Design basis accident conditions

Reactor dynamics in normal and design basis accident conditions... 

The control room is shielded against radiation so that continued occupancy under design basis accident conditions is possible. In the event that the control room becomes inaccessible, it is possible to bring the reactor from power range operation to cold shutdown conditions by utilizing alternative controls and equipment that are available outside the control room. 

The function of the control room area ventilation system (HVAC) is to provide a controlled environment for the comfort and safety of the ccmtrol room personnel and to ensure the operability of control room components during normal operation, anticipated operational transients and design basis accident conditions. In the event of a failure in all the redundant trains of the HVAC system, the increase of temperature in the control room area can lead in a short time to malfunctions of the electronic equipment in the control cabinets. 

USNRC Generic Letter 96-06 and Generic Letter 96-06 Supplement 1, Assurance of equipment operability and containment integrity under design basis accident conditions. 

Ventilation systems with appropriate filtration should be considered and should be provided if necessary for use in normal operation and in design basis accident conditions. This may include a separate subsystem with charcoal filters for use in accident conditions. In the case of a beyond design basis accident involving a release of fission products, the ventilation system may also be used for the mitigation of consequences. In many research reactors, ventilation systems are essential for the fulfilment of the confinement function. 

The design basis of a plant specifies the necessary capabilities of the plant to cope with a specified range of operational states and design basis accident conditions, in compliance with the defined requirements for radiation protection. The design basis typically includes the specification for normal operation, the conditions created by PIEs, important assumptions and, in some cases, the particular methods of analysis. 

It should be ensured that the systems important to safety are capable of performing their safety functions when required in normal operations, external events and anticipated operational conditions, and in and after design basis accident conditions. This is fundamental to preventing the release of radioactive materials and to preventing or mitigating radiological consequences for human health and the environment if it occurs. 

Examples of hazardous enviromnental conditions arising from design basis accident conditions which could cause failure of equipment are the radiological conditions and steam conditions associated with pipe breaks, including breaks of the reactor coolant system. Examples of potentially hazardous process conditions include high velocity two phase flow, high levels of vibration or debris laden process... 

The time available for planned operator action from the onset of an anticipated operational occurrence or design basis accident conditions varies among Member States, ranging between 10 and 30 min. This period depends upon such factors as the complexity of the decision, the displays available, the need to distinguish between different PIEs and the consequences of a wrong decision. 

The Requirements for Design require (Ref. [1], para. 6.84) that the design be such as to minimize the likelihood that operator action could defeat the effectiveness of the protection system in normal operations and anticipated operational occurrences, but not to negate correct operator actions under design basis accident conditions. 

I C systems important to safety that are required to be available for use at all times in operational states or design basis accident conditions should be connected to a non-interruptible power supply. The performance requirements of non-intermptible power supplies should satisfy the requirements of the system that they power. 

The display facilities should cover appropriate variables, in consistency with the assumptions of the safety analysis and with the information needed by the operator for operational states and design basis accident conditions. The accuracy and range of displays should be consistent with the assumptions of the safety analysis. 

A description should be provided of the environmental conditions in which each component has to operate, including normal conditions, anticipated operational occurrences and design basis accident conditions. 

Connected systems are those systems that are connected directly to the RCS or, for some PWR designs, to the secondary side of the steam generators. Together with other systems and components, connected systems perform their functions to ensure the integrity of the RCS in normal operation or following anticipated transients or in design basis accident conditions. The systems that perform these safety functions include ... 

The primary objective of the RCSASs is to ensure that an adequate flow and quality of coolant are available to remove heat from the core in aU operational states and in and following design basis accident conditions. The RCSASs may also be used to mitigate the consequences of design basis accidents and beyond design basis accidents. 

To provide and maintain a sufficient reactor coolant inventory for core cooling in aU operational states and under design basis accident conditions, and to transfer the heat generated to the ultimate heat sink ... 

In the design of SSCs of the RCSASs important to safety, account should be taken of all external hazards such as seismic hazards (for further information see under Seismic considerations in this section, paras 3.24-3.27), tornadoes, missiles, floods and hurricanes that may possibly be encountered in all operational states and in design basis accident conditions. 

All pressure retaining components of the RCSASs should be designed with an appropriate safety margin to ensure that the pressure boundary will not be breached and that the fuel design limits will not be exceeded in operational states or in design basis accident conditions. 

In the design and location of safety and/or relief valves in the RCS, its pressurizer (in PWRs) and other interconnected vessels (if any), account should be taken of the single failure criterion so that the pressure boundary of the RCS can be maintained within the design limits in all operational states and in design basis accident conditions. 

---