Abstract: The present invention is a containment tank or a generator set including an outer tank and an inner flexible tank that offers a storage area for diesel fuel for a diesel engine powered generator set. So that the generator set diesel engine have ready source of clean diesel fuel the inner flexible tank provides a corrosion-free storage area that will not corrode or contaminate the diesel fuel. The outer tank is designed to provide a structure that not only protects the inner tank from damage, but acts as a support structure for the diesel engine generator set and other components (enclosure, switchgear, etc.) that are needed for the generator sets protection from wind, rain or snow or to act as control devices for diesel engine generator set. Also, since the design of the inner tank is flexible, it can be replaced easily through a manway formed on the outer tank without having to disconnect or disassemble the generator set.