It’s one of the oldest nuclear reactors in Japan, a boiling-water unit made by General Electric, just a couple of weeks shy of 40 years old and close to retirement age. On Saturday, something went very wrong. The explosion at Fukushima Daiichi’s Unit 1 rattled an already shaken nation and renewed anxiety that the exquisite technology of nuclear power may yet be overmatched by the natural violence of the earth.

There is no confirmation of a meltdown, but the situation is exceedingly tense in a nation uniquely sensitive to the dangers of radiation — no other country knows the pain of being attacked with atomic weapons. Japan prides itself on the nation’s preparation for calamity but now finds itself tested by a technological crisis piled mercilessly upon a horrific natural disaster.

The incident at Fukushima Daiichi has been felt across the planet’s nuclear power industry.

“Obviously, any time you have an incident at a nuclear plant that involves any kind of damage or an explosion, it’s not good,” said Mitch Singer, spokesman for the Nuclear Energy Institute, the industry’s lobbying arm. “But in the scheme of things, is it a disaster? We don’t think so.”

The explosion was not nuclear. Industry officials said it was created by the release of hydrogen gas that mixed with oxygen and exploded.

The building around the reactor vessel is partially destroyed, but Japanese officials say the primary vessel and the reactor core within are intact.

“If the reactor vessel is breached . . . then this radioactive stuff starts coming out in copious amounts,” said Robert Alvarez, a former senior adviser to the Department of Energy who studies nuclear power at the Institute for Policy Studies in Washington.

The three operating nuclear units at the complex all shut down automatically when the earthquake hit. But stopping a fission reactor isn’t quite like throwing a switch. The reactors use nuclear fission to boil water, create steam, power turbines and generate electricity. But Friday the Fukushima complex was hit by a double whammy: violent shaking from the historic magnitude-8.9 earthquake, and then the battering-ram tsunami that crashed ashore.

During the shutdown, rods dropped into the reactor core to absorb neutrons and halt all nuclear fission. But radioactive elements continue to give off heat through radioactive decay.

To sap that heat, the plant needed power to circulate cooling water through the core. The temblor, however, knocked out the electrical grid, and the tsunami took the backup generators   offline. Engineers then switched to batteries. But the batteries have limited duration, and the backup generators were brought in.

The term “meltdown” does not necessarily mean that the entire nuclear core has turned into a molten glob of metal and ceramic. It can be any event in which the core overheats and damages the apparatus. Such an event carries with it the danger of a release of radiation into the environment.

The Japanese nuclear regulatory agency reported a jump in radiation near the main gate of the complex over the course of five hours Saturday. Another spike in radiation, including the radioactive isotope cesium-137, was reported at a nearby observation post. Japanese authorities later said the radiation had dropped again. But the cesium-137 was the equivalent of a flare, a dramatic sign of something seriously awry.

The cesium-137 could come only from the radioactive fuel within the reactor core, Alvarez said. That means it escaped either from the reactor vessel itself, which would indicate a catastrophic breach, or from piping outside the reactor.

The nuclear power industry speaks of “defense in depth,” a concept of multiple layers of containment and backup plans to ensure that, even if something goes wrong, catastrophe won’t ensue. The failure of these systems in Japan carries an echo of the BP oil spill disaster, in which backup safety devices and redundancies turned out to be unequal to the unfolding blowout.

“The problem with the BP event is that they didn’t have a Plan B,” said Alex Marion, vice president of nuclear operations for the Nuclear Energy Institute. “We have, I would say, sufficient defense in depth. We have Plan B, C, D and possibly E.”

Tom Clements, southeastern nuclear campaign coordinator for Friends of the Earth, said the damaged Japanese reactor was of a design “haunted by questions about its ability to survive a severe accident.”

Clements added, “We’re against nuclear power because of this very reason: a nuclear power accident can turn into a disaster of huge proportions in just a short period of time, and it’s not worth taking that risk.”

The nuclear industry has had two previous high-profile calamities: a partial meltdown in 1979 at the Three Mile Island plant in Pennsylvania, and the far more disastrous 1986 fire at a plant near the Ukrainian city of Chernobyl.

As at Fukushima, the Three Mile Island accident was triggered by a disruption of water flow to the reactor. Several instruments failed and operators did not realize that pressure was building inside the reactor. A heavy secondary containment shield ultimately prevented all but a tiny amount of radiation from escaping into the environment.

The Chernobyl disaster, in contrast, was caused by a crude reactor design and at least six fatally flawed decisions by operators during a risky test. A huge power spike and the bad decisions drove the reactor out of control. An explosion then blew the reactor apart and spewed radioactive debris for a week.

Unlike U.S. and Japanese nuclear plants, Chernobyl lacked the heavy shielding that eventually halted the Three Mile Island disaster — and that all of Japan desperately hopes will prevent Fukushima Daiichi’s Unit 1 from melting down.

Staff writer Brian Vastag contributed to this report.