Fuel. Uranium is the basic fuel. Usually pellets of uranium oxide (UO2) are arranged in tubes to form fuel rods. The rods are arranged into fuel assemblies in the reactor core
The core of the reactor contains all of the nuclear fuel and generates all of the heat. It contains low-enriched uranium (<5% U-235), control systems, and structural materials. The core can contain hundreds of thousands of individual fuel pins.
The coolant is the material that passes through the core, transferring the heat from the fuel to a turbine. It could be water, heavy-water, liquid sodium, helium, or something else.
The turbine transfers the heat from the coolant to electricity, just like in a fossil-fuel plant.
- A neutron moderator reduces the speed of fast neutrons, thereby turning them into thermal neutrons capable of sustaining a nuclear chain reaction.
- Commonly-used moderators are
- regular (light) water(in 74.8% of the world’s reactors),
- solid graphite(20% of reactors),
- heavy water(5% of reactors) and
Control Rods or Reactivity control
- Control rods are used in nuclear reactors to control the fission rate of uranium and plutonium. They are composed of chemical elements such as boron, silver, indium and cadmium that are capable of absorbing many neutrons without themselves fissioning.
- Absorbing more neutrons in a control rod means that there are fewer neutrons available to cause fission.
Difference between Moderator and controlling Road
- Moderators slow down neutrons
- Control Rods absorb neutrons
- A critical mass is the smallest amount of fissile material needed for a sustained nuclear chain reaction.
- The critical mass of a fissionable material depends upon its nuclear properties, its density, its shape, its enrichment, its purity, its temperature, and its surroundings.
- When a nuclear chain reaction in a mass of fissile material is self-sustaining, the mass is said to be in a critical state
- Criticality is a nuclear term that refers to the balance of neutrons in the system.Balance of neutrons can be achieved using moderators and control rods.
- “Subcritical” refers to a system where the loss rate of neutrons is greater than the production rate of neutrons and therefore the neutron population decreases as time goes on. during shutdown, the reactor is placed in a subcritical configuration so that the neutron population and power decreases.
- “Supercritical” refers to a system where the production rate of neutrons is greater than the loss rate of neutrons and therefore the neutron population increases. When a reactor is starting up, the neutron population is increased slowly in a controlled manner, so that more neutrons are produced than are lost, and the nuclear reactor becomes supercritical.
- “Critical”-When the neutron population remains constant, this means there is a perfect balance between production rate and loss rate, and the nuclear system is said to be “critical.” when a reactor is said to have “gone critical,” it actually means it is in a stable configuration producing a constant power.
Nuclear poison (or neutron poison)
In reactor physics, a substance (other than fissionable material) that has a large capacity for absorbing neutrons in the vicinity of the reactor core. This effect may be undesirable in some reactor applications because it may prevent or disrupt the fission chain reaction, thereby affecting normal operation. However, neutron-absorbing materials (commonly known as “poisons”) are intentionally inserted into some types of reactors to decrease the reactivity of their initial fresh fuel load. (Adding poisons, such as control rods or boron, is described as adding “negative reactivity” to the reactor.)
Types of Reactors
There are many different kinds of nuclear fuel forms and cooling materials can be used in a nuclear reactor. As a result, there are thousands of different possible nuclear reactor designs. Here, we will discuss Reactors—
Pressurized Water Reactor
- The most common type of reactor. The PWR uses regular old water as a coolant. The primary cooling water is kept at very high pressure so it does not boil. It goes through a heat exchanger, transferring heat to a secondary coolant loop, which then spins the turbine. They could possibly burn thorium or plutonium fuel as well. In contrast to a boiling water reactor, pressure in the primary coolant loop prevents the water from boiling within the reactor.
- Very stable due to their tendency to produce less power as temperatures increase. Easier to operate from a stability standpoint.
- PWRs are compact reactors that fit well in nuclear submarines and nuclear ships.
- The coolant water must be highly pressurized to remain liquid at high temperatures.
- This requires high strength piping and a heavy pressure vessel and hence increases construction costs.
- The higher pressure can increase the consequences of a loss-of-coolant accident.
- The high temperature water coolant with boric acid dissolved in it is corrosive to carbon steel (but not stainless steel) and can lead to radiation exposure.
- It is necessary to enrich [2-5%] the uranium fuel, which significantly increases the costs of fuel production.
- Can’t breed new fuel — susceptible to “uranium shortage”
Boiling Water Reactor
- Second most common, the BWR is similar to the PWR in many ways. However, they only have one coolant loop. The hot nuclear fuel boils water as it goes out the top of the reactor, where the steam heads over to the turbine to spin it.
- It is the second most common type of electricity-generating nuclear reactor after the pressurized water reactor (PWR).
- The main difference between a BWR and PWR is that in a BWR, the reactor core heats water, which turns to steam and then drives a steam turbine. In a PWR, the reactor core heats water, which does not boil.
- This hot water then exchanges heat with a lower pressure water system, which turns to steam and drives the turbine.
- Simpler plumbing reduces costs
- Power levels can be increased simply by speeding up the jet pumps, giving less boiled water and more moderation. Thus, load-following is simple and easy.
- Lower risk (probability) of a rupture causing loss of coolant compared to a PWR.
- Can operate at lower core power density levels using natural circulation without forced flow.
- BWRs do not use boric acid to control fission burn-up to avoid the production of tritium leading to less possibility of corrosion within the reactor vessel and piping.
- BWRs are ideally suited for peaceful uses like power generation, and desalinization, due to low cost, simplicity, and safety focus, which come at the expense of larger size and slightly lower thermal efficiency.
- With liquid and gaseous water in the system, many weird transients are possible, making safety analysis difficult
- Primary coolant is in direct contact with turbines, so if a fuel rod had a leak, radioactive material could be placed on the turbine. This complicates maintenance as the staff must be dressed for radioactive environments.
- Control rods are inserted from below for current BWR designs. In case of power failure, the reactor core can undergo significant damage and turn catastrophic.
- Can’t breed new fuel — susceptible to “uranium shortage”
- Does not typically perform well in station blackout events, as in Fukushima.
Pressurized Heavy-Water Reactor (PHWR)
- Uses heavy water (deuterium oxide D2O) as its coolant and neutron moderator.
- The heavy water coolant is kept under pressure, allowing it to be heated to higher temperatures without boiling, much as in a pressurized water reactor.
- While heavy water is significantly more expensive than ordinary light water, it creates greatly enhanced neutron economy, allowing the reactor to operate without fuel-enrichment facilities (offsetting the additional expense of the heavy water) and enhancing the ability of the reactor to make use of alternate fuel cycles.
Advantages of Pressurized Heavy-Water Reactor (PHWR)
- It can be operated without expensive uranium enrichment facilities.
- The mechanical arrangement places most of the moderator at lower temperatures. The resulting thermal neutrons are “more thermal” making PHWR more efficient. So, PHWR uses fuel more efficiently.
- Since unenriched uranium fuel accumulates a lower density of fission products than enriched uranium fuel, it generates less heat, allowing more compact storage.
Disadvantages of Pressurized Heavy-Water Reactor (PHWR)
- The reduced energy content of natural uranium as compared to enriched uranium necessitates more frequent replacement of fuel.
The increased rate of fuel movement through the reactor also results in higher volumes of spent fuel than in LWRs employing enriched uranium.
Prepared By: Akshita