**Typical nuclear radii** are of the order **10 ^{−14} m**. Assuming spherical shape, nuclear radii can be calculated according to the following formula:

r = r_{0} . A^{1/3}

where r_{0} = 1.2 x 10^{-15 }m = 1.2 fm

If we use this approximation, we, therefore, expect the **geometrical cross-sections** of nuclei to be of the order of πr^{2} or **4.5×10 ^{−30 }m² for hydrogen** nuclei or

**1.74×10**nuclei.

^{−28}m² for^{238}USince there are many nuclear reactions from the incident particle point of view but, in nuclear reactor physics, neutron-nuclear reactions are of particular interest. In this case, the neutron cross-section must be defined.

**The volume of an atom** is about **15 orders of magnitude** **larger **than the volume of a nucleus. For **uranium atom**, the **Van der Waals radius** is about **186 pm = 1.86 ×10 ^{−10}m**. The Van der Waals radius, r

_{w}, of an atom is the radius of an imaginary hard-sphere representing the distance of closest approach for another atom. Assuming spherical shape, the uranium atom has a volume of about

**26.9 ×10**. But this “huge” space is occupied primarily by electrons because the

^{−30}m^{3}**nucleus**occupies only about

**1721×10**of space. These electrons together weigh only a fraction (let say 0.05%) of the entire atom.

^{−45}m^{3}