Explain the fundamental differences in neutron behavior between a thermal and a fast reactor and how these differences impact micro reactor design.

The fundamental differences in neutron behavior between thermal and fast reactors stem from the energy levels at which the neutrons primarily interact with the fissile material, and this distinction profoundly impacts reactor design, including that of micro reactors.

In a thermal reactor, neutrons are slowed down through interactions with a moderator material (like water, graphite, or heavy water) to energies of about 0.025 electron volts (eV). These "thermal" neutrons are much more likely to cause fission in fissile materials like Uranium-235. The reason is that the probability, or cross-section, for fission is significantly higher at these lower energies for these fissile isotopes. The design implication here is that a thermal reactor needs a moderator to slow down the neutrons and make them more effective at causing fission. This design typically includes a substantial volume of moderator material, leading to a relatively larger core size. Control mechanisms often focus on regulating neutron absorption, and the fuel cycle is generally geared towards managing relatively lower energy neutron interactions. Thermal micro reactors often utilize water as both a coolant and a moderator, simplifying the design and operation. They are also more sensitive to changes in temperature due to the impact of temperature on the behavior of thermal neutrons in the moderator.

On the other hand, a fast reactor operates using neutrons at high energy levels, typically in the range of 100 keV to several MeV. These "fast" neutrons are not deliberately slowed down by a moderator. Fission with fast neutrons is typically less probable than with thermal neutrons for common fissile materials. Fast reactors, therefore, necessitate using fuel with a much higher enrichment of fissile material, and a higher neutron flux is required to maintain a sustained chain reaction. In contrast to thermal reactors, fast reactors do not need large volumes of moderator, leading to smaller, more compact core designs. They also use coolants like liquid sodium or lead, which do not significantly slow down neutrons. The design challenge lies in ensuring efficient heat removal and handling the higher neutron fluxes. A key advantage of fast reactors is that they can utilize fertile materials like Uranium-238 to breed more fissile material, enabling more efficient fuel cycles and reducing waste. Fast micro reactors, though less common, can achieve very high power densities and are suitable for certain specialized applications. However, they present additional complexities in terms of coolant handling and core materials selection, and require more robust neutron shielding as well. Control mechanisms for fast reactors must be effective across a broad energy spectrum, and the core design must also consider the effects of fast neutron capture in structural materials.

In micro reactor design, the choice between thermal and fast neutron spectra has significant impacts on various aspects, including reactor size, fuel enrichment, coolant selection, safety mechanisms, and economic viability. Thermal micro reactors might be easier to design due to their use of readily available materials, while fast reactors, albeit more challenging to design, offer a more efficient use of fuel and can operate at higher power densities. For example, a thermal micro reactor may have a core filled with slightly enriched uranium fuel rods immersed in water which serves as both coolant and moderator. Its control system might use boron control rods that absorb thermal neutrons to manage the chain reaction. By contrast, a fast micro reactor could have a small, compact core with highly enriched uranium and plutonium fuel cooled by liquid sodium. Its control mechanisms might rely more on geometry and reflector properties that effect the higher energy neutrons in the core. The design considerations ultimately depend on the specific application, the size and power requirements of the micro reactor, and the need for fuel cycle efficiency and safety concerns.