Why can’t uranium-238 be used for fission?

Uranium-238, a radioactive isotope of uranium, is not commonly used for nuclear fission due to its inability to sustain a chain reaction. Unlike uranium-235, which readily undergoes fission when bombarded with neutrons, uranium-238 has a lower probability of undergoing fission upon neutron absorption.

Furthermore, the majority of reactors are designed to use enriched uranium-235 as fuel, as it is more easily fissioned compared to uranium-238. The higher proportion of uranium-235 in enriched uranium allows for a self-sustained chain reaction to occur, making it the preferred choice for nuclear power generation.

Uranium-238 is a common isotope of uranium that is found in abundance in nature. However, despite its abundance, uranium-238 cannot be directly used for fission reactions in nuclear power plants. In this article, we will explore the reasons behind this limitation.

The Nature of Uranium-238

Uranium-238, or U-238 for short, is a radioactive isotope with a long half-life of about 4.5 billion years. It undergoes alpha decay, emitting alpha particles and transforming into thorium-234. This decay process is not suitable for sustaining a controlled chain reaction necessary for fission reactions.

Key Differences: Uranium-235 vs. Uranium-238

To understand why U-238 cannot sustain a fission reaction, it is essential to differentiate it from uranium-235 (U-235). Unlike U-238, U-235 is a fissile isotope that can undergo a chain reaction when bombarded with neutrons.

Nuclear Properties

U-235 possesses certain nuclear properties that make it suitable for sustaining a fission reaction. Its nucleus is more prone to fission compared to U-238. Specifically, U-235 has a higher probability of undergoing induced fission when struck by a slow-moving neutron. On the other hand, U-238 has a lower probability of sustaining fission, even with the same neutron.

Natural Abundance and Enrichment

Another crucial difference lies in the natural abundance of these isotopes. U-238 is far more abundant in nature, accounting for about 99.3% of natural uranium, while U-235 comprises only about 0.7%. This disparity necessitates the enrichment process to increase the concentration of U-235, making it suitable for fission reactions.

Neutron Capture and Fertile Material

U-238 can capture a neutron and transform into U-239, which undergoes beta decay and transforms into plutonium-239. Plutonium-239 is a fissile material and can sustain a controlled chain fission reaction. This process is utilized in breeder reactors, where U-238 is converted into plutonium-239. However, reprocessing and additional steps are required to extract the plutonium-239 for use as fuel.

While uranium-238 is abundant and plays a crucial role in nuclear reactors as a fertile material, it is not directly suitable for fission reactions. The key differences between U-238 and U-235, including nuclear properties, natural abundance, and the ability to sustain fission chain reactions, make U-235 the preferred isotope for nuclear power generation. Understanding these distinctions helps us recognize the challenges and requirements involved in harnessing the power of nuclear energy.

Uranium-238 cannot be used for fission due to its stable nature and inability to sustain a chain reaction. Unlike uranium-235, which is fissionable, uranium-238 requires additional processes, such as enrichment or breeding, to be used as a fuel in nuclear reactors.

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