand in hand with human sustainability is the right way forward.

Sources

There are a great many textbooks out there addressing energy analysis and energy sustainability. The most accessible and accurate one I have found -- which is also free (as in free beer) -- is by David MacKay and entitled "Sustainable Energy - without the hot air". I am recommending it so strongly because it was recommended by my father Dr John Peet, a retired chemical engineering professor whose research for the past forty years has centred on energy analysis and energy sustainability. MacKay's book is available online in different formats at withouthotair.com. When I use material or examples from this book I will reference it as [MacKay].

In discussing nuclear power and nuclear accidents, I will use some material from a public lecture "Understanding the radioactivity at Fukushima" by Ben Monreal of UCSB Physics. I will reference these excerpts as [Monreal].

Other sources used only once will be referred to as they occur.

Nuclear power

Here is some good advice about not thinking in terms of binaries:-

We made the mistake of lumping nuclear energy in with nuclear weapons, as if all things nuclear were evil. I think that's as big a mistake as if you lumped nuclear medicine in with nuclear weapons.
-- Patrick Moore, former Director of Greenpeace International.

What kinds of radiation are there?

Alpha -- these are helium nuclei. They are heavy (in subatomic particle terms) and therefore don't penetrate air or denser substances very far, so are called short-range. The most harmful ingestion route is inhalation, because the alpha particles cannot penetrate significantly into clothing or skin. Example alpha emitters: uranium-235, polonium-210, radon-222, and radium-226. (Nomenclature note: the number refers to how many neutrons are in the nucleus; which chemical element we have is determined by how many protons are in the nucleus.)
Beta -- these are just electrons or positrons, and their emission involves anti/neutrinos, via the weak force. They are lightweight and moderately penetrating. They can injure skin if there is prolonged exposure. Example beta emitters: strontium-90, carbon-14, tritium (hydrogen-3), potassium-40.
Gamma and X-ray -- these are photons that have sufficiently high energy that they can (like alphas and betas) ionize atoms and do DNA damage. They weigh nothing, can travel many feet in air and many inches in human tissue, and can penetrate clothing. These sources need heavy shielding. Example gamma emitters: iodine-131, caesium-137, cobalt-60, radium-226, technetium-99m.
Neutron -- these are dangerous directly and also because nuclei in the body can capture impinging neutrons and become radioactive. Neutron sources also require a lot of shielding.

Nuclear reactors currently in service on planet Earth are called fission reactors. Nuclear fission occurs when the nucleus splits into (usually two, sometimes three) daughter nuclei, releasing energy and often neutrons in the process. Nuclear fission can occur spontaneously in heavy isotopes, but in a nuclear reactor or bomb the process is driven by bombardment with neutrons. For some isotopes, each nuclear fission produces at least as many neutrons as were needed for the initial bombardment, enabling a nuclear chain reaction to occur.

[a pictorial representation of a nuclear chain reaction in which the neutron coming from one fission decay triggers another fission, etc.]

Such materials are called fissile. Out-of-control fission chain reactions are for bombs only, so in nuclear reactors the fission chain reaction must be carefully controlled. Fission involves the strong nuclear force and produces a lot of energy (about 200MeV per fission). This is why nuclear reactor design is at least as complicated and expensive as rocket science.

The power of the strong nuclear force

Here is an example from [MacKay] illustrating how powerful nuclear fission reactions are compared to other ways of generating energy. In one of his book chapters, he discusses a British example of energy usage. Each average Briton consumes about 16kg of fossil fuels per day, composed of 4kg coal, 4kg oil, and 8kg gas. This fossil fuel habit creates 11 tons of waste carbon dioxide per year, or about 30kg/day. How much natural uranium would be required to produce the same amount of energy as burning these fossil fuels? The answer is 2 grams. Yes, you read that correctly -- just two grams. The amount of ore required to produce this fuel is about 200 grams, and the resulting nuclear waste weighs about a quarter of a gram. Nuclear waste is nasty stuff because it contains alpha, beta and gamma radiation sources and because the variety of daughter elements makes the chemistry very challenging as well. However, the amount of waste is staggeringly smaller than the fossil fuel total, and it does not contribute any greenhouse gases.

