CANADIAN NUCLEAR SOCIETY SOCIÉTÉ NUCLÉAIRE CANADIENNE

Nuclear Questions & Answers

In June 1995, a group of Grade 8 students from École O'Kelly School, Canadian Forces Base Shilo, visited Whiteshell Laboratories and the Underground Research Laboratories of Atomic Energy of Canada Limited (AECL). The students had won a province-wide competition held by the Manitoba Branch of the Canadian Nuclear Society (CNS). As part of their entry, the students had compiled a list of questions for us to answer. Dr. Dave Wren, a research chemist and past chair of the CNS Manitoba Branch, wrote the following set of answers for the students. They, and all other contest entrants, received copies of the question and answer sheet. Since then we have received other requests for this sheet, so here it is on the net.

Morgan Brown
Chair, Manitoba Branch of the Canadian Nuclear Society
September 1996

Questions from the O'Kelly School, Shilo, Manitoba About the Nuclear Industry

The nuclear industry and nuclear power are subjects that are frequently mentioned in the news, often in connection with concerns regarding their safety and the wisdom of producing electricity using nuclear power. Concern regarding any industrial activity is always valid. Any large-scale human activity, if carried out with insufficient regard for its impact on the environment and people, has the potential to cause serious harm. The nuclear industry from its birth has been much more aware of this potential than probably any other industry in the world and has designed its reactors and other facilities to meet stringent public expectations for safety and environmental protection.

For someone who does not actually work within the nuclear industry, there are naturally a number of very good questions that arise. Answers to these questions can help to understand how the nuclear industry works and how it acts to meet those tough public expectations.

The following are a number of excellent questions about the nuclear industry and radiation that were raised by a group of Grade 8 students from Shilo, Manitoba. The questions penetrate to many of the important issues about nuclear power and radiation. Brief answers have been provided for these questions, but interested students are encouraged to read further for more information. Some of the answers are also a little bit technical. This is hard to avoid if you really want to understand the answers.

1. How many Canadians are employed in the nuclear power industry?

There are about 28,000 people directly employed within the nuclear industry in Canada. These people fall into three groups. The first is the people who work in the uranium mining and processing industry. Canada is the world's largest producer of uranium. In 1993 we mined about 28% of the total uranium produced in the world, worth about $600 million. There are about 2,200 people were employed in the uranium production industry.

The second group is the people that operate nuclear reactors to produce electricity. There are about 13,100 people employed by Ontario Hydro, Hydro Quebec and New Brunswick Power in their nuclear divisions. By far the largest number (12,000) are employed by Ontario Hydro because it operates 19 nuclear reactors compared to one each for Hydro Quebec and New Brunswick Power.

The third group is the people who perform research, design reactors and build and supply components and services for the nuclear industry. The total here is about 13,000, including about 4,500 people employed by AECL in nuclear research, and reactor design and construction, plus about 8,500 employed in private industry across Canada.

Reference: "Nuclear Power in Canada - The CANDU System", pg. 12.

2. How is the nuclear industry regulated?

All countries of the world with nuclear power programs have a regulatory agency that sets the regulations and standards for the industry in that country. In Canada, the Atomic Energy Control Board (AECB) has the responsibility for setting and enforcing the regulations and standards for all activities involving the use of radioactive materials. This includes not only the operation of nuclear power reactors, but also the operation of nuclear research laboratories and research reactors, the use of medical radioisotopes in hospitals and laboratories, the operation of irradiation facilities and equipment, the industrial use of radiation sources, the mining of uranium, and the management of radioactive wastes. The AECB is a federal government agency that is completely independent of the owners and operators of nuclear reactors and other facilities. It is completely separate from AECL, which is a crown corporation that designs and builds nuclear reactors.

The AECB sets its regulations based on publicly acceptable standards for safety taking into account the experience and the standards set around the world. It also relies on the work of international scientific committees which establish guidelines for estimating the health risks of exposure to low levels of radiation. These are based on extensive medical research and reviews of historical radiation exposures.

The goal of AECB regulation is to ensure that public safety and environmental protection is maintained. The Board sets its limits for radiation exposure based on safety reviews and risk studies. These limits ensure that the health risk to the public and to nuclear industry workers from radiation is well below the other normal health risks that we face in our daily lives. One of the ways it ensures this is by setting the limits for radiation exposure well below the levels of naturally occuring background radiation that we all receive.

