Environmental plutonium — creation of the universe to twenty-first century mankind

Abstract

Gigatonne amounts of 244Pu and 239Pu were probably formed in supernova explosions during the creation of the universe and there is ongoing production of 239Pu by spontaneous fission of 238U, and thus plutonium must be regarded as a natural radioelement which has been present on earth during the whole of the evolution of life. The cosmogenic 244Pu largely decayed away over the first 3000 million years of the earth's existence, leaving 239Pu as the predominant isotope at a constant base-level of about 100 amol kg-1 of the earth's crust. This natural plutonium is relatively immobile, but a base-load of <2 amol has probably been present in the human body since the emergence of Homo sapiens. Human activities since 1945 contributed to the release of man-made 238,239 & 240Pu into the environment and this has increased the human body content by about 5 orders of magnitude to 130 fmol. Similarly low concentrations of plutonium will be found in plants and animals. These ultra-trace amounts of plutonium correspond to the deposition of < 1 atom in every 500 cells within the critical tissues of the human body and raise fundamental questions about the chemical speciation and the biochemical behaviour of plutonium compared to those of the chemically-related natural radioelements thorium and uranium which are present at concentrations of hundreds of atoms per cell.