How is the SPR process effectively halted?

Halting the SPR (Storage Pressure Relief) process is effectively done by closing all three level control shutoff valves. These valves are critical components in maintaining the operational integrity of the system, and their closure prevents any further flow of hazardous materials. When these valves are shut, it eliminates the pathway for substances to pass through, thereby stopping the SPR process decisively.

The other options do not effectively stop the process. Initiating fuel flow contradicts the goal of halting the SPR process, as it would introduce more fuel into the system rather than stopping it. Increasing tank pressure can exacerbate the situation by potentially causing over-pressurization and does not contribute to stopping the process. Activating the fire suppression system, while important for safety in case of a fire, does not directly halt the SPR process itself. Thus, closing the level control shutoff valves is the most effective method for ensuring the safe and complete cessation of the SPR process.