Page 258 - Dynamics and Control of Nuclear Reactors

APPENDIX C Basic reactor physics 259

The most probable fission fragments are Cs-140 (Cesium) and Rb-93 (Rubid-
ium). For example, the reactions involving these fission fragments are given as
follows:
235 1 236 140 93 1
92 U + n ! 92 U ∗ ! 55 Cs + Rb +3 n + Energy 200 MeVð Þ (C.7)
0
0
37
The high-speed fission fragments lose energy by interaction with the molecules of
the surrounding medium (fuel, structure, moderator, etc.), thus converting kinetic
energy (KE) to thermal energy. There is also heating due to gamma radiation and
slowing down of neutrons to lower energy levels. An average of υ¼2.43 neutrons
are produced per fission induced by thermal neutrons in U-235.
Avogadro’s Number (AN): is the number of molecules per gram mole or atoms
per gram atom of a substance. One gram-atom is the quantity of substance in grams,
numerically equal to its atomic mass. Avogadro’s Number is numerically equal to
23
AN¼6.023 10 atoms per gram-atom.

Example C.2
The number density (N) of U-235 atoms in natural uranium is the number of atoms of U-235
3
per cm . The number density is given by
N ¼ ANÞ ρe=m
ð
where
3
ρ¼material density, gm/cm
e¼enrichment [U-235/(U-235+U-238)]
m¼gram U per gram atom U.

The number density of U-235 in 1% enriched uranium (with a material density of
3
19.0 g/cm is [3].

23
19:0 gramU=cm 3 6:023 10 ð atomsU=gram atomUÞ atomsU 235
N 235 ¼ 0:01
238 gramU=gram atomUÞ atomU
ð
3
20
¼ 4:80 10 atoms U-235=cm :
Some energy equivalents are as follows:
1 eV is the amount of kinetic energy imparted to an electron when accelerated
through a potential difference of 1 V.
1eV¼1.602 10 19 J
1 cal¼4.184 J.
The absorption of a neutron by the U-235 nucleus has the form
92 U + n ! 92 U ∗ (C.8)
235 1 236
0
This results in extra internal energy of the product, because the sum of masses of the
two interacting particles is greater than that of a normal U-236 nucleus (at ground
state). This excess energy is sufficient to cause nuclear fission (electrostatic repul-
sion dominates nuclear attraction).