The AECB enforces its regulations through the control of licenses for various activities using radioactive materials. For example, the activities of AECL's research laboratories are reviewed by the AECB to ensure that safety goals are met. The licenses for these laboratories are normally issued for a two year period. For nuclear reactors, there is a stringent licensing review before permission is given to build a reactor, and periodic reviews to maintain an operating license. There are actually AECB staff located at every nuclear power reactor to help ensure that regulations are met.

Reference: "Up Front - Power under Control: Regulating the Nuclear Industry in Canada"

3. How safe will the disposal vaults be?

AECL has developed a concept for the disposal of used nuclear fuel that is now being reviewed by a Federal Environmental Assessment and Review Office (FEARO) Panel. The concept is that spent fuel bundles will be placed inside titanium metal drums (because titanium will not corrode (rust) easily) and these drums will be put into a vault in granite rock about 500 to 1000 m down in the ground (a sort of special reverse mine where we put things into the ground instead of taking them out). Clay and other special materials will be used to seal around the drums to slow down the movement of water in the rock (yes, there is water in cracks in granite) to slow the rusting of the drums and dissolution of any of the fuel. This will prevent the radioactive atoms in the used fuel from travelling in the water to the surface where they could possibly harm people.

This concept will be extremely safe. We measure safety in terms of risk - how likely is someone to experience an accident or develop an illness. The disposal of used nuclear fuel will be much less risky than other things that we normally do. AECL's disposal concept has been designed to meet tough requirements set by the Atomic Energy Control Board. The most important is that there be less than 1 chance in 1,000,000 that someone living very close to the disposal vault will develop a fatal cancer in any year. The actual vault is expected to do much better than this. Even this low risk assumes that very low levels of radiation can cause cancer. This assumption is not proven. The risk is so low because the radiation dose to someone living near the disposal vault is expected to be about 1,000,000 time less than the radiation dose you get everyday from the natural radioactivity in the rocks and soil of the earth. You should worry more about getting hit by lightning.

Reference: "Regulating Nuclear Fuel Waste", pg. 13.

4. How many accident reports do you have filed on an average within a year from Canada alone?

The Atomic Energy Control Board keeps records of all of the unexpected events that occur all over Canada associated with the use of radioactivity. It classifies these events according to the different levels of seriousness. In 1994 there were a total of 200 different events reported to the AECB (from 22 reactors). Of these only 1 was considered to be a serious incident. In this event a pipe broke at a reactor and quite a lot of heavy water spilled onto the floor. It was not considered a serious accident because the reactor was shut down normally and there was no radioactivity released to the environment. Canada has actually had only one serious accident ever. This happened at the NRX research reactor in the very early days of nuclear development (1952). (see also http:://http://titan.fanshawec.on.ca/people /jedicke/nrx.htm)

The other 199 events reported in 1994 were all minor. About 120 reports were made when something unexpected happened (for example an unusual signal from an instrument). Some reports were made, as required, if a reactor broke a minor operating rule. There were about 45 reports on unplanned, or potential, radiation exposures.

The nuclear power stations report all of their own events to the AECB, but the AECB checks up on their honesty. As part of this checking, the AECB actually has some people who are present every day at every power station in Canada. A report on all the events is written by the AECB every three months and is available to the public.

Reference: Annual reports are available from the Atomic Energy Control Board,
(613) 995-5894,Box 1046, Ottawa, Ontario, K1P 5S9
http:://www.gc.ca/aecb/docs/ar95/eng/menu.htm

5. Is plutonium sold from nuclear plants?

There is absolutely no plutonium sold from any CANDU power reactors.

Plutonium is naturally produced in all nuclear power reactors as a result of the capture of neutrons by uranium atoms. This converts the natural isotope uranium-238 into plutonium-239, but most of this plutonium is actually burnt in the reactor. About half of the heat generated in a CANDU reactor comes from burning the plutonium that is formed.

