Ignalina Handbook

5. Accident Management on Site

 

The essence of nuclear safety is to prevent a loss of control of the nuclear chain reaction and to prevent the possibility for the formation of a critical mass during operation, reloading, transportation and storing of nuclear fuel, provision of heat-transfer and reliable cooling as well as confining the radioactive materials in prescribed boundaries. Nuclear safety at the plant is ensured by maintaining normal operation and by involving technical and organizational measure. The NPP design foresees a system for the inspection of failure detection in all barriers to radioactive release. The mentioned system is intended to assure safe operation .

The essence of radiological safety is to prevent radiation exposure of plant personnel, environment and the general public in excess of authorized limits.

 

5.1 Handling of Deviations, Incidents and Accidents

 

Nuclear power plant states may be classified into operational states and abnormal events. The operational states include both normal operation and deviation from normal operation or anticipated operational occurrences. The normal operation is an operation of a nuclear power plant within specific operational limits and conditions including shutdown, power operation, shutting down, starting, maintenance, testing and refueling. Deviations are all operational processes departing from normal operation which are expected to occur once or several times during the operating life of the plant and which do not cause any significant damage to items important to safety nor lead to abnormal events.

Abnormal events refer to all fault conditions which lead to unplanned shutdown. Abnormal events relevant to safety may be classified into incidents and accidents. Incident is an abnormal event, when the reactor safety systems are activated but allowing more or less immediate return to normal operation. Accident is a state defined under accident conditions or severe accident. Accident conditions are departure from operational states in which the releases of radioactive materials are kept to acceptable limits by appropriate design features. These departures do not include severe accidents. Definition of severe accident was given in Chapter 2.

Those departures of process parameters which can lead to an abnormal event during operation are considered as limits of safe operation. Table 5.1 presents the values of the RBMK-1500 reactor parameters which, if reached during operation on load, are considered as exceeding the operational safety of the plant.

 

Table 5.1 Limits of safe operation of Ignalina RBMK-1500 plant [14]

Parameter	Variation limit
Reactivity margin in the effective manual control rods, pcs	below 30
Reactor power, MW	over 4800
Fuel channel power, MW	over 4.25
Fuel linear rating, W/cm	over 485
Critical heat flux margin coefficient in the fuel channel	less than 1
Maximum linear fuel rating coefficient	less than 1
Graphite temperature margin coefficient	less than 1
Design graphite stack temperature, oC	over 760
Excess pressure in the steam separator, MPa	over 7.95
Water flow in control rod cooling circuit, m3/s	less than 0.256
Water flow in the CPS channel with inserted rod, m3/s	less than 0.83× 10-3
Heat-up rate of the reactor and MCC, oC/h	exceeding 30
Cool down rate of the reactor and MCC, oC/h	exceeding 30
I-131 activity level in MCC water for maximum allowable number of failed rods in accordance with the "Regulations for Nuclear Safety of the Nuclear Power Plant Reactors" PBYa RU AC-89, Bq/kg	over 3.7× 105

 

Deviations are controlled by the reactor main operating and control systems. Necessary organizational measures are also established to prevent exceeding of the limit and violation of the requirements for safe operation of the plant. Automatic reactor protection devices against unallowable change of the parameters and failures of the power unit equipment are activated on signals and setpoints. When indications of deviation from process parameters occur, the necessary actions including reac-tor shutdown and power drop to a safe level are taken.

In emergencies, the staff is generally guided by special plant procedures [11-13], which describe actions to be taken for elimination and management of the accident and/or mitigation the accident consequences. Established procedures provide a basis for a suitable operator response to abnormal event. Operators are trained to take advantage of time requiring no immediate operator action to recognize and identify causes of the abnormal plant response. These procedures cover design basis accidents. The main task of the operating personnel in case of an incident or accident is to prevent the development of a design basic accident into a severe accident. For this purpose, any available main or safety systems must be used. By the use of these resources, the following three main safety functions should be performed:

• provision of reactor subcriticality and shutdown;
• provision of heat transfer from the reactor and reliable core cooling;
• confining of the radioactive materials into prescribed boundaries, ensuring integrity of fuel channels, MCC, feedwater and steam pipes as well as the integrity of ACS compartments.

The main tasks of the operators for elimination of accidents are as following:

• identifying the cause of the accident by activating reactor scram systems, safety interlocks, alarm systems, indications, changes of process parameters and reports of operating personnel;
• detecting the accident location and evaluation of the scale of the accident;
• preventing accident development, isolation of damaged equipment;
• ensuring nuclear, radiation and fire safety;
• ensuring safety of the on-site personnel;
• preventing accident progression and eliminating the effects;
• preventing violation of the requirements for safe operation of the equipment not enveloped by the accident;
• minimizing size and effects of the radioactive contamination of compartments, unit, site and environment;
• surveying conditions of isolated equipment and possibilities to return it to service;
• ensuring the reliable core cooling.

The main safety systems described in Chapter 3 should always be ready to perform their functions during operation on load. According to [11-13], all actions that are necessary for elimination of accident conditions and switching should be performed by two persons with obligatory mutual confirmation of actions to minimize the probability of errors [13].