The Chernobyl sarcophagus30 May 2001
The sarcophagus at Chernobyl was constructed to protect the damaged reactor and to prevent radioactive contamination seeping out into the environment.
In 1986, the Russian authorities commissioned the construction of a sarcophagus above the damaged reactor in order to prevent the radioactivity present in the lava and the remaining structures of the reactor from dispersing into the environment, and to prevent rainwater from penetrating into what was left of the reactor and contaminating the ground.
The sarcophagus was built on the two remaining walls of Unit 4, one of which is shared with Unit 3. The other two walls were erected on the debris of the reactor building. The sarcophagus consists of beams and large metal plates which, given the very high dose levels near the reactor, were lowered into place using cranes with no opportunity to check joints and fixings. In addition, the high level of radiation to which the structures are subject may destabilise them. This situation precludes the performance of specific diagnostic tests on the medium- and long-term solidity of the sarcophagus.
In addition to this building, a separating wall was built in the turbine hall to isolate the turbines in unit 4 from those in unit 3. In the auxiliary building which separated these two units, the compartments used to unit 4 were filled with concrete, thus providing radiation protection over 6m thick.
On the northern side of reactor 4, a 50m high protective wall supported by four giant steps was built to reinforce the sarcophagus and provide protection against radiation. The bottom of this wall, known as the ‘waterfall’ wall, is 20m thick.
The sarcophagus roof is made of sheets of corrugated iron, rests on 1.2m diameter steel tubes placed on four enormous beams (two B1s and two B2s) anchored on the eastern and western walls of the reactor.
To the south of reactor 4, two pillars were constructed on the foundations of the destroyed reactor to support another wall built of beams and plates.
A considerable percentage of the 190t of reactor fuel is still in the sarcophagus. The surface of the lava created by the fusion of the fuel and the buildings (100-130t, including 50-80t of fuel) is now at ambient temperature. In addition, the lower reactor compartments contain 3000m3 of a mixture of the water used to extinguish the fire, and rainwater.
In addition to the collapse of the sarcophagus, there are two other risks. The first is the risk of a criticality event. Experts agree that the risk of a criticality event cannot be completely ruled out, even though a chain reaction seems highly improbable. The second risk is that of the re-suspension in the atmosphere of radioactive dusts due to the decomposition of the lava into dust. The total mass of radioactive dusts present in the sarcophagus is estimated at 30t with an activity level of 1017Bq, caused mainly by strontium-90 (47%) and caesium-137 (30%). To improve this situation, a polyvinyl solution is periodically sprayed into the sarcophagus to fix the dust. Following weekly spraying during the initial years after the accident, this operation is now carried out on a monthly basis.
Reinforcement of the sarcophagus
The collapse of the sarcophagus would create an emission of radioactive dust which would lead to the exposure of on-site personnel, but would affect only very slightly the nearest populations living in the town of Slavoutich, some 50km east of the Chernobyl plant.
The works carried out to reinforce and stabilise the sarcophagus are designed to reduce as far as possible the leaching of radioactive dusts from the sarcophagus and to reduce the risk of its collapse in both the medium- and long-term and in the event of a natural disaster.
A list of 15 reinforcements was drawn up and studied by the operator. Given the difficulties inherent in working on the sarcophagus and the associated risks to personnel and the environment, only the highest priority works were carried out by stabilisation of the ventilation chimney shared by reactors 3 and 4, and reinforcement of the concrete structures of the beams supporting the roof of the sarcophagus. These beam reinforcing works were carried out between March and December 1999.
The Shelter Implementation Plan was launched in 1998, covering an eight-year period by a G7 working group of experts in nuclear safety and the Ukrainian government. The project is funded by the EBRD by $760 million, including $50 million provided by the Ukraine government. The plan has two principal objectives:
the stabilisation of the sarcophagus and the implementation of measures to protect workers and the environment. Within this framework, five main areas of work have been prioritised: the reduction of the probability of destruction of the sarcophagus, the reduction of the consequences of its possible destruction, the improvement of facility safety (criticality control, contaminated water management, characterisation of materials containing fuel), the improvement of safety for workers and the environment and the development and implementation of a strategy to ensure long-term safety.
The plan is being implemented by a Chernobyl team assisted by a project management unit bringing together Bechtel, Battelle, and EdF. This structure was set up principally to define the programme of basic tasks required to achieve the objectives of the plan and to obtain the necessary authorisations from the Ukrainian Nuclear Regulatory Department (NRD). To carry out this brief, the NRD is using Ukrainian experts from the State Scientific Technical Centre of Nuclear and Radiation Safety (SSTC) and foreign experts from Scientech and Riskaudit.
