Types of Reactors

Light-water reactors (LWRs) are a type of power reactor that utilize ordinary water for both cooling and moderation. There are two primary variations: pressurized-water reactors (PWRs) and boiling-water reactors (BWRs).
In a PWR, high-pressure and high-temperature water is used to extract heat from the reactor core and is then directed to a steam generator. Within the steam generator, the heat from the primary loop is transferred to a secondary loop, which also contains water but at lower pressure. Initially, the water in the secondary loop is below the boiling point, but as it absorbs heat from the primary loop, it becomes saturated and eventually slightly superheated. The steam produced in this process is then utilized as the driving force in a steam-turbine cycle.

Pros

Reactor design features a strong negative void coefficient, causing cooling when water bubbles, as the moderator, essential for sustaining the chain reaction, is also the coolant.
A secondary loop effectively keeps radioactive materials separate from turbines, simplifying maintenance procedures.
Extensive operational experience has informed ongoing optimization of designs and procedures.

Cons

Rapid escape of pressurized coolant occurs if a pipe breaks, requiring extensive backup cooling systems.
Inability to breed new fuel leaves the system vulnerable to a potential uranium shortage.

Pressurized-water reactor

A Boiling Water Reactor (BWR) functions by employing a direct power cycle approach. Within the core, water undergoes boiling at a moderate pressure level. The resultant saturated steam exits the core and proceeds through a sequence of separators and dryers situated within the reactor vessel, facilitating its transition into a superheated state. This superheated water vapor is subsequently utilized as the operational medium for driving the steam turbine.

Pros

Streamlining plumbing cuts costs.
Boosting power levels is as easy as ramping up jet pump speeds, resulting in a higher proportion of moderated water and less boiling.
Achieving load-following is straightforward and uncomplicated.
Extensive operational experience has led to significant optimization of designs and procedures.

Cons

The presence of both liquid and gaseous water in the system introduces various unpredictable transients, posing challenges for safety analysis.
Direct contact between the primary coolant and turbines means that in the event of a fuel rod leak, radioactive material could contaminate the turbine, necessitating maintenance staff to work in radioactive environments.
Inability to breed new fuel leaves the system vulnerable to potential uranium shortages.
Performance during station blackout events, as seen in Fukushima, is typically subpar, further highlighting safety concerns.

CANDU reactors (pressurized heavy water reactor)

The reactor utilizes unenriched uranium as its fuel, while heavy water functions as both a coolant and a neutron moderator. The heavy water is maintained under high pressure, enabling it to reach elevated temperatures without boiling, similar to the setup in a pressurized water reactor.

Pros

Minimal uranium enrichment needed.
Refuelable during operation, maintaining high capacity factors (assuming fuel handling machines are operational).
Highly flexible, capable of utilizing various fuel types.

Cons

Certain variants exhibit positive coolant temperature coefficients, raising safety apprehensions.
Neutron absorption within deuterium results in the generation of tritium, a radioactive substance prone to occasional leakage in minor amounts.
Theoretical modifications could potentially accelerate the production of weapons-grade plutonium marginally quicker than traditional reactors.

Sodium Cooled Fast Reactor

These reactors utilize liquid sodium metal for cooling. Sodium, being heavier than hydrogen, accelerates neutron movement, resulting in fast reactions. They can employ either metal or oxide fuel and have the capability to burn a diverse range of fuels.

Pros

Can breed its own fuel, effectively eliminating any concerns about uranium shortages.
Can burn its own waste
Metallic fuel and excellent thermal properties of sodium allow for passively safe operation — the reactor will shut itself down safely without any backup-systems working (or people around), only relying on physics.

Cons

Sodium coolant, which is used in these reactors, can react dangerously with both air and water. This means that if there are any leaks in the pipes, it can lead to sodium fires, which are very hazardous.
Dealing with these sodium fires can be managed through engineering solutions, but they still pose a significant challenge for these reactors, causing setbacks in their operation and safety.
To fully dispose of nuclear waste, these reactors require reprocessing facilities. However, these facilities also have the potential to be misused for nuclear proliferation purposes.
The surplus neutrons utilized in these reactors for resource utilization can also be covertly exploited to produce plutonium, which could be used in the development of nuclear weapons.

Fast reactors

Fast reactors are a class of advanced nuclear reactors that have some key advantages over traditional reactors in safety, sustainability, and waste. While traditional reactors contain moderators to slow down neutrons after they’re emitted, fast reactors keep their neutrons moving quickly (hence the name). Fast neutrons can unlock the energy in the dominant isotope of uranium (U238) and thus extend known fuel resources by around 200x. Your average thermal neutron moves around at about 2200 m/s while a fast neutron might be cruising well above 9 million m/s, which is about 3% of the speed of light.

Pros

Fast reactors get more neutrons out of their primary fuel than thermal reactors, so many can be used to breed new fuel, vastly enhancing the sustainability of nuclear power.
Fast reactors are capable of destroying the longest-lived nuclear waste, transforming it to waste that decays to harmlessness in centuries rather than hundreds of millennia.
Fast reactors typically use liquid metal coolants rather than water. These have superior heat-transfer properties and allow natural circulation to remove the heat in even severe accident scenarios. The result: if something goes very wrong at the plant, and none of the operators are awake, AND none of the control rods work, the reactor can just naturally shut itself down. This comes with a con (see cons).
Fast reactors can employ metallic fuel rather than oxides (thanks to chemical compatibility with the liquid metal coolant). Since metal has very high thermal conductivity, the reactor can shut itself down without surpassing temperature limits. This enhances the safety of these reactors significantly.

Cons

Fast reactors are incredibly efficient, using up to 200 times fewer resources compared to other types. However, they have a higher initial requirement of fissile atoms, needing at least three times more to kickstart the process. This is why thermal reactors were developed first, as they required fewer fissile atoms to begin operation.
Fast reactors operate on quicker time scales than thermal reactors, mainly because they have fewer delayed neutrons. This means they can undergo unexpected changes more rapidly compared to their counterparts.
One concerning aspect of fast reactors is the potential for bubbles to form in the coolant. Unlike traditional reactors where bubbles help cool the system, in fast reactors, they can actually cause the reactor to heat up further. This creates a positive feedback loop where more heat generates more bubbles, leading to even more heat. However, this feedback loop is manageable due to the presence of strong negative feedback mechanisms.
Fast reactors rely on bizarre coolants derived from heavy atoms to keep the neutrons moving at high speeds. The most common coolant used is liquid sodium, although it is highly reactive with both air and water. Another option is the liquid lead-bismuth eutectic, which also presents its own challenges. These unconventional materials require extra caution and may necessitate stricter tolerances in various systems, potentially increasing overall costs.

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