The light-water reactors used in most of the other countries of the world use fuel enriched with the fissionable isotope uranium-235. A fissionable isotope will break apart when it absorbs a neutron and release energy (and a few more neutrons). When this fuel can no longer burn well, partly because some of the uranium-235 is used up, it makes sense to reprocess the fuel and take out the remaining uranium-235 and plutonium to use in making new fuel. This does not make much economic sense for CANDU reactors because the spent CANDU fuel does not have as much leftover uranium-235. Not all light-water reactor owners and countries reprocess their fuel because this is still quite expensive. Those that do, get back the uranium-235 and plutonium for use in their own reactors. Nobody sells plutonium from reactor fuel to make bombs.

Reference: "Nuclear Facts - Does Canada Contribute to Nuclear Weapons Proliferation?"

6. What are the chances of terrorists getting nuclear waste to make weapons?

The chances are very, very, very, very small.

A CANDU nuclear reactor 'burns' natural uranium (that is just uranium taken from the ground and made into nuclear fuel bundles with no 'enrichment', or addition of extra uranium-235). Natural uranium has about 0.7% uranium-235, light-water reactor fuel has about 3% uranium-235. After a period of time in a reactor, much of the uranium-235 is used up and the fuel cannot produce as much energy so it must be discarded. The spent, or used, fuel also now contains some plutonium as described in Question 5. As well, splitting uranium atoms also produced new atoms, fission products, in the fuel. Some of these 'poison' the fuel so that it will not 'burn' well and some are very radioactive.

To make a nuclear bomb, it is necessary collect some fairly pure uranium-235 or plutonium-239 as a metal. This is hard to get from used fuel for three reasons. First you must separate the uranium or plutonium isotopes to get the ones that you want (different isotopes are chemically the same but have different numbers of neutrons in the nucleus). A power reactor makes not only plutonium-239 which can be used in making nuclear weapons, but also a whole series of other plutonium isotopes, including Pu-238, Pu-239, Pu-240, Pu-241, Pu-242, and Pu-243. Some of these are very bad for making nuclear bombs. If you don't separate the isotopes and get everything very pure, you can't make a bomb.

Separating different isotopes of the same chemical element is very, very, expensive because the isotopes are chemically identical. To get enough material to make a bomb would take a very long time or require a very large amount of equipment. Only governments can really afford to do this and it is difficult to keep secret.

The second reason is that used fuel is highly radioactive (because of the fission products) and must be handled in special shielded facilities, like in special handling facilities called hot cells. These are not something the average terrorist has on hand in his garage. A terrorist would get sick from radiation before he could make a bomb.

Finally you need to get quite a bit of used fuel to get enough uranium or plutonium to make a bomb. Terrorists would need to steal a large amount which would be hard to do.

Reference: "The Status of Safeguarding 600MW(e) CANDU Reactors"

7. How can nuclear radiation cause birth defects?

Radiation can cause birth defects by affecting the chemistry in the sperm or egg cells of people. You can think of radiation as a tiny bullet that shoots into a cell (see the discussion on radiation in Question 8). This bullet smacks into some of the chemical molecules in a cell as it slows down and eventually stops. The result is that some of the molecules are broken apart. Sometimes the broken chemical is the DNA molecule in the nucleus of the cell. This molecule is the blueprint that makes us what we are.

DNA molecules suffer tiny breaks all the time during normal life in the cell and there are other special molecules that normally fix up these breaks as good as new. Sometimes the repairs don't work well and there is a permanent change in the molecule. Most the time these new changes don't matter or the cell just dies. Sometimes (very rarely) the change turns the cell into a cancer cell.

If the DNA change happens in a sperm cell or an egg cell, then there may be a birth defect if the damaged sperm fertilizes a normal egg (or vice versa). A baby will develop with a change in the DNA blueprint. Most of the time these changes won't matter but sometimes we may see a real difference that we call a birth defect.

It is important to remember that small changes in DNA happen all the time from a number of causes, of which radiation is only one. These changes are not all bad or even noticeable. Exposure to radiation is only likely to lead to important birth defects when the amount of radiation is so high that the cell repair team cannot fix all the damage and there are very many changes made to DNA molecules.

References:"Regulating Nuclear Fuel Waste", pg. 26 and 27.
Nuclear Information Series "Radiation is Part of Your Life", pg. 20-22

8. What are the different kinds of radiation?

Radiation generally refers to the invisible transportation of energy from one place to another. Radiation includes the movement of very small, fast particles and the movement of electromagnetic waves. We experience the second as light (visible radiation) and heat (infrared radiation) and use electromagnetic radiation for radios, microwaves and other applications.

