On Monday, the International Atomic Energy Association issued a new rating classifying Japan's crippled Fukushima Daiichi nuclear power station in the same category as the most serious nuclear disaster--Russia's Chernobyl crisis in 1986. Even though the Japanese spokesperson for the Nuclear and Industrial Safety Agency took pains to differentiate the crisis from Chernobyl, Fukushima is now classified as a level 7 alongside it. In the most basic terms, what does that mean?

Here's a brief explainer to guide through the upgraded severity level on the International Nuclear and Radiological Event Scale. First things first:

What is the International Nuclear and Radiological Event Scale?

The IAEA, the worldwide nuclear watchdog organization which has been monitoring the Fukushima crisis, created the scale as a simple way to explain to the public the severity of these types of highly technical incidents in 1989. The scale, pictured below, classifies events in two groups: "Incidents" (levels 1-3) or "Accidents" (levels 4-7). And though it's an attempt to standardize nuclear safety classifications, the New York Times explains, the IAEA leaves it up to the "country where the accident occurs to calculate a rating based on complicated criteria."



[Sources: IAEA website, INES PDF]

Where has Fukushima been classified on that scale?

Since the first earthquake hit Japan nearly a month ago, Fukushima has been classified as an "accident with wider consequences" or a level 5. If you've been reading about Fukushima since the disaster began on March 11th, you'll note that the primary reference points in news accounts about the crisis tend to refer to two other major accidents: the 1979 Three Mile Island (level 5) crisis in the U.S. and the 1986 Chernobyl disaster in Russia (level 7). Up until yesterday, Fukushima was classified on the same level as Three Mile Island.

What's the difference between a level 5, 6 and 7 accident?

The chart above designates a level 5 as an "accident with wider consequences," level 6 as a "serious accident" and level 7 as a "major accident." On the INES PDF fact sheet, however, it explains the hallmarks of a level 5 as the "limited release of radioactive material likely to require implementation of some planned countermeasures." Level 6 is differentiated by the release of "significant" radioactive material released, and level 7 is explained as the "major release of radioactive material with widespread health and environmental effects requiring implementation of planned and extended countermeasures."

Why has Fukushima been upgraded to a level 7?

On Monday, prior to the upgrade, Kyodo News had described an incident at Fukushima in which Japan's Nuclear Safety Commission noticed that, "at some point" the plant had been releasing 10,000 terabecquerels levels of radioactive materials per hour for "several hours." The news outlet said that authorities who were keeping track of the amount of high amount of radiation released were considering upgrading the severity level. When the severity level was upgraded, Japan's Nuclear and Industrial Safety spokesman announced at a press conference that, "based on the cumulative data we've gathered, we can finally give an estimate of total radioactive materials emitted."

Does this mean conditions at the plant are getting worse?

No, it doesn't. "In fact, conditions appear to be markedly more stable than in the days following the Tohoku earthquake and tsunami, although not completely under control," stressed a BBC News analysis. Essentially, officials concluded that--based on the total amount of radiation released since the accident began--the severity level needed to be upgraded.

If the level 7 classification puts Fukushima on the same level as Chernobyl, how similar are the crises?

Nuclear experts will be debating this for awhile. But currently, Japanese officials are taking pains to stress the differences between Fukushima and Chernobyl. "It is quite different from Chernobyl," said Japan's spokesman for Japan's Nuclear and Industrial Safety Agency. "First, the amount of released radiation is about a tenth of Chernobyl," and also "at Chernobyl, the nuclear reactor itself exploded." Still, the New York Times has noted a Tokyo Electric Power Company official in a recent news conference saying that "the radiation leak has not stopped completely and our concern is that it could eventually exceed Chernobyl."