Some Facts About Neutron and Reactor Physics - The Risks of Nuclear Energy Technology - page 15

10000

U-235 fission
U-238 fission
Pu-239 fission

1000

100

10

1

0.1

0.01

0.01 0.1 1 10 100 1000 10000 100000 1E+006 1E+007

Energy, eV

Fig. 2.3 Microscopic fission cross sections (measured in barn) for U-235, U-238 and Pu-239 [ 13 ]

10000

U-238 capture

1000

100

10

1

0.1

0.01

0.001

0.01 0.1 1 10 100 100 1000 10000 100000 1E+006 1E+007

Energy, eV

Fig. 2.4 Microscopic capture cross section for U-238 [ 13 ]

is very important for the fuel-Doppler-temperature coefficient, which determines—
among other temperature coefficients—the safety characteristics of nuclear power
reactors [ 12 , 18 ].
The microscopic fission cross sections of U-235 or Pu-239 (Fig. 2.3 ) become
higher—with the exception of the energy range where the resonances occur—at
low neutron energies (about < 0.1 eV).