Resolution of Issues Arising Failure Con

The 1986 accident at the Chernobyl Nuclear Power Station in the Ukraine occurred in Reactor Unit 4. An identical Reactor Unit 3, situated in the same building complex, is still operating and provides a major source of electricity to the Kiev region. Between Unit 3 and Unit 4 there is a link building, 'V' block, which carries a number of reactor support systems.
An analysis of the link budding, 'V' block, between Units 3 and 4 of the Chernobyl Nuclear Power Plant was carried out by the Alliance consortium in 1995, as part of their work on the design of a new enclosure for Unit 4. This showed that the structure would be severely overloaded and likely to fail under the conditions of the maximum credible earthquake, which occurs once in 10 000 years.
The present study has been carried out to evaluate the consequences of structural failure of the 'V' block, due to seismic or other cause on the operation of Unit 3. In particular, the aim was to determine if Unit 3 could be shut down safely, and maintained in a safe shut down state, in the event of structural failure of 'V' block.
The work has been carried out by the Consultant, based upon both earlier Alliance work and new sources of information from Ukraine.

The project aimed at re-assessing the conclusions drawn by a previous contractor (Alliance Consortium) on the stability of the 'V' block of ChNPP in relation to external hazards, on the basis of newly available evidences and information.
It has to be reminded that the structural assessment was carried out in two big steps, as during the final project meeting, Ukrainian Consultants supported the existence of additional reinforcing structures in the 'V'-block, which were not considered in the first step of the study.Therefore, a new assessment had to be carried out, in order to reach a complete agreement among the Parties and a full consistency with all the available documents.

In particular, the project carried out the following tasks:

• Assessment of the actual conditions of the structures of the 'V' block after the 1986 accident at the Unit 4
• Review of the external hazard assessment carried out by the previous Consultant
• New seismic analysis of the 'V' block, completed with a structural re-assessment
• New analysis of the consequences of the 'V' block failure in terms of nuclear safety and radioactive release into the environment
• Collection of recommendation for structural strengthening and improvement of the nuclear safety of Unit 3.

Seismic scenario

The hazard scenario was confirmed, even if at the upper bound of the conservative range (Pga = 0.18 g).
The structural assessment showed that under the maximum credible earthquake (10 000 year return period) there is insufficient reinforcement in the main structural members of the building above the 18m level to withstand the loads. Therefore the Consultant concluded that structural failure of the critical 'U' shaped walls will take place during the specified seismic event. Moreover, as these elements are predicted to take a large proportion of the calculated seismic loading on 'V' block, failure would substantially increase loading on the remaining load bearing elements (primarily the reinforced concrete building column and beam framing), which would in turn be expected to fail under the resulting massive increase in load.

The analysis of the seismic displacements and their comparison with the width of the expansion joints between the 'V' block and Unit 3 made the Consultant conclude that there is a real risk of hammering during earthquake, with consequent, serious, damage to the structures.
Finally, the loads on the foundations are sufficiently high to cause tilting of the building. Liquefaction of the soil below the building cannot be ruled out. These effects would all occur, but to a much lesser extent, during a design earthquake which has a return period of 100 years.
In conclusion, the Consultant stated that partial or total collapse of the 'V' block structure under the specified seismic loading is likely.

Tornado

A study has also been carried out of the vulnerability of 'V' block to a tornado with a probability of occurrence of once in 30 000 years. This shows that the ventilation stack on the building could collapse, but the building itself would not be seriously damaged. The precast concrete cladding is not considered to be vulnerable to localised damage, unless fixings are inadequate due to poor workmanship or deterioration. It was not possible to determine the quality of the existing fixings within the scope of this study as this would require too detailed site investigation.

Explosion

The explosion hazard was not considered as insufficiently covered by the available documentation.

Consequences of external hazard on the nuclear safety of Unit 3
The 'V' block hosts some important systems/functions, such as:

• Parts of the intermediate cooling system
• Part of the service water system
• Active effluent treatment installations
• Equipment decontamination facilities
• Ventilation systems
• Gaseous discharge treatment plant
• Maintenance workshops
• Ventilation exhaust stacks.

The detailed analysis of the external hazard scenarios on these systems/functions highlighted the following:

• In case of an earthquake, the reactor will be safely shut down, even in the event of a complete loss of off-site power. It will be possible to continue to remove heat provided that the Service Water System continues to work. It is likely that the main service water pipe, which passes through the building below ground level, will remain operational under most earthquake scenarios, but could fail in the event of liquefaction of the soil below the 'V' block foundations. Should this occur, cooling would be removed from the feedwater and emergency core cooling pumps, leading eventually to loss of coolant to the core. It would also lead to the prevention of the removal of decay heat by the service water to the reactor lake.
• 'V' block provides the systems for cleaning up water supplies from radioactive contamination and for the extraction and filtration of contaminated air. The loss of the building would cause a release of liquids which would be equivalent to 12 cubic meter of low level waste, and the release of contaminated air at a lower level than would be the normal case from the ventilation stack. Neither of these scenarios would have any effect beyond the site boundary, but would provide an extra hazard for site personnel.
• The effect of toppling of the ventilation stack either due to earthquake or due to tornado depends on the direction in which it falls. If it falls in the direction of Unit 3, it will still be possible to safely shut down the reactor. The most damage would be produced if it fell onto the turbine building, resulting in possible fire and loss of coolant to the reactor.

In conclusion, a comprehensive list of recommendations was issued to mitigate the effects of an earthquake on the 'V' block.

The second phase of the structural assessment was based on verbal communications with the Local Subcontractor (LS).

From the results of this assessment it was concluded that the presence of major structural walls of the type and configuration described by the LS would significantly improve the performance of the 'V' block structure in case of a seismic event. As a result, the conclusions of the first phase study would be modified, excluding any major collapse, hammering and soil liquefaction in case of the considered external hazard types.

However these conclusions were strictly connected to many assumptions on structural layout, strength and soil properties made by the LS, and definitely not assessed by the Consultant.

The final set of recommendations was developed as in the following, in the case of confirmation of the existence of the “additional structures”:

• Provide documentary evidence of the existence of the additional structural walls in the 'V' block
• Reinforcement of the support structure to the ventilation stack
• Reinstate the clear separation (now filled with bricks) between buildings to improve seismic performance
• Consider local strengthening of the soft storeys of 'V' block above the 18m level in the east-west direction, which constitute a major zone of weakness of the structure to lateral seismic loading
• Carry out a survey of the supports for the service water main pipe work and if necessary consider means to enable it to withstand a major earthquake, including soil liquefaction
• Consider installation of additional isolating valves within the Service Water System
• Consider measures aimed at improving the reliability of cooling water supplies. A study should be carried out into the possibility of providing a diverse supply of service water.

Comments

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

All the involved experts agreed that the two studies fully exploited the available data.
The project met the objectives stated in the TORs.

This management of the project was very sound and full agreement was reached between the Contractor and the Local Institution who participated to the project.

Further Information

Further information on the project results could be sought from the beneficiary organizations.

The final report of the project is available at the EC/JRC-IE-TSSTP archive.