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Nuclear Safety(Nuclear Accidents, Security,radiation)

Nuclear Safety

Safety is closely linked with Security, and in the nuclear field also with Safeguards.

Safety focuses on unintended conditions or events leading to radiological releases from authorized activities. It relates mainly to intrinsic problems or hazards.

Security focuses on the intentional misuse of nuclear or other radioactive materials by non-state elements to cause harm. It relates mainly to external threats to materials or facilities.

Safeguards focus on restraining activities by states that could lead to acquisition of nuclear weapons. It concerns mainly materials and equipment in relation to rogue governments.

Nuclear safety includes the actions taken to prevent nuclear and radiation accidents or to limit their consequences. This covers nuclear power plants as well as all other nuclear facilities, the transportation of nuclear materials, the use and storage of nuclear materials for medical, power, industry, and military uses. In addition, there are safety issues involved in products created with radioactive materials. Some of the products are legacy ones (such as watch faces), others, like smoke detectors, are still being produced.

Nuclear weapon safety, as well as the safety of military research involving nuclear materials, is generally handled by separate agencies than civilian safety, for various reasons, including secrecy.

A nuclear power plant cannot blow up like a bomb. A bomb converts a large part of its U-235 or plutonium into fission fragments in about 10^-8 seconds and then flies apart. This depends on the fact that a bomb is a very compact object, so the neutrons don't have far to go to hit another fissionable atom. A power plant is much too big to convert an important part of its fissionable material before it has generated enough heat to fly apart. This fact is based on the fundamental physics of how fast fission neutrons travel. Therefore, it doesn't depend on the particular design of the plant.

The History of Nuclear Safety

From the very beginning of the development of nuclear reactors; safety has been an important consideration. On December 2, 1942, when the first atomic reactor was brought to criticality, Enrico Fermi had already made safety an important part of the experiment. In addition to a shutoff rod, other emergency procedures for shutting down the pile were prepared in advance. Fermi also considered the safety aspects of reactor operation. Shortly before the reactor was expected to reach criticality, Fermi noted the mounting tension of the crew. To make sure that the operation was carried out in a calm and considered manner, he directed that the experiment be shut down and that all adjourn for lunch. At the very beginning, with such leadership in safety, it is no wonder that the operation of reactors to date has such an impressive track record.

The first ten years of nuclear power development were devoted to demonstrating that power reactors could be designed, built, and operated; the second ten years were devoted to showing that power reactors might be operated economically; the third decade saw the rise of a viable commercial industry, the fourth decade, punctuated by the accident at Three Mile Island, was a mix of rapid commercial growth coupled with increasing government regulation, the fifth decade, despite the Chernobyl accident, was highlighted by a reaffirmation by the nuclear industry to providing a safe source of electrical generation and serious public skepticism. The nuclear industry is now well into the sixth decade of nuclear power, public skepticism is still a major factor guiding the future of nuclear power both in the United States and internationally. For the industry to survive, nuclear advocates will have to be devoted to restoring the public’s faith in the integrity of the industry and be willing to participate in a healthy debate of the issues. Meanwhile, industry technologists are continuing their devotion to operating reactors economically, advancing improvements And development new system designs. Reactor safety has played a significant role in these developments and will continue to do so.

Chernobyl Disaster in Europe

However, to a lesser extent, a power plant can still blow up if it is sufficiently badly designed and operated. The Chernobyl plant reached 150 times its normal power level before its water turned to high-pressure steam and blew the plant apart, thus extinguishing the nuclear reaction. This only took a few seconds.

As a result, it was a rather small disaster. 31 people died. Cave-ins in coalmines often kill hundreds. However, about 20 square miles of land became uninhabitable for a long time. This isn't a lot.

Fall-out from the Chernobyl explosion will contribute an increase to the incidence of cancer all over Europe. How much of an increase is disputed. Since the increase will be very small in proportion to the amount of cancer, we probably won't know from experience.

The largest estimates are in the low thousands, which would make Chernobyl a disaster comparable to the Bhopal chemical plant or the Texas City explosion of a shipload of ammonium nitrate or the Halifax disaster during World War I. On the other hand these large estimates are small compared to the number who have died in each of several recent large earthquakes in countries using stone or adobe or sod houses.

It is comparable to the number killed in coal mining accidents in the Soviet Union over the years Chernobyl was operating.

The large estimates depend on the linear hypothesis which is almost certainly wrong but which is used for regulatory purposes because it is so conservative. The estimates are probably too high by a substantial factor, maybe 10, maybe 100.

However, a recent survey indicates a greatly increased rate of thyroid cancer in children (including three deaths) j in Belarus since the accident. I don't know the total number of cases, which would permit comparing Chernobyl with other accidents.

The Chernobyl accident depended on the specific characteristics of the RBMK reactors, of which the Soviets built 16 before switching to designs more like those used in the rest of the world. The relevant features of RBMK reactors include "positive void co-efficient of reactivity". This means that if the reactor gets too hot and some of the water turns to steam, the rate of the nuclear reaction increases. In most other power reactors, the void coefficient is negative. If some water boils the reactor tends to stop.

RBMK reactors don't have containment shells designed to prevent radioactive materials from getting out.

