Ensuring the long-term safety of the Shelter which surrounds the destroyed Unit 4 reactor at Chernobyl is a crucial part of dealing with the aftermath of the 1986 disaster. The huge quantity of radioactive substances enclosed within the Shelter and the poor reliability of its structural elements and enclosing structures make the facility a radiation hazard, unlikely to be able to withstand exposure to extreme internal or external effects. Such an incident could lead to the collapse of the structural elements and the release of large quantities of radioactive dust, with significant radioecological and radiological consequences.
The fact that ignition and fire are among such conditions is confirmed by the fire of 1993 (in area 805/3) when a temperature increase inside the facility led to a ten-fold increase in radioactive aerosol emission levels.
The situation is also complicated by the particular conditions of the Shelter, where most areas are inaccessible because of the high radiation levels and the presence of exposed fuel containing masses (FCM) representing a nuclear hazard.
SOURCES OF FIRE HAZARD IN THE SHELTER
Existing sources of fire ignition and combustion
The main source of ignition and fire is the large quantity of combustible materials accumulated in the Shelter. Experts assess this quantity at approximately 2000 tonnes. The materials include:
• Electric cable insulation, approximately 250 tonnes
• Plastic floor coverings, over 160 tonnes
• Paint finishes (walls and equipment)
• Graphite, approximately 320 m3
• Auxiliary wooden structural components
• Turbine oil
• Combustible waste
• Dust, approximately 30 tonnes
• Organic products of dust suppression, approximately 5 tonnes (polyvinyl alcohol, latex etc)
• Roofing materials.
At present, the distribution of combustible materials and the extent of their radioactive contamination are known only very approximately.
Highly conflicting assessments have been made of dangerously explosive concentrations of a number of these items, such as graphite dust and dry polyvinyl alcohol.
In the circumstances, the fire status of the Shelter could be described as follows:
The Shelter represents the gravest fire hazard, and provisions for its fire protection should be based on special standards and regulations making proper allowance for the associated potentially high nuclear and radiation hazards.
Possible causes of ignition and fire
There is no detailed analysis of the effects of a range of initiating events on the ignition, propagation and consequences of Shelter fires.
Internal initiating events to be considered include:
• Short circuits and disruptions in the operation of electrical systems and earthing devices, including events caused by the collapse of structural elements and internal flooding.
• Local temperature increases due to spontaneous fission.
• The formation of pyrophoric materials and composites and their accumulation in supercritical quantities.
• Failure to observe fire safety regulations when working with naked flame.
• Sabotage.
External initiating events to be considered include:
• Storms and lightning.
• Extreme natural events leading to the collapse of structural elements and enclosing structures (extreme precipitation, hurricanes, tornadoes, earthquakes etc).
• Extreme situations of technogenic origin (external explosions and fires, aircraft collision etc).
The likelihood of fires in the current condition of the Shelter
An issue of crucial importance in arriving at an objective assessment of the fire hazard represented by the Shelter and creating an optimal and effective fire protection system (involving minimal material and financial expenditure and minimum dose loads) is the assessment of the likelihood of fires, their extent and consequences.
Existing assessments are not more than expert opinions, and have not been confirmed by the relevant research. No methodology exists for carrying out analyses of this kind specifically for the conditions inside the Shelter.
In the twelve years of the Shelter’s existence, there have been six fires:
• 14.02.88: Ignition of cables in area 201/3 caused by a failure to observe fire safety regulations during electric welding.
• 19.09.88: Boards and building debris in the ventilation duct in area 207/4 caught fire during electric welding.
• 17.10.88: Rags in area 402/3 caught fire as a result of failure to observe regulations on work with naked flame.
• 09.11.92: Ignition of an oscillograph power transformer in the “Finish” system area, G-359/1.
• 14.01.93: Ignition of wooden sleepers in area 805/3 caused by prolonged exposure to the heat of an electric light.
• 23.02.1996: Ignition of building debris in 284/4 during work with naked flame.
In the fire of January 1993, which lasted for over four hours, aerosol emission through the ventilation duct increased approximately tenfold, and the concentration of active alpha nuclides in the direction of the wind increased from 1.1 x 10-17 Ci/l to 1.1 x 10-16 Ci/l.
In a number of papers, the probability of fires is estimated at 0.5 per year. This estimate is much higher than the standard figure of 4 x 10-4 per year.
It must again be stressed that the lack of a representative analytical method with which to assess the probability of fires in the Shelter and assess the severity of their consequences means that any conclusions reached could be excessively conservative and could result in inefficient use of the material, financial and human resources available for improving its fire safety.
Possible adverse consequences of Shelter fires
Ignition and fire can have the following adverse consequences:
• Loss of stability of internal structural elements and enclosing structures, and its collapse, accompanied by the emission of significant quantities of radioactive substances into the environment.
• Disruption of system operation, down to total loss of nuclear and radiation safety control and maintenance systems and their support systems, and most importantly of the electricity supply.
