**3.3 High burnup**

One of the many advantages of fast reactor technology is the ability to "burn" to a greater extent than thermal reactor. The average burnup for a typical light water reactor is 45,000 MWD/MTHM. EBR-II demonstrated 20 atom % burnup which is the equivalent of 190,000 MWD/MTHM. These burnups are possible primarily because of the fast neutron spectrum present in the reactor. Along with the energy extracted from the fission of U-235, the fast spectrum transmutes the U-238 to higher order actinides. Those elements are subsequently fissioned, releasing energy rather than creating a problematic waste issue. The transmutation process does happen in thermal spectrum reactors, but to a far lesser extent. Given this, the extractable energy from fast reactors is fundamentally limited by the structural materials of the fuel and how long they can serve the engineering requirements under significant irradiation.