In the nuclear industry, radiation usually refers to 'ionizing' radiation. This radiation includes fast-moving particles and light waves that have enough energy to separate electrons from atoms to create ions (charged particles). There are basically four kinds of ionizing radiation associated with nuclear reactors:

• beta particles,
• alpha particles,
• gamma rays, and
• neutrons

A beta particle is just a fancy name for a high-energy (a very fast moving) electron. Fast electrons can slam into atoms and knock out other electrons as they slow down. These collisions create new ions, more speeding (but now a little slower) electrons and can break apart chemical molecules. Electrons are small, so they can penetrate into things, but they also bounce easily off the electrons and nuclei of atoms so they can't go very deep. Beams of high-energy electrons are used industrially to sterilize medical equipment and can irradiate food (if it is not too thick).

Alpha particle is a fancy name for a helium atom nucleus (two protons and two neutrons together) that is moving very fast. This is a relatively big, heavy particle that can't go very deep into anything (tin foil will stop an alpha particle), but it can do a lot of damage getting there.

Gamma rays are very energetic electromagnetic radiation. They can move a long distance into materials before stopping and require the most shielding. Eventually a gamma ray will collide with an atom to knock out a high-energy electron, and this then behaves like a beta particle. X-rays behave like gamma rays, but are weaker.

Neutrons are only produced during the fission of uranium in a reactor. They start out moving fast and slow down as they collide with the nuclei of atoms. These collisions can break molecules apart. Eventually the neutrons slow down and they are either absorbed into another nucleus (e.g., in changing uranium-238 to plutonium-239) or they fall apart into a proton and an electron.

A fifth category of ionizing radiation that has nothing to do with nuclear reactors is cosmic rays. We are continuously being bombarded with ionizing radiation from space, an array of fast particles and light rays. These come from the fusion reactions in our sun and the other stars of the galaxy. The amount is small and we don't notice its effects. If we fly high in the sky in an airplane, we actually see more cosmic radiation because we are above the thick atmosphere that absorbs more of the cosmic rays. A trip across the Atlantic Ocean will give you about the same ionizing radiation as getting a medical x-ray.

References: "Regulating Nuclear Fuel Waste", pg. 6-7.
Nuclear Information Series "Radiation is Part of Your Life", pg. 4-14.

9. Is irradiated food radioactive?

No.

Food is irradiated with either gamma rays (normally produced by the radioactive decay of cobalt-60 or cesium-137) or high-energy electrons (produced by an electron accelerator). The amount of radiation that the food receives is designed to kill most of the bacteria and other organisms in the food. This ensures that the food is not harmful (it is a form of sterilization). It also allows food to stay fresh longer because it kills some of the agents that cause food to spoil.

Most radioactive atoms are made by the absorption of one or more neutrons in a nuclear reactor or by the fission of uranium (or plutonium) in a reactor.

Reference: Nuclear Facts "Why Food Irradiation?"

10. Are Canadians insured against nuclear power plant accidents?

Yes.

Canada has a law, the Nuclear Liability Act, that requires nuclear power station operators to have $75 million in insurance against accidents. This would pay the public for any damages resulting from an accident. If an accident caused more than $75 million worth of damage, then the federal government would become responsible for paying extra compensation. This ensures that individuals are protected against losses.

Reference: Nuclear Facts "Are Canadians Insured Against Nuclear Power Plant Accidents?"

11. What is a meltdown?

A 'meltdown' refers to a very unlikely accident in a nuclear reactor. To understand, it helps to know a little bit about nuclear reactors. Reactors are just very big sources of heat. They typically produce about 3,000,000,000 watts of heat when operating. About 1/3 of this is converted to electricity and the rest is thrown away as waste heat (as a result of the thermodynamic laws of nature). The waste heat must be removed by very effective cooling systems that use a lot of cold water. When you turn a reactor off, you stop the fission process that produces most of that heat energy. However, there is still lots of heat being produced in the uranium fuel from the radioactive decay of the fission product atoms made by the nuclear fission. When you first stop a reactor this 'decay heat' is still about 3,000,000 watts. To think about these large numbers, remember that a light bulb uses about 100 watts and small room-heater uses about 1000 watts.

