The explosion of Unit 4 at the Chernobyl nuclear power plant in 1986 left the reactor destroyed, with some 180 metric tons of irradiated fuel exposed to the atmosphere. In an attempt to prevent the escape of additional radiation, the Ukrainians built a concrete sarcophagus over the unit. The sarcophagus, called a ukrytie, or shelter, by the Ukrainians, was begun in May 1986 and completed in November of that year.
The sarcophagus was erected in part using remote construction methods--because of the high radiation fields--and without full information on the strength of the original building, which meant that its structural integrity could not be gauged.
Between 1987 and 1991, Ukrainian and Russian scientists conducted research at the sarcophagus to determine the location and physical state of the irradiated fuel. The scientists found three forms of fuel, widely distributed: core fragments, which had been thrown to the upper floors of the unit by the force of the explosion; a congealed form of vitrified fuel, sand, concrete and metal structures known as Chernobylite; and about 10 metric tons of radioactive dust from one to several microns in size.
In addition to the approximately 180 metric tons of fuel or fuel-containing materials, the scientists identified 64,000 cubic meters of radioactive building materials, 10,000 metric tons of metal structures and 800-1,000 metric tons of radioactive water in the destroyed unit.
A number of fuel assemblies were ejected from the reactor, falling up to one-quarter mile away. Some were bulldozed up and moved to the sarcophagus, and some were buried where they landed.
Over this period, the Ukrainians mapped the location of radiation fields within the unit, measured radiation and temperature levels within the sarcophagus, monitored site ground water and nearby rivers, and reinforced internal structures that had been badly damaged by the accident to prevent further failures.
The sarcophagus is not leak tight. Rainwater can enter and radionuclides can escape.
The 10 metric tons of radioactive dust within the sarcophagus represent a major threat to public health and the environment. The fear is that the movement or collapse of an internal structure--like the 1,000-metric ton reactor lid sitting on edge in the mouth of the reactor vessel--could stir up the dust, which could then be propelled by heat and air currents into the atmosphere. In 1988, for instance, drilling equipment was accidentally dropped, sending up a thick cloud of dust and forcing the evacuation of the sarcophagus. The Ukrainians have installed a system for sprinkling resins to control dust within the structure, and it reportedly works.
The high radiation levels within the sarcophagus contribute to the problems with dust and structural integrity. The magma containing molten fuel is disintegrating in the high radiation fields, providing even more radioactive dust. And the reactor's original concrete and other support structures are losing mechanical strength. The Ukrainians have attempted to deal with this problem through structural reinforcements, not always with success.
In one reported case, a load-bearing I-beam rests on a wall without a plate to spread the load. With no margin of safety where the I-beam rests, heavy snow or high winds could overload the wall, causing it and the roof of the sarcophagus to collapse. No generally accepted technique for reinforcing the wall has yet been developed.
At a conference in Ukraine in December 1994, officials reportedly said that one of the weak points in the structure had been repaired and the sarcophagus could operate for another 10 years.
There is also a concern about earthquakes, which could topple an internal structure and--in the case of a 1990 quake--create new vents in an already cracked structure. Since 1990, the Ukrainians have used a dust suppresser to periodically spray neutron absorbers inside the central hall, where much of the irradiated debris is located.
The Ukrainians reportedly admit that it is difficult for them to determine the stability of the structure using traditional monitoring instruments because some 40 percent of the reactor building within the sarcophagus is inaccessible owing to high radiation levels.
Water poses problems, too. The basement of the reactor building is reportedly flooded with 20,000-25,000 liters (5,283-6,604 gallons) of water, and outside burial pits containing debris from the accident are flooded. The leaching by water of radionuclides from the fuel-containing materials is a growing problem, and prevention of groundwater contamination is a high priority. Yet according to a Ukrainian nuclear industry official, the ground-water at the Chernobyl site is being contaminated.
Ukrainian authorities had planned to deal with the lack of leak tightness by eliminating about 70,000 square meters (83,720 square yards) of vents in the sarcophagus, but they have postponed the work for a year because of a lack of money.
According to an official from Germany's GRS, it is unlikely that a large portion of the mass of fuel inside the destroyed reactor would go critical. But the Chernobylite is apparently starting to be transformed into a water-soluble, pumice-like substance. As particles of this substance are lifted into the air by heat-generated convection currents, the amount of radioactive dust inside the reactor building will increase.