Other less extensive nuclear accidents

The Three Mile Island accident destroyed the reactor, but the core itself remained confined. Radioactive gases were vented, but there is no accepted evidence that this harmed the public. Fault trees for possible failures have been generated and studied. However, there could be something not taken into account.

At the end of 1998 there were 9012 civilian power reactor years of experience throughout the world, and Chernobyl is the only nuclear power plant accident harming the public. The U.S. Navy has been powering ships with nuclear reactors for 50 years and has had no nuclear accidents.

In 1999 Japanese technicians mixing up fuel for an experimental reactor violated the safety procedures and created a critical mass of uranium, which caused an increasing nuclear reaction until the container with the mixture boiled over and stopped the reaction. Three people were hospitalized, two of whom died. The press, especially AFP, which is anti-nuclear, billed this as the worst nuclear accident since Chernobyl in 1986. Losing two people in 13 years isn't much. That's good for an energy source.

It is amicable to note that nothing is perfectly safe, but there are acceptable standards and risk tolerance in every effort and nuclear power are safe enough to be relied upon as a source of energy.

Safeguarding Nuclear Materials against Terror

Every country wanting to make bombs has succeeded as far as is known. None have used material produced in power reactors. (Plutonium produced in RBMK reactors may have been used in Soviet weapons. The RBMK was designed as a dual-purpose reactor suitable both for power production and bomb production. For this it was necessary to be able to replace fuel rods while the reactor was operating, and this made the reactor too big for a containment structure, and this is what allowed the radioactivity to spread.)

If the fuel rods are kept in the reactor for the two years or so required for economical power generation, much of the Pu-239 atoms produced absorb another neutron and become Pu-240. It is more expensive to separate the Pu-240 from the Pu-239 than to get Pu-239 from a special purpose reactor in which the fuel rods are removed after a short time. The Pu-240 makes the bomb fizzle if there is very much of it.

It seems that some of the Russian PU-239 of which samples were sold in Germany was pure enough so that some isotope separation process was probably used after the plutonium was extracted from the fuel rods.

Nuclear fuel can be diverted to make nuclear weapons. This is the most serious issue associated with nuclear energy and the most difficult to address, as the example of Iran shows. But just because nuclear technology can be put to evil purposes is not an argument to ban its use.

Radiation Effects on the body

Certain body parts are more specifically affected by exposure to different types of radiation sources. Several factors are involved in determining the potential health effects of exposure to radiation. These include:

  • The size of the dose (amount of energy deposited in the body)

  • The ability of the radiation to harm human tissue

  • Which organs are affected

The most important factor is the amount of the dose - the amount of energy actually deposited in your body. The more energy absorbed by cells, the greater the biological damage. Health physicists refer to the amount of energy absorbed by the body as the radiation dose. The absorbed dose, the amount of energy absorbed per gram of body tissue, is usually measured in units called rads.

Another unit of radiation is the rem, or roentgen equivalent in man. To convert rads to rems, the number of rads is multiplied by a number that reflects the potential for damage caused by a type of radiation. For beta, gamma and X-ray radiation, this number is generally one. For some neutrons, protons, or alpha particles, the number is twenty.


The losing of hair quickly and in clumps occurs with radiation exposure at 200 rems or higher.


Since brain cells do not reproduce, they won't be damaged directly unless the exposure is 5,000 rems or greater. Like the heart, radiation kills nerve cells and small blood vessels, and can cause seizures and immediate death.


The certain body parts are more specifically affected by exposure to different types of radiation sources. The thyroid gland is susceptible to radioactive iodine. In sufficient amounts, radioactive iodine can destroy all or part of the thyroid. By taking potassium iodide can reduce the effects of exposure.

Blood System

When a person is exposed to around 100 rems, the blood's lymphocyte cell count will be reduced, leaving the victim more susceptible to infection. This is often refered to as mild radiation sickness. Early symptoms of radiation sickness mimic those of flu and may go unnoticed unless a blood count is done.According to data from Hiroshima and Nagaski, show that symptoms may persist for up to 10 years and may also have an increased long-term risk for leukemia and lymphoma.

Intense exposure to radioactive material at 1,000 to 5,000 rems would do immediate damage to small blood vessels and probably cause heart failure and death directly.

Gastrointestinal Tract

Radiation damage to the intestinal tract lining will cause nausea, bloody vomiting and diarrhea. This is occurs when the victim's exposure is 200 rems or more. The radiation will begin to destroy the cells in the body that divide rapidly. These including blood, GI tract, reproductive and hair cells, and harms their DNA and RNA of surviving cells.

Reproductive Tract

Because reproductive tract cells divide rapidly, these areas of the body can be damaged at rem levels as low as 200. Long-term, some radiation sickness victims will become sterile.

Dose-rem Effects
5-20 Possible late effects; possible chromosomal damage.
20-100 Temporary reduction in white blood cells.
100-200 Mild radiation sickness within a few hours: vomiting, diarrhea, fatigue; reduction in resistance to infection.
200-300 Serious radiation sickness effects as in 100-200 rem and hemorrhage; exposure is a Lethal Dose to 10-35% of the population after 30 days (LD 10-35/30).
300-400 Serious radiation sickness; also marrow and intestine destruction; LD 50-70/30.
400-1000 Acute illness, early death; LD 60-95/30.
1000-5000 Acute illness, early death in days; LD 100/10.


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