Emergency Ventilation Systems for Nuclear Power Plants

Emergency Ventilation Systems

Function

The typical nuclear plant has a number of Emergency or Safeguards Ventilation Systems. The following are the types of systems used with their main functions.

Containment Emergency Cooling Systems

To remove heat from the containment in the event of any incident which could result in the temperature reaching temperatures of 250 to 300 F.

Filtration Systems

To divert exhaust paths, normally routed directly to the environment, through special filters in order to reduce the amount of any radioactive materials within levels allowed by regulation

Pressure Differential Systems

To maintain pressure differentials outside the containment higher to reduce the likelihood of leakage from the containment to the reactor or auxiliary building, and subsequently from those buildings to the turbine building.

Cooling Systems

To cool emergency equipment when it is operating, e,g, diesel generators, electrical motors and supply breakers.

Components and Flow Paths

Containment Emergency Cooling Systems

These systems usually have Fan Cooler Units which have a internal fan which can draw the hot moist air (that may be present during postulated accidents) across radiator type coils that are cooled by some external source, e.g. the Essential Service Water System. The cooled exhaust from these units is usually routed through a Duct to the top of containment where the air would be hotter. Separate Fans in the exhaust duct, sometimes called Dome Recirculation Fans, direct the flow to the containment dome.

The figure below for the Kewaunee plant containment ventilation systems illustrates the flow paths and components used.

Courtesy US Nuclear Regulatory Commission

Filtration Systems

These systems usually consist of Fans that draw the air from the potentially contaminated areas through PAC Filter Units. The PAC unit consists of 3 filter units -

P - Particulate filter that removes large particles (sometimes called roughing filters)

A - Absolute filter that removes over 99.5 % of particles

C - Charcoal filter that removes radioactive iodine (and to a limited extent radioactive gases)

After passing through the PAC unit, the exhaust may be routed to an exhaust stack.

Such systems will normally be actuated by radiation monitoring sensors mounted in the exhaust ducts. In addition, they will automatically start any time the emergency core cooling systems start or containment isolation is required.

In conjunction with these "special" or recirculation fans starting, the normal ventilation system equipment will shutdown with appropriate fans tripping and duct dampers closing or opening so that the filtration system is the only one operating.

The Maine Yankee plant diagram below illustrates this type of system.

Courtesy US Nuclear Regulatory Commission

Pressure Differential Systems

Pressure differential systems depend on the flow rates of the various buildings' supply and exhaust fans. Such systems also take into account certain sizes of openings in the buildings. Because of this operators may be required to keep a log of all building openings and their size. The supply and exhaust fans are sized to create pressure differentials between the various buildings.

Turbine Building > Auxiliary or Reactor Building

Cooling Systems

Cooling systems usually consist of one of the following:

Unit Coolers, locally mounted by emergency equipment, that have internal fans that draw local air across cooling coils and direct the cool exhaust toward the motor, circuit breaker, or electrical supply panel of concern.

Air Conditioning Units as may be used for the Control Room or locally mounted to cool electrical panels or equipment

Fans which direct air from outside or specially designated areas to cool motors, diesels, or other equipment needing cooling. This air may be cooled by passing across Radiators which are in turn cooled by other systems.