22.6 Nuclear Reactors | Nuclear Physics | Physics 12

A nuclear reactor is a device in which a controlled, self-sustaining nuclear chain reaction is maintained to produce energy. The most common type is the water-moderated (thermal) reactor.

Components of a Water-Moderated Power Reactor

1. Fuel Rods

The fuel consists of enriched uranium (see Uranium Enrichment below), typically in the form of uranium dioxide ( $UO_{2}$ ) pellets sealed inside metal tubes called fuel rods. The fissile isotope $_{92}^{235} U$ undergoes fission when struck by a slow (thermal) neutron:

$_{92}^{235} U +_{0}^{1} n \to fission fragments + 2 - 3_{0}^{1} n + energy (200 MeV)$

2. Moderator

Fission neutrons are released at high speeds. Fast neutrons are much less likely to cause further fission in $_{92}^{235} U$ . The moderator slows these fast neutrons down to thermal speeds (~0.025 eV) through elastic collisions.

In a water-moderated reactor, ordinary water ( $H_{2} O$ ) or heavy water ( $D_{2} O$ ) acts as the moderator.
Graphite is another common moderator.
Heavy water is preferred because it has a low neutron-absorption cross-section, meaning it slows neutrons without absorbing too many of them.

3. Control Rods

Control rods are made of neutron-absorbing materials such as Cadmium (Cd) or Boron (B). They regulate the chain reaction:

Inserting control rods deeper into the core absorbs more neutrons → reaction rate decreases.
Withdrawing control rods allows more neutrons to cause fission → reaction rate increases.
For steady power output, the multiplication factor $k = 1$ (critical state).
To shut down the reactor (SCRAM), rods are fully inserted → $k < 1$ .

4. Coolant

The coolant (usually pressurised water) flows through the reactor core and absorbs the thermal energy produced by fission. It carries this heat to a heat exchanger (steam generator), where it converts water in a secondary circuit into steam. The steam drives a turbine connected to an electrical generator.

5. Pressure Vessel

The reactor core (fuel rods, moderator, control rods, and coolant) is enclosed in a thick steel pressure vessel that withstands the high pressures of the coolant.

6. Biological Shielding

Surrounding the pressure vessel is several metres of high-density concrete (biological shielding). Its purpose is to absorb and block:

Intense gamma radiation
Neutron flux

This protects workers and the surrounding environment from harmful radiation.

Uranium Enrichment

Natural uranium has the following isotopic composition:

Isotope	Abundance	Fissile?
$_{92}^{238} U$	~99.3%	No (fertile)
$_{92}^{235} U$	~0.7%	Yes

Only $_{92}^{235} U$ undergoes fission with thermal neutrons. At 0.7% concentration, a sustained chain reaction is difficult to achieve in most reactor designs.

Uranium enrichment is the industrial process of increasing the proportion of $_{92}^{235} U$ relative to $_{92}^{238} U$ :

Reactor-grade enrichment: 3–5% $_{92}^{235} U$ — used in power reactors.
Weapons-grade enrichment: >90% $_{92}^{235} U$ — used in nuclear weapons.

How Enrichment Works

The most common method is gas centrifuge separation. Uranium is converted to uranium hexafluoride gas ( $UF_{6}$ ). Because $^{235} UF_{6}$ is very slightly lighter than $^{238} UF_{6}$ , high-speed centrifuges can separate them. Many stages (a cascade) are needed to achieve the desired enrichment level.

Energy Flow in a Nuclear Power Plant

$Fission (nuclear energy) \to Thermal energy (coolant) \to Steam (turbine) \to Electrical energy (generator)$

A nuclear reactor is a device in which a controlled, self-sustaining nuclear chain reaction is maintained to produce energy. The most common type is the water-moderated (thermal) reactor.

Components of a Water-Moderated Power Reactor

1. Fuel Rods

$_{92}^{235} U +_{0}^{1} n \to fission fragments + 2 - 3_{0}^{1} n + energy (200 MeV)$

2. Moderator

In a water-moderated reactor, ordinary water ( $H_{2} O$ ) or heavy water ( $D_{2} O$ ) acts as the moderator.
Graphite is another common moderator.
Heavy water is preferred because it has a low neutron-absorption cross-section, meaning it slows neutrons without absorbing too many of them.

3. Control Rods

Control rods are made of neutron-absorbing materials such as Cadmium (Cd) or Boron (B). They regulate the chain reaction:

Inserting control rods deeper into the core absorbs more neutrons → reaction rate decreases.
Withdrawing control rods allows more neutrons to cause fission → reaction rate increases.
For steady power output, the multiplication factor $k = 1$ (critical state).
To shut down the reactor (SCRAM), rods are fully inserted → $k < 1$ .

4. Coolant

5. Pressure Vessel

The reactor core (fuel rods, moderator, control rods, and coolant) is enclosed in a thick steel pressure vessel that withstands the high pressures of the coolant.

6. Biological Shielding

Surrounding the pressure vessel is several metres of high-density concrete (biological shielding). Its purpose is to absorb and block:

Intense gamma radiation
Neutron flux

This protects workers and the surrounding environment from harmful radiation.

Uranium Enrichment

Natural uranium has the following isotopic composition:

Isotope	Abundance	Fissile?
$_{92}^{238} U$	~99.3%	No (fertile)
$_{92}^{235} U$	~0.7%	Yes

Only $_{92}^{235} U$ undergoes fission with thermal neutrons. At 0.7% concentration, a sustained chain reaction is difficult to achieve in most reactor designs.

Uranium enrichment is the industrial process of increasing the proportion of $_{92}^{235} U$ relative to $_{92}^{238} U$ :

Reactor-grade enrichment: 3–5% $_{92}^{235} U$ — used in power reactors.
Weapons-grade enrichment: >90% $_{92}^{235} U$ — used in nuclear weapons.

How Enrichment Works

Energy Flow in a Nuclear Power Plant

$Fission (nuclear energy) \to Thermal energy (coolant) \to Steam (turbine) \to Electrical energy (generator)$