• Violent movements of air masses with entrained dust formations, with the resultant increase in the emission of radioactive substances through non-leaktight enclosures and through ordinary ventilation ducts.
• Accelerated dust formation.
• Accelerated degradation of fuel-containing masses, migration of fissile isotopes through the air and their accumulation in dangerous quantities.
• A threat to personnel from the fire, high temperatures, and the generation and propagation of fumes and toxic combustion products.
• Increased radiation exposure of personnel and members of the special fire brigades putting out fires.
• Loss of property.
In addition to the above, indirect consequences of ignition and fire, especially those associated with the use of specific fire extinguishing materials, must also be considered. In particular, the use of water to extinguish fires could have the following adverse consequences:
• Accelerated fission reaction, and even spontaneous fission due to neutron moderation and washing out of efficient neutron absorbers.
• Leakage of contaminated water through spaces in structural elements and foundations, and the attendant contamination of the surrounding area and the operating units, and of surface and underground water sources.
• Accelerated degradation of structural elements (corrosion, leaching out).
• Accelerated degradation of FCM, with the formation of aqueous solutions of uranium and plutonium salts, their migration and concentration in stagnant areas.
Special features of the current fire protection system
When the Shelter was being designed and built, fire safety was not dealt with as a separate issue. The situation was complicated by the nature of the Shelter, which meant that existing fire safety rules and regulations could not be applied to it.
In addition, the Shelter lacks various fire protection facilities, or has them, but not in working order (or only partly in working order). These include:
• Fire detection and alarm systems.
• Automatic fire extinguishing systems in the most dangerous areas.
• Fire and combustion product confinement systems.
• Passive fire protection devices.
Therefore, the Shelter’s fire protection as it exists does not fully comply with fire safety objectives and the principles of their implementation.
IMPLEMENTING THE PRINCIPLES OF FIRE PROTECTION
Comprehensive analysis of the fire hazard represented by the Shelter
The task of carrying out a comprehensive analysis of the fire hazard represented by the Shelter has not lost its urgency. Such an analysis should include:
• A credible determination of the quantities and locations of combustible and dangerously explosive materials.
• A study of the true fire resistance of the Shelter’s structural elements and components, with recommendations on bringing it up to the required level.
• A study of the mechanisms of formation and accumulation of explosive and fire-hazardous substances and materials, and development of preventive measures.
• Identification and zoning of explosion and fire hazardous sections and areas.
• A study of the effect of internal and external phenomena, both natural and man-made, on the fire safety of the Shelter.
• A study of the status of systems allowing early detection and suppression of ignition, fire fighting facilities and facilities for reducing the consequences of fires to a minimum, with recommendations on improving these systems and facilities.
• Assessment of the likely scale of possible ignitions and fires and their consequences, and of their impact on the nuclear and radiation safety of the facility.
• Development of a representative method of assessment of the probability of fires whose consequences could expose personnel, the public and the environment to greater than normal radiation levels.
Fire prevention
The principle of fire prevention is the most important and highest priority element of the efforts to improve the Shelter’s fire protection. It should be based on the following set of measures:
• Identifying and as far as possible removing combustible and toxic materials, with due regard for the level of their radioactive contamination.
• Analysing and if necessary increasing the fire resistance of structural elements, enclosures and cable runs by coating them with special fire-resistant and non-combustion sustaining materials and compounds, resistant to ionising radiation and the temperatures and humidity levels inside the Shelter.
• Rebuilding the existing monitoring and diagnostic and process systems and their power supplies to make them fire and explosion safe in the high humidity environment of the Shelter, and constructing an effective and reliable earthing system.
• Replacing the existing cabling with fire-resistant and non-combustion sustaining cables, rearranging cables in accordance with fire safety requirements, to include physical separation and passive fire protection of cable runs.
• Developing and implementing measures to prevent the possible collapse of structural elements from affecting the condition of live cables and equipment (or restrict the impact as much as possible), in order to reduce the probability of short-circuiting and ignition.
• Developing and implementing measures to detect and prevent the formation of critical accumulations of dangerously explosive substances and mixtures inside the Shelter, and improving dust suppression methods accordingly.
• Developing and implementing methods of protecting the Shelter from fires occurring at the Chernobyl’s operating units, assessing the fire resistance of the relevant partition walls and protective barriers, and increasing it as necessary.
• Improving lightning protection.
• Introducing stricter regulation and monitoring of the use of naked flames inside the Shelter.
• Increasing the physical security of the Shelter so as to prevent unauthorised operations and sabotage.
• Strengthening a fire safety culture among staff.
Early detection and suppression of ignition
At present, the Shelter has no facilities for early detection of ignition.
Most specialists consider that it should be provided with an early ignition and fire detection and alarm system as soon as possible.
The structure and type of the system and the number and location of detectors and alarms should be selected after a special analysis, based on the following principles:
• The system should monitor areas representing the greatest fire hazard (with due allowance for their accessibility in terms of radiation levels) and areas containing nuclear and radiation safety diagnostic and maintenance equipment.
