Why Moderator are necessary in Nuclear Reactor.
Importance of Moderators:
1. Neutron Velocity Reduction: Fast neutrons released during fission reactions have higher velocities and are less likely to cause further fission. Moderators, usually composed of materials like water, graphite, or heavy water, slow down these neutrons, increasing their chances of interacting with fissile nuclei.
2. Increased Cross-Sectional Area: Slower neutrons have a larger cross-sectional area, making them more likely to be captured by fissile nuclei. This increases the likelihood of sustaining a chain reaction.
3. Thermal Neutron Utilization: Many fissile isotopes, like uranium-235, exhibit higher fission probabilities when bombarded by thermal neutrons. Moderators help in achieving a thermal neutron spectrum, optimizing the fission process.
4. Control of Reactor Power: Moderators contribute to controlling the reactor's power output. By adjusting the concentration or placement of moderators, reactor operators can regulate the rate of fission reactions and maintain a stable and safe operating condition.
Types of Moderators:
1. Light Water (H2O): Ordinary water serves as a moderator in pressurized water reactors (PWRs). It is effective in slowing down neutrons and transferring heat.
2. Heavy Water (D2O): Heavy water is used as a moderator in some reactors. It not only slows down neutrons efficiently but also absorbs fewer neutrons compared to light water, allowing for a more efficient use of fissile material.
3. Graphite: Graphite, a form of carbon, is employed as a moderator in some reactors. It effectively slows down neutrons and has the advantage of low neutron absorption.
In summary, moderators are indispensable in nuclear reactors as they facilitate the sustained release of energy by slowing down fast neutrons, increasing their chances of causing additional fission reactions. The choice of moderator material depends on reactor design and the desired characteristics of the nuclear chain reaction.
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