New route to deal with Chernobyl’s waste30 October 2000
The best way to approach the waste remaining inside the destroyed reactor at Chernobyl may be from underneath, according to a Russian proposal.
It has been suggested that the Chernobyl Sarcophagus should be transformed into a temporary storage facility for radioactive material. The proposal has been developed from an early design that was published in 1998 (see NEI, November 1998, p22). This proposal includes several novelties in engineering solutions, and it will require a smaller funding.
The biggest task involved in transforming the Sarcophagus to a storage facility is the removal of fuel-containing materials. In order to remove all of this material all the other collections of fallen objects should also be dispatched, simultaneously removing other radioactive materials and items.
The exposure dose rate inside the Sarcophagus has decreased considerably during the last decade and a half. Thanks to this circumstance, it has become possible to use high performance machinery with cabs that protect the operators from the effects of ionising radiation.
STRENGTH AND STABILITY
The strongest and most stable structures of the Sarcophagus as it currently exists are the northern cascade wall and the western counterfort one. Other elements that are considered strong and stable are the wall at the eastern side of the sarcophagus, which separates the destroyed unit 4 and the adjoining unit 3, and the two ventilation shafts that were not destroyed.
Elements such as the wall along axis 50 and the carcass between axes 50 and 51 supporting the cover should be categorised as insufficiently strong and stable. The same is the case with supports for the ‘Mammoth’ beam. These supports rest on the basement, which in fact is the destroyed structures of the deaeration stack filled with concrete and sand. This basement is about 6m thick, and its strength is unknown.
The most likely accident that could befall the Sarcophagus – in fact almost the only significant event – would be the fall or partial fall of the reactor cover. This cover – a concrete shield several feet in depth – is held at an acute angle above the remains of the reactor core. A fall could be precipitated by a sufficiently-large movement of one of the supports of the Mammoth beam, of the wall along axis 50, or the carcass between axes 50 and 51.
At one time the ‘Alliance’ joint venture company designed an arch that could be installed over the Sarcophagus to allow decommissioning to be carried out. The cost amounted to r3 million. The project-based cost of this construction was estimated at $1.4 billion, and the project was finally rejected because of the arch’s high cost.
In fact, the strength and stability of the Sarcophagus would not be inferior to those proposed by Alliance for the arch, if a stronger and more stable cover were to be established on its reliable walls, with the connections sealed properly.
The choice of approach
To start the work of dismantling and removing the contents of the Sarcophagus, a tunnel must be driven into the structure. The tunnel’s cross-section must be large enough to allow the passage of high capacity machinery of large size and removal of large fragments of equipment and structural elements. Which route should this tunnel take?
Approaches from the top of the reactor and from the side of the operational third unit are senseless.
It is not feasible to construct a tunnel in the reinforced radioactive concrete mass of the cascade wall, some 40m thick.
Construction of a tunnel through the counterfort wall is also a difficult and dangerous task. This is because possible fall of this wall along axis 50, the carcass between axes 50 and 51, and the cover.
Finally, construction of the tunnel from the machine room would be also dangerous, because of the possibility of a shift of one of the Mammoth beam supports and the resulting fall of the cover.
There is therefore only one safe and feasible way to construct the tunnel: from beneath. In this route the tunnel passes via the sub-reactor compartments, through the concrete basement of the destroyed unit four. This concrete material is 2m thick.
Before radioactive materials can be removed from the Sarcophagus there must be a preparation phase.
Beneath the Sarcophagus, in front of the sub-reactor compartments, facilities must be constructed for radioactive materials to be received and processed. These facilities should be connected to the ground surface and the inside of the Sarcophagus compartments by tunnels, which in the latter case would pass through the concrete basement of unit 4. The upper boundary of the waste reception facilties would be at the bottom boundary of the support plate of the unit.
Maintainable equipment must be provided to process the waste and to move waste and machinery in both directions through the tunnels.
The waste reception facilities will be used to separate material according to its physical state, chemical composition, and types of activity encountered. They will also be used for the decontamination of objects delivered, destruction of large parts of equipment and structural elements to fragments of appropriate sizes; filling the transportation devices and containers with materials and objects for their delivery along the tunnels to a suitable processing facility; and delivery of materials and objects with low radioactivity levels to the ground surface for intermediate storage at the site.
According to the information from participants in the accident cleanup, there were originally plans to construct a concrete wall around the plant, which would be extended under the surface to a depth of 30m. This would be deeper than the upper sand layer, reaching approximately the middle of the marl layer, which has a filtration coefficient of 2.5x10-4-1.8x10-2m/day. The entire wall length would be 8.4km, but only 2.1km was ever constructed. Although the wall was not completed the groundwater level within the site appears to be much higher than that in adjacent areas.
Before the premises for radioactive material reception and the tunnels can be constructed, the below-ground concrete barrier must be finished. In addition, several boreholes must be drilled in a regular array over the site and the groundwater must be pumped out to an appropriate level. Lowering the groundwater level would facilitate the construction of tunnels and reception premises.