From below the Sarcophagus29 October 1998
A method for dismantling the destroyed Chernobyl reactor and its Sarcophagus is proposed, involving tunnelling from underneath the plant. Coming from below should be the safest approach, causing the least disturbance to the structure particularly during operations to remove the highly radioactive material. This will then facilitate the next stages: removing the remaining less active material and the Sarcophagus, and disposing of the waste.
The destroyed reactor 4 at Chernobyl and its Sarcophagus, the temporary structure (also known as “Object” or “Shelter” in Russian terminology) which was built to contain the radioactive material, represents a source of radioactive contamination for the environment. Transforming this into an ecologically safe site requires first removing the radioactive material, then dismantling and removing all remaining structures and finally disposing of the waste in accordance with the international regulations now in force. How to accomplish this in the safest way, minimising the risk to the environment and limiting the exposure to workers, will have to be decided.
The state of the Sarcophagus and distribution of fuel-containing material was published in 1992 by the R&D Institute for Power Technologies (NIKIET) (see panel). The conclusions have not substantially changed – particularly that the structure will continue to function as expected for a full 30 years. Nevertheless, given the risks posed by the fuel and a collapse of the structure, it is desirable to make the site ecologically safe as soon as possible.
Of particular concern is the amount of water which comes mostly from moist air entering through the openings in the Sarcophagus and condensing on the internal surfaces. There is no leak-proof basement beneath the Sarcophagus, which would be nearly impossible to construct. Therefore, radioactive substances washed-off by the water can spread to the ground water.
UNDERGROUND RECEPTION HALL
The removal method proposed involves approaching the problem literally from below – to access radioactive material from underneath the Sarcophagus and transport it to processing facilities which must first be constructed and commissioned.
Of particular importance is the construction of a hall for receipt of radioactive material below the Sarcophagus, with tunnels connecting it to the (ground) surface as well as to the internal cavity of the Sarcophagus. A drive system with remotely controlled and maintainable equipment will transport the robotics systems used for the operation and return the radioactive material back to the reception hall.
A tunnel cutter will be able to work through the ferroconcrete structures below the +9.00 m level which are known to be fully preserved. It should therefore be possible to identify the most convenient directions to tunnel within the Sarcophagus.
In the reception hall, the radioactive material will be subdivided according to their physical state and type of radioactive waste. Radioactive materials and other items will be loaded into transport containers and transferred along the tunnels to the processing facility according to the method applied. The materials and items with no or very low levels of radioactivity can be dispatched to the surface by other tunnels to be stored at the plant site.
The arrangement of the reception hall beneath the Sarcophagus enables the personnel, protected by the intact concrete basement, to safely approach the lower rooms and sections where the highly radioactive fuel-containing materials are located. To facilitate the removal of radioactive material, a network of the rooms, divided into radiation zones, will be created for preliminary material separation, packaging and dispatching. Initial efforts will be focused on removing fuel-containing material; it will then be much easier to deal with the larger quantities of lower activity material.
Because the dismantling will be made from below, some materials will be transferred to the reception room by their own gravity. For example, contaminated water will be drained along the tunnel and pumped to the vessels of the liquid radioactive waste decontamination system.
Radioactive substances have to be removed according to the international regulations now in force. With this objective in view, some structures have to be erected, such as processing and storage facilities for radioactive wastes; remotely controlled and maintainable equipment must also be provided.
The processing and storage methods used will be determined by the materials’ physical state, chemical composition, quantities, radioactivity levels, requirements for stabilisation, the methods chosen for volume reduction, the number and types of barriers needed to prevent the release of material during storage and many other factors. Of course, all of these should be taken into account in the development of work specifications for designing the facilities.
The existing Sarcophagus has first to be turned into an ecologically compatible system and kept in a safe state until radioactive materials are completely removed. It must remain a major barrier preventing the release of radioactive dust when work is carried out demolishing large structures within and removing the fragments. This means, first of all, reliably sealing openings in the walls and ensuring that the seals are well maintained and remain undisturbed.
