( CNN ) -- Japanese authorities are expected to announce Friday that a `` cold shutdown '' has been achieved at the Fukushima Daiichi nuclear power plant . That would mean the plant 's crippled reactors have stayed at temperatures below the boiling point for some time . The announcement is a symbolic milestone . But experts say it will take years -- perhaps decades -- to fully clean up the worst nuclear disaster since Chernobyl . The plume of radioactive particles that spewed from Fukushima Daiichi -- where reactor cooling systems failed in the aftermath of Japan 's devastating earthquake and tsunami in March -- displaced about 80,000 people who lived within a 20-kilometer ( 12.5-mile ) radius of the plant , as well as residents of one village as far as 40 kilometers to the northwest . The government has yet to determine when those evacuated can return to their homes . Significant work -- with significant risks -- remains to be done at the plant . `` This is far from over , and the work will go on for a long time , '' Tokyo Electric Power Co. President Toshio Nishizawa told CNN this week . Citing government sources , Japan 's Kyodo News agency reported Thursday that scrapping crippled reactors at the plant could take up to 40 years . In October , a spokesman for Tokyo Electric -- which owns the plant -- said temperatures in the three reactors where meltdowns occurred had already been brought down below 100 degrees Celsius ( 212 degrees Fahrenheit ) , but the company would have to maintain those conditions for some time before declaring the reactors in cold shutdown . `` They 're making this out to be some big milestone , some big thing . But the reality of this is that it 's not , '' said Michael Friedlander , a former senior operator at U.S. nuclear power plants . `` The problem is , with these reactors and the condition that they 're in , to suggest that they 're in cold shutdown really does n't do justice to the situation . They 're no safer today than they were basically in June . '' Officials could start removing spent fuel rods from the facility next year , but it could be up to a decade before they are able to access the reactor vessels , he said . `` They 'll probably spend two or three years studying it , '' he said . Jack DeVine , who helped lead the cleanup of the Three Mile Island nuclear plant after the 1979 partial meltdown there , said the cold shutdown likely means officials may have one less thing to worry about . If they do n't have to use as much water to cool the reactors , they also do n't have to worry about what to do with contaminated cooling water -- one of many problems that have plagued workers at the crippled plant . `` It 's not like there 's a dramatic difference . It 's just a gradual incremental difference that does make life easier , '' he said . The earthquake and tsunami on March 11 killed more than 15,000 people in northeastern Japan . The country was on edge for weeks as cooling systems failed and utility and government employees scrambled to prevent a nuclear catastrophe at the six-reactor Daiichi plant , located about 240 kilometers ( 150 miles ) north of Tokyo . Hydrogen explosions blew apart the No . 1 and No . 3 reactor housings , while another hydrogen blast is suspected to have damaged the No . 2 reactor . Fires believed caused by heat from the No . 4 spent fuel pool damaged that unit 's reactor building . Japan 's Nuclear and Industrial Safety Agency eventually categorized the accident as a level-7 event -- the highest level on an international scale for nuclear disasters -- putting it on par with the 1986 Chernobyl disaster . `` We are going to check each reactor properly and determine what is going to happen to those left , '' Nishizawa , the Tokyo Electric president , said Wednesday . Removing spent fuel rods is the next step , but officials need to further survey the area before that happens , he said . `` We are considering sending a robot into the fuel tanks to really have a good idea ( about ) the situation . This will be necessary when we take out the fuel , '' Nishizawa said . `` But I do n't believe what we see will be 180 degrees different from our simulations . But as we say , seeing once is better than hearing 100 times , so we will have a good look at what 's happening inside . '' At Three Mile Island , where damage was less severe and only one reactor was affected , it took two and half years before officials were able to get inside the reactor to assess it , DeVine said . At Fukushima Daiichi , that process will likely take much longer , he said . `` I think in terms of what 's ahead , it 's still the daunting task that it 's always been , which is first of all , figuring out where everything is . ... They 're going to be searching for and finding pieces of fuel , chunks of molten material that have now cooled , through all of that rubble , '' he said . CNN 's Catherine E. Shoichet and Matt Smith in Atlanta , and Licia Yee in Tokyo contributed to this report .
The announcement means temperatures have fallen below the boiling point . Experts say significant work -- with significant risks -- needs to be done . Kyodo News : Scrapping crippled reactors will take several years . Many people who lived near the nuclear plant remain displaced .