Exposure of sarcophagus workers
The Institut de Protection et de Surete Nucléaire (IPSN) does not officially have any accurate dosimetric data relating to the persons working on and in the sarcophagus. However, information gathered as part of the assistance being provided to the Ukrainian safety authority enables us to estimate the level of exposure of the workers during the stabilisation work already carried out.
The reinforcing works on the two beams involved some 300 people. The corresponding collective dose was 3.5 person-sieverts: 10% for the locating and preparatory works, and 90% for the work itself.
•The preparatory works involved approximately 100 people. The highest individual doses remained below 15mSv, with 10 people being exposed to between 10-15mSv.
•The stabilisation works themselves involved 200 people. About 20 people were exposed to between 30-40mSv.
According to the data available, no workers exceeded the individual limit dose of 40mSv accepted by the Ukrainian authorities for this work. The level of internal exposure is not known exactly, but does not appear to be significant for these works.
The collective target dose published in the 15 reinforcements on the sarcophagus is 25mSv. By way of illustration, the collective dose for the operation of nuclear plants in France is about 70mSv per year.
As part of its brief to provide technical support to the Ukrainian safety authority, the IPSN is paying particular attention to consideration of the radioprotection elements of the safety analyses carried out during these works. In this field, the main areas for improvement identified by the IPSN in terms of radioprotection are:
•The estimation and monitoring of the internal exposure of workers and the adequate provision of measures designed to protect the respiratory tract.
•The application of the principle of radioprotection optimisation (ALARA) designed to reduce exposure.
•The drawing up of procedures and guidelines specifying the radioprotection measures implemented on the ground.
Within the framework of the ‘Franco-German Chernobyl Initiative’, the IPSN and its German counterpart, GRS, are working with the Chernobyl Centre, the plant operator, the ISTC (the ‘sarcophagus’ Scientific and Technical Interdisciplinary Centre of the Ukrainian Academy of Sciences), the NISK (Ukrainian Academy of Engineering Sciences), and the Moscow Kurchatov Institute have developed a database on the condition and safety of the sarcophagus.
This database will help to improve the estimation of radiological risks inside and around the building and to validate the current radioprotection measures. A unique source of data for future engineering projects, the database should also help to define a long-term strategy for the future of the sarcophagus and contribute to the implementation of the EBRD managed Shelter Implementation Plan.
The contracts relating to this project were signed in 1998 between the IPSN and the GRS on one side, and the Russian and Ukrainian institutes on the other. Since then, most of the data has been collected from the various participants, validated and prepared for entry into the database.
Thus constructed, the database can be consulted using a 3D software package which will eventually allow the user to make a ‘virtual visit’ of the sarcophagus and its surrounding area in order to facilitate location and information searches. This information will be available, depending on its nature, in the form of tables of data, graphs, text, plans, photographs or films.
More precisely, this database revolves around four projects to be developed under the responsibility of the various institutes as follows:
•The construction and equipment of the sarcophagus and related infrastructures (NIISK). This project is designed to bring together the technical documentation relating to the construction of reactor 4 and the buildings constructed on this reactor after the incident and to draw up an inventory of the equipment located in this structure.
•The radiological situation inside the sarcophagus (Kurchatov Institute). This project is designed to compile the results of the measures carried out by this Institute in the various zones of the building in order to determine dose levels, contamination levels and to locate the radioactive emissions.
•Evaluation of the radiological situation near the sarcophagus (ISTC). This takes into account the developments due to the decrease in radioactivity, but also the transport of radioelements by air/water.
•Compilation of data to the quantities, activities and characteristics of the radioactive substances present inside the sarcophagus (Kurchatov Institute). These substances are made up of the debris from the core of the reactor which was covered with sand and materials dropped by helicopter just after the accident and the radioactive magma which ran down to the bottom of the building, where it solidified.
In June 1999, an initial version of the database was sent to the Chernobyl Centre. In November 2000, the IPSN installed at Fontenay-aux-Roses an updated version of the software which, although it contained only part of the data, was nevertheless representative of a final version. This version is currently being tested.
Other projects at Chernobyl
The decommissioning of Chernobyl will be carried out over a period of many years. Having been shutdown, the nuclear facilities will be emptied of their radioactive contents and then dismantled. These operations will be performed with a view to the potential future reuse of the site, possibly subject to certain conditions. The final extent of dismantling will in part depend on the eventual use of the plant, and could range from the simple closure of the plant to the large-scale elimination of radioactivity to allow the site to be reused.
