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.)

Understanding Uranium-235 is crucial for grasping the complexities of nuclear science. Uranium-235 (U-235) is an isotope of uranium that is particularly important in nuclear technology because it is the only naturally occurring isotope that is fissile - this means it can sustain a nuclear chain reaction, which is a series of fission processes.

During fission, a neutron collides with the nucleus of a U-235 atom, which then splits into smaller nuclei, additional free neutrons, and a significant amount of energy. For example, the equation for a simple fission reaction of U-235 may resemble the following: \[^{235}U + n \rightarrow ^{92}Kr + ^{141}Ba + 3n + Energy\]
This liberated energy is harnessed in nuclear reactors and weapons. It must be noted that while U-235 is essential for generating power, it also poses risks if not properly contained and controlled, as demonstrated tragically in the 1997 Russian accident.

Neutron reflection plays a pivotal role in both enhancing and controlling nuclear reactions. Materials that can reflect neutrons, such as copper in the mentioned accident, effectively bounce neutrons back into the fissile mass. This bouncing back can increase the likelihood of additional fissions occurring since more neutrons are retained within the U-235 mass.

The concept of neutron reflectors is often utilized in nuclear reactors to help sustain or contain the chain reaction. In the case of the accident, the copper shell surrounding the uranium inadvertently became a neutron reflector, which led to an uncontrolled escalation of the chain reaction since the additional neutrons caused more atoms to undergo fission. This aspect of neutron reflection is pivotal for understanding both the design of nuclear reactors and the potential causes of nuclear fission accidents.

A nuclear chain reaction occurs when one single nuclear event causes an average of one or more subsequent nuclear reactions, creating a self-propagating series of these reactions. The reaction can be represented as follows:
One neutron induces the fission of a U-235 atom, which then releases additional neutrons that can trigger further fission events. This can be simplified as: \[1 \text{ fission } \rightarrow \text{ neutron emission } \rightarrow \text{ more fissions}\]

To achieve a controlled chain reaction, as seen in nuclear reactors, the number of neutrons that cause subsequent fissions must be regulated. However, in an uncontrolled environment, like the one in the Russian incident, too many neutrons can lead to an exponential increase in reactions, releasing an immense amount of energy in a short period, and resulting in a disastrous outcome.

Radiation damage is the destructive effect on materials and living tissue due to exposure to high levels of ionizing radiation. When an uncontrolled nuclear chain reaction occurs, it emits not only neutrons but also other forms of radiation, such as gamma rays, which can cause severe damage to the human body at the cellular and DNA level.

Radiation can instantly kill cells, create free radicals that cause long-term damage, and increase the risk of cancer by causing mutations in DNA. Acute radiation sickness, which the scientist in the 1997 accident suffered, is a rapid-onset condition caused by exposure to high doses of radiation, leading to symptoms ranging from nausea and vomiting to skin burns and ultimately, if the exposure is high enough, death. This is why safety protocols in nuclear facilities are stringent, to prevent such exposure as much as possible.