Centrifuges with significant amounts of contamination have been observed during routine radiation safety inspections. While it is not uncommon for centrifuges to become contaminated, it is unusual for them to become so contaminated as to be reading in excess of 1 mRem/hour at one inch from the surface with a geiger counter. The following provides guidelines for researchers who use centrifuges in their procedures with radioactive material to reduce exposure.
The minimization of exposure from centrifuges contaminated with radioactive material can be divided into three distinct approaches:
- Minimize the release of radioactive material in the centrifuge during the centrifugation of tubes containing radioactive material,
- Routinely decontaminate the centrifuge to remove contamination deposited in the centrifuge during use, and
- Provide shielding for the centrifuge to minimized exposure from the contamination.
The Best method for minimizing exposure is to prevent the centrifuge from becoming contaminated in the first place. While it is difficult to do, you can reduce the amount of contamination by using micro-tubes with screw caps and "O" rings. The cap with the "O" ring reduces leakage from the micro-tube. An alternative to the screw cap is a micro-tube with a locking cap. Fisher Scientific and Research Products International Corp. are two companies that provide such micro-tubes. Micro-tubes with screw caps can be found:
- on page 141 of Research Products International's 2000 catalog, and
- on pages 317 to 320 of Fisher Scientific's 2000/01 catalog.
Another approach to minimizing contamination is to obtain a centrifuge which accepts a rotor with a sealed lid. The sealed lid provides containment of the aerosols preventing contamination of the inside of the centrifuge. Example of such a centrifuge can be found on page 293 of the Fisher Scientific 2000/01 catalog.
Routine decontamination of the centrifuge is the second most reliable means of reducing exposure from contamination in a centrifuge. The centrifuge should be monitored with a geiger counter on a monthly basis and decontaminated if dose rates are greater then 0.1 mRem/hour at one inch from the surface. This may involve removing the rotor to clean and soak it to remove the contamination.
If all else fails, you can shield the centrifuge to reduce exposure to researchers. In most cases simply closing the lid on the centrifuge will significantly reduce exposure from the contamination. However, when the lid is up additional shielding will be necessary. If your laboratory is working with beta emitters such as P-32 you can place a Plexiglas shield in front of the centrifuge. For gamma and x-ray emitters such as I-125 and Cr-51 you will need to place a lead shield or leaded glass shield in front of the centrifuge.
I encourage researchers to use the prevention and decontamination methods over the shielding method. While shielding can be effective, it allows the problem to become worse. I noted in one laboratory during a routine inspection that the dose rates from a centrifuge were in excess of 15 mRem/hr at one inch from the surface. This is not an acceptable radiation level under any circumstance.
For additional information regarding contaminated centrifuges see Surveying Equipment for Repair.