Define the term "thermal stress" in the context of boiler operation.

Thermal stress specifically refers to the stress that occurs in materials due to differences in temperature within the material. In the context of boiler operation, as the boiler heats up or cools down, varying temperatures can create temperature gradients. These gradients lead to differential expansion or contraction within the material of the boiler, which in turn can generate internal stresses.

For instance, the exterior of a boiler might heat up more quickly than the interior, causing the outer layers to expand. If the inner layers do not expand at the same rate, this can create tension or stress points. If not properly managed, these thermal stresses can lead to material fatigue, cracking, or even catastrophic failure of the boiler. Understanding thermal stress is crucial for maintaining the integrity and safety of boiler systems, especially during start-up or shut-down operations, where significant temperature changes occur.