Secondly, a powerful system of ventilation is needed. It must incorporate air purification equipment providing highly efficient decontamination and an exhaust ventilation plant with several stacks to remove air from the reception hall as large volumes of air will flow from the Sarcophagus via the tunnels. This purification complex must have 100% redundancy.
Together, the sealing of the Sarcophagus and the ventilation system will prevent radioactive dust being released into the atmosphere during the work.
After the ventilation system is turned on for the first time, considerable amounts of dust suspended in the air and deposited on surfaces will be removed. Operating continuously, the system must be designed to remove dust even if the unstable upper slab above what remains of the core, collapses, raising a massive amount of dust.
Radioactive material will be transferred to the reception hall by the robotics system that must be designed for high reliability. This system will incorporate different robots and tools for handling a wide variety of applications (welding, cutting and manipulating equipment with a capability of demolishing large structures), remote controls, drive motors, transport and transfer mechanisms, devices for dust suppression and collection, actuators for loading/unloading operations etc. It can be provided with a shielded operator cab allowing the team to function much more effectively, eg to constantly watch the state of load-carrying structures of the Sarcophagus (which may have to be strengthened with concrete reinforcement or by repairs). This will help maintain the safety of the structure until the radioactive material removal is finished.
After all these materials are removed, the Sarcophagus can be dismantled and taken away.
The facilities necessary for receiving and processing the radioactive material and one possible arrangement are shown here.
The removal operation should only be started after all these facilities are completed and commissioned. During construction, the workers will only be affected by the background radiation around the site. The highest exposure would be expected during wall sealing operations.
Some of these facilities could later be used for decommissioning the remaining units at the station as well.
WHY GO FROM BELOW?
The question can, of course, be asked: why dismantle from beneath, rather than from the side (lateral) of the top?
The main reason is that neither of the alternative approaches really provides the possibility of safely approaching the most highly radioactive materials for their removal. To organise a lateral or top-down operation, it is unavoidable that the strength and integrity of the Sarcophagus will be impaired in creating a transport path for the removal. These activities will also be accompanied by a considerable dose burden for the personnel which will also effect the quality of the work.
Dismantling from beneath allows the use of the material’s own weight for their transfer. Any of the alternatives would require much larger energy for the transport of non-packaged materials, with various obstacles to overcome.
It would be also impossible to organise proper exhaust ventilation as effectively as in the case from beneath.
Furthermore, with the other options, lower activity material would have to be removed first. As these materials provide a shield against the higher activity from fuel-containing material, which are mostly located in the middle and at the lower levels, personnel would be subjected to increasing dose rates during material removal.
In addition, going in laterally or from the top greatly complicates the setting up of a network of rooms within the Sarcophagus for preliminary separation.
Ultimately, the method proposed here, dismantling from beneath, makes a complete solution feasible – the total removal of the destroyed reactor and Sarcophagus while ensuring a low level of collective dose, the main advantage of the method.
|Sarcophagus report from Bulletin for Public Information No1*|
|• The earthquake that occurred in May 1990 corresponded to 3.5 - 4 points on the Richter scale (in the area of the plant). The investigation of the Sarcophagus structures after the earthquake has shown that its design-basis life time of 30 years remained a quite realistic one. • The estimated total area of openings in the Object’s walls is 1000 m2. • In the course of the accident “liquidation” period (before the Sarcophagus was constructed), a part of the fuel subassemblies were removed from the plant site; however, most except for those remaining in the reactor vault and adjacent rooms are covered up by the cascade wall. • From 6 to 10 t of finely dispersed fuel appear to be distributed over many rooms; however, the largest amount of that fuel has been detected in the central hall. • Most of the fuel is confined in hardened lava-type formations that spread through many of the rooms under the reactor during the accident. • The fuel in the lava masses is in the form of finely dispersed particles embedded into a glass-like matrix. Radiochemical effects gradually degrade the masses, increasing the risk of the migration of the fuel particles. • There are 800 to 1000 t of water in the remaining rooms.|