On September 22, 1994, analysts conducting a bioassay review at the Rocky Flats Site determined that a security officer had received an indication of an uptake of plutonium and assigned him a committed effective dose equivalent of 430 millirem. After an intensive investigation, facility personnel determined that the uptake indication was inaccurate because an offsite laboratory had used a contaminated reference solution in the analysis of these samples. (ORPS Report RFO--WSIR-PROTFORCE- 1994-0010)
Radiation protection personnel at Rocky Flats routinely send bioassay samples to a commercial laboratory located in Hanford, Washington. For each sample, the laboratory identifies the isotopes, their activity, and calculates internal radiation doses. To determine the plutonium-239 activity, laboratory analysts combine a sample with a reference solution that contains a known quantity of plutonium-242 and then place it in a radiation activity counter. Both plutonium isotopes emit alpha radiation that have discrete energy values. If a sample contains plutonium-239, then a high count rate will occur at the energy value for the alpha radiation it emits. The plutonium-242 provides both an energy and activity reference point during the counting process. Since the analysts know the quantity of plutonium-242 in the combined sample, they can compare the count rates to determine the quantity of plutonium-239.
Internal Dosimetry department personnel reviewed the work history of the security officer, and could not identify any events that would have caused an intake of plutonium. With this information, and with a high number of subsequent sample results indicating the presence of plutonium, EG&G managers contacted the laboratory and learned that the reference solution was contaminated with plutonium-238 and -239. The source of the contamination is under review by EG&G and laboratory personnel. Radiation protection personnel indicated that EG&G qualified the laboratory because it used draft ANSI standard N13.30 for internal dosimetry analysis. EG&G personnel attempt to verify the accuracy of a positive sample result by reviewing trended routine bioassay data.
This event illustrates the importance trending sample data to ensure the quality of analyses from offsite laboratories. Additional quality assurance can be obtained by performing periodic inspections of qualified offsite laboratories. Audits can ensure that the quality of the laboratory's procedures and equipment is maintained.
For more information on this event, contact Fred Jaeger, DOE health physicist, at (303) 966-3006.