The world on Tuesday marks a quarter century since the world's worst nuclear disaster at Chernobyl in Ukraine, haunted by fears over the safety of atomic energy after the Japan earthquake.
In the early hours of April 26, 1986, workers at the Chernobyl atomic power station were carrying out a test on reactor four when operating errors and design flaws sparked successive explosions.
The radioactive debris landed around the reactor, creating an apocalyptic scene in the surrounding area, while material also blew into the neighbouring Soviet republics of Belarus and Russia and further into western Europe.
Two workers were killed by the explosion and 28 other rescuers and staff died of radiation exposure in the next months. Tens of thousands needed to be evacuated and fears remain of the scale of damage to people's health.
Russian President Dmitry Medvedev annonced he will make a landmark visit to Chernobyl on Tuesday to take part in the memorial ceremonies, where he is expected to be joined by his Ukrainian counterpart Viktor Yanukovych.
Russian Orthodox Patriarch Kirill will hold a service in the Kiev region in the early hours of Tuesday, striking a bell at 1:23 am local time (2223 GMT) -- the time when the explosion went off -- to formally mark the start of remembrance ceremonies.
He will then head to the affected zone to hold an Easter service at a chapel in the settlement of Chernobyl and then a service by a memorial next to the disused power station itself.
But the anniversary has gained an eerily contemporary resonance after the earthquake in Japan which damaged reactors at the Fukushima Daiichi power plant and prompted leaks of radiation.
Japan has placed the disaster on the maximum seven on an international scale of atomic crises, the same level as Chernobyl, and the troubles at Fukushima have prompted many questions about whether atomic power is too great a risk.
The operator of Fukushima, Tokyo Electrical Power Co. (TEPCO), has also come under fire over its information policy, an echo of the disastrous reluctance of the Soviet authorities to admit the truth over Chernobyl in 1986.
Moscow stayed silent on the Chernobyl disaster for three days, with the official news agency TASS only reporting an accident at Chernobyl on April 28 after the Forsmark nuclear plant in Sweden reported unusually high radiation.
In 1986 and 1987, the Soviet government sent over half a million rescue workers (liquidators), to clear up the power station and decontaminate the surrounding area, many not fully aware of the scale of the calamity.
"I think that our modern states must see the main lesson of what happened at Chernobyl and the most recent Japanese tragedy as the necessity to tell people the truth," Medvedev told a meeting of liquidators in the Kremlin.
"The world is so fragile and we are so connected that any attempts to hide the truth, to gloss over a situation, to make it more optimistic, will end with tragedy and cost the lives of people."
But despite the notoriety of Chernobyl, controversy has raged for years even between the UN's own agencies over the number of deaths directly caused by the disaster, with estimates ranging from tens of thousands to dozens.
Some experts have said the worst health legacy of Chernobyl is mental rather than physical, with those affected traumatised by the memory of April 1986, forced relocation and the sense that they are victims of nuclear catastrophe.
In 2005, several UN agencies including the World Health Organisation, said in a report a total of 4,000 people could eventually die as a result of the radiation exposure.
But the UN Scientific Committee on Atomic Radiation (UNSCEAR) says other than the 30 confirmed deaths in the immediate aftermath only 19 ARS (Acute Radiation Syndrome) survivors had died by 2006 for various reasons.
Other than 6,000 cases of thyroid cancer -- a usually treatable condition -- from contaminated milk there was "no persuasive evidence" of any other effect on the general population from radiation, it said in a report in February.
But environmental campaign group Greenpeace in 2006 accused the UN agencies of grossly underestimating the toll, saying there would be an estimated 93,000 fatal cancer cases caused by Chernobyl.
After the disaster, the Soviet authorities put up a supposedly temporary concrete shelter to protect the destroyed reactor but there have long been worries about its durability.
A new sarcophagus is being built nearby and is scheduled to be erected over the reactor in the next years.
But astonishingly, the European Bank for Reconstruction and Development (EBRD), which is running the project, has yet to win full funding for its completion.
The conference last week secured 550 million euros ($785 million) in new pledges, short of the 740 million euros still needed.
Chernobyl continued producing energy until well after the collapse of the Soviet Union. Reactor number two shut after a fire in 1991, reactor number one closed in 1997 but reactor number three continued working right up until December 2000.
Explore further: Physicist creates ice cream that changes colors as it's licked