The decommissioning programme includes construction of various facilities on the site to be used for the storage of irradiated fuels, the treatment of liquid waste, the treatment of solid waste resulting from the dismantling of the reactors and the storage of the products of these treatments.
Spent fuel storage facility
The spent fuel from the shutdown reactors are currently stored in the reactor cores, the tanks adjoining the reactors, and an old storage facility. Construction of a new facility is planned to dry store these fuels for 100 years using the Nuhoms procedure.
The fuel storage facility will consist of two main buildings.
•The first will house a workshop to be used for breaking up the fuel assemblies using the Nuhoms procedure. This building will also be used to store the control rods from the reactors.
•The second will be made up of a group of modular passive cooling cells in which the containers will be stored, each cell containing one container.
The nominal processing capacity of the workshop will be 2500 assemblies per year over ten years. On the basis of current figures, there will be 256 modular cells.
The project was started in June 1999, and work is currently underway on the building foundations. According to the construction schedule, the operating licence is due to be granted at the start of 2003. The assemblies will be transferred to the storage facility over ten years. The estimated cost of the project is E80 million.
Liquid waste treatment facility
Low- and medium-activity liquid waste (treated water from the tanks, waste from the four reactors) are stored on the power station site in two sets of troughs. The first consists of five external troughs each with a capacity of 5000m3; the second of nine 1000m3 troughs built in individual cells. In total, 26,000m3 of liquid waste are currently stored (out of a capacity of 34,000m3). Some of the troughs also contain solids, in particular, resins.
A workshop is currently being built to treat this liquid waste and transform it into parcels of solid waste.
Initially, the main treatment operations will be to pump the waste out of the current troughs into a reception pond in the treatment workshop. This will be a delicate operation, since it was not planned for when the 14 troughs were built. As a result, the pumping operation requires the installation of ventilated boxes containing the transfer and rinsing equipment above the troughs and the making of connections to the existing network of transfer pipes. In particular, it will be necessary to terrace the domes of the 5000m3 troughs and punch openings in them.
Next, the waste will be concentrated and mixed with a binder to immobilise it. The parcels produced in this manner will then be stored in a surface storage centre due to be built near the Chernobyl site.
The distillates from the waste concentration stages will be reused to resuspend the solid particles in the troughs, or purified to meet the conditions under which they can be transferred back to the plant and disposed of.
It is planned that the waste from dismantling Chernobyl will also be treated in this workshop.
Preparation of the site where this workshop is to be built started in 2000. According to the planned production schedule, the operating licence is due to be granted at the end of 2002.
The processing of the existing liquid waste should take place over ten years. The estimated cost of this project is 25 million Euros.
Solid waste processing facility
It is planned to construct a new solid waste treatment facility. The low- and medium-activity solid waste in question comes from the four reactors (metal, concrete, plastic, wood, paper), and is currently stored in a dilapidated building divided into three compartments. The first two, each containing 1000m3, are practically full. The third (1800m3) which contains the most active waste is only 18% full. The exact nature of this waste is relatively unknown, and consequently special precautions will have to be taken during the treatment operations.
Part of this waste has been covered with concrete, and its retrieval will involve the use of various removal techniques including the drilling of bore holes in the three compartments and remote removal. The daily waste retrieval rate will be 3m3. The estimated total quantity of waste is about 2350m3.
The treatment of the waste from the dismantling of the power plant is also scheduled to be carried out in this new treatment facility.
The waste will be separated into various categories according to its physical properties and radiological characteristics. Waste not suitable for surface storage (the most active waste) will be temporarily stored in leak-proof containers on the site while awaiting subsequent treatment.
Following appropriate treatment, the least active waste will be clad in concrete and placed in containers. The workshop’s daily production capacity should be 20m3 of processed waste.
The final surface storage of the low- and medium-activity waste parcels will be carried out in concrete-lined trenches. The storage site will be located within the power plant exclusion zone. It will be necessary to draw up acceptance criteria for the waste parcels on this site based on an impact study and in conjunction with the properties of the parcels produced in the liquid and solid waste processing workshops.
The solid waste removal and treatment facilities and the final storage of the waste parcels will be financed by the TACIS
Invitations to tender for the construction of the facilities are currently being evaluated. Given the progress of this phase, the selection of contracting teams should take place soon, enabling removal and processing operations to start in late 2003/early 2004. The removal and treatment of waste will last for at least five years.