Page 28 - PR 2014 2016 09 Nuclear Reactors and Fuel Cycle

272 Nuclear Reactors and Fuel Cycle | Progress Report

ing the nuclear fuel useful life to be extended. tration between 1 and 2.5%. These differences
in proportion are due to higher absorption
Burnable poison is first of all a tool for long- cross section of Erbium.
term control of a nuclear reactor. Its function is
basically to control the neutron population. For The study of densification using these two ma-
this purpose, the chemical element employed terials followed the same route. The powders
should react with neutrons arising from nucle- were mixed mechanically and pressed in order
ar fission of the uranium atom and besides, the to their green density reach 50% of theoretical
isotope formed (son of nuclear reaction) should density of the mixture.
be unable to perform this function. Because of
this feature, the term burnable is established, Experimentally, it was found that additions of
as this material will lose the ability to absorb up to 4% of Er O increase shrinkage to higher
2 3
neutrons, while the fuel reactivity decreases. values than for pure UO , showing that erbi-
2
um acts as a sintering aid agent and allows
For this purpose, the two burnable poisons its use without the need of additives. On the
commercially used with the nuclear fuels are other hand, Gadolinium Oxide, when added
Gadolinium and Erbium. Both burnable poi- to 7% in proportion by mass, causes a signifi-
sons belongs to rare earths family. They are cant drop in shrinkage. It was possible to get
added to fuel in powder form as oxides, with best results using nano-gadolínia, increasing
chemical formula Gd O and Er O . The first one the shrinkage and also allowing to use it for
2 3 2 3
is used with mass concentration between 6 to the manufacture of nuclear fuels without the
10%. Erbium oxide is used with mass concen- need of additives.

Figure 37. X-ray diffraction pattern of
uranium silicide sample, and calculated
diffractogram by the Rietveld Method.

Instituto de Pesquisas Energéticas e Nucleares