A thermal nuclear reactor uses a moderator to lower the kinetic energy of fast-moving neutrons - AQA - A-Level Physics - Question 6 - 2019 - Paper 2

After each collision, a neutron loses 63% of its kinetic energy. Therefore, after one collision, the new energy is:

$E_1 = 0.37 \times E_0$

For 5 collisions, the kinetic energy is calculated using:

$E_5 = E_0 \times (0.37)^5$

Substituting $E_0 = 2.0 \, MeV$:

$E_5 = 2.0 \, MeV \times (0.37)^5 \approx 0.25 \, MeV$

The number of collisions required for a neutron to lose energy to a specific level is influenced by the nucleon number of the moderator. A higher nucleon number can lead to a greater likelihood of effective collisions, as more nuclei are available to interact with the neutron. Thus, moderators with a higher nucleon number can decrease neutron energy more efficiently.

To calculate the energy released during the fission process, we use the mass-energy equivalence principle:

1. Calculate the mass difference:

$\Delta m = (mass_{initial}) - (mass_{final})$

Where:

• $mass_{initial} = 235.044 + 1.0087 \, u$
• $mass_{final} = 141.930 + 89.908 + 4(1.0087) \, u$

2. Substituting values: $\Delta m = (236.0527 - 141.930 - 89.908 - 4.0348) \, u \approx 0.0809 \, u$

3. Convert mass difference to energy using: $E = \Delta m \cdot c^2 = 0.0809 \, u \cdot 931.5 \, MeV/u \approx 75.4 \, MeV$

1. Low Greenhouse Gas Emissions: Nuclear power generates electricity with minimal greenhouse gas emissions, contributing to climate change mitigation.
2. High Energy Density: Nuclear fuel contains a large amount of energy in a small volume compared to fossil fuels.
3. Reliability and Consistency: Nuclear power plants provide stable and continuous energy supply, unlike some renewable energy sources that may be intermittent.