Question

In 1997 a scientist at a nuclear research center in Russia placed a thin shell of copper on a sphere of highly enriched uranium-235. Suddenly, there was a huge burst of radiation, which turned the air blue. Three days later, the scientist died of radiation damage. Explain what caused the accident. (Hint: Copper is an effective metal for reflecting neutrons.)

Short answer

The cause of the accident was a criticality incident, a uncontrolled nuclear chain reaction, triggered by placing copper, a reflector of neutrons, on a sphere of uranium-235, a fissionable material. This led to the reflection of neutrons back into the uranium, inducing more fission and producing a huge, lethal burst of radiation.

Step-by-step solution

Understanding the Material Involved

First, it's crucial to understand the materials involved. Uranium-235 is a radioactive isotope used in nuclear reactors and weapons. When a slow-moving neutron hits it, it can split, releasing energy and additional neutrons which can then cause other uranium-235 atoms to split, creating a chain reaction. Copper, on the other hand, is a reflector of neutrons. When a neutron hits a copper atom, it can bounce back, similar to light reflecting from a mirror.

Analyzing the Incident

By placing a shell of copper around a sphere of uranium-235, the neutron-firing uranium atoms were encased. As a result, many neutrons that would have escaped into the surrounding environment instead reflected back into the uranium sphere.

The Cause of the Accident

These reflected neutrons caused more uranium-235 atoms to split, increasing the rate of fission, and causing a rapid chain reaction. This reaction released a large amount of radiation energy, which manifested as a burst of radiation, turning the air blue. This is known as a criticality incident. The scientist was exposed to this high-intensity, short-term radiation which ultimately led to lethal radiation damage.

Key concepts

uranium-235

Uranium-235 is a notable isotope utilized in nuclear reactions. It plays a critical role in both nuclear reactors for energy generation and in the production of nuclear weapons. Its significance lies in its ability to sustain a chain reaction. When a neutron hits a uranium-235 atom, it splits, or undergoes nuclear fission. This splitting releases a considerable amount of energy as well as more neutrons.

• Energy Release: The fission process of uranium-235 results in the release of energy, explaining its use in nuclear power.
• Chain Reaction: The neutrons released from each fission event can strike other uranium-235 atoms, causing them to fission as well, perpetuating the cycle.

Proper control of the reaction is essential, as an uncontrolled chain reaction can escalate rapidly, leading to dangerous situations like explosions or severe radiation release.

neutron reflection

Neutron reflection is a process critical to understanding nuclear fission reactions, particularly how they can be intensified. Metals such as copper can effectively reflect neutrons back into a reactive core, increasing the possibility of further interactions with fissile material like uranium-235.

• Reflection Process: When a neutron collides with a copper atom, it bounces back, much like how light reflects from a surface.
• Reactivity: By reflecting neutrons back into the uranium-235, the chances of additional fission events are heightened.

In the context of the 1997 incident, the copper shell acted as a reflective barrier, preventing the escape of neutrons and redirecting them back into the uranium, thereby intensifying the reaction and leading to the critical incident.

criticality incident

A criticality incident refers to an uncontrolled nuclear chain reaction that results in an unexpected and dangerous release of radiation. This phenomenon occurs when there is an accumulation of sufficient fissile material, coupled with conditions that foster a self-sustaining fission reaction.

• Chain Reaction: The incident typically involves rapid successions of fission reactions, producing intense radiation.
• Radiation Burst: Such incidents can emit a massive amount of radiation in a very short period, posing severe health risks and even turning the surrounding air blue due to ionization.

The 1997 incident at the Russian nuclear research center exemplified a criticality accident. The scientist's use of a copper shell significantly increased the fission reactions within the uranium-235 by reflecting neutrons that would typically escape, leading to a dangerous burst of radiation that cost him his life.