This restricted theoretical note extends a structural framework for spectral shielding into the domain of hazardous material containment. While shielding focuses on routing-dominant regimes that minimize irreversible interaction, many real-world systems operate in participation-dominant regimes where interaction cannot be eliminated. We introduce the principle of structural retention, framing containment as the managed continuation of participation rather than as a failure of shielding. Structural retention emphasizes distributed, bounded, and predictable engagement of energy and matter to ensure long-term stability under sustained exposure. The framework links spectral selectability to containment reasoning, enabling predictive assessment of where participation will occur and how it may be structurally accommodated without escalation or loss of control. No implementation details, material specifications, or containment architectures are disclosed. References to applied systems serve only to illustrate how the theoretical framework may inform assessment and design logic.