Rahim Ghanooni, M.S, CHMM Co-2 Decontamination Air Monitoring User Group Las Vegas, NV May 2010

Transcription

1 CO-2 Decontamination Rahim Ghanooni, M.S, CHMM Air Monitoring User Group Las Vegas, NV May 2010

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4 The dry ice blast cleaning and decontamination process describes a technology that utilizes pellets of extruded dry ice as the cleaning media. These extruded pellets are directed at surfaces to be cleaned much the same as with other commercial blast cleaning processes such as abrasive grit, steel shot, glass or plastic beads or liquid abrasive materials. Upon contacting the surface, the dry ice pellets undergo a sublimation process and through this process remove contaminants that are trapped in high efficiency filtration units for collection and disposal.

5 CO-2 decontamination is a non-destructive process that allows for decontamination of equipment that can not be decontaminated by any other methods. This technology is non conductive, non abrasive on most materials, uses non toxic materials, is considered non hazardous and does not generate any secondary or mixed waste residues. When properly applied, dry ice blast decontamination is a totally dry process and environmentally safe. Examples of materials suitable for CO-2 decontamination include such items as hand tools, lead shielding materials including bricks, blankets and sheet, valve components, pump parts, mirror insulation, electrical equipment (motor control centers, switchgear, motor and generator windings) and glass.

6 Unlike most materials, dry ice, the solid form of carbon dioxide, passes from the solid state directly to the gaseous state without going through a liquid phase. This unusual property is called sublimation and this characteristic makes dry ice a highly versatile blast medium. The dry ice blast cleaning technology works very similarly to a commercial blast operation such as corn cob, walnut shell or other abrasive blast technologies. Through these technologies, kinetic energy, transfers energy to the surface being clean and mechanically removes (or abrades) the surface for the removal of the contaminant. The system evaluated here uses 99.9% pure liquid nitrogen as the propellant. The liquid nitrogen is converted to high pressure nitrogen gas in an "air to air" heat exchanger.

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8 ADVANTAGES Time Saving Radwaste shipment High Decontamination Factor (DF) Waste Generation Dose Reduction

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11 SYSTEM DESCRIPTION

12 Both nitrogen and carbon dioxide are asphyxiates and when the decontamination chamber is properly ventilated are of secondary concern to airborne radioactive materials. A calculation was performed to determine the expected oxygen level during blasting, as a result of oxygen displacement by gaseous CO-2.

13 The equation was derived from quantitative relationship between mediums in secular equilibrium. N N Integrate the above equation from zero to some value N N N N 0 0

14 which is equal to amount of CO2 build up, will results. CO 2 = K [1-exp(- t)]/ EQ-1 Where CO 2 = Quantity of CO 2 in the booth at time t K= Rate of CO 2 introductions into booth (lb/sec) = Ventilation, fraction removal (Sec-1) t= Time from start of process (Sec)

15 The maximum amount of CO 2 in the booth can be determined by setting t= CO 2 max (lb) = K (lb/sec) / EQ 2 The following parameters are used for the calculation. a) V (Volume of booth) = (12) x (8)x (10) = 960 (ft 3 ) b) K (CO 2 usage) = 150 lb/hr = lb/sec b) (Air removal rate ) = 1500 (cfm), obtained from actual flow measurements made on the decontamination chamber. = [1500 (ft 3 /min) /60 sec] / 960 ft 3 =0.026 Sec Substituting the above parameter in equation 2, will result. CO 2 max (lb) = (0.042/0.026)=1.6 lbs Maximum amount of CO 2 in the booth

16 Since CO2 acts as an ideal gas, therefore the volume can be determine from PV=nRT EQ-3 Where P = Pressure in atmosphere (1 Atom) V = Volume (ft 3 ) R = Gas constant (atom ft 3 /lb-mole 0 K) T = Temperature ( 0 K) = (273+21) = K n = Moles of CO 2 = [1.6/44]=0.036 Substituting the above parameter in equation 3, and solve for volume, V=[[1.6 lbs/44 lbs/lbs.mole] * [1.3 atom ft 3 /lb-mole 0 K] * K ]]/1atm V= 13.9 ft 3 Volume of CO 2 in the booth

17 Since the volume of the booth is 960 ft3, therefore the percent of CO 2 in the booth is %V= (13.9/960 )100 =1.44% Concentration of O 2 in booth can be determine O 2 = [Ambient Concentration - Displaced Concentration] EQ-4 Substituting the above parameter in equation 4, O 2 = [ (0.198 x )]

18 O 2 = 19.79% Percent Oxygen in the booth An oxygen sensor was placed inside the booth during the first week of operation, which confirmed our calculations. After the first week of operation it was determined that monitoring was no longer necessary and the sensor was removed. On the basis of these calculations is was determined that Airline respiratory protection is not needed based on oxygen deficiency.

19 Contamination (dpm-100cm -2 ) X 1000 AMUG 2010 Pre and Post Decontamination Smear Pre Decon Smear Post Decon

20 Activity (uci/cc) AMUG 2010 Airborne Activity in the Booth 3.00E E E E E E E Contamination (mrad)

21 AMUG 2010 OXYGEN LEVEL CONCLUSION The use of CO-2 decontamination unit is successful and exceeds all expectations. Cost Efective ALARA benefits have been realized from dose reductions both in time spent decontaminating equipment as well as reducing radiation dose rates for workers. Many items were successfully radiologically free released for unrestricted use. Radioactive waste generation was significantly reduced by eliminating secondary waste streams normally generated as a byproduct of other decontamination methods. The HEPA filter volume generated through CO-2 decontamination is insignificant.

22 OXYGEN LEVEL Rahim Ghanooni Health Physicists