The G7 countries and Ukraine have agreed on a plan to reduce the risk of a second explosion at the Chernobyl nuclear power plant's destroyed reactor. The $780 million project, expected to shift into high gear this summer, is supposed to resolve lingering doubts about the safety of the site of the world's worst nuclear accident.
On 26 April 1986, Chernobyl's reactor number 4 exploded, spewing radioactive debris that has triggered a rash of thyroid cancers in children of Belarus, Russia, and Ukraine. After the explosion, much of the radioactive fuel from the damaged reactor flowed like lava into the rooms below the reactor chamber, where it solidified. However, gaps in the protective "sarcophagus" built over the ruins to keep the dangerous debris from dispersing have allowed an estimated 3000 cubic meters of water to leak into the reactor's innards. Ukrainian scientists and some Western experts have warned that the water, by providing the moderator needed for a nuclear reactor to operate, could spur an uncontrolled chain reaction in the remaining fuel--perhaps triggering a second explosion.
In a series of closed meetings last year, a G7 expert team assessed this risk and proposed ways to reduce it. Among the warning signs they considered were at least three incidents since 1990 in which detectors in the sarcophagus measured an increase in neutrons from the fuel masses, a potential sign that the fuel was going "critical." However, says a State Department official involved in the meetings, "the general consensus is that the probability of criticality is almost nil." Remove the water, she says, "and it is nil." But she adds that the participants also considered other dangers. One scenario is that a major earthquake could crumble the sarcophagus and "change the configuration" of the fuel masses, perhaps increasing the criticality risk.
The G7 plan involves some 22 projects that, by 2005, will drain the radioactive water, build a new sarcophagus over the current one, and develop robots that can withstand the intense radiation in parts of the sarcophagus. By providing core samples of the fuel masses, the robots should help engineers get a better idea of the amount of fuel, its configuration, and potential criticality. The plan, to be administered by the European Bank for Reconstruction and Development, also calls for buying Western neutron detectors to install in the current sarcophagus to monitor potential danger while the work goes forward.