[en] This report describes the mechanical and thermal-hydraulic design of a thoria shim rod system, a key element in achieving neutron economic control in the seed-blanket light water breeder reactor. These thoria shim rods, in conjunction with a thoria/urania seed and thoria blanket core pattern which levels core lifetime reactivity variations, almost entirely eliminate the need for parasitic neutron absorption for reactivity control during full power operation. The individual finger-type shim rods are similar in size and shape to commercial reactor control rods, and are similarly mounted in a cluster and driven by the same basic type of control rod drive mechanism (except that no fast insertion or scram function is required). Compared to conventional control rods, the shim rods have substantially higher heat production and the zircaloy shim rod cladding may be subject to greater wear from rod motion. The shim rods are sized to maximize thoria content consistent with adequate structural strength and flow for cooling, and are suported and configured to minimize flow-induced vibration, excessive wear and thermal hot spots. Performance predictions indicate that the shim rods and associated components will have performance and safety margins comparable to a commercial pressurized water reactor and, in addition, in the event of failure of shim rod components, the reactor could safely continue to operate