The safety priority is to prevent any accident by sound design, quality of manufacture and safe operation, and especially to prevent severe accidents. Nuclear power plants are designed so that the public is protected from a wide range of malfunctions. Nevertheless, the possibility that prevention might fail and a severe accident happen, cannot be ignored and such accidents have always been taken into account in considering safety. The first part of the safety approach is accident management, to stop accidents developing and the second is to contain any radioactive materials. The last is emergency planning if there is a risk of a significant release of radioactivity to the environment. Accident management is a constituent of normal operation. Accident management starts functioning in the course of an accident as specified in emergency procedures. The main tasks are as follows:
The main objectives of accident mitigation are protection of the population and plant personnel as well as protection of the plant equipment and premises. The emergency response plan [15] is designed for protection of the personnel as well as confinement and mitigation of the radiation accident at the Ignalina NPP. This is a basic document for taking organizational engineering, medical, evacuative and other actions. This plan is valid for all on-site personnel, the fire protection staff, the security guards and the attached persons. The actions required by the plant shall be taken on-site and within the exclusion zone (3 kilometer in radius). The existing plan based on old regulations will be revised during 1997.
5.4.1 Types of Radiation Accidents
A radiation accident at the Ignalina NPP is defined as an infringement of the normal operation in which release of radioactive materials and ionizing radiation goes beyond the specified limits and which requires to stop the operation of the facility/equipment containing ionizing radiation sources. Accidents are classified according to the spread of involved radiation materials or ionizing radiation into three types: on-site or local, off-site or area and general accidents [15]. To enable an early start-up of the emergency organisation, on-site as well as off-site, technical criteria are under development which will identify the level of radiation accident at an early stage.
Local accident is an infringement of plant operation in which on-site release of radioactive materials and ionizing radiation goes beyond the normal operation limits specified for equipment, process systems, facilities and buildings. Certain actions have to be taken to protect the plant personnel.
Area accident is an infringement of plant operation in which off-site release of radioactive materials and ionizing radiation within the exclusion zone exceeds the specified normal operation limits. Radiation exposure of personnel and contamination of plant facilities, buildings and territory may occur and go beyond the permissible limits. Actions have to be taken to protect the plant personnel.
General accident is an infringement of plant operation in which off-site release of radioactive materials and ionizing radiation outside the exclusion zone exceeds the specified normal operation limits. Radiation exposure of plant personnel and population may exceed the specified limits. Actions have to be taken to protect the plant personnel and population.
5.4.2 Technical Priorities for Recovering Reactor Safety
Reactor design and accident safety measures allow for design basis accidents. Severe accidents, however, cannot be ruled out. Accident management implementation procedures deal primarily with bringing these beyond design basis accidents under control. Severe accidents will at some point involve a mismatch between heat production and removal, that is, either an excess of power or inadequate cooling. Consequently, the two key safety functions are important here. The first is the reactivity control system which regulates the power rate and is needed to stop the chain reaction. The second is the cooling system and its power supply, with special attention to the residual heat removal system, which is needed after the reactor has been shutdown.
In the event of severe accident the fuel pellets, cladding of fuel rods, pressure tubes and pipelines of primary circuit could be damaged already during the initial stage of the accident. Only accident confinement system could prevent release of radioactive material to the environment. In this case, radiological consequences of accident would depend on the capability of this system to withstand elevated temperatures and pressure for the duration of the accident. It is therefore very important to have technical means for the reduction of pressure and release filtering. For the RBMK-1500 reactors to perform these functions and to recover reactor safety the following engineered safety systems should be ready to perform their functions:
5.4.3 Measures to Protect Vital Equipment and Buildings
The emergency response plan identifies the plant capabilities and planned actions to protect the equipment important to safety. This includes the capability to detect and fight fires, the support from external emergency response organizations, interface with the main control room emergency operating procedure, and the capability to repair damaged equipment and systems.
