Sarisa - How do nuclear reactors work?

How do Nuclear reactors work?

Nuclear energy - all generated by the 440 nuclear reactors across 30 countries existing today - contributes to approximately 10% of all the world’s electricity. [1] It provides a relatively environmentally friendly source of non-renewable power, but of course, not without its downsides. But how do they all work? What is the driving force behind the production of its electricity? What are the common issues?

HOW DOES NUCLEAR FISSION WORK?

The electricity produced by nuclear reactors is driven by a simple reaction called nuclear fission. This refers to the process by which neutrons - upon contact and at the correct speed - split uranium or plutonium isotopes, thereby releasing heat and more high-energy neutrons. This heat is then converted into steam that is used to spin turbines for electricity. As for the neutrons released: there should be an average of 1 neutron produced per reaction (more or less in accordance with the reaction rate) though additional neutrons are sometimes released which go on to produce more reactions. [2]. The three usable fuels used in nuclear reactors are uranium, plutonium and thorium; The most commonly used material is uranium. [3]. However, only 0.7% of the actual fissile U-235 makes up the natural uranium. The rest consists of mainly isotope U-238 and very little of U-234. This is an issue, as the 0.7% of isotope U-235 found in natural uranium is not enough to power nuclear reactors. It has to be at a concentration of 2-5% to support any continuous nuclear chain reaction. So, how is this dealt with? The uranium is enriched. [4]

NOTE: The size and weight of each isotope differs slightly, meaning that they can be separated if in the right conditions. (This ties in later with the methods of isotope separation).

ENRICHMENT OF URANIUM

Because natural uranium alone cannot act as a fuel, the percentage of U-235 has to be increased for it to be available for use. Commercial enriched uranium is called enriched uranium oxide. First, the natural uranium must be sent to a plant that produces uranium oxide - also known as “yellowcake” - and then combined with hydrogen fluoride and fluorine gas to form uranium hexafluoride. [5] The reason this is done is because uranium hexafluoride has properties that make it ideal for enrichment due to its volatility (it is able to change state easily without a great change in kinetic energy needed) [6]. From here, there are two enrichment processes to obtain enriched uranium oxide: gaseous diffusion and gas centrifugation. [7]

Gaseous diffusion: this process forces uranium hexafluoride through semi-permeable membranes that separate the isotopes.

Based on Graham’s Law: the rate of effusion of a gas is inversely proportional to the square root of its molecular mass. The lighter particles would therefore pass the semi-permeable membranes faster than the heavier ones.
Because, for some reason, I couldn’t find the ratio of effusion rate for U-235 and U-238 online, here are my calculations (which may be incorrect!): 1.006:1 (The rate of reaction for U-235 is faster, though by not that much)
But because there are only very slight differences in the molecular mass of U-235 and U-238, this means that the process has to be repeated several times (the passing through of the UF6 through the diffuser) for any sufficient high separation to occur.
The products of each repetition go on to be used as the input for over thousands of stages; this is called a cascade. The specific number of stages in a cascade depends on the level of enrichment desired.
But because it is difficult to maintain the necessary conditions for UF6 to remain in a gaseous state, gaseous diffusion is considered the most time-consuming and expensive process for enriching uranium.[8]

Gas centrifugation: this process relies on the principle of centripetal force for depleted uranium to exit as waste, and to extract the isotope U-235 as the product

Relies on the principle of centripetal force: The centripetal force is the force required to keep a body in motion in a curved path, and that is directed toward the of the rotation. [9] The principle states that a body in curved motion exerts an equal and opposite force on the other body (centrifugal force).
Uranium hexafluoride is fed into the tank
The particles of different masses (in this case, U-235 and U-238) separate in a gradient along the radius of the centrifuge.
Denser isotopes move downwards toward to bottom of the centrifuge (Uranium depleted of U-235) and exits the waste stream
Enriched U-235 is less affected and exits the product stream into the next tank
The process is repeated in a cascade to achieve higher concentrations [10]

MODERATORS

In addition to the enrichment of uranium as a fuel, moderators are also used to absorb some of the neutrons’ kinetic energy and slow them down so that the fissile nuclei can absorb the neutrons (this is different to the function of absorbers, which absorb the neutrons themselves) [12]. Commonly used for moderators are heavy water, light water and graphite. In traditional nuclear reactors, the moderator is the same thing as the coolant: water. When fast neutrons strike the hydrogen atoms in H2O, they slow down. [13] This works to ensure that nuclear fission reaction rates are being controlled.

