I understand that this pan is a container that is part of the fuel tank. I searched for translations in IATE and I only got the French one: "bac flottant". In French a "bac" can be either a tray (es: bandeja) or a box, container. Then I checked the term in TermCat (a Catalan terminology centre) in the Industry and Energy section and, although this exact term doesn't appear, other terms with "pan" in them do appear, and have translations in Spanish as "barca", "caldera" or "cámara", of which I would choose "cámara". But you may choose another one depending on the context. Finally, every term containing "floating" has its direct translation in Spanish as "flotante".