The job of a nuclear reactor is to turn nuclear fuel (fissile material like uranium or thorium) into fission products and the all-important heat which is used to generate electricity. Most of the reactor power comes from primary fission events; only a small percentage comes from later fission product decays. Neutron capture on reactor materials, water, or air can create induced radioactivity, making further messes in the nuclear waste mix. The heat coming from a nuclear reactor can be converted into electricity in a multitude of different ways, which is why there are several different basic types of nuclear reactor design. The Fukushima Dai-ichi reactors, for example, were a 1970s Boiling Water Reactor (BWR) design. Personally, I prefer the more modern proliferation-resistant designs such as pebble bed reactors or Molten Salt Reactors (MSRs) burning thorium fuel. Even Ontario's older-but-not-ancient CANDU reactors have superior passive safety features to BWRs.

Radiation damage

Only ionizing radiation (alpha, beta, gamma, X, neutron) can hurt you. Even so, for millions of years humans have lived with low doses of radiation from Earth's natural radioactivity (mostly in uranium-238, thorium-232 and potassium-40) and from cosmic rays. Why is this? Even the most punchy particle of ionizing radiation will not damage every atom it comes across in the human body. Some examples: [Monreal] points out that alpha decays common in minor actinides (produced from neutron capture on fuel) damage only every 10th atom passed, while beta and gamma decays damage about every 3000th atom passed. In other words, ionization rarely leads to DNA damage. Also, human cells have extremely effective DNA repair mechanisms operating even without impinging ionizing radiation, so that DNA damage rarely results in changed cells. Changed cells, in turn, rarely result in cancer. So one impinging subatomic particle which can ionize a hydrogen atom in your right eye is not likely to give you eye cancer. The issue is how many you absorb over what time period.

Different units are bandied around in discussing damage from ionizing radiation. The one we will focus on is the Sievert, which denotes a biological effect of absorbing one Joule of energy per kilogram of human tissue. It corresponds to absorbing roughly [Monreal] ten billion Cs-137 decays per gram of body mass. [Monreal] lists the extra cancer cases per 10000 people after receiving a dose of 1 Sievert as follows: breast 7, stomach 5, lung 4, leukaemia 3, colon 2, thyroid 1.6; this means that a 1 Sievert dose is a risk you would go out of your way to avoid, like texting while driving. But a single dose of a few milliSievert should not make you lose sleep.

milliSieverts

Here are some sample doses to help you count your milliSieverts.

For single doses:-

What	How many mSv?
Dental X-ray	0.005–0.03 mSv
People within 16 km of Three Mile Island during accident	0.08 mSv
Mammogram	2 mSv
Brain CT scan	0.8–5 mSv
ICRP limit for volunteers averting major nuclear escalation	500 mSv
ICRP limit for volunteers rescuing lives or preventing serious injuries	1000 mSv

For ongoing doses:-

What	How many mSv?
Average individual background radiation dose	0.00023 mSv/hour
Highest dose rate measured in Finland during Chernobyl	0.0005 mSv/hour
Living near a nuclear power station	0.0001–0.01 mSv/year
Living near a coal-fired power station	0.0003 mSv/year
Sleeping next to a human for 8 hours every night	0.02 mSv/year
Cosmic rays at sea level	0.24 mSv/year
Ground radiation	0.28 mSv/year
Natural radiation in the human body	0.40 mSv/year
Maximum acceptable dose for the public from any man made facility	1 mSv/year
New York-Tokyo flights for airline crew	9 mSv/year
Current average dose limit for nuclear workers	20 mSv/year
Dose from background radiation in parts of Iran, India and Europe	50 mSv/year
Dose from smoking 30 cigarettes a day	60–80 mSv/year

Cartoonist xkcd has provided a brilliant visual representation of relative harms of different dosages of ionizing radiation for those wanting more visual detail. Click on the picture for a higher-resolution image.

[xkcd's radiation dosage chart. I apologize here to people using screen readers: this is sufficiently complicated that it would take several pages to provide a textual description of all the components. However, all the most pertinent information is already listed in the above milliSievert dose lists.]