• It should operate continuously, and have its own independent power supply. It should not be sensitive to common-mode failure and should allow periodic checks and servicing without loss of its main function.
• Detectors and other components should be suitable for Shelter conditions (high radiation levels, humidity etc).
• The system should be highly reliable and have high noise stability.
• It should include a special ignition and fire annunciation board located in the control room.
• Audible and visual signalling of emergencies should be provided, with indication of the location (zone) of the ignition point.
• The system should include provision for initiating the emergency signal manually, with push button control boxes located in the most dangerous and safety-significant areas.
Containment of fire and combustion products
The most effective means of containment is zoning of areas and facilities and separating them with fire-resistant barriers. Zoning (sectioning) of the Shelter must take into account radiation levels and the division of areas into manned, partially manned and unmanned areas.
A separate issue, which is important in the Shelter environment, is smoke extraction and the containment of combustion products. There is a clear need for smoke extracting ventilation systems fitted with efficient combustion product and radioactive substance absorbing and filtering devices.
Fire fighting equipment
Currently, virtually the only fixed fire extinguishing system available at the Shelter is a system known as the “dry line”. It is based on the pre-disaster process water feed line and fire pumps. It consists of a 200 mm fire main running along the perimeter of the facility, with dual fire hose connection valves placed every 120-150 m. There is a piping arrangement for feeding water units into some areas of the deaerator stack and machine hall.
Another urgent task is the selection (and development) of special fire extinguishing equipment and materials for use in the Shelter. Stand-alone gas, foam, powder and aerosol fire extinguishers are probably the best candidates.
Fire-fighting equipment suitable for dealing with serious accidents caused by the collapse of external structural elements should also be provided. This should consist first of all of equipment capable of delivering fire-extinguishing media over distances of up to 100 m. The accident of April 1986 is direct proof of the need for such equipment.
Fire management
Fire management means constant preparedness of the management and staff of the Shelter, the Shelter’s own fire brigade and other public service fire brigades for dealing with fires and minimising their effects. It can be achieved through the implementation of the following measures:
• Familiarising personnel with fire safety regulations and with action required to detect and fight fires.
• Training voluntary fire fighting teams.
• Developing and implementing communication and interaction systems to link site personnel with public service fire brigades.
• Establishing stores of personal protection equipment, fire fighting appliances and materials.
• Special permits for public service fire brigade personnel, to cover fire fighting in electric power and radiation hazardous areas.
• Establishing an operations centre to manage the operations of voluntary and public service fire brigades.
• Preparing a site fire fighting plan, and ensuring permanent readiness for its implementation.
The preparation of fire protection standards and instructions
Existing standards and instructions are not totally applicable to the Shelter, and therefore the task of adapting existing documentation or developing special fire safety standards and instructions for the Shelter remains on the agenda.
The documentation system should have as its basis a document entitled “General Rules and Regulations for Ensuring the Fire Safety of the Shelter”.
This standard should be used to draw up detailed rules, instructions and methods for specific applications.
Implementing the principles of fire protection during the various stages of work on transforming the Shelter
The conclusions and recommendations presented in this paper relate principally to the Shelter stabilisation phase. It is clear that the process of transformation of the Shelter into a guaranteed safe system will require the concept of its fire protection to be modified. The content of the measures taken to implement the principle of defence in depth will be defined as specific plans and projects for the transformation of the Shelter are developed.
IMPLEMENTATION OF THE CONCEPTUAL DOCUMENT ON FIRE PROTECTION OF THE SHELTER
The above approaches are all set out in the Conceptual Document on Fire Protection of the Shelter Facility, approved by the Standing Committee on Technogenic and Environmental Safety and Emergency Situations in December 1996.
A Programme of Work to implement the provisions of the Conceptual Document has now been drawn up and approved. However, difficulties with financing have made it impossible to implement the programme in full. Programme items completed in 1998 included:
• Work on modernisation of the Shelter’s power supply system.
• Coating electric cables and cable conduits with fireproofing compounds.
• Work has begun on the “General Rules and Regulations for Ensuring the Fire Safety of the Shelter”.
• Work has begun on correcting the existing operations manuals and the fire fighting plan.
• A start has been made on basic fire fighting training of personnel.
• A design for a Fire Safety training room has been prepared.
• Terms of Reference have been prepared and contracts are being signed covering the following areas:
* A thorough analysis of the fire safety of the various areas of the Shelter.
* Calculation of the required fire resistance of supporting and enclosing structures in the fire zones of areas in use.
* Preparation of technical proposals on increasing the Shelter’s fire safety.
Furthermore, as part of the TACIS programme, the SIP plan (Chernobyl Unit 4 Shelter Implementation Plan) includes Task 16, “Industrial safety and fire protection infrastructure, and access control”.
The SIP plan includes analysis of dangerous fire sources, enhancement of fire protection of the site, fire prevention measures, fire protection of work under way at the Shelter, improvement of fire detection and extinguishing systems, and the identification and implementation of methods of removal of combustible materials.
