Radial Peaking Factor ($P_{radial}$): Ratio of peak to average power density across the core radius.

To flatten: Vary enrichment radially (lower enrichment at periphery), use burnable poisons, or place control rods strategically.

Local Peaking Factor ($P_{local}$): Peak-to-average power density within a specific fuel assembly or near structural components.

Caused by water gaps, moderator gaps, presence of control rods, or structural materials.
Can be reduced by burnable poisons (e.g., Gadolinium) or specific fuel rod design.

High peaking factors necessitate conservative design limits (e.g., lower average LHR) to ensure the hottest spot does not exceed safety limits.

Batch Refueling (e.g., LWRs):
- A fraction (e.g., 1/3 or 1/4) of the core's fuel assemblies is replaced at scheduled intervals (typically 12-18 months).
- Requires reactor shutdown, depressurization, and cooling down.
- Fuel assemblies are shuffled to optimize burnup and power distribution (e.g., moving partially spent fuel to less reactive regions).
- Common patterns: Out-in (fresh fuel at periphery), In-out (fresh fuel at center), Low-leakage (fresh fuel away from periphery).
Online Refueling (e.g., PHWRs):
- Fuel bundles are replaced while the reactor is at full power.
- Advantages: Higher capacity factor, continuous optimization of power distribution, no costly shutdowns for refueling.
- Disadvantages: Increased complexity of refueling machinery, potential for fuel handling accidents.
- Chequerboard/Scatter Pattern Refueling: Bundles are moved from low-power zones to high-power zones to achieve uniform burnup and power distribution.
- Example: A channel might contain 8 bundles. Bundles are typically pushed in from one end, and spent bundles are discharged from the other.
- Fuel management strategy aims to reduce neutron leakage and improve overall neutron economy.

Redundancy:
- Multiple identical components or systems performing the same safety function.
- Ensures that if one component fails, others can still perform the function (e.g., multiple safety injection pumps).
- Provides fault tolerance.
Diversity:
- Use of different principles, technologies, or sensing mechanisms to perform the same safety function.
- Protects against common-mode failures (where a single event or design flaw could disable all redundant systems).
- Example: Using both mechanical and hydraulic scram systems, or different types of sensors for a critical parameter.
These principles are fundamental in designing robust reactor protection and safety systems.

L/D Ratio of Pellets: Length-to-diameter ratio of fuel pellets. Important for managing thermal expansion and preventing pellet bowing.
Pellet-Clad Chemical Interaction (PCCI): Chemical reactions between fuel and clad, often mediated by fission products, leading to clad degradation.
Fuel Cycle Length: Duration between refueling outages (for batch refueling) or the average time a fuel bundle stays in the core (for online refueling).
Capacity Factor: Ratio of actual power produced to maximum possible power over a period. Online refueling enhances capacit