(UPDATED) Japanese officials announced on Tuesday morning that they were planning to raise the event level at the Fukushima Daiichi nuclear power plant from 5 to the maximum level of 7, the highest on the international scale for nuclear incidents and the same level assigned to the 1986 disaster at Chernobyl in the Ukraine. 
 
 The decision was made after Japan’s nuclear safety body determined that at one point after the March 11 earthquake, the plant was releasing 10,000 terabecquerels of iodine-131 for several hours; level 7 accidents are defined as releasing tens of thousands of terabecquerels. “The INES [International Nuclear and Radiological Event Scale] rating itself is not an indicator of a daily phenomena but the assessment after careful consideration and calculation on the event that happened in the past,” Ken Morita of Japan’s Nuclear and Industrial Safety Agency (NISA) tells TIME. 
 
 NISA also noted that the amount of radioactive material being released at Fukushima today is less than 1 terabecquerel. The agency said that to date, Fukushima has released only about 10% of the total radiation released 25 years ago at Chernobyl, or about 1.8 million terabecquerels. About 30 people, mostly workers, died in the immediate aftermath of Chernobyl, though the U.N. has estimated that the long-term death toll due to exposure could eventually be as high as 4,000. 
 
 (More on TIME.com: See the top 10 environmental disasters.) 
 
 INES, designed in 1989 by the International Atomic Energy Agency (IAEA) and the Nuclear Energy Agency of the Organisation for Economic Co-operation and Development, ranges from 1 (anomaly) to 7 (major accident). The scale is intended to help easily communicate with the public to indicate the seriousness of a nuclear event. Chernobyl is the only other event to have been given a 7, an accident classified as having a major radioactive release with widespread impact on the environment and public health. According to INES, “Such a release would result in the possibility of acute health effects; delayed health effects over a wide area, possibly involving more than one country; long-term environmental consequences.” (Read the IAEA’s glossary of short- to long-term health effects of radiation exposure here.) 
 
 Besides Chernobyl, the only other event that has come close to a 7 was a 1957 accident at a fuel-processing plant (where spent nuclear fuel is recycled into new fuel) in Russia, in which an off-site release of radiation prompted preventive evacuations. The Three Mile Island accident in the U.S. in 1978, in which a reactor core was severely damaged, but the off-site release of radioactivity was limited, was classified as a 5. Almost all reported events at nuclear facilities are a level 3 or lower, according to INES. 
 
 Tuesday’s announcement comes on the back of a minor fire spotted by workers outside Fukushima’s Reactor 4 on Tuesday morning, shortly after the second of three major aftershocks hit the beleaguered northeast in a span of 24 hours. Three people in Iwaki died in landslides triggered by the 7.1-magnitude aftershock on Monday night. The government also expanded the exclusion zone around Fukushima on Monday to include several towns within a 19-mile (30 km) radius where residents had been told they could remain at home but were recommended to stay indoors. The towns added to the mandatory evacuation zone were found to have high levels of radiation. 
 
 Meanwhile, Greenpeace said that in a survey conducted in Fukushima last week, its team of experts found radiation levels 75 times higher than the government recommendation in 11 samples of vegetables from gardens and small farms. The environmental group also announced that it found radiation levels equivalent to an annual exposure of 5 millisieverts — the evacuation threshold for Chernobyl — at a playground in Fukushima City, pop. 300,000. Greenpeace is urging the government to delay the start of the school year. 
 
 Though raising Fukushima’s level to 7 may not herald any immediate worsening of events, it is sure to add to many residents’ growing concern — and feeling of helplessness — over what could happen at dozens of other nuclear reactors spread across this seismic archipelago. On Sunday, more than 17,000 people protested at two demonstrations in Tokyo against nuclear power. It was the first time that Yohei Nakamura, 45, had ever been to a protest. “For a long time, I’ve been suspicious of nuclear power, but now I realize it’s a serious problem,” he said amid the crowds carrying placards and shouting slogans. He said anti-nuclear demonstrations were undercovered in the Japanese press because of the influence of the Tokyo Electric Power Co., which owns Fukushima. “TEPCO is one of the most powerful companies in Japan,” Nakamura said. “They use a tremendous amount of money for advertising. If the mass media shows anti-nuclear-power activities like demonstrations, they risk losing TEPCO as an advertiser.” 
 