A big problem could develop in a reactor if the flow of cooling water is stopped. For that reason, there are lots of separate safety systems to prevent that from happening, and to make sure that reactors shut off very fast. Nuclear reactor designs like to use the belt and suspenders and safety pin approach to keeping their pants on.

The light-water reactor designs, used in the United States and most other countries in the world, have all of their nuclear fuel inside one large steel pot of cooling water. If this pot were to dry out, the decay heat could make the fuel so hot that it would melt the bottom out of the pot. This would be a 'meltdown' accident.

In Canadian CANDU reactors, the nuclear fuel is distributed in pipes that run through a separate large tank of water. Even if the normal cooling water in the pipes is stopped, this tank of water will keep the fuel from melting. It is much more difficult to imagine an accident in a CANDU reactor that would cause a 'meltdown'. It is not strictly speaking impossible but you have to imagine that an awful lot of things go wrong that should not occur. Even so, that is why we build CANDU reactors with very strong containment buildings so that accidents which damage the reactors do not harm the public.

In thinking about 'meltdowns' and other reactor accidents, it is important to remember that these are very unlikely things to occur. Scientists and engineers with very active imaginations sit around trying to think of everything that could possibly go wrong and then think of how to prevent that. The regulators like the AECB look over their shoulders and ask difficult questions to make sure things are safe.

Reference: Nuclear Facts "How Safe are Nuclear Reactors?", "How Does a Nuclear Reactor Work?"

12. Is a nuclear reactor like a nuclear bomb?

Nuclear power plants can not explode like a nuclear bomb because of the way that they are made. In a nuclear bomb, energy is released in a runaway chain reaction. Neutrons released from the fission of one uranium (or plutonium) atom cause the fission of more atoms, and so on. For this to work well, the uranium atoms need to be very close together, so bombs use pure uranium metal. Unfortunately the heat from the fission reactions will try to make to bomb fly apart, so you have to work very hard to get a big bang before the bomb falls apart and the chain reaction stops.

In a nuclear reactor, the uranium starts out spaced well apart in separate fuel elements. The arrangement is designed so that there is a well-controlled chain-reaction that cannot run away. This is partly managed through the use of different control systems (such as control rods). It is possible to imagine an accident where the control system fails and the reactor tries to increase the speed of its chain-reaction. However, it cannot explode like a bomb. Instead, the fuel would just heat up for a short time before it damaged the reactor and fell apart. In the worst possible case, the result would be a mess and a pile of hot fuel that could not keep a runaway chain reaction going.

If there were such an accident, very hot fuel might contact cold water and make lots of steam very rapidly. This might look like an explosion and could do some damage inside a reactor containment building. This is what happened at the reactor in Chernobyl. The difference between there and Canada is that CANDU reactors are surrounded by a containment building that would hold the steam in, while the Chernobyl reactor just had a weak building shell that could not hold in the steam.

Reference: Nuclear Facts "How does a Nuclear Reactor Work?"

13. Why is water a good temporary shield for radioactivity?

Water is an excellent permanent shield for radiation. A radiation shield is just a layer of material that can absorb radiation and prevent it from harming anyone. All materials will absorb radiation. Some materials are better than others. Normally 'heavier' elements such as lead are best because they have a large nucleus and lots of electrons to absorb the energy of the radiation and slow down particles. The denser the material, the thinner that it needs to be to act as an effective shield. Air is a very poor shield because it is not very dense at all.

Water is an excellent choice because it is very cheap and because it is a liquid. This means that it can be used to completely fill a tank around a radiation source without leaving any air gaps that would not stop radiation. Water is used in CANDU nuclear power stations as a radiation shield in two places. The reactor itself sits in a large tank of water (we actually call it the Shield Tank). This allows people to work safely quite near the reactor, even when it is operating. The used fuel from a reactor sits safely on the bottom of a large pool of water (it looks like a swimming pool). The water is so deep that it is safe to stand beside the pool and see the used fuel on the bottom.

Water and other materials are good permanent shields because they are not used up when they absorb radiation. Instead the radiation is just converted into heat, which can easily be removed from water.