Spilled Gas Clearance Time

Noble gases such as Xenon in the air present an external source of radiation exposure that must be calculated. Many commercially available dosimeters and survey instruments are not capable of accurately measuring worker doses from immersion in noble gases.

Should an accidental release of Xenon-133 occur into either the imaging room or the hot lab, the nuclear medicine technologists would immediately remove themselves and the patient, if feasible or applicable, from the room. The door to the room should be immediately closed to minimize leakage to surrounding unrestricted areas. The room will remain vacant until such a time as the room ventilation would dilute the concentration of released Xenon-133 to levels below the Derived Air Concentration (DAC) for restricted areas, i.e., 1 x 10^-4 microcuries/milliliter for Xenon as specified by the NRC in Table 1 of Appendix B to Part 20.

Because normal room ventilation is usually not sufficient to ensure timely clearance of spilled gas, the calculations described below should be done to determine the amount of time a room should be cleared in case of a gas spill. This clearance time should be posted in the room.

Collect the following data:

1. Highest activity of gas in a single container that could possibly be released,

in microcuries (µCi):

A:  

2. Sum of airflow supply from each vent in the room, in milliliters(mL)/min

(if different during heating and cooling seasons, use the lesser value):

S:  

3. The total room air exhaust rate determined by measuring, in ml/min, the

airflow to each exhaust vent in the room. (The exhaust should be vented

and not recirculated within the facility.) This may be either the normal

air exhaust or a specially installed gas exhaust system:

Q:  

4. The Derived Air Concentrations (DAC) in restricted and unrestricted areas. For

Xenon-133, the DAC values are 1 x 10^-4 µCi/mL in restricted areas and 5 x 10⁷µCi/mL

in unrestricted areas.

C:  

5. The room air volume in milliliters(mL). One cubic foot (cft) is approximately 28,300ml:

V:  

For each room calculate the following:

1. Determine the net airflow in the room (Q – S). The value must be positive to ensure

the room is at negative pressure:

Q – S:  

2. The evacuation time in minutes is calculated as follows: T= V/Q x ln (A/CV):

T:  

Example Calculation:

Room dimensions of 12 × 12 × 8 feet, an exhaust rate of 115.2 cft/min, and a gas “spill” involving the largest activity of Xe-133 used is 20 mCi, the room evacuation time would be:

V = 12 × 12 × 8 ft × 28,300 mL/cft = 3.26 × 10⁷ mL;

Q = 115.2 cft/min × 28,300 mL/cft = 3.26 × 10⁶ mL/min;

A = 20 mCi = 2 × 10⁴ µCi;

C = 1 × 10^–4 µCi/mL; and

Licensees should perform this calculation for their specific facility layout and the maximum activity of Xenon-133 likely to be used. Note: Some licensees may choose not to evacuate the room, because the radiation dose is minimal from a “spill” of a standard activity administration of Xenon-133 in a properly ventilated room.

References:

Jeffry A. Siegel, PhD, Society of Nuclear Medicine. (2001).Nuclear Regulatory Commission Regulation of Nuclear Medicine; Guide for Diagnostic Nuclear Medicine. p32-41.

NRC Regulations (10 CFR) PART 20—STANDARDS FOR PROTECTION AGAINST RADIATION