A Critical Look at NORM In The Monazite Cycle

摘要

Thorium, cerium, lanthanum and other rare earths have been extracted from monazitesands for a long time due to several reasons. At the end of the XIX century monazite sands fromBrazilian beaches were concentrated gravimetrically to be clandestinely transported to Europe tobe used in the manufacture of fabric bags to be burned. Those bags when burning had theirbrightness enhanced by a mixture of thorium, lanthanum and cerium. The thorium oxide (ThO_2)percentages found in monazite concentrates from several parts of the world range from 1 to 11%.While not burned or processed chemically monazite concentrate can be considered hazardous onlydue to gamma radiation and ~(220)Rn(thoron) inhalation by those who stay very close to storagepiles. In old monazite plants the thoron concentration in air reached levels higher than 20kBq.m~(-3).Fortunately, the industrial hygiene improved with time and today's monazite separation plants donot present such high thoron concentrations. Old and recent data from rare earth processing plantssuggest that occupational annual doses indoors may be up to two orders of magnitude higher thanthe worldwide average effective dose reported by the United Nations Scientific Committee on theEffects of the Atomic Radiation (UNSCEAR) due to the inhalation of thoron. Consumer products,like fabric bags used in gas or kerosene lamps have ~(232)Thconcentrations of the order of 150Bq.g~(-1).Remnants of the monazite industrial cycle can present ~(228)Raactivity concentrations as high as5.0kBq.g~(-1). Piles of monazite concentrates were kept for strategic reasons during cold war times inseveral places of the territories of the United States and the former Soviet Union. Cleanup at thosesites may be hazardous mostly due to high concentrations of thoron daughters that may be inhaledby workers. The paper will take a critical look at the naturally occurring radioactive materials(NORM) extant in the monazite cycle and its remnants.