 — With reporting by Lucy Birmingham / Tokyo 
 
 See pictures of Japan’s nuclear wasteland. ||||| Japan raised the crisis level at its crippled nuclear plant Tuesday to a severity on par with the 1986 Chernobyl disaster, citing high overall radiation leaks that have contaminated the air, tap water, vegetables and seawater. 
 
 Buddhist monks, Japan Self-Defense Force personnel firefighters and other relief workers observe a moment of silence on Monday, April 11, 2011, exactly one month after the devastating earthquake hit northeastern... (Associated Press) 
 
 Buddhist monks pray in an area destroyed by a tsunami in Natori, Miyagi Prefecture, northern Japan Monday, April 11, 2011, one month to the day after an earthquake and tsunami devastated the city. (AP... (Associated Press) 
 
 Members of the Japan's Ground Self-Denfence Force, left, and firemen take a moment of silence in an area destroyed by tsunami in Natori, Miyagi Prefecture, northern Japan, Monday, April 11, 2011, one... (Associated Press) 
 
 Japanese stop for a moment of silence at 2:46pm, exactly a month after a massive earthquake struck the area in the port town of Kesennuma, Miyagi Prefecture, Japan, Monday, April 11, 2011.(AP Photo/Sergey... (Associated Press) 
 
 Volunteers cook noodles for evacuees at an evacuation center in Soma, Fukushima Prefecture, northeastern Japan, Monday, April 11, 2011, a month after an earthquake and tsunami struck the east coast. ... (Associated Press) 
 
 A survivor takes rest after she took a moment of silence at 2:46 p.m., exactly a month after a massive earthquake struck the area in Kamaishi, Iwate prefecture, Japan, Monday, April 11, 2011. (AP Photo/Lee... (Associated Press) 
 
 A broken clock sits in the debris a month after a massive earthquake struck the area in Kamaishi, Iwate prefecture, Japan, Monday, April 11, 2011. (AP Photo/Lee Jin-man) (Associated Press) 
 
 A survivor woman wipes her tear after she takes a moment of silence at 2:46pm, exactly a month after a massive earthquake struck the area in Kamaishi, Iwate prefecture, Monday, April 11, 2011. (AP Photo/Lee... (Associated Press) 
 
 Japanese police officers make their way through the rubble at an area devastated by the March 11 earthquake and tsunami in the port town of Kesennuma, Miyagi prefecture, Japan, on Monday, April 11, 2011.... (Associated Press) 
 
 An eldery Japaneese man observes the area devastated by the March 11 earthquake and tsunami in the port town of Kesennuma, Miyagi prefecture, Japan, on Monday, April 11, 2011. Exactly a month ago today... (Associated Press) 
 
 A Japanese man throws the garbage out of a window of his house just next to the grounded ship at an area devastated by the March 11 earthquake and tsunami in the port town of Kesennuma, Miyagi prefecture,... (Associated Press) 
 
 A man walks along the grounded ships at an area devastated by the March 11 earthquake and tsunami in the port town of Kesennuma, Miyagi prefecture, Japan, Monday, April 11, 2011. Exactly a month ago... (Associated Press) 
 
 A Japanese family, who refused to give their family name, cleans an area around their house next to the grounded ship at an area devastated by the March 11 earthquake and tsunami in the port town of Kesennuma,... (Associated Press) 
 
 A Japanese family walks along the flooded street at an area devastated by the March 11 earthquake and tsunami in the port town of Kesennuma, Miyagi prefecture, Japan, on Monday, April 11, 2011. Exactly... (Associated Press) 
 
 Greenpeace activists and other environmentalists light candles amidst hundreds of paper cranes at the Heroes' Monument in solidarity to the Japanese disaster victims at suburban Quezon city, northeast... (Associated Press) 
 
 This Sunday, April 10, 2011 photo released by Tokyo Electric Power Co. (TEPCO) shows orange colored floats suspending"silt fence," that was installed under water to help prevent contaminated water from... (Associated Press) 
 
 Japanese nuclear regulators said they raised the rating from 5 to 7 _ the highest level on an international scale of nuclear accidents overseen by the International Atomic Energy Agency _ after new assessments of radiation leaks from the Fukushima Dai-ichi plant since it was disabled by the March 11 tsunami. 
 
