What is the main reason why the η value must be increased in a light water moderated reactor?

In a light water moderated reactor, increasing the η (eta) value, which represents the neutron economy or the ratio of fission neutrons produced to those absorbed, is crucial for optimizing the reactor's efficiency. A higher η indicates that a larger fraction of neutrons produced through fission events is available to sustain further fission reactions, which is essential for maintaining a critical chain reaction.

Fissile material, such as uranium-235 or plutonium-239, plays a critical role in this process. By increasing the proportion of fissile material, the chances of those neutrons inducing further fission reactions within the reactor core are elevated. This effectively enhances the overall reactivity and fuels the energy production, leading to a more efficient reactor operation.

In contrast, reducing neutron leakage, improving coolant properties, or decreasing system heat capacity, while important aspects of reactor design and operation, do not directly align with the fundamental goal of increasing the η value in relation to optimizing fissile material usage. The focus on fissile material ensures that the reactor can maintain a sustainable and efficient fission process, making this the primary reason for the emphasis on increasing η in light water moderated reactors.