Two Ukrainian academicians reported in December 1994 that the possibility of explosion or fire within the sarcophagus is increasing. They said several fires have already occurred, and one--in 1993--burned for several hours and increased the radioactive discharge from the reactor building tenfold. The academicians concluded that a large fire could cause a radioactive release, in the form of fuel dust, on a scale similar to that of the 1986 accident. They urged the development and implementation of an integrated fire detection and suppression system for the sarcophagus.
In 1991, the Soviet government initiated a study of the costs, risks, time scales and implications of three options for dealing with the weakened sarcophagus: Build a new, separate structure over the existing sarcophagus; fill the existing sarcophagus with a special concrete; or decommission the reactor buildings and sarcophagus.
The Organization for Economic Cooperation and Development's Nuclear Energy Agency (NEA) agreed to provide experts in nuclear safety and waste management to help the Soviet panel evaluate the options.
In early 1992, the panel concluded that filling the sarcophagus with special concrete was the preferable option, but Moscow admitted that the newly independent Ukraine might not follow its advice. In fact, Ukrainian politicians said that the rapidly weakening structure needed to be either enclosed in a protective shell or, preferably, removed from the site.
In July 1992, the Ukrainian government announced an international competition for the best project to provide a second "shelter" for the destroyed reactor that would last for 100 years or more. The aim is to first contain and then eliminate the destroyed reactor and all radioactive equipment, structures and materials.
The deadline for proposals--originally Dec. 31, 1992--was extended to April 26, 1993. A jury of scientists from Ukraine, Russia and Belarus awarded second prize to a French consortium; there was no first prize.
At a "Sarcophagus Safety-94" meeting held in Ukraine in March 1994, 172 nuclear experts from 12 countries gathered to discuss the deteriorating sarcophagus. They agreed to establish a standing group of experts from various countries to develop recommendations for future action.
Participants also received details of the tendering process for a European Union feasibility study on dealing with the sarcophagus. The study will be funded from the 3 million ECU ($3.78 million) earmarked by the EU's TACIS program for improving the safety of the sarcophagus.
The same month, following a visit to the Chernobyl plant, a team of experts from the International Atomic Energy Agency (IAEA) noted the "technically confirmed accelerated deterioration of the shelter which, if it collapses, would have serious consequences."
In August, the EU awarded a 3-million-ECU contract to the Alliance Group to study the feasibility of strengthening the existing sarcophagus and building a new shelter over it. The group consists of two U.K. companies, three French companies and a German company. Under the terms of the contract, it must review all concepts that were finalists in the 1993 Ukrainian government competition, select an option and carry out a cost and design study within eight months of contract award.
In September 1994, the U.S. company Orbit Technologies announced that its special ceramic silicone foam had been approved by Ukraine for testing at the sarcophagus. The foam could be used to seal the sarcophagus and the fuel-containing materials inside it, preventing the release of radioactive aerosols and other soluble forms of radionuclides.
In March 1995, EU officials presented the results of the first phase of the Alliance Group's feasibility study to Russian, Ukrainian and Western participants. The group concluded that the high level of radioactivity inside the existing sarcophagus required the construction of a new shelter over it that is leaktight and would permit the dismantling of the structures beneath it. The new shelter must be built over Unit 3 as well as the destroyed Unit 4, which would require the decommissioning of Unit 3. According to the group, the existing sarcophagus is unstable and could collapse under external forces, especially earthquakes. Long-term stabilization of the existing structure is not a feasible option.
The second phase of the study, which included a detailed examination of strengthening the existing sarcophagus, development of a design for the new shelter, identifying requirements for dismantling Unit 4, studying nuclear waste issues, drawing up a project management plan and estimating total cost, was completed in mid-1995. In July, the Alliance Group presented two options: the construction of a new shelter over units 3 and 4, and the construction of a new shelter for Unit 4 alone.
The estimated cost--$1.6 billion--would cover provisional stabilization work on the existing sarcophagus, construction of a new shelter, and project management. The group proposed a two-stage funding system for the project. The first stage would be the creation of an International Fund for Chernobyl, a financial institution devoted to collecting public and private grants to finance the project. The second stage--which would entail the dismantling of the sarcophagus over 20 years following the 10-year construction period--would be funded in such ways as siphoning off a percentage of revenues from energy taxes or lotteries.
Ukrainian nuclear experts and officials have been critical of some aspects of the group's proposal, but they have expressed a willingness to cooperate with the group in finding a solution. The biggest obstacle to progress is the absence of funding.
November 1995