Compact designs have a limited lifetime for the graphite moderator and fuel / breeding loop separator. Under the influence of fast neutrons, the graphite first shrinks, then expands indefinitely until it becomes very weak and can crack, creating mechanical problems and causing the graphite to absorb enough fission products to poison the reaction. The 1960 two-fluid design had an estimated graphite replacement period of four years. Eliminating graphite from sealed piping was a major incentive to switch to a single-fluid design. Replacing this large central part requires remotely operated equipment. MSR designs have to arrange for this replacement. In a molten salt reactor, virtually all of the fuel and fission products can be piped to a holding tank. Only a fraction of one percent of the fission products end up in the graphite, primarily due to fission products slamming into the graphite. This makes the graphite surface radioactive, and without recycling/removal of at least the surface layer, creates a fairly bulky waste stream. Removing the surface layer and recycling the remainder of the graphite would solve this issue. Several techniques exist to recycle or dispose of nuclear moderator graphite. Graphite is inert and immobile at low temperatures, so it can be readily stored or buried if required. At least one design used graphite balls (pebbles) floating in salt, which could be removed and inspected continuously without shutting down the reactor. Reducing power density increases graphite lifetime. By comparison, solid-fueled reactors typically replace 1/3 of the fuel elements, including all of the highly radioactive fission products therein, every 12 to 24 months. This is routinely done under a protecting and cooling column layer of water. Graphite-caused positive reactivity feedback – When graphite heats up, it increases U-233 fission, causing an undesirable positive feedback. The LFTR design must avoid certain combinations of graphite and salt and certain core geometries. If this problem is addressed by employing adequate graphite and thus a well-thermalized spectrum, it is difficult to reach break-even breeding. The alternative of using little or no graphite results in a faster neutron spectrum. This requires a large fissile inventory and radiation damage increases. Limited plutonium solubility – Fluorides of plutonium, americium and curium occur as trifluorides, which means they have three fluorine atoms attached (, , ). Such trifluorides have a limited solubility in the FLiBe carrier salt. This complicates startup, especially for a compact design that uses a smaller primary