Question: What type of acid was dropped from helicopters after the Chernobyl disaster to curb any risk of explosion?
About the article "Chernobyl disaster", section "Debris removal".

Candidate passages:
The worst of the radioactive debris was collected inside what was left of the reactor, much of it shoveled in by liquidators wearing heavy protective gear (dubbed "bio-robots" by the military); these workers could only spend a maximum of 40 seconds at a time working on the rooftops of the surrounding buildings because of the extremely high doses of radiation given off by the blocks of graphite and other debris.
The reactor itself was covered with bags of sand, lead and boric acid dropped from helicopters: some 5,000 metric tons of material were dropped during the week that followed the accident.
At the time there was still fear that the reactor could re-enter a self-sustaining nuclear chain-reaction and explode again, and a new containment structure was planned to prevent rain entering and triggering such an explosion, and to prevent further release of radioactive material.
This was the largest civil engineering task in history, involving a quarter of a million construction workers who all reached their official lifetime limits of radiation.
By December 1986, a large concrete sarcophagus had been erected to seal off the reactor and its contents.
A unique "clean up" medal was given to the workers.
Many of the vehicles used by the "liquidators" remain parked in a field in the Chernobyl area.
During the construction of the sarcophagus, a scientific team re-entered the reactor as part of an investigation dubbed "Complex Expedition", to locate and contain nuclear fuel in a way that could not lead to another explosion.
These scientists manually collected cold fuel rods, but great heat was still emanating from the core.
Rates of radiation in different parts of the building were monitored by drilling holes into the reactor and inserting long metal detector tubes.
The scientists were exposed to high levels of radiation and radioactive dust.
After six months of investigation, in December 1986, they discovered with the help of a remote camera an intensely radioactive mass in the basement of Unit Four, more than two metres wide and weighing hundreds of tons, which they called "the elephant's foot" for its wrinkled appearance.
The mass was composed of sand, glass and a large amount of nuclear fuel that had escaped from the reactor.
The concrete beneath the reactor was steaming hot, and was breached by solidified lava and spectacular unknown crystalline forms termed chernobylite.
It was concluded that there was no further risk of explosion.
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 city after the disaster.
Chernobyl was evacuated soon after the disaster.
The base of operations for the administration and monitoring of the Chernobyl Exclusion Zone was moved from Pripyat to Chernobyl.
Chernobyl currently contains offices for the State Agency of Ukraine on the Exclusion Zone Management and accommodation for visitors.
Apartment blocks have been re-purposed as accommodation for employees of the State Agency.
Because of regulations implemented to limit exposure, workers in the Chernobyl Exclusion Zone are limited in the number of days per week or weeks per month they stay in Chernobyl. 
The Truth About Chernobyl is a 1991 book by Grigori Medvedev.
Medvedev served as deputy chief engineer at the No.
1 reactor unit of the Chernobyl Nuclear Power Plant in the 1970s.
At the time of the Chernobyl disaster in 1986, Medvedev was deputy director of the main industrial department in the Soviet Ministry of Energy dealing with the construction of nuclear power stations.
Since Medvedev knew the Chernobyl plant well, he was sent back as a special investigator immediately after the 1986 catastrophe.
In his book, Medvedev provides extensive first-hand testimony, based on many interviews, describing minute by minute precisely what was and was not done both before and after the explosion.
It has been described as a tragic tale of pervasive, institutionalized, bureaucratic incompetence leading up to the accident; and heroic, heartbreaking sacrifice among those who had to deal with the emergency afterwards.
In 1991 it was awarded the Los Angeles Times Book Prize, Science and technology. 
Friends of Chernobyl's Children (UK) is a charity founded in 1995 that brings children, who are at risk, from Belarus to the United Kingdom for a month every year.
The charity is named after the Chernobyl disaster of 1986.
It is one of the oldest of the 20 Chernobyl-named charities registered with the Charity Commission.
Each year over 1,000 children come over to Britain from orphanages or disadvantaged homes.
They are also given a full year's supply of vitamins and medicine for them and, if money allows, other members of their family.
The founder and director of the charity, Olwyn Keogh, was awarded an MBE in 2007 "For services to child victims of the Chernobyl disaster in Belarus".