 The new ranking signifies a "major accident" that includes widespread effects on the environment and health, according to the Vienna-based IAEA. But Japanese officials played down any health effects and stressed that the harm caused by Chernobyl still far outweighs that caused by the Fukushima plant. 
 
 The revision came a day after the government added five communities to a list of places people should leave to avoid long-term radiation exposure. A 12-mile (20-kilometer) radius already had been cleared around the plant. 
 
 The news was received with chagrin by residents in Iitate, one of the five communities, where high levels of radiation have been detected in the soil. The village of 6,200 people is about 40 kilometers from the Fukushima plant. 
 
 "It's very shocking to me," said Miyuki Ichisawa, 52, who runs a coffee shop in Iitate. "Now the government is officially telling us this accident is at the same level of Chernobyl." 
 
 Japanese officials said the leaks from the Fukushima plant so far amount to a tenth of the radiation emitted in the Chernobyl disaster, but said they eventually could exceed Chernobyl's emissions if the crisis continues. 
 
 "This reconfirms that this is an extremely major disaster. We are very sorry to the public, people living near the nuclear complex and the international community for causing such a serious accident," said Chief Cabinet Secretary Yukio Edano. 
 
 But Edano told reporters there was no "direct health damage" so far from the crisis. "The accident itself is really serious, but we have set our priority so as not to cause health damage." 
 
 Hironobu Unesaki, a nuclear physicist at Kyoto University Research Reactor Institute, said the revision was not a cause for worry, that it had to do with the overall release of radiation and was not directly linked to health dangers. He said most of the radiation was released early in the crisis and that the reactors still have mostly intact containment vessels surrounding their nuclear cores. 
 
 The change was "not directly connected to the environmental and health effects," Unesaki said. "Judging from all the measurement data, it is quite under control. It doesn't mean that a significant amount of release is now continuing." 
 
 Prime Minister Naoto Kan, in a national television address, urged the public not to panic and to focus on recovering from the disaster. 
 
 "Right now, the situation of the nuclear reactors at the Fukushima plant has been stabilizing step by step. The amount of radiation leaks is on the decline," he said. "But we are not at the stage yet where we can let our guards down." 
 
 Continued aftershocks following the 9.0-magnitude megaquake on March 11 are impeding work on stabilizing the Fukushima plant _ the latest a 6.3-magnitude one Tuesday that prompted plant operator Tokyo Electric Power Co., or TEPCO, to temporarily pull back workers. 
 
 Officials from Japan's Nuclear and Industrial Safety Agency said that the cumulative amount of radioactive particles released into the atmosphere since the incident had reached levels that apply to a Level 7 incident. Other factors included damage to the plant's buildings and accumulated radiation levels for its workers. 
 
 "We have refrained from making announcements until we have reliable data," said NISA spokesman Hidehiko Nishiyama said. "The announcement is being made now because it became possible to look at and check the accumulated data assessed in two different ways," he said, referring to measurements from NISA and Japan's Nuclear Security Council. 
 
 NISA and the NSC have been measuring emissions of radioactive iodine-131 and cesium-137, a heavier element with a much longer half-life. Based on an average of their estimates and a formula that converts elements into a common radioactive measure, the equivalent of about 500,000 terabecquerels of radiation from iodine-131 has been released into the atmosphere since the crisis began. 
 
 That well exceeds the Level 7 threshold of the International Nuclear and Radiological Event Scale of "several tens of thousands of terabecquerels" of iodine-131. A terabecquerel equals a trillion becquerels, a measure for radiation emissions. 
 
 The government says the Chernobyl incident released 5.2 million terabecquerels into the air _ about 10 times that of the Fukushima plant. 
 
