The defense in depth concept that is used in constructing nuclear plants is characterized by what?

The defense in depth concept in nuclear plant construction is fundamentally about implementing multiple layers of safety measures to prevent accidents and mitigate their consequences. The choice of repeated layers of thick shielding materials is central to this concept, as it involves both physical barriers and safety protocols designed to protect the environment and the public from radiation exposure.

In nuclear facilities, shielding materials are crucial for absorbing and attenuating radiation emitted from the reactor core and other radioactive sources. By employing layers of thick shielding, the design reduces the likelihood of radiation leaks, thus enhancing the overall safety of the facility. This layered approach means that even if one barrier fails, additional layers still provide protection, aligning perfectly with the principles of defense in depth, which emphasize redundancy and multiple safeguards.

The other options don't encapsulate the essence of the defense in depth philosophy: assuming accidents won’t happen neglects the emphasis on preparedness and mitigation; preventing explosions is part of safety measures but not the primary focus on radiation protection; and while keeping fissionable materials separate is crucial for preventing criticality accidents, it does not reflect the comprehensive aspect of defense in depth that integrates various safety features. Thus, the choice of repeated layers of thick shielding materials accurately represents the core concept of defense in depth in nuclear plant safety.