Fire safety assurance is one of the key aspects of accident prevention and mitigation of accidents as well as protection of the general public and plant equipment. The design requirements for existing fire protection system are quite stringent, but functioning should be improved. Automatic fire protection system is designed for detection, preventing and coping with fires at Ignalina NPP. This system includes automatic fire alarm system, automatic fire detection and fighting systems, fire confinement and smoke removal systems as well as the ventilation system. The automatic fire alarm system is installed in areas housing electrical equipment, repair shops and store rooms. Special attention is given to rooms housing electrical equipment of safety systems. Automatic fire detection and fighting system includes foam and water fire-fighting subsystems. The foam fire-fighting subsystem is designed to protect oil system equipment inside the reactor and turbine generator buildings. The water fire-fighting subsystem is designed to protect cable rooms, cable ducts and unit transformers. Both subsystems consist of two independent pump stations interconnected by pipelines. Fire confinement system is designed for automatic closure of air inflow/extract dampers in the area where the fire started. The fire is confined to one room. The smoke removal system is designed to remove smoke from the cable vault in which the fire was initiated. The general concept of ventilation of areas with potential formation of explosive materials consists in diluting these materials to non-explosive concent-rations and discharging them into the atmosphere.
On-site emergency response unit is set up based on the production principle. When an indication of the accident becomes evident, the plant shift supervisor takes a decision on emergency shutdown of the unit, takes actions to limit the size of the accident in accordance with operating procedures for process equipment, and coordinates actions of the department’s shift managers. Special units are meant for emergency repair and performing specific activity when coping with accidents at the plant (restoration of reactor, turbine and electric equipment, instrumentation and automatic devices, decontamination of plant personnel, etc.). Special Units comprise personnel of the appropriate workshops (Figure 5.2). As well as the operating staff, special departmental units, the fire brigade and the town hospital are involved in mitigation of accident consequences. If necessary, other national fire brigades and medical institutions are called for. Based on the situation the Governmental Commission for Emergency Situations can draw in additional civil defence resources and other national institutes.
5.4.4 Measures to Protect Plant Personnel
The major action to protect the personnel is to timely diagnose the accident. The following main provisions are aimed at protecting the personnel in case of an accident:
Fig. 5.2 Plant special branch unit organization
The Director General has authority for implementation of the emergency response plan through the Emergency Operating Center and services. The time for the managers to assemble is 20 minutes on working hours and 2 hours outside working hours. The maximum time for the emergency response staff to get ready is 1 hour on working hours, and 6 hours outside working hours. Progress actions reports shall be submitted to the Emergency Operating Center every 30 minutes and, if the situation changes considerably, then immediately. By order of the Plant Shift Supervisor the on-site plant personnel (about 3000 people) have to evacuate to the shelter, which can accommodate 800 people, to the underground plant rooms having the minimum radiation level or other gathering places for evacuation. The shelter work in three operating modes: mere ventilation, ventilation with filters and air regeneration. The shelter is capable of reducing radiation by a factor of 1000 and sustaining overpressure up to 0.2 MPa. It is equipped with emergency power supply, a first aid point and water reserves. If an accident is declared, the personnel in the accident zone have to act as follows: put on personal protective equipment, take stable iodine tablets available at workplaces, undergo sanitary treatment and change clothes if contacted some contaminated process liquid, follow the reports of the radiation protection department shift supervisor on radiation situation and the orders of the direct managers, the radiation protection shift supervisor and the Emergency Operating Center.
Evacuation of the on-site personnel to Visaginas is planned in the optional manner: during office hours and during non-office hours. Estimated evacuation time for 2500 people is 72 minutes. There are about 230 people working at the plant during non-office hours. These people will be evacuated by shift transport within 60 minutes.
The emergency response system is currently under revision. The emergency response organization will be based on the plant responsibility, while the task of civil defence forces will be to cope with protection of the public outside the plant.