 If the leaks continue, the amount of radioactivity released in Fukushima could eventually exceed the amount emitted by Chernobyl, a possibility that Naoki Tsunoda, a TEPCO spokesman, said the company considers "extremely low." 
 
 In Chernobyl, in the Ukraine, a reactor exploded on April 26, 1986, spewing a cloud of radiation over much of the Northern Hemisphere. A zone about 19 miles (30 kilometers) around the plant was declared uninhabitable, although some plant workers still live there for short periods and a few hundred other people have returned despite government encouragement to stay away. 
 
 In 2005, the Chernobyl Forum _ a group comprising the International Atomic Energy Agency and several other U.N. groups _ said fewer than 50 deaths could be confirmed as being connected to Chernobyl. It also said the number of radiation-related deaths among the 600,000 people who helped deal with the aftermath of the accident would ultimately be around 4,000. 
 
 The U.N. health agency, however, has said about 9,300 people are likely to die of cancers caused by radiation. Some groups, including Greenpeace, have put the numbers 10 times higher. 
 
 The Fukushima plant was damaged in a massive tsunami that knocked out cooling systems and backup diesel generators, leading to explosions at three reactors and a fire at a fourth that was undergoing regular maintenance and was empty of fuel. 
 
 The magnitude-9.0 earthquake that caused the tsunami immediately stopped the three reactors, but overheated cores and a lack of cooling functions led to further damage. 
 
 Engineers have pumped water into the damaged reactors to cool them down, but leaks have resulted in the pooling of tons of contaminated, radioactive water that has prevented workers from conducting further repairs. 
 
 A month after the disaster, more than 145,000 people are still living in shelters. The quake and tsunami are believed to have killed more than 25,000 people, but many of those bodies were swept out to sea and more than half of those feared dead are still listed as missing. 
 
 ___ 
 
 Associated Press writers Shino Yuasa and Noriko Kitano in Tokyo and Eric Talmadge in Soma contributed to this report. ||||| (See Correction & Amplification item below.) 
 
 TOKYO—The Japanese government raised its assessment of the monthlong crisis at its Fukushima Daiichi nuclear power plant to the highest severity level by international standards—a rating only conferred so far upon the Chernobyl accident. 
 
 Japanese officials raised its nuclear crisis to the level of Chernobyl. WSJ's Yumiko Ono reports. Also Jerry Seib reports on a new era of dialogue in Washington as debt ceiling and budget talks heat up again. And Donald Trump gets serious about 2012. 
 
 Past Nuclear Incidents See a timeline of how the Fukushima nuclear incident compares to other nuclear incidents. View Interactive More photos and interactive graphics 
 
 Japan's nuclear regulators said the plant has likely released so much radiation into the environment that it must boost the accident's severity rating on the International Nuclear Event scale to a 7 from 5 currently. That is the same level reached by the Chernobyl nuclear disaster in the former Soviet Union, which struck almost exactly 25 years ago, on April 26, 1986. 
 
 "Based on the cumulative data we've gathered, we can finally give an estimate of total radioactive materials emitted,'' Hidehiko Nishiyama, spokesman for Japan's Nuclear and Industrial Safety Agency, said at a press conference Tuesday. 
 
 Even as they upgraded their assessment of the situation, Japanese officials went to lengths to say that the problem they are struggling to contain isn't anywhere near the disaster of Chernobyl. 
 
 "It is quite different from Chernobyl," said Mr. Nishiyama. "First, the amount of released radiation is about a tenth of Chernobyl," he said, adding that while there were 29 deaths resulting from short-term exposure to high doses of radiation at Chernobyl, there were no such deaths at Fukushima. 
 
 Enlarge Image Close ZUMAPRESS.com Japanese police searched for victims inside the deserted evacuation zone in Minamisoma in Fukushima Prefecture. 
 
 "At Chernobyl, the nuclear reactor itself exploded," he said, adding that at the Fukushima plant, the pressure vessel and the containment vessel were largely intact. 
 
