Question

Which type or types of nuclear reactors have these characteristics? \(\begin{array}{l}{\text { (a) Can use natural uranium as a fuel }} \\ {\text { (b) Does not use a moderator }} \\ {\text { (c) Can be refueled without shutting down }}\end{array}\)

Short answer

The Fast Breeder Reactor (FBR) is the type of nuclear reactor that fulfills all three given characteristics: it can use natural uranium as a fuel, does not use a moderator, and can be refueled without shutting down.

Step-by-step solution

Type of reactor with characteristics (a), (b), and (c)

The only type of nuclear reactor that fulfills all three given characteristics is the Fast Breeder Reactor (FBR). Let's review each characteristic for the FBR: a) Can use natural uranium as a fuel: FBRs can use natural uranium as well as a mixture of uranium and plutonium as a fuel. Fast neutrons are more likely to cause fission in natural uranium than in enriched uranium, making it suitable for FBRs. b) Does not use a moderator: FBRs don't use a moderator because they rely on fast neutrons for fission reactions. Instead, they use a coolant, usually liquid sodium, to transfer heat from the reactor core to the power cycle. The coolant also helps to slow down the neutrons without moderating their energy levels to the thermal range. c) Can be refueled without shutting down: FBRs have a continuous fueling process, known as on-load refueling, which allows fuel to be added and removed without stopping the reactor. This increases the plant's uptime and operational efficiency. In conclusion, the Fast Breeder Reactor (FBR) is the type of nuclear reactor that fits all three given characteristics.

Key concepts

Natural Uranium Fuel

Fast Breeder Reactors (FBRs) have the advantage of utilizing natural uranium as a fuel. This is possible because they rely on fast neutrons, which are more effective in causing fission in isotopes found in natural uranium. Natural uranium primarily consists of two isotopes: uranium-238 (U-238) and a small amount of uranium-235 (U-235). Most conventional reactors use enriched uranium, which has a higher concentration of U-235, the fissile material that easily undergoes fission. In contrast, FBRs can exploit U-238 by breeding it into plutonium-239 (Pu-239), another fissile material.

This capability allows FBRs to make better use of the Earth's uranium resources. By turning non-fissile U-238 into Pu-239, FBRs essentially "breed" fuel as they operate, thus creating a sustainable fuel cycle. This efficient use of natural uranium helps in maximizing energy extraction and minimizing waste.

No Moderator Nuclear Reactor

Unlike traditional nuclear reactors, Fast Breeder Reactors do not use a moderator. Moderators in nuclear reactors work to slow down neutrons to thermal speeds, where they have a higher probability of causing fission in U-235. However, FBRs are designed to operate with fast neutrons. These fast-moving neutrons have enough energy to induce fission directly in certain materials, including the plutonium-239 bred within the reactor.

Instead of a moderator, FBRs utilize a coolant to transport heat from the reactor core. Typically, liquid sodium serves as this coolant in FBRs due to its superior heat conduction properties and ability to operate without slowing down neutrons significantly. While sodium is effective, it does require careful handling due to its reactivity with water and air. Overall, the absence of a moderator allows FBRs to efficiently harness the energy from fast neutrons and sustain the breeding of new fuel.

On-Load Refueling

One of the standout features of Fast Breeder Reactors is their ability to perform on-load refueling. This means that FBRs can have their fuel replaced or refueled without being shut down. On-load refueling greatly enhances the operational efficiency of FBR plants and maximizes electricity generation uptime.

The continuous refueling process of FBRs involves the removal of spent fuel and addition of fresh fuel elements during reactor operations. This approach eliminates the need for lengthy shutdown periods typically required for refueling in conventional reactors. As a result, FBRs can maintain more consistent power output and reduce downtime costs, favoring their application in long-term energy planning.

• Maintains consistent electricity production
• Minimizes costly shutdown periods
• Optimizes resource utilization

On-load refueling is just another reason why FBRs are seen as a promising technology for sustainable nuclear energy.