BK5 3 Nuclear Energy

Nuclear Fission

A nuclear reaction in which a heavy nucleus splits into lighter nuclei, releasing a huge amount of energy and neutrons. Example: U-235 undergoing fission when bombarded by a slow-moving neutron.

Spontaneous vs Non-Spontaneous Nuclear Reaction (HKEAA Trap)

A radioactive decay (e.g., alpha/beta decay) involves ONE initial particle and is spontaneous.
Fission/fusion involving MORE THAN ONE reacting particle (e.g., U-235 + neutron) is NOT spontaneous.
(Trap: Students often fail to realize multiple reacting particles mean the reaction requires a trigger).

Conditions for Sustained Chain Reaction

1. The mass of the fissile material must be greater than or equal to the critical mass.
   2. At least one neutron on average from each fission must cause another fission.
   (HKEAA Trap: Do not just write "slow neutrons are produced" - fission produces fast neutrons that trigger further reactions).

Critical Mass

The minimum amount of fissile material required for a sustained nuclear chain reaction. If the mass is smaller than the critical mass, too many neutrons escape without hitting a fissile nucleus.

Nuclear Fusion

A nuclear reaction in which two light nuclei join together to form a heavier nucleus, releasing a huge amount of energy. It is the primary source of solar energy in the Sun's core.

Why Nuclear Fusion Requires Extremely High Temperatures (~10^7 K)

To provide the light nuclei with extremely high kinetic energy to overcome the strong electrostatic repulsion between the positively charged nuclei, allowing them to get close enough for the strong nuclear force to bind them.
(HKEAA Trap: Do NOT write "to melt the nuclei". The work done to overcome repulsion becomes electric potential energy, not heat or nuclear energy).

Mass-Energy Relationship Formula delta E = delta m * c^2,
what is e,m,c

where delta E is energy released (in J), delta m is mass defect or loss in mass (in kg), and c is the speed of light in vacuum (3.00 x 10^8 m/s).
[IN DSE FORMULA SHEET]

Mass Defect / Binding Energy Concept (HKEAA Trap)

The rest mass of a bound nucleus is ALWAYS STRICTLY LESS than the combined rest mass of its separated free nucleons. The "loss in mass" is converted and released as energy during formation.
(Trap: Common misconception is believing a nucleus is heavier because it contains 'binding energy'. Binding energy represents a lower energy state).

Calculating Mass Defect in Nuclear Fission (HKEAA Trap) (wtf is this)

delta m = Total mass of reactants - Total mass of products.
(Trap: Always remember to include the incident neutron's mass on the reactant side when calculating the total initial mass, e.g., Reactants = U-235 + 1n).

Electron-volt (eV) Definition

One electron-volt is the kinetic energy gained by an electron when it is accelerated across a potential difference of 1 V. 1 eV = 1.60 x 10^-19 J.
[NOT IN DSE FORMULA SHEET - MUST MEMORIZE/DERIVE]

Atomic Mass Unit (u) Definition

1 u is defined as 1/12 of the mass of a carbon-12 atom. 1 u = 1.661 x 10^-27 kg.
[NOT IN DSE FORMULA SHEET - MUST MEMORIZE/DERIVE]

Energy Equivalent of 1 u

1 u of mass defect is equivalent to 931 MeV of energy.
(Trap: Be careful not to confuse MeV (10^6 eV) with eV when doing unit conversions between u, MeV, and J).
[NOT IN DSE FORMULA SHEET - MUST MEMORIZE/DERIVE]

Controlled Nuclear Fission vs Fusion (Environmental Impact)

Fission generates electricity but produces long-lived radioactive nuclear waste requiring deep underground storage. Fusion uses abundant fuel (e.g., deuterium from seawater), has non-radioactive major products (helium-4), but technology is still under development due to the difficulty of building reactors to withstand ~10^7 K plasma.

Power Delivered by a Radioactive Source (HKEAA Trap) (wtf is this)

Power P = (Activity or Rate of decay) * (Energy released per decay).
P = A * delta E.
(Trap: Do NOT use the total number of atoms N in the calculation; you MUST use the Activity A = kN). [NOT IN DSE FORMULA SHEET - MUST MEMORIZE/DERIVE]

Calculating Mass Loss Rate from Power (HKEAA Trap) (wtf is this)

Power P = (delta m / delta t) * c^2.
(Trap: A common mistake is forgetting to square the speed of light 'c' when computing mass defect rate from power, e.g., using P = m/t / c instead of c^2). [NOT IN DSE FORMULA SHEET - MUST MEMORIZE/DERIVE]

Conservation Laws in Nuclear Reactions

Total electric charge (atomic number Z) and total nucleon number (mass number A) are strictly conserved in all nuclear reactions. Momentum is also strictly conserved.

Role of Moderators / Water in Nuclear Fission (HKEAA Trap)

Fission of U-235 produces fast neutrons, which are poorly absorbed by U-235. Water acts as a moderator to slow down these fast neutrons into slow neutrons so they can easily be captured to trigger further fission. If water dries up, the supply of slow neutrons ceases, and the chain reaction stops. (Trap: Do not write "neutrons are not energetic enough" - they are too fast without a moderator).