 Still, Fukushima Daiichi operator Tokyo Electric Power Co. warned Tuesday that since the Fukushima Daiichi plant is still releasing radioactive materials, the total level of radiation released could eventually exceed that of Chernobyl, a spokesman said. 
 
 The new assessment comes as Japan admits that the effects of the Fukushima Daiichi nuclear accident—which has already caused the evacuation of tens of thousands of people and spread radiation through groundwater and farms over a broad section of eastern Japan—are likely to be long-lasting and grave. The accident was precipitated by the massive March 11 earthquake and tsunami, which knocked out Fukushima Daiichi's power and cooling systems, causing several of the reactors to overheat. 
 
 The International Nuclear Event scale, whose development is coordinated by the International Atomic Energy Agency, measures the severity of accidents based on how much radiation is released, the degree of damage to the nuclear cores and how widespread and long-lasting the effects are likely to be. 
 
 Level 5—the previous level given the Fukushima Daiichi accident—indicates a "limited release'' of radioactive materials requiring "some planned countermeasures.'' The 1979 Three Mile Island accident in Pennsylvania was rated a 5. 
 
 Enlarge Image Close European Pressphoto Agency The reactor building of Unit 1-2 at Fukushima Daiichi Nuclear Power Station on Sunday. 
 
 Enlarge Image Close ZUMAPRESS.com Houses collapsed by the earthquake and tsunami in Minamisoma. 
 
 Level 7 labels this "a major accident," the most serious on the international scale. It means high levels of radiation have been released, and that the amount of time needed to bring the plant under control will require an extended period. But not all "major accidents" are equal in severity. 
 
 The decision to upgrade formally the severity of the accident came a day after Japan broadened the 12-mile nuclear evacuation zone around the plant to include all or part of five towns and villages that housed tens of thousands of people before the disaster, a sign that officials now see the long-term risks as far higher than originally estimated. 
 
 And the crisis appears far from over, with constant reminders that efforts to bring the crippled reactors under control are far from complete. Operator Tepco scrambled to keep reactors stable in the wake of another big earthquake Monday and a battery fire Tuesday morning, signs of how vulnerable the plant remains a month after the quake. 
 
 Experts have predicted it could take months for Tepco to bring Fukushima Daiichi's reactors truly under control, and years to clean up the plant itself. 
 
 Japanese nuclear regulators determined that after the accident, the plant has likely released tens of thousands of terabecquerels—or a mind-boggling tens of thousands of trillions of becquerels—of radiation in the immediate area. That's a level that's been recorded only during the Chernobyl accident. 
 
 While the new assessment puts Fukushima on a par with Chernobyl, there are key differences between the two, suggesting the Ukraine disaster was still far more serious. 
 
 The Japanese government said the monthlong crisis at its Fukushima Daiichi nuclear power plant is on par with Chernobyl in terms of severity. WSJ's Mariko Sanchanta and Yumiko Ono discuss the public's reaction to the news. 
 
 In the case of Chernobyl, a graphite fire burned uncontrolled for days, spewing out radioactive smoke that spread around the world. Fukushima, unlike Chernobyl, has a containment structure, which, even if damaged, has meant that the Japanese accident has shown "much, much, much lower'' traces of far-flung radiation, Wolfgang Weiss, chair of the United Nations Scientific Committee on the Effects of Atomic Radiation, said in Vienna last week. 
 
 The release from Fukushima of tens of thousands of terabecquerels of iodine-131, while huge, appears to be smaller than the 5.2 million terabecquerels released from Chernobyl. Japanese government officials said the radiation release was between 370,000 and 630,000 terabecquerels so far from Fukushima. The permissible level of iodine-131 for vegetables and fish is 2,000 becquerels per kilogram, or just a tiny fraction of what has been released. 
 
 A 2005 United Nations study said up to 4,000 people could eventually die from radiation exposure to Chernobyl. 
 
 In Japan, so far, a handful of workers have been hospitalized, but they were released a few days later, and regulators said they showed no signs of lasting injury. 
 
