PLUTONIUM — SEABORG 209 



It is of interest to give a brief description of the complex nuclear 

 reactions which lead to the production of this isotope in the uranium- 

 graphite chain-reacting units. As is now well known, the nuclear 

 chain-reaction depends on the following reaction of the isotope U^^^ 

 with neutrons : 



U'^°-f/i > fission products -f neutrons -f- energy (1) 



The neutrons liberated in this fission are fast neutrons, and in the 

 case of the natural uranium constituting such chain-reacting units 

 these would react preferentially with the large amount of IJ-^^ 

 (99.3 percent by weight) and not leave enough neutrons to react with 

 the U^^^ to maintain the chain-reaction unless special precautions are 

 taken. If the neutrons are reduced in energy this situation no longer 

 applies, as the probability of the fission reaction increases markedly 

 with decrease in neutron energy. In the uranium-graphite lattice 

 structure, in which lumps of uranium are interspersed in a graphite 

 matrix, the graphite slows down the fission neutrons without captur- 

 ing them. The fission neutrons which originally escaped from the 

 uranium then return from the graphite to the uranium, and after the 

 proper ptroportion undergo reaction 1 again in order to perpetuate the 

 chain, the majority of the remainder undergo the following reaction : 



U^38.^^ , U239 , J^p239 , p^239 (3) 



leading to the production of plutonium. 



However, a certain proportion of the fission neutrons have a suffi- 

 ciently high energy to produce the following reaction: 



W'+n >W+2n (3) 



before they escape from the uranium into the graphite. The IJ-^" 

 formed in this manner decays to the above-mentioned Np^^^, thus lead- 

 ing to the production of this isotope in the uranium in addition to the 

 primary desired product, Pu-^", and the fission byproducts. In a 

 uranium-graphite pile the Np-^^ is produced at a rate corresponding to 

 the order of 0.1 percent of that of the primary product, Pu^^^. 



Some of the isotope Np^^'' has been recovered by suitable modification 

 of the chemical separation process used at Hanford. As a result of 

 this work and these special runs, several hundred milligrams of the 

 isotope Np^^^ have been recovered and made available for the investiga- 

 tion of its chemical properties. 



Using this material, it has been possible to make an intensive study 

 of the chemical properties of neptunium, leading to the establishment 

 of its oxidation states and the properties of a large number of its 

 compounds. This work has shown that neptunium has the oxidation 

 states VI, V, IV, and III with a general shift in stability toward the 

 lower oxidation states as compared to uranium. 



