STABILISATION OF EXISTING SHELTER

The radiological consequences of the explosion, followed by fire, on 26th April 1986, of the Unit 4 Reactor at the Chernobyl Nuclear Power Plant had a considerable and long lasting impact on the international public opinion.

The sarcophagus (the Ukrityie), erected in haste between May and November 1986, in order to contain as quickly as possible the radioactive ruins of Unit 4, after few years showed all the inevitable signs of imperfection due to extreme radiological conditions and rapidity with which it was built. Monitoring and analysis of the Ukritiye showed that the initial expectation of a 30 year lifetime was unlikely to be achieved.

The ruins contained in the Ukrityie included over seven hundred kilos of trans uranium elements, especially plutonium 239, whose very long half life (24 000 years), required in the medium-term their recovery and storage in repositories followed by secure disposal.
In the following years, the stability of the Ukrityie needed to be carefully assessed, before a second shelter was constructed. The management of waste and the minimization of radiation doses to both workers and public posed additional issues. Also the long term storage and disposal requirements needed to be assessed to ensure consistency of approach and compliance with international regulatory requirements.

Therefore in June 1992 the Ukrainian Government organized an international competition calling for proposals to transform the existing Ukritiye into an ecologically safe system. The competition named “Kiev 92” attracted a total of 394 proposals from the international community. The quality and experience of the participants demonstrated the willingness of States and of the major organizations to cooperate with Ukraine, meanwhile become independent from USSR, in the management of the requirements to meet the challenges of Chernobyl Unit 4.

In 1994 the European Commission took the decision to provide funding for a Feasibility Study for a second shelter (Shelter 2) to maintain the initiatives started by the Ukrainian Government. Therefore a call for proposals was organized in April 1994 by DGI within the TACIS framework. The Alliance consortium was awarded that contract. It was funded among most of the finalists of the Kiev competition, other European Organizations, and Ukrainian and Russian Organizations and Institutes.
The principal objective of this Feasibility of Study was to determine the optimum concept for protecting the environment from uncontrolled releases of nuclear materials arising form the damaged remains of Chernobyl Unit 4. To achieve this objective the study was broken into 3 distinct phases:

• Phase 1 - Option studies
• Phase 2 - Concept definition
• Phase 3 - Concept validation and review

Only Phase 1 and 2 objectives were covered by this project.

The final report of Phase 1 and 2 was issued in July 1995.

The main objective of Phase 1 report was to review the existing information related to Chernobyl Unit 4 and associated structures. Based upon the review of existing conditions, the final report outlined the options considered in achieving the principal objectives of the whole TACIS assistance to Chernobyl, which included the stabilisation of the existing shelter and the construction of a second shelter (Shelter 2). In particular, the Shelter 2 was requested to meet the following requirements/characteristics:

• Durability: at least for 100 years;
• Monitoring of safety related parameters throughout the life;
• Safety and security: to be guaranteed for 100 years;
• Health physics: to be provided at the site to comply with state-of-the-art regulations;
• Decontamination facilities: to be provided to confine contamination;
• Power supply at the site;
• Communication system available;
• Water supply on site;
• Waste treatment and disposal (for non radioactive waste);
• On site roads, including emergency routes;
• On site lighting;
• Fencing and marking the site perimeter;
• Site security;
• Radioactive waste treatment and storage facilities;
• Compliance with regulations.

The study was based on a strict technical evaluation of the different options. The final report of Phase 1 was based on a methodical and exhaustive analysis of the possible and acceptable risks. After a detailed description of this approach, the report examined the options related to:

• Waste management;
• Stabilization of Ukritiye;
• Dismantling/Cleaning up;
• Shelter 2 design.

The aim of the Phase 2 report was to define in more detail the feasibility of constructing a second shelter around the damaged Unit 4 reactor at Chernobyl. The report detailed the work performed in developing the Shelter 2 design recommended during the Phase 1 study. In particular, the following issues were covered:

• Structural design considerations;
• The location of the East Wall;
• The construction methodology for the Shelter 2 structure;
• Site Organization and logistics;
• The management of radioactive wastes;
• Cost estimation & scheduling.

achievements (as requested in the TORs)

During Phase 1, the Consultant carried out the following tasks:

• Obtain, review, assess and validate the information regarding the reactor 4 and its environment;
• Carry out option studies to evaluate the structural, environmental, and financial programme, the safety consequences and occupational exposure of various aspects of the shelter 2, such as (among the others):
  • how to stabilise the existing structures to achieve both stability and radiological safety, access, maintenance, etc.;
  • how to retrieve the content of Reactor 4, assess its radioactivity and determine appropriate waste management actions;
  • whether to site the waste assaying, sentencing and primary processing facilities within the new Shelter 2, in a separate but adjoining structure, or in a separate linked structure connected by a tunnel or covered passage;
  • how to preserve the integrity of Unit 3 containing the operational Reactor 3 given that there are a number of structural and services connections between the units.

In relation to the waste management, the study concluded the following:

• The stabilisation of Ukritiye and the construction of Shelter 2 cannot commence until a safe storage or disposal route is available for the radioactive waste generated.
• The Ukritiye site and surrounding areas cannot be considered as a suitable site for the disposal of radioactive waste due to the unfavourable geological conditions. The only credible solution to the problem is to remove the radioactive waste and to dispose the material in a way that does not create an undue risk to the environment for now or at any time in the future.
• The management of radioactive waste can be split into 3 phases with stabilisation:
  • The construction phase where the majority of the waste will be soil. The facilities required at this stage will include a soil treatment plant, liquid effluent treatment plant and storage facilities for LILW-SL and LILW-LL;
  • The stabilisation and dismantling phases where the majority of the waste will be concrete and steel. The additional facilities required at this stage will be volume reduction equipment including super compaction, melting and incineration which will be installed in the Waste Management Building. This building will also contain grouting facilities for packaging the waste prior to final disposal.

In relation to the stabilisation of the existing Ukritiye, the study concluded the following:

• There were still crucial information to be gathered in order to feed Phase 2, namely: soil properties, earthquake site hazard, drawings, report of the structural damage, GA drawings of the buildings adjacent to Unit 4, original design basis of the Unit 4 and new loads applied during and after the accident.
• The results of stability analyses on the existing structures clarified that the Block b, B and Gamma will fail in the case of an earthquake of intensity 0.18g to 0.09g together with the West buttress wall.
• Concepts for stabilisation of the existing structures were developed for all the most critical areas (seven) of the Ukrityie.
• Additional concepts were developed on the confinement of rainwater, ground water, of the water inside the Ukrityie, the containment of air and dust, the mitigation of the radiological risk in case of collapse of the Ukrityie.

In relation to the dismantling of the damaged and contaminated structures inside the Ukritiye, the study concluded the following:

• In the short term further studies were needed to establish the compatibility of the construction planning of Shelter 2 with the short-term dismantling operations.
• In the long term the data gathering and assessment had to be pursued since much information was still missing, among which the exact location of part of the fuel. Moreover, the possibility of replacing the roof by a light structure or reinforcing it should be studied as well.
• A plan for equipment adapted to the working conditions expected on the site should be prepared. A preliminary task was established for remote handling equipment.

In relation to the design of the Shelter 2, the study concluded the following:

• It is possible to obtain a partial containment of the Ukritiye without prejudice to the date of final shutdown of Unit 3. Final containment can only be ensured after total shutdown of unit 3.
• The 100 year lifespan required for Shelter 2 means that a light construction is not sufficient. The thickness of the structure walls is dictated by the degree of radiation protection to be provided for the maintenance personnel and means that a heavy structure must be erected.
• It is uncertain whether the existing buildings can contribute to containment of Shelter 2 or serve as bearing points and also guarantee the 100 year lifespan.
• Closure of Shelter 2 at its East boundary with the existing buildings seems to have no other reasonably technical solution than to extend Shelter 2 to cover unit 3.
• Of the 19 proposals for the Shelter 2 selected by the ”Kiev 92” competition, only two groups were retained, namely three Box-Type structures (composed by walls and roof, partially supported by the existing foundations) and three Arch-Type structures (one single arch above the Ukrityie, with independent foundations). A complex set of criteria was developed, dealing with the design life, construction, and structural performance and one arch solution was selected as the most promising.
• According to the results of the studies on hydrology, groundwater radioactivity and geology, a detailed analysis of the proposed six solutions for the underground containment was carried out. The solutions included groundwater flow management, complete containment using tunnelling, complete containment using mining, wall containment only, pumping and “do nothing”(for reference). Comparison criteria were set up in terms of safety requirements, radiological safety, ground conditions, site conditions and foundation technology. The preferred option appeared to be the confinement of the groundwater flow using groundwater management in the form of diaphragm or slurry walls and/or other means.

Site infrastructures were also covered by the study, in order to provide security, fire safety, radiological protection, power supply, management of raw materials and effluents, monitoring and control, communications, and emergency response. No problems were identified by the Consultant in the feasibility of the relevant infrastructures.
In general, the phase 1 study concluded the following:

• The existing Ukritiye is not stable and collapse may occur during a seismic event. Long-term stabilisation of Ukritiye is not feasible. In particular it was concluded that under seismic loadings (intensity 7, PGA at 0.18 g) the structure will collapse. During Phase 2 a detailed proposals for the short-term stabilisation of Ukritiye is needed, even if these short-term solutions should only be considered as temporary solutions prior to Shelter 2 construction.
• Block “B” adjacent to Ukritiye is not stable: the Block B is unstable under seismic loadings and it was considered impracticable to stabilise it with Unit 3 in operation. It is also considered impractical to construct an East Wall within the boundaries adjacent to Unit 4. Therefore any possible solutions for an East wall will require the closure of Unit 3. It was concluded that any design for Shelter 2 should consider the structural dependency between the Ukritiye and the Block B.
• The construction of Shelter 2 is considered an urgent requirement. The most suitable configuration for such structure was identified. Moreover, formal approval of Shelter 2 design criteria are required before starting Phase 2: such design criteria should be subjected to review by all the stakeholders.
• Prior to the construction of Shelter 2, Waste Management Facilities are also required. Large quantities of radioactive waste will be produced, which will require an integrated Waste Management approach to be established. Radioactive wastes arising from the pre-construction operations will require sorting, monitoring and either storage or treatment and disposal. Moreover, regulations for the classification and disposal of radioactive wastes should be agreed and formally approved before Shelter 2 construction commences.
• Prior to the construction of Shelter 2 some short-term dismantling operations may be required. These include the excavation of foundations and the removal of radioactive material from parts of the pioneer wall. Work may also be required to dismantle parts of the Turbine Hall. Therefore it was recommended that detailed studies were carried out to assess the short-term requirements for dismantling operations.

During the phase 2, the consultant carried out the following tasks:

• Review and amend as necessary the recommendations set out in Phase;
• Complete the data collection campaign;
• Develop the options approved at the end of Phase I into a comprehensive concept definition report.

Phase 2 concluded the following:

• Availability of technical information on the existing conditions of site and structures will play a major role in the future project and construction work and should be organised now. In particular, standards and codes, design data, site data should be improved.
• Stabilisation of the structures is one of the most difficult tasks. It will be a complex combination of structural strengthening concepts, some of which have been predefined in the study. However, the strengthening concepts should be studied in detail to refine their design and to optimise their consequences with respect to the stability of the structures.
• The total dose uptake parameter for the stabilisation work requires detailed analyses and the development of appropriate construction technologies and research to reduce as much as possible this effect.
• The improvement of the water exclusion by enhanced sealing of the Ukrityie is to be carried out on both the roof and the walls of the Ukritiye.
• A new concept of textile fabric cover could be applied to improve the containment of Ukritiye for water exclusion and against aerosol dispersion.
• The option of pumping out the water that is inside Ukritiye to manage the liquid waste should be applied to reduce the risk of its dispersion into the groundwater.
• New research is necessary to find technologies to hinder the production of radioactive dust inside the Ukritiye.
• The Emergency system against the dispersion of dust has to be developed and installed to prevent non permissible consequences in case of major collapse.
• An emergency system to improve the protection of the personnel in case of criticality should be installed inside the Ukritiye. Specific robotic technologies must be used to reduce the dose uptake during installation.
• A centralised control and monitoring system, as defined in this section, must be installed to control and monitor all the pertinent parameters with respect to the safety and the security inside and around Ukritiye. The installation of this system will also require appropriate remote technologies.
• The project of stabilisation will have complex interfaces with Site development, with Shelter 2, with the existing workshops and with the temporary and future facilities for the waste management. These interfaces need a specific Organisation to maintain the safety.
• New methods and new technologies have to be considered in the studies and applied to the construction work of stabilisation.

The final report suggests the composition of a project management group and identifies the main project phases, namely: a preliminary phase (for the definition of roles and responsibilities and reference standards), the setting of the funding, the definition of the objectives of the projects and the FSAR, the preliminary design, the contracting of the work packages, the commissioning of the Shelter 2 and its operation.

The timeframe was 15 years, and the relevant costs 1400 million USD (1995 prices).
Stabilisation (20 years required, partially overlapping with the construction of the Shelter 2), waste storage and waste treatment were excluded from this estimate.

Comments

(quality of the results, lesson learnt, recommendations for follow-up)
The project met the objectives stated in the TORs.
This management of the project was very sound, the technical conclusions very well described and the involvement of the Ukrainian Organisations paved the way for a fully successful implementation of the stabilisation project.