Project U/TSO/UNit4-Protocol: Chernobyl unit

After the 1986 accident at the Unit 4 at the Chernobyl site, Ukrainian and Russian authorities constructed a new “containment" for the damaged reactor. This structure (the Ukritiye) was erected in great haste and under extremely arduous conditions. Subsequent monitoring and analysis of the Ukritiye has shown that the initial expectation of a 30 year lifetime was unlikely to be achieved.
The International Community and the Ukrainian Authorities agreed on a combination of short and long term measures for the reparation of the damaged structures and their safe operation or encasement. In particular, the short term phase was guided by the estimated service life time of the structural components built just after the accident. Therefore, the short term phase was about 15 to 20 years beginning in 1996, taking into account upgrading and/or at least maintenance of the global stability of the sarcophagus. The long term phase is expected to follow the short term phase. It is assumed that by the beginning of this long term phase the main radioactive inventory will have been removed from the present operating nuclear facilities.

One of the first priorities for any safety assessment and upgrading of the Ukritiye was to develop a set of safety objectives for such “special” facility and for the encasement (the so called “Shelter structure”), for the short term and long term measures, based on both a general analysis of external hazards and a tentative classification of the main risks posed to the environment, population and workers.

These safety objectives supported the definition of "Design criteria" by the team of international experts in charge of the design assessment (see relevant PRSs).
In general terms, safety objectives mean general requirements (of the regulatory administration) for the design, construction and operation of anything related to the object "Shelter". The main goal of these safety objectives is to allow the operator and designers to define solutions so that the hazards should remain acceptable regarding their radiological consequences for the workers, the population (including future generations) and the environment.

More specifically, the safety objectives had to address the fact that the sarcophagus (Shelter including the remains of unit 4) is not an operating nuclear facility but a temporarily confined destroyed reactor, and that there is also a post-accidental contamination of the environment. Despite of this special nature, all activities should meet the generic recommendations for dose limits to workers and population.

The objective of this contract was the development of preliminary safety objectives for the safety assessment of the sarcophagus and its stabilization.

The first task is the analysis of the potential consequences of the accident scenarios: however, due to the fact that it is not possible to demonstrate the stability of the present Shelter, it was necessary to include a collapse scenario among the initiating events.

A complete set of safety objectives was developed and collected in a document titled “Safety objectives, principles and criteria concerning the shelter stabilization and its transformation to a safe condition”, delivered to the NRA.

The document defined a framework to assess that the radiological consequences for the workers, the population and the environment remain acceptable in normal and accidental conditions. In this respect, it was recognized that there is no difference between the radiological protection objectives for the short term phase and for the long term phase. The analysis of the Ukrainian standards confirmed a full agreement between them and the international practice.

A number of objectives and principles were formulated, namely:

• safety culture
• operating organization responsibility
• safety state regulation
• approved engineering and technical experience
• quality assurance
• human factors
• evaluation and verification of safety
• experience of operation and study on safety
• defense in depth
• prevention of accidents
• accident mitigations
• protection of people and the environment
• protection outside national boundaries
• protection of the future generations
• burden for the future generations
• A set of criteria was also provided to meet the above mentioned principles.

In relation to the reference scenarios to be considered in the safety assessment of the Shelter, the reduction of the dust release (e.g by stabilization of the Shelter and dust suppression) and/or the limitation of the radiological consequences for the workers on the site was considered one of the most relevant. Therefore, the main safety objective for the short term measures was identified in the urgent upgrading of the Shelter.

Another scenario described in the project outcome refers to the stability of the so called “Block B". This building separates unit 3 from the former unit 4. Consequences of events on the sarcophagus must not endanger the stability, neither of "Block B" nor of unit 3 as long as these units contain a significant amount of radioactive materials. It was concluded that if such events cannot be controlled, the shut down and the removal of the main radioactive sources from unit 3 and from "Block B" should be implemented in the shortest delay.

The inventory of the radiological situation inside the sarcophagus was also addressed in the study. In fact, the information that has been gathered up to the project start-up was sufficient to draw the conclusion that extensive work on the sarcophagus would require performing prior restoration tasks in its immediate surroundings e.g. the shielding, or removal of the radioactive materials buried around the sarcophagus.

The concept for the restoration of the site was the last addressed scenario. Bearing in mind that the aim is to utilize the entire area again some time in the future, plans for the decontamination had to be drawn up and different options for the decontamination had to be assessed. The analysis included in particular: the elimination of the dumps with fuel-containing masses (FCM) and of those where materials contaminated with fission and activation products are buried, the development of a waste management policy, and the planning and preparation of interim storage facilities for the recovered contaminated masses and FCM. Only the results of such investigations might show if this objective can be reached or to what extent it has to be modified.

Therefore, the final report developed a step by step restoration concept for the site as a whole, also involving the analysis of the tasks to be implemented in the close surroundings of the Shelter. Due to the presence of two nuclear power plants in operation and other nuclear installations (e.g. spent fuel pools) at the site, it was recommended that the short term phase also evaluated the risks resulting from the operation of those facilities, to be compared with the risk posed by the sarcophagus itself.

Finally a special section of the final report provides a reference list of initiating external events (design basis) that have to be considered for the upgrading of the sarcophagus. The reference exceedance probability was chosen at 1E10-4/y. Events which could be excluded on the basis of either distance or probability were neglected.

For human induced events, a more precise classification was provided to include human activities that can be forbidden, relocated, or strictly regulated. Moreover, all hazards linked to human activities related to the stabilisation of the sarcophagus were included.

In conclusion, the documents provided some tentative values for the design basis, the exceedance probabilities, references to suitable applicable standards and procedures. It covered climatic scenarios, straight and rotational wind, snow, rain, lightning, earthquakes, raising of the water table, aircraft crash, dropped loads external fire and explosions.

A number of internal events was also discussed and reference provided for the development of the relevant design basis: internal explosions, fire, flood, dropped loads, criticality and corrosion respectively.

Comments

(quality of the results, lesson learned, recommendations for follow-up)

The project met the objectives stated in the TORs and provided invaluable support and input to all the safety relevant activities performed at the ChNPP site since 1996 on.