ABSORBERS

A variety of different chemical elements are incorporated into the control rods of nuclear reactors to control reaction rates through the absorption of neutrons: boron, cadmium, indium, silver, etc. [14] They can be either withdrawn to increase reaction rate, or can reduce it by insertion. Unlike moderators, they absorb whole neutrons. Absorbers with a large neutron absorption cross-section may be referred to as a “neutron poison”; These are unlikely to be used unless initially working with highly reactive fuels.

TYPES OF REACTORS

There are six types of reactors: Magnox, AGR, PWR, BWR, CANDU, and RBMK. Here are the basic distinctions between them. [15]

Some Clarifications:
Heavy-water vs. Light-water

Light water (H2O): Contains one proton, and is less dense than heavy water.

Heavy water (D2O): Denser than light water, as it contains both a proton and a neutron. It is an efficient moderator and absorbs less neutrons. [16]

Magnox (Magnesium Alloy Graphite Moderated Gas Cooled Uranium Oxide Reactor)

The Magnox reactor - a type of gas-cooled reactor - is designed to run on natural uranium, with graphite as a moderator and CO2 as the coolant. An advantage of the Magnox reactor is its ability for on-load refueling; The core is open on one end, so that fuels could be removed or added in while the reactor is still running. [17]

AGR (Advanced Gas Reactor)

The AGR is a gas-cooled reactor developed further from the earlier Magnox design. It still uses graphite as a neutron moderator and CO2 as a coolant, but the main difference is that the AGR operates at a higher temperature. [18]

PWR
The Pressurized Water Reactor is one of two light water reactors that makes use of light water both as a coolant and a moderator.

BWR

The Boiling Water Reactor is another type of light water reactor. The difference is that the reactor core boils water, which turns to steam and then drives the turbine. [19]

CANDU (Canada Deuterium Uranium)

The CANDU reactor is a pressurized heavy-water reactor (a type of reactor that uses heavy water as both its coolant and moderator. “The heavy water is kept under pressure which increases its boiling point so that it can operate at high temperatures without boiling,” - Energyeducation.ca.) The deuterium refers to its use of heavy water (deuterium oxide) and the uranium to its original use of uranium as a reactor fuel. [20]

RBMK (Реактор Большой Мощности Канальный, РбМК)

The RBMK reactor - standing for Reaktor Bolshoy Moshchnosty Kanalny, and meaning high-power channel-reactor - is a water cooled, graphite moderated reactor (hence its other name: Light Water Graphite Reactor). It is widely known as the reactor with the design flaws responsible for the Chernobyl incident. [21]

COMMON ISSUES
Buildup of nuclear reactor “neutron poisons” that slow down reaction rates.

One of the byproducts of fission in uranium atoms (decay product from fission-produced Tellurium-135) is iodine-135. Isotope I-135 has a half life of 6.57 hours, and further decays into Xenon-135. [22] Xe-135 is the strongest known neutron absorber, with a neutron absorption cross-section (a measure of the probability of neutron capture) of 3 million barns, compared to 400-600 barns for uranium fission events. [23] This means that a buildup of this nuclear poison may cause the temporary disabling of a nuclear reactor for several hours, as there are not enough neutrons to produce fission reactions. This is colloquially referred to as being stuck in the xenon pit or iodine hole. A build-up of short-lived neutron poisons is called reactor poisoning, while a build-up of long-lived neutron poisons is called reactor slagging. [24]