Scientists have detected nuclear reactions taking place underneath the new confinement that isolates the destroyed reactor of the Chernobyl Nuclear Power Plant. Buried uranium fuel poses a potential hazard. Specialists are trying to establish whether the reactions stop on their own or require urgent action, an article posted on Science magazine says.
According to Anatoly Doroshenko of the Institute for the Safety of Nuclear Power Plants, sensors track a growing number of neutrons, indicating nuclear fission reactions are underway. Experts can not rule out a new accident, but the neutron flux grows slowly, and it may take several years before the worst-case scenario occurs. Scientists will thus have time to come up with extraordinary measures to counter the threat.
On April 26, 1986, when part of the reactor core of the fourth block melted, uranium fuel rods, their zirconium cladding, graphite control rods and fire sand formed corium - a lava-like fuel-containing material (FCM) or nuclear lava. It flowed into the basement of the reactor hall, where it froze having formed an "elephant's leg" - an extremely radioactive mass. Ten minutes near this substance is lethal. The Chernobyl corium contains 170 tons of radioactive uranium, but radioactivity decreases over time.
The first safe confinement, called the Shelter, allowed rainwater to seep in. Water slows down neutrons, which raises the likelihood of their interaction with uranium nuclei. The process led to a sharp increase in the neutron flux as a result of decay.
Scientists could originally handle this problem by spraying neutron-absorbing gadolinium nitrates, but the spray could not penetrate into the basement. It was believed that the New Safe Confinement would stabilize the situation, but the neutron flux continued to grow in some places anyway. For example, in room 305/2, where tons of FCM are buried under the rubble, the neutron flux has doubled in four years.
Specialists have established that the increase in radioactivity is due to the drying of FCM, which paradoxically increases the probability for uranium nuclei to absorb free neutrons. Experts fear that the fission reaction will accelerate exponentially, which may lead to an uncontrolled release of nuclear energy. In this case the Shelter will collapse, and the space underneath the confinement will be filled with radioactive dust.
Scientists consider an opportunity to develop a robot that would be able to withstand the intense radiation long enough to drill holes in the FCM and insert boron cylinders that will absorb free neutrons.