Conceptual Reactor Physics Design of a Lead-Bismuth-Cooled Critical Actinide Burner

Abstract

Destruction of actