What is a primary risk associated with on-line refueling in reactors?

The primary risk associated with on-line refueling in reactors revolves around the increased risk of accidents due to changes in core configuration. During on-line refueling, elements of the reactor core are replaced or rearranged without taking the reactor offline. This process can lead to dynamic changes in the reactor’s core configuration and neutron flux distribution, potentially introducing instability.

As fuel assemblies are removed and new ones are inserted, the core's balance and reactivity are altered, which may lead to situations where control is harder to maintain. If these changes are not managed correctly, there could be an inadvertent reactivity excursion, which poses a significant risk to safe reactor operation. Additionally, the presence of moving parts and the handling of radioactive materials in an active environment heightens the hazard of accidents compared to traditional planned refueling outages where the reactor is shut down and systems are in a stable condition.

This understanding underscores the importance of having stringent safety protocols and monitoring systems during on-line refueling operations to mitigate risks associated with these dynamic changes.