The UREX process separates uranium from transuranic wastes (TRU) and fission products (FP). Nuclear reactors require fissile isotopes that will absorb neutrons and break apart into smaller nuclei while releasing a large amount of energy as well as multiple neutrons. Fissile isotopes in spent fuel include not only 235U, but also 239Pu, 241Pu, and several isotopes of americium (Am) and curium (Cm).
TRU contains the actinides with atomic numbers greater than that of uranium. This includes Pu, Np, Am, and Cm. When TRU is separated from uranium, the TRU still poses a significant risk of sustaining a chain reaction. This is quantified through the effective neutron multiplication factor, keff.
To prevent TRU from becoming critical (sustaining a chain reaction), keff must be maintained at a value of less than 1. The presence of neutron poisons (Sm, Xe, B, Hf, Cd, etc.) will decrease keff. Neutron poisons are found in fission products. The presence of neutron moderators (H, C, Be) or materials that reflect neutrons will enhance keff.
To assess keff, Monte Carlo simulation codes are used. The concentration of TRU, process salts, and fission products along with the geometry of the mixture and surrounding reflective material are inputs to these codes
