Emergency Ventilation Systems for Nuclear Power Plants

Normal and Emergency Ventilation Systems

Function

The typical nuclear plant has numerous ventilation systems that have traditional and/or emergency functions. Examples of such systems and their safety significance are noted. Hyperlinks lead to system flow diagrams.

BWR	PWR	Safety-Related
Primary Containment Ventilation	Containment Fan Coil Units & Dome Fans	x
Drywell Cooling	Containment Cooling
Reactor Building Ventilation	Auxiliary Building
Standby Gas Treatment	Auxiliary Building Special Ventilation (Zone SV)	x
	Shield Building Ventilation	x
Control Room Air Conditioning	Control Room Air Conditioning
Emergency Filtration	Control Room Post Accident Recirculation	x
	Safeguards Ventilation	x
Diesel Generator Ventilation	Diesel Generator Ventilation	x
Diesel Generator Building or Room Ventilation	Diesel Generator Building or Room Ventilation
Turbine Building Ventilation	Turbine Building Ventilation
Screenhouse Ventilation	Screenhouse Ventilation	x
Radwaste Building Ventilation

Ventilation systems consist of ducts and dampers. Chillers, air conditioning units, or fan coil units may be used to cool the air passing through the ducts. Fans may be used to circulate the air. Filters may be used to remove impurities from the air. In general terms, the functions of these systems are:

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.

Ventilation duct and Airlock in Containment

Control Rod Drive Cooling Ventilation above Reactor

Fan Coil Unit (closeup)>>>

Fans (e.g. Standby Gas Treatment)

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.


Shield Building Vent Filter and Fan	Control Room Filter Unit

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.