 Japanese suppliers impacted by last month's quake are struggling to hang onto their business amid shortages of key components and electricity. Companies that rely on that hard-hit supply network are also rethinking their exposure to Japan. 
 
 On Edge | Worries over Fukushima Daiichi Enlarge Image Close 
 
 Earthquake in Japan View Interactive 
 
 There are, however, regular reports in the Japanese press of elevated radiation exposure for the workers trying to contain Fukushima, and it could be months, or years, before the real impact is known. The same is true for the population in and around the plant. 
 
 Officials said they expanded the original evacuation zone because the accident had lasted longer than expected. 
 
 "Japan has been doing drills for possible nuclear accidents, but they assumed that the accidents would be resolved in about 10 days," said Mr. Nishiyama, the spokesman. "We are now dealing with a crisis of a historic proportion. This has necessitated different kinds of responses than initially planned." 
 
 Even in announcing the expanded evacuation zone, Japanese officials said residents of the affected areas weren't in danger of surpassing government exposure limits anytime soon and that they have about a month to move. 
 
 Testing by Japanese, U.S. and IAEA officials shows that the radioactive contamination is spreading unevenly from the plant, creating what are known as hot spots due to wind, topography and other natural conditions that show a higher density of radioactive material compared with some areas closer to the plant. 
 
 The move will present major logistical hurdles for communities already battered by the March 11 earthquake and tsunami that damaged the plant as well as much of the surrounding countryside. The area includes towns and villages with a population totaling about 115,000 people before the crisis, though the number of people affected is likely to be far less because the government's order applies only to particular hot spots believed to have higher radiation levels, not a set radius from the plant. 
 
 Meanwhile, efforts to stabilize reactors at Fukushima Daiichi continue to be dogged by setbacks and scares, in a sign of how fragile the situation on the ground remains. On Tuesday morning, Tepco said there was a small fire at a battery unit outside reactor No. 4, which was put out shortly after being reported. 
 
 On Monday, a 7.1 magnitude quake centered in coastal Fukushima temporarily shut down power supply and makeshift cooling systems to three reactors at the plant, causing the evacuation of workers to the compound's command center. The systems remained down for nearly an hour while the evacuation remained in effect, keeping workers from switching to emergency power generators. 
 
 Tepco said the suspension didn't appear to have caused significant safety issues. But the scramble to restore power served as a reminder of how aftershocks and the risk of tsunami could upset the delicate efforts to stabilize the problems at the plant. 
 
 —Mitsuru Obe contributed to this article. 
 
 Correction & Amplification: 
 
 The Three Mile Island accident occurred in 1979. An earlier version of this article incorrectly said it happened in 1978.
– Japan has made the decision to raise the Fukushima Dai-ichi nuclear disaster severity level from 5 to 7. That obviously means "worse." But what else does it mean? Japan finally has an estimate on how much radiation has been released: The level is an indication of the total radioactive materials emitted. Data has revealed that the cumulative amount of radioactive particles released into the atmosphere had reached levels that apply to a Level 7 incident, reports the AP. So what is that estimate? According to two estimates, the equivalent of about 500,000 terabecquerels of radiation from iodine-131 has been released. The Level 7 threshold is "several tens of thousands of terabecquerels." But all 7s are not created alike: The Wall Street Journal notes that officials were careful to say this this is still not a Chernobyl-size disaster. "The amount of released radiation is about a tenth of Chernobyl"—about 5.2 million terabecquerels—said a rep for Japan's Nuclear and Industrial Safety Agency. But it could still top Chernobyl in one regard: TEPCO today warned that the plant continues to release radioactive materials, and that the total levels emitted could eventually top those released by Chernobyl. Level 7 also indicates widespread effects on the environment and health: The Telegraph noted that Japan's safety commission now says radiation in excess of the amount considered safe for humans during an entire year has been found as far as 37 miles from the plant; the evacuation zone is only 18 miles. Is there another big one we haven't been talking about? Sort of. Besides Chernobyl, Time notes that a 1957 accident in Kyshtym, Russia, was the only one that's come close to being a 7; it was a 6. Most nuclear accidents tend to be a 3 or less.