What is defined as the space between the primary tank and secondary tank?

The term that defines the space between the primary tank and the secondary tank is "interstice." This area is crucial for leak detection and prevention in double-walled tank systems used for storing hazardous substances. The interstitial space allows for monitoring any potential leaks from the primary tank; if there is a breach, any leaked material will collect in the interstitial area, thereby preventing it from contaminating the surrounding environment.

Understandably, the other terms referred to in the question relate to different concepts in tank management and environmental safety. For instance, the containment zone typically refers to an area specifically designed to contain spills and prevent environmental contamination, while the secondary layer might describe the outer protective layer itself but does not specifically address the space between the two tanks. A vapor gap suggests an empty space that might relate to gas containment but does not accurately describe the interstitial space pertinent to secondary containment systems.

Thus, "interstice" is the correct terminology that specifically addresses the space used for monitoring and potentially capturing any leaks from the primary tank, ensuring the integrity of hazardous material storage systems.