CHERNOBYL UNIT 4 : ACTIONS 2 + 4

After the accident on Chernobyl Unit 4 in 1986, the Soviet authorities constructed a confinement building, known as the Ukritiye. It was constructed in a very short time under emergency conditions to confine the damaged unit, and was not expected to function in the long term.

The Ukrainian authorities initiated in 1992 an international tender for proposals for the safe enclosure of the damaged unit. Following the tender, the European Commission awarded a contract to the European consortium “Alliance” on a feasibility study for the safe enclosure of Unit 4 (see Contract 22721). This study was reported in July 1995. The Alliance study compiled and verified many existing data relating to Ukritiye, evaluated the options for safe enclosure and presented a recommended design for a structure.

The present study was aimed to consider alternative solutions. The requirements for the study were developed in accordance with the protocol signed on 11 September 1995 between the Ukrainian Authorities and the European Commission. This protocol required a number of actions to be taken in order to redefine the safety criteria and the design objectives for works both to stabilise the Ukritiye and for the conversion of the Ukritiye into a safe ecological condition.

The main objective of the short term measures to be proposed was to reduce the risk to the workers, the public and the environment. The measures had to consider:

• Reduction of accidental collapse potential (stabilisation of the Ukritiye)
• Reduction of accidental release consequences
• Fuel containing material, radioactive material management
• Transformation of the Ukritiye into a safe operating condition

The objective of the Long Term Measures was to determine an optimum alternative solution - in safety and in economic terms - for the conversion of Ukritiye into an environmentally safe condition.
The Contractor (Trischler und Partner GmbH, in collaboration with other European companies, Initec s.a., Ismes spa and Ove Arup & Partners), was responsible for the project management, including the integration of the “Alliance”, not only for providing the background information relating to the Alliance studies, but also for integrating and coordinating the local organisations and the Counterpart.
The following Ukrainian and Russian organisations were also involved in the study on a sub contractual basis: Ukrainian Scientific and Research Institute of Building Constructions (NIISK), Interdisciplinary Scientific and Technical Centre “Shelter” (ISTC), Interatom instrument, Atomenergostrojproekt and Research Institute of Designing and Experimental Technologies (VNIPIET) from St. Petersburg.

The project was divided into three phases. The end of each phase was a project milestone:

• Phase 1. Data finding and site model.
• Phase 2. Alternative study.
  • 2.1. Scenario screening
  • 2.2. Design criteria
  • 2.3. Feasibility study of alternatives
• Phase 3. Comparison, conclusion, recommendations.

The contract was signed on 4 April 1996 and was completed by 4 February 1997.

An International Expert Team, comprising specialists from EC countries, the United States, Ukraine and Russia, and representatives from Japan collaboratively executed the project. The work was divided into two main parts. The first part included extensive investigation of a number of alternative solutions, referred to as Scenarios, and defining potential short and long term measures based on the knowledge and understanding developed during the investigation of the various scenarios. The second part involved the development of a recommended course of action and an elaboration of the execution framework.

Scenario Investigation

Five scenarios were investigated, including subdivisions and variants within them, as follows:

1. Scenario 1: Base line. Continue the status quo, except organisational measures. This scenario was investigated for comparison purposes (Coordinator: INITEC/Spain).
2. Scenario 2: Short term measures. Implement immediate short term measures and continue them in the future as long term measures (Coordinator: Alliance, with main contribution from Ukrainian expert organisations).
3. Scenario 3: New Shelter. Implement immediate short term measures and a shelter suitable to allow early or longer term removal of the radioactive inventory inside the shelter. This shelter had to meet the requirements for containment until the removal of radioactive inventory could be implemented.
   • Scenario 3a: Light weight shelter for early removal of inventory (Coordinator: Ove Arup & Partners/UK).
   • Scenario 3b: Long lived shelter for deferred removal of inventory (Coordinator: Alliance).
4. Scenario 4: Earth shelter. Implement immediate short term measures and shield the radioactive inventory with bulk material, e.g. sand, in order to optimise and stabilise the present Ukritiye before implementing a small shelter for confinement and for an early or deferred removal (Coordinator: Trischler und Partner).
5. Scenario 5: Cemented material fill. Implement immediate short term measures and shield the radioactive inventory with cemented material fill in order to optimise and stabilise the present Ukritiye before implementing a small shelter for confinement and to proceed to early or deferred removal (Coordinator: Ismes/Italy).
   • Scenario 5 option 1.a: Cemented material fill.
   • Scenario 5 option 1.b: Cemented material fill & Cable stayed bridge/Roof truss.
   • Scenario 5 option 1.c: Cemented material fill & Cable stayed bridge confinement.
   • Scenario 5 option 2: Concrete Monolith (with main contribution from VNIPIET).

Safety Objectives and Design Criteria

On the basis of documents prepared by Riskaudit and the Ukrainian NRA, and on the basis of safety and operational Ukrainian regulations and requirements, safety objectives were summarised in order to derive design criteria for the purpose of Short and Long Term Measures. These safety objectives and design criteria were issued in one separate document. This document was concluded to be acceptable by the NRA for the purpose of the project.

Development of recommended course of action

All scenarios were reviewed by an International Expert Team and the knowledge and understanding gained during the investigation process were utilised to develop a comprehensive compilation of potential actions. From this compilation, a list of short term measures was developed and long term measures were considered in a frame concept, involving a stepwise approach for achieving an environmentally safe condition. The recommended course of action was intended to address the short and long term risks in an organised framework of steps leading to the transformation of the damaged unit into an environmentally safe condition, as summarised below:

Phase 1: Immediate actions and other Short Term Measures

• Task 1.1. Reduce collapse accident probability by structural stabilisation (to be coordinated with Tasks 2.1. and 2.2.)
• Task 1.2. Reduce collapse accident consequences (to be coordinated with Tasks 2.1. and 2.2.)
• Task 1.3. Increase nuclear safety by criticality control and contained water management (as a part of an integrated monitoring system)
• Task 1.4. Increase workers’ and industrial safety (appropriate monitoring and safety equipment are necessary)

Phase 2: Preparation of the long term conversion into an environmentally safe site

• Task 2.1. Provide safer accesses by shielding and stabilising as far as possible with cemented material fill and dust fixatives (to be scheduled in parallel with implementation of Phase 1)
• Task 2.2. Provide a confinement and remove unstable upper parts. Alternatively the confinement should be designed to take a collapse
• Task 2.3. Develop a removal strategy and optionally implement an early partial removal

Phase 3: Conversion into an environmentally safe site

• Task 3.1. Convert the site into a safe structure (to be scheduled in parallel with implementation of Phase 1)
• Task 3.2. Control and maintain the safe structure until removal (to be scheduled in parallel with implementation of Phase 1)
• Task 3.3. Removal of remaining inventory when appropriate and necessary.

Estimated cost and schedule

The cost estimations and time scheduling were made on the basis of conceptual approaches. More refined figures would have to be elaborated within the proposed course of actions. The estimated costs for Phases 1 and 2, without early removal of accessible FCM (Fuel Containing Materials), were approximately 510-735 million US$. With early removal of accessible FCM the estimated combined Phase 1 and 2 costs were approximately 735-1460 million US$.

It was proposed that Phase 1 could be implemented between 1997 and 2001, while Phase 2 could start in 1997 and be completed by 2004. If partial FCM removal would be undertaken, Phase 2 could extend up to 2025 and latest to the life time of the confinement structure. Phase 3 could start at any time after completion of Phase 2, i.e. after 2004 or after accessible FCM removal, and could be completed within four years.

Criticality study

Special expert effort was addressed to consider the criticality risk for the development of a proposal to convert Ukritiye into an ecologically safe condition. The final report “Evaluation, Control and Mitigation of Criticality Risk” was issued by the Contractor on 13 December 1996. Analytical studies concluded that a critical configuration is theoretically possible but extremely unlikely.

The requirements for a reliable monitoring system which enabled unambiguous interpretation of incidents with increased neutron count rates were defined in the report. Recommendations were given for the FCM characterisation to improve the data base for criticality evaluations. Recommendations for mitigation measures were also presented.