The Reality of 400 Grams of Uranium-235: Can It Produce a Yield of 5 KT?

The question of whether 400 grams of uranium-235 (235U) can produce a yield of 5 kilotons (KT) nuclear explosion has long been a topic of fascination and inquiry, particularly among those with an interest in nuclear physics and weapons development. This article will explore the scientific basis for understanding the feasibility of such a scenario.

Understanding Critical Mass

To begin, it is essential to understand the concept of critical mass. The critical mass is the minimum amount of fissile material needed to sustain a chain reaction. For uranium-235, this is the essential element in a nuclear weapon, as it undergoes fission when hit by a neutron, releasing energy and additional neutrons.

According to the Law of Large Numbers, as the size of a nuclear weapon increases, its efficiency (yield per unit mass) also increases. This is because larger masses have better control over neutron escape, leading to a higher probability of sustaining a chain reaction. On average, the yield of uranium-235 per kilogram is around 17 kt, leading to the conclusion that 400 grams would yield approximately 6.8 kt under ideal conditions.

Practical Considerations

While the mathematical calculations suggest that 400 grams could potentially produce a yield of 6.8 kt, this is purely on the basis of theoretical mass. In reality, achieving a self-sustaining chain reaction with such a small amount of uranium-235 is almost impossible due to several technical limitations.

Firstly, it has been established that the minimum mass needed for a bare sphere of highly enriched uranium-235 at room temperature is around 52 kg (some sources suggest 47 kg). This critical mass is further reduced by the addition of a neutron reflector, such as beryllium, which can lower the required mass to approximately 15 kg. These figures represent the minimum for a simple, non-boosted bomb, meaning a bomb with no added thermonuclear material to enhance the reaction.

Secondly, the efficiency of the Little Boy bomb, the uranium-based bomb dropped on Hiroshima, which required about 64 kg (141 lbs) of uranium with an enrichment level of ~80%, highlights the inefficiency of using such a small amount of uranium. Despite its inefficiency, this amount was deemed sufficient for a practical and devastating explosion.

Conclusion

Based on scientific understanding, it is clear that 400 grams of uranium-235 is entirely insufficient to produce a yield of 5 kt or more without additional boosting techniques. The small mass would not only struggle to achieve criticality but would also face significant design and technical challenges in ensuring a stable and controllable chain reaction.

The reality is that achieving the desired yield from such a small amount of fissile material is not feasible using conventional methods and would require extreme technical advancements and large-scale resources.