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Gadolinium is a naturally-occurring chemical element with atomic number 64 which means there are 64 protons and 64 electrons in the atomic structure. The chemical symbol for gadolinium is Gd. Gadolinium belongs to rare earth elements (a set of seventeen chemical elements in the periodic table).

Gadolinium was first discovered in 1880 by Jean Charles Galissard de Marignac. It is named for mineral (gadolinite), which is named for Finnish chemist Johan Gadolin.

Natural gadolinium consists of six stable isotopes, 154Gd (2.18%), 155Gd (14.8%), 156Gd (20.5%), 157Gd (15.7%), 158Gd (24.8%) and 160Gd (21.9%) and one radioisotope 152Gd (0.2%) with half-life of 1.1 x 1014 y.

In the nuclear industry, gadolinium is commonly used as a neutron absorber due to the very high neutron absorption cross-section of two isotopes 155Gd and 157Gd. Their absorption cross-sections are the highest among all stable isotopes. 155Gd has 61 000 barns for thermal neutrons (for 0.025 eV neutron) and 157Gd has even 254 000 barns. For this reason, gadolinium is widely used as a burnable absorber, commonly used in fresh fuel to compensate for an excess of reactivity of reactor core. In comparison with other burnable absorbers, gadolinium behaves like a completely black material. Therefore gadolinium is very effective in compensation of the excess of reactivity. Still, on the other hand, improper distribution of Gd-burnable absorbers may lead to the unevenness of neutron-flux density in the reactor core.

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Gadolinium 155 and 157. Comparison of radiative capture cross-sections.
Source: JANIS (Java-based Nuclear Data Information Software); The JEFF-3.1.1 Nuclear Data Library
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The effect of gadolinium burnable absorbers (BA) can be demonstrated on boron letdown curves. At the beginning of a specific fuel cycle, the critical concentration of boric acid in the reactor core without burnable absorbers (blue curve) significantly differs from the critical concentration of boric acid in the reactor core with burnable absorbers (red curve). The difference is dependent on the amount of BA used.

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