Mainstream media reporting initially compared the explosions and loss of control at the Fukushima nuclear plants to the Level Five event at Three Mile Island (TMI). As Fukushima spiraled out of control the media turned to Chernobyl, the undisputed Level Seven nuclear event. A recent article from Reuters concerning nuclear ratings reform mentions only those two events as if nothing else even remotely as serious has ever happened.

Few outlets mention the fire at Windscale, the ravaged test site Semipalatinsk, the hydrogen bomb contamination at Palomares or the nuclear waste explosion at Kyshtym. In A Survey of the World's Radioactive No-Go Zones, Der Spiegel does describe many of the nuclear events that the atomic power industry would probably like us to forget, such as 1949's radiation release at the Hanford Site in Washington State: 

... 240 square miles are uninhabitable due to the radioactivity that has seeped into the soil and ground water: uranium, cesium, strontium, plutonium and other deadly radio-nuclides. ... More than 100,000 spent fuel rods -- 2,300 tons of them -- still sit in leaky basins close to the Columbia River.

The plant is also notorious for the so-called "Green Run" -- the deliberate release of a highly radioactive cloud from the T-plant, the world's largest plutonium factory at the time. The radiation was almost 1,000 times worse than that released during the 1979 meltdown at Three Mile Island in Pennsylvania, the worst nuclear accident in American history. Fallout from the experiment drifted all the way to California. People wondered why they suddenly got sick. Studies would eventually show that some babies at Hanford were radiated twice as much as the children of Chernobyl.

Hanford doesn't appear on the usual lists of nuclear or radiation accidents, and I haven't found a nuclear disaster Level rating for it, but sixty years later, it is an ongoing cleanup effort that will be more expensive than TMI. That should tell us something.

1957's Windscale in Great Britain is officially a Level 5, but somehow seems worse than TMI: 

... by the Irish Sea, the British had hurriedly built two atomic reactors after World War II for power production and to make weapons-grade plutonium.The speed of construction carried a great cost. In 1955, 251 workers were exposed to radiation during repair work. Then, on Oct. 10 1957, a reactor core began to burn. In an attempt to extinguish the fire, a radioactive cloud was released, followed by a second one the next day. The radiation reached as far as Switzerland. The fires were only brought under control after two days.
The authorities attempted to cover up the accident, initially saying only that there had been an incident, but that the workers involved had been able to scrub away the radiation with soap and water. The only warning was that cow's milk in a radius of 200 miles from the reactor should not be consumed. In reality, the population surrounding the reactor received radiation doses 10 times higher than that seen as permissible for a lifetime.
According to official figures, 33 people were killed by the after-effects of the disaster, with more than 200 diagnosed with thyroid cancer. To this day, 15 tons of damage fuel rods are still stored on site as is radioactive ash and mud, leftover from the fire. The reactor is now to be dismantled using a robot built exclusively for the project. In all, it is set to cost some 500 million pounds.

1957's Kyshtym in the USSR is officially a Level 6, but data was officially concealed for decades:

One of the worst nuclear accidents took place on Sep. 29, 1957, but was only made public years later. On that day, a tank containing 80 tons of highly-radioactive liquid waste exploded at the Mayak plutonium plant in the southern Urals, 15 kilometers east of the Russian city of Kyshtym. The blast produced a radioactive cloud that was about 300 kilometers long and 40 kilometers wide, and which traveled northeast. The radiation did not reach Europe, but was at the same level of that released during the Chernobyl disaster in 1986. About 15,000 people who lived in the area were evacuated, and the houses located in a 25-kilometer zone surrounding the location were destroyed. No one was allowed to go back. The plutonium production at the plant, which also delivered the material for the Soviet Union's first atomic bomb, was not discontinued.
It wasn't until the 1970s that information about the catastrophe leaked to the West. The Soviet regime first admitted it in 1989. The number of deaths and details of the long-term effects remain unknown. The 150-square-kilometer area over which the radioactive cloud dispersed remains closed off to this day and entry is forbidden.

In one of those odd coincidences, we set up a guest bed this week in a room where we keep many of our books. I happened to glance at the shelves and saw a book, Nuclear Disaster in the Urals written in 1979 by dissident biologist Zhores Medvedev. We pick up a lot of books at the thrift shop, and a lot at library sales, so I must have bought it and forgotten it years ago. Medvedev caused an international sensation in the 1970s by mentioning the Kyshtym disaster in a scientific paper. He was immediately attacked by ... the British!, in the person of Chairman John Hill of the UK Atomic Energy Authority who was deeply skeptical that there was an explosion, that nuclear waste could even explode, and all that rot. Medvedev suspected that the British were still smarting over Windscale. When the CIA also expressed doubts. Mevedev felt compelled to prove his assertions, and wrote this book.

In his book, Medvedev said that the scale of the disaster was generally understood in the USSR, but that scientists just weren't allowed to mention anything to do with radiation in scientific papers due to both military secrecy and Lysenkoism. Once Khruschev and Lysenko were out of power, secrecy was relaxed just a bit, and scientists began cautiously publishing studies involving radioactivity, though leaving out specifics to avoid repercussions.

For example, one study concerned radioactive contamination in two very large lakes. Though the lakes aren't named, Medvedev uses the types of fish present, and not present, the lack of running water supply, ice cover, etc., to infer that the lakes were almost certainly in the Urals. From their size, he deduces that they must have been accidentally contaminated as no one would intentionally contaminate such an important food source. He estimated the intensity of the contamination by the ratio of the cesium isotope, which collects in muscles, to the strontium isotope, which collects in the bones.

As we saw with the Democracy Now debate over Chernobyl casualties beween Helen Caldicott and Georges Monbiot, UN and IAEA authorities won't accept non-peer reviewed, non-English studies as evidence. Even though it is officially a Level 6, Kyshtym may well have generated worse contamination than Fukushima ever will.