Explain the terms nuclear fission and specific heat capacity - Leaving Cert Physics - Question 8 - 2018

A moderator increases the rate of fission by slowing down the neutrons produced during the fission reaction. This increases the likelihood of these slower neutrons being absorbed by other fissile nuclei, promoting more fission reactions.

A moderator works by reducing the kinetic energy of fast neutrons. This is achieved through collisions between the neutrons and the atoms of the moderator material, effectively slowing them down to thermal velocities where they can efficiently sustain the fission chain reaction.

The energy absorbed by the water can be calculated using the formula:

$E = m imes c imes heta$

Where:

• $m = 5000 ext{ kg}$
• $c = 2.23 imes 10^6 ext{ J/kg}$ (specific latent heat of vaporisation)
• $heta = 70 ext{ K}$ (temperature increase)

Substituting in these values:

$E = (5000 ext{ kg}) imes (4180 ext{ J/kg K}) imes (70 ext{ K})$

Calculating:
$E = 1.26 imes 10^{10} ext{ J}$

$^{235}_{92}U + n \rightarrow ^{139}_{56}Ba + ^{94}_{36}Kr + 3n$

To calculate the energy released during fission, we can use the mass-energy equivalence formula:

$E = mc^2$

Where the mass loss in this reaction is given as 3.0 x 10^{-28} kg. Substituting in the value for c (the speed of light, approximately $3.00 imes 10^8 ext{ m/s}$):

$E = (3.0 imes 10^{-28} ext{ kg}) \times (3.00 imes 10^8 ext{ m/s})^2$

After calculations, we find:

$E = 2.74 imes 10^{-11} ext{ J}$

Converting this to MeV (1 J = $6.242 imes 10^{12}$ MeV), we get:

$E = 171 ext{ MeV}$

Fusion reactors are not currently a practical source of energy because the conditions required for fusion to occur—extremely high temperatures and pressures—are difficult to achieve and maintain. Moreover, the energy input needed to create and sustain these conditions often exceeds the energy produced.

One major advantage of fusion reactors over fission reactors is that they produce significantly less radioactive waste, making them more environmentally friendly. Additionally, the fuel sources for fusion (like isotopes of hydrogen) are more abundant than those for fission.