Exploring the Temperature of Half-Critical Nuclear Masses
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Understanding Half-Critical Masses
It's astonishing to think that one can safely sit near a sub-critical mass of nuclear weapon-grade material while enjoying a cup of coffee. However, the situation drastically changes if two half-critical masses are brought together—potentially leading to catastrophic consequences.
The infamous "Demon Core" at Los Alamos National Laboratory serves as a stark reminder of the dangers involved. In a functional nuclear bomb, it's crucial to quickly compress the material to prevent a fizz-out—an event where nuclear material disperses without detonating. Thus, keeping two sub-critical masses at a safe distance is imperative, especially since they might be positioned closely in bomb pre-triggers.
What is a Half-Critical Mass?
While "half-critical mass" isn't a formally recognized term in nuclear physics, it refers to a mass of fissile material that is less than the critical mass required for a sustained nuclear chain reaction. The temperature of this mass is influenced by its size, shape, purity, and any moderators or reflectors present that affect neutron behavior.
When below critical mass, an accidental fission event or neutron introduction won't initiate a self-sustaining chain reaction. Nevertheless, there will still be some radioactivity and heat generated. In practical scenarios, sub-critical fissile masses usually don't get significantly warm due to fission alone. The heat primarily comes from the decay of radioactive isotopes within the material, which typically doesn't elevate the temperature to a noticeable degree.
Quick Summary?
Essentially, a half-critical mass doesn't significantly rise in temperature above the ambient level. The conditions required for a self-sustaining reaction are so finely tuned that a half-critical mass remains largely inert.
However, in controlled environments like reactors, sub-critical masses can be arranged to produce considerable heat through induced fission events, necessitating an external neutron source or other stimulation.
It's essential to recognize that "critical mass" is not a fixed value; it varies based on factors like geometry, density, and the presence of materials that either reflect or absorb neutrons. Therefore, determining a specific temperature for a "half-critical mass" is complex.
Risk of Criticality Excursions
Even if a mass is sub-critical, improper handling can result in a "criticality excursion," leading to a brief, self-limiting chain reaction. Such events pose serious dangers, potentially releasing substantial radiation and heat, although they generally do not sustain for extended periods. Historical cases have been documented and studied to understand the risks associated with handling fissile materials.
Historical Incidents
Several notable incidents have occurred involving what are often referred to as "criticality excursions." These events can lead to significant radiation exposure without causing a nuclear explosion, which is characteristic of a weapon.
The Demon Core (1945/1946)
This plutonium sphere was intended for a potential atomic bomb on Japan and later used for criticality experiments at Los Alamos. Two tragic accidents involved:
- Harry Daghlian (1945): While experimenting to determine the critical mass, Daghlian accidentally dropped a tungsten carbide brick onto the core, pushing it into a supercritical state. He quickly removed the brick but suffered a lethal dose of radiation, passing away 25 days later.
- Louis Slotin (1946): In a similar experiment, a slip of Slotin's screwdriver led to a criticality event when the reflector halves closed. He managed to separate them quickly but received a fatal radiation dose, dying nine days later.
Cecil Kelley Criticality Accident (1958)
At Los Alamos, Kelley was exposed to a criticality excursion while mixing plutonium with various chemicals in an unapproved vat, leading to critical conditions and his eventual death 35 hours later.
Tokaimura Criticality Accident (1999)
In Japan, workers inadvertently caused a critical condition by pouring excessive enriched uranium into a tank, resulting in the deaths of two individuals and exposure for many others.
Temperature Behavior During Incidents
During these incidents, temperatures would have spiked rapidly but were self-limiting due to the physics involved. As the material heats up, it expands, halting the reaction. While exact temperatures remain unknown, criticality excursions can produce enough heat to melt or vaporize materials, depending on specific circumstances.
These incidents have prompted extensive study and the establishment of strict safety protocols for handling fissile materials.
Initial Temperature Considerations
Initially, a half-critical mass would be at ambient temperature, with decay processes generating some heat. However, this is usually insufficient to make the material noticeably warm unless part of a larger assembly designed for fission, akin to a nuclear reactor.
A half-critical mass of fissile material, while not capable of sustaining a chain reaction, does exhibit some radioactivity based on its isotopic composition and purity. This includes spontaneous fission events and various decay processes. Some fissile isotopes have a slight probability of undergoing spontaneous fission, producing smaller nuclei, neutrons, and energy release. However, in a sub-critical state, most neutrons escape or are absorbed without causing further fission, resulting in low activity.
Real-world fissile material typically contains a mix of isotopes, and not all isotopes are fissile. For instance, natural uranium is predominantly U-238, which is not fissile but can be converted into fissile material like Pu-239 through neutron capture. Thus, the material's behavior depends on its mass and isotopic composition.
The Physics Behind Criticality
It's remarkable how humanity has mastered atomic energy to coexist with materials that can unleash massive destruction under certain conditions. A sub-critical or “half-critical” mass of fissile materials like uranium or plutonium exemplifies this balance. While harboring immense potential energy, these materials remain stable and relatively safe as long as they stay below their critical mass.
This sub-critical state prevents a self-sustaining chain reaction, allowing for the safe storage, transportation, and handling of these materials without immediate risk of nuclear chain reactions. This intricate balance between stability and potential chaos highlights the complexities of atomic physics and human ingenuity.
The term "criticality" in nuclear physics signifies the moment when conditions are just right for sustaining a nuclear chain reaction. When fissile materials like uranium-235 undergo fission, they break into smaller elements and release energy as heat. This process also emits additional neutrons—typically around 2 to 3 per fission event—which can trigger further fissions, setting the stage for a chain reaction.
In a sub-critical mass, the number of fissions and released neutrons isn't sufficient to sustain this reaction. While some neutrons may escape or be absorbed, leading to minimal radioactive decay and heat production, nothing explosive or runaway occurs. In a critical mass, the release of neutrons balances out to sustain the reaction at a steady rate, as seen in controlled environments like nuclear reactors.
Conversely, in a super-critical state, each fission event releases enough neutrons to instigate multiple subsequent fissions, causing exponential growth in the reaction and releasing immense energy rapidly—this is the principle behind nuclear weapons.
Key numbers to remember: In a sub-critical mass, fewer than one neutron will trigger another fission; in a critical mass, exactly one will; and in a supercritical mass, more than one will, leading to exponential increases in the reaction.