As Japan assesses the damage of Thursday's dangerous leak of radioactive material at a uranium processing plant, authorities worry over the long term implications of the potentially lethal radiation leak.

Forty-nine people were hospitalized after the leak, suffering from varying degrees of radiation sickness. Three people were critically exposed and remained in intensive care Friday. The nuclear accident is being called the worst in the country's history.

Unlike the well-known disaster at Chernobyl, Thursday's incident happened not at a nuclear power plant, but at a facility that processes uranium for nuclear reactors.

Employees are reported to have loaded 16 kg of uranium into a container, nearly eight times the normal amount. Such an accident can cause a "criticality." That's a nuclear reaction, similar to what occurs all the time within a nuclear power plant. Outside the contained environment of a nuclear power plant, an "unplanned" nuclear reaction is extremely hazardous.

A nuclear reaction, or nuclear fission, occurs when neutrons hit uranium, causing atoms to split. When atoms split, they release huge amounts of energy. That energy causes a self-sustaining reaction that keeps on going, and potentially cannot be controlled.

The "chain reaction" effect was heightened by the presence of water at the plant in Japan. Workers spent much of the night draining coolant water that had helped feed a "chain reaction" of uranium.

In Thursday's accident, radioactive particles were released into the atmosphere. There were also gasses released. Most of the poisons will be dissipated into the atmosphere, and will go down to a relatively low concentration. However, some of them will end up on the ground, and then on people's skin.

Although radiation levels in the surrounding towns are back to normal, and more than 300,000 people were allowed to come out of their homes and unseal their windows and doors, people living closest to the nuclear processing plant are not yet allowed to return home.

About 150 people are being kept from their homes as work crews in radiation suits begin a clean-up operation to rid the area near the plant of residual radioactivity.

The government has also asked farmers not to harvest vegetables from their fields in the area. Fisherman are also unable to catch fish from the rivers.

The hospitalized include fire fighters and workers from a nearby golf course but the majority are plant workers who battled through the night to douse a fission chain-reaction that caused the radiation leak.

Workers brought the chain-reaction under control by smothering it with special chemicals.In doing so they exposed themselves to potentially lethal doses of radiation.

About 310,000 residents in nine cities, towns and villages near the plant spent a tense night and day inside their homes.

The accident happened early Thursday when workers at the plant in Tokaimura, 140 kilometres northeast of Tokyo were mixing uranium with nitric acid.

They accidentally poured eight times too much uranium into the mixing tank, setting off a nuclear chain reaction.

Workers say there was a blue flash and radiation detection alarms sounded.

Radiation levels shot up to 15,000 times their normal level. The three workers who are in critical condition were exposed to radiation levels 4,000 times higher than normal.

Police are investigating for possible criminal negligence related to the accident. The president of the company which runs the nuclear plant has apologized for the incident, admitting that the company may have neglected some regulations.

Officials spent more than 24 hours worrying about chemical reactions continuing inside the plant, the same kind of reactions that occur when uranium is heating up to be used as nuclear fuel. As it gets hotter, it creates more radiation.

For much of Friday officials battled the strong possibility the accident could reach a level of a "criticality incident."

Criticality occurs when a nuclear chain reaction becomes self-sustaining, meaning it will continue to grow without any more fuel being added.

Tony Sinclair, a mechanical industrial engineer at the University of Toronto, told CBC News the danger may not be over yet.

"It probably means that when the criticalility incident originally existed, there was a tremendous burst released, and that initial burst has now subsided," Sinclair said.

"There could still be a critical reaction going on, but perhaps at a lower level."

Russia and the United States have put together a team of experts, and are waiting for Japan to ask them to fly in.

To add to the problems, rain fell overnight, washing radioactive material onto cars, houses and people.

Tokaimura has a population of around 34,000 people and is home to 15 nuclear-related facilities.

Accidents have plagued the Japanese nuclear power industry.

Two years ago, a fire in a radioactive waste storage area at the plant released contaminated smoke into the air.

The environmental organization Greenpeace says the accident is a symptom of what it calls "a safety crisis" facing Japan's nuclear industry.

Given the level of public anger over the accident, it is expected that the government will launch a full inquiry.