The P-Area is a historic tailings retention facility retained by a series of upstream constructed dams, with starter dams constructed in the early 1960s. In 1964, a 1.52 m wide, 1.83 m tall, 670 m long rock tunnel was constructed from the downstream toe of one of the perimeter dams to the central area of the planned tailings pond. In 1982, the decant tower experienced a structural fracture attributed to ice loading, resulting in an uncontrolled release of tailings, which prompted the immediate and emergency decommissioning of the tunnel. The initial decommissioning strategy was necessarily reactive due to the urgency of the event. A plug was installed in the damaged tower, and a secondary plug was placed near the tunnel portal. Over time, tailings deposition prevented direct inspection of the tower plug, and a subsequent risk evaluation identified that the secondary portal plug alone would not be adequate should the tower plug degrade or the tunnel become reconnected to pond water. To enhance long-term safety and reliability, a new design program was initiated, incorporating lifecycle-based engineering principles, closure requirements, and provisions for future inspection and monitoring. The work included the development of engineered remedial and risk reduction measures to ensure long term stability. This paper presents the design methodology, and technical considerations involved in re evaluating and upgrading the decommissioned infrastructure.