Gadolinium Oxide

Gd₂O₃

Absorbers

Density: 7.41 g/cm³

Gadolinium oxide burnable absorber. Gd-155 and Gd-157 have the largest thermal neutron capture cross sections of any stable nuclides (~61,000 and ~254,000 barns).

Composition

Nuclide / Element	Fraction
Gd-152	0.000800
Gd-154	0.008720
Gd-155	0.059200
Gd-156	0.081880
Gd-157	0.062600
Gd-158	0.099360
Gd-160	0.087440
O-16	0.600000

Code output

MCNP

c Gadolinium Oxide (Gd2O3)
c Density: 7.41 g/cm3 (use -7.41 on cell card)
c Gd2O3 = 2 Gd + 3 O = 5 atoms, natural Gd isotopics
m35   64152.80c  0.000800  $ Gd-152
      64154.80c  0.008720  $ Gd-154
      64155.80c  0.059200  $ Gd-155
      64156.80c  0.081880  $ Gd-156
      64157.80c  0.062600  $ Gd-157
      64158.80c  0.099360  $ Gd-158
      64160.80c  0.087440  $ Gd-160
       8016.80c  0.600000  $ O-16

Notes

Gd-155 (σ_a ≈ 61,000 b) and Gd-157 (σ_a ≈ 254,000 b) are the primary absorbing isotopes. Used as a burnable poison mixed directly into UO2 fuel pellets (typically 2–10 wt% Gd2O3).

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