 The amount formed depends greatly upon the nature of the fuel used and the conditions under which it was used.For instance, the use of MOX fuel (239Pu in a 238U matrix) is likely to lead to the production of more 241Am and heavier nuclides than a uranium/thorium based fuel (233U in a 232Th matrix).For highly enriched fuels used in marine reactors and research reactors, the isotope inventory will vary based on in-core fuel management and reactor operating conditions.