The anomalies, incidents, and accidents of our nuclear world

By Shelley DuBois, reporter

FORTUNE — The nuclear crisis in Japan, the aftermath of an 9.0 magnitude earthquake, including a 7.1 magnitude aftershock yesterday, and a tsunami on March 11, adds to a long list of major nuclear accidents, all of which stem from some combination of human error, insufficient safety procedure, or outdated equipment.

We won’t know the extent of the damage from this nuclear crisis until well after the ruined reactors are cooled and buried, maybe not even then. Even now — 28 days after the tsunami — Japan’s government has disclosed very little information about what has happened at the scene of the accident, the Tokyo Electric Power Company’s (TEPCO) Fukushima Daiichi Nuclear Power Station. Workers there, however, are known to be still trying to cool the damaged reactors. According to the International Atomic Energy Agency (IAEA), TEPCO will have to release over 10,000 tons of low-level radioactive water into the Pacific Ocean.

Eventually, this Japanese incident will be assigned a final numerical rating according to a scale devised by the IAEA. The scale, called INES (for International Nuclear and Radiological Event Scale), breaks down the severity of nuclear events into bloodless, regimented categories. Each event ultimately gets a ranking from 1 to 7.

A rating of 1 is applied to “an anomaly.” That includes situations in which a person is exposed to a higher level of radiation than the annual limit set by regulators, but does not necessarily develop health problems.

An “anomaly” can also mean that a safety problem at a facility could have resulted in greater damage than it did or that radioactive material has been stolen or transported incorrectly. For example, in Texas in 2006, the driver of a Ford truck containing a 50-pound radiography camera left his keys in the truck. It was stolen and then found a couple of days later. This incident rated a 1 on the INES scale.

Level 2 is an “incident” and Level 3 is a “serious incident.” Which designation applies depends on the extent of security problems, the number of people exposed to radiation, and the dosage they receive.

Levels 4-7 on the INES scale are officially called “accidents” and often, but not always, signal that one or more fatalities from radiation exposure have occurred. The INES ranking scale doesn’t cover deaths at nuclear facilities from non-radioactive chemical leaks or explosions.

Japan itself had a Level 4 accident in 1999, when three workers preparing fuel for a reactor at a Japan Nuclear Fuel Conversion plant triggered a nuclear chain reaction, called a criticality. Two workers subsequently died. An investigation of the accident found that the company had changed its work procedures for preparing nuclear fuel three years earlier, streamlining these but also increasing the likelihood of their causing a criticality.

Today’s Japanese nuclear disaster is for the moment rated level 5, which signifies that there is severe damage to the reactor core and that members of the public will probably be exposed to some level of radiation.

Level 5 is also the designation for America’s infamous 1979 event known as Three Mile Island, denoting a facility near Harrisburg, Pennsylvania that was owned by Metropolitan Edison (Exelon Corporation owns it now.) In this crisis, a reactor at the plant automatically shut down in response to a malfunction that caused it to run too hot. The plant’s operators responded incorrectly to the situation and stopped cooling the reactor, which overheated, suffered damage, and released radioactive material inside the plant.

“The accident caught federal and state authorities off-guard,” said a report by the United States Nuclear Regulatory Commission, which attributed the accident to “a combination of personnel error, design deficiencies, and component failures.”

No one died at the Three Mile Island accident, but the facility incurred nearly $1 billion of damages and clean-up costs.

History’s second-most damaging nuclear accident occurred in 1957 at Russia’s Mayak plant in the Kyshtym province. Caused by the explosion of a tank that contained radioactive waste, this crisis is recorded as a Level 6 “serious accident” on the INES scale. The explosion spewed roughly 75 tons of radioactive waste into the atmosphere, according to the World Nuclear Association, and caused an estimated 200 people to develop fatal cancer.

There has only been one Level 7 “major accident,” and that, of course, was at Chernobyl, located in what is now called Ukraine. This disaster occurred in 1986 when operators, ignoring safety regulations, switched off the electrical control system governing a reactor that was already flawed. The reactor then proceeded to reach an unstable state.

A surge of power caused a chain reaction that set the reactor core on fire. It burned for 10 days and released a cloud of radioactive matter that spread over a wide area of the USSR.

At the plant, 134 emergency workers were exposed to high doses of radiation that killed 28 of them in that same year, 1986.

To that toll of about 30 deaths must be added the indirect effects of Chernobyl—deaths caused, for example, by that radioactive cloud, which is estimated to have exposed more than 5 million people to abnormal doses of radiation.

Children were particular victims. Around 4000 people who were children in 1986 had by 2002 developed thyroid cancer from the exposure, according to a report issued jointly by several parties including the IAEA, World Health Organization and the governments of Russia, Belarus and Ukraine. Many of the children developed cancer from drinking irradiated milk produced at dairies in the contaminated area.

The overall death toll from Chernobyl remains uncertain, says that same report: “It is impossible to assess reliably, with any precision, numbers of fatal cancers caused by radiation exposure due to Chernobyl accident.”

The death toll from nuclear events in the energy industry should not be overstated, a point implicitly made in a 2010 joint report by the OECD and the Nuclear Energy Agency. The report states that 2,259 people in the OECD’s 32 countries died from coal industry-related accidents between 1969 and 2000 and that 3,713 people died from major oil industry-related accidents during the same period.

Even so, Japan’s accident has caused some political leaders to pause the progression of nuclear power. Government officials in Germany and China, for example, are reassessing their countries’ approval process for new nuclear projects.

That doesn’t mean there will necessarily be a long-term diminution in nuclear projects. The International Energy Agency’s World Energy Outlook for 2010 predicts that primary energy demand will grow by 36% between 2008 and 2035 and that nuclear power’s share of the energy mix will meanwhile increase from 6% to 8%.

Indisputably, the world will continue to need more fuel. The nuclear accident in Japan casts into sharp relief the pressure on regulators and energy producers to face one of this generation’s greatest challenges: meeting the spiking global energy demand while simultaneously keeping people safe.

More from Fortune: