40x is meant to refer to the use of fuel. That is a LWR or BWR typically has a burn up fraction of 1-5% or thereabouts. While a burn and bread design can use 40% or more of both the fuel and fertile elements. Of course this is also the problem. No fuel element/pellet has ever been rated with such a high burn up fraction, where volume and gas build up can compromise the element. The cost of validating fuel elements is one of the bigger costs in terms of time and hence money for a new design.
Otherwise the design seems like a good one. Uses depleted uranium as the main stock of fuel, gives the most of the benefits of reprocessing without the reprocessing.
