ETL 1110-3-498
24 Feb 99
The total required TFA floor space is determined from the button-up period, the number of people
sheltered, and the required floor area per person. Generally, large open areas such as common
areas, multipurpose areas, gymnasiums, etc., provide the most efficient floor area for protecting a
large number of personnel. The TFA envelope should include bathroom facilities and, if possible,
kitchen facilities.
C-6. Airlock Requirements.
Personnel that ingress and egress the TFA during CP operations must process through an airlock.
The airlock maintains TFA overpressure, prevents the migration of airborne contaminants into the
TFA, and purges contaminants from personnel before they enter the TFA. The number of airlocks
required depends on the number of personnel that ingress and egress during a given time period.
a. Stand-Alone Airlock. For existing facilities, the stand-alone two-stage airlock discussed
in Appendix F will be used. This airlock is designed to process two people into the facility within
8 minutes with 4 minutes in the first stage and an additional 4 minutes in the second stage. An
integral 94 L/s (200 cfm) air filtration unit is used to provide the necessary air purge rate upon
which the dwell time is based.
b. Integral Airlock. For new facilities, the integral single-stage airlock discussed in
Appendix F will be incorporated into the vestibule area. The airflow purge rate through the single-
stage airlock will provide a 3 log reduction in contaminants based on equation C-1.
(eq C-1)
where:
Q = airflow, cubic meters per minute
V = airlock volume, cubic meters
T = purge time, minutes
Airlock airflow is driven by internal overpressure from the TFA. Uncontaminated air from the
TFA will enter the airlock near the ceiling and exhaust near the floor. The airflow rate through the
TFA will be monitored by an airflow measuring station or measuring device that can be field
calibrated, and the airflow rate will be displayed at a CP control panel. The airflow rate through
the airlock will be controlled by a flow control damper and backdraft damper. A warning light will
illuminate when the airflow rate drops below design values. The static air pressure drop from the
TFA to the airlock and from the airlock to the exterior will not exceed 25 Pa (0.1 inch wg) at each
location, giving a total airlock static pressure drop of not more than 50 Pa (0.2 inch wg).
C-7. Toxic-Free Area Overpressure.
C-3
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