In Vivo Calibration Measurements for Mixed Oxide Nuclear Fuel

Mixed oxide (MOX) nuclear fuel containing enriched uranium and isotopes of plutonium presents a challenge for monitoring occupational exposure using direct in vivo measurements because photons from U and Am will confound evaluation of plutonium in the lungs. MOX is a unique blend of enriched uranium and plutonium that can be used in place of traditional enriched uranium fuel in commercial pressurized water reactors. The plutonium in MOX fuel may be derived from either surplus weapons-grade plutonium or reprocessed commercial nuclear fuel. Thus, the final isotopic composition of MOX depends upon the origin of the plutonium, how well chemical and radiological impurities are removed from the plutonium prior to fuel fabrication, the time between fuel fabrication and use, and (for spent fuel) the total time of irradiation in the reactor. Occupational exposure to MOX will lead to significantly different risk depending upon where in the fuel cycle exposure occurs. Calibration of an array of high resolution detectors to perform in vivo measurements for MOX in the lungs was performed using the Lawrence Livermore thoracic phantom and a set of lungs containing a homogeneously distributed mixture of U, U, Am, and Pu to simulate conditions that might arise following inhalation of a typical MOX aerosol. Spectrum deconvolution was able to separate and identify x-rays associated with plutonium decay as long as the chest wall thickness was less than a few centimeters. With a chest wall thickness greater than a few centimeters, the U L X-rays from Pu were attenuated and could not be detected. Because there is no standard isotopic composition for MOX fuel, evaluation of results obtained by in vivo measurement will be very challenging. Although photons from U are easily detected in vivo following accidental intake involving MOX fuel, plutonium will be significantly more difficulty to evaluate and will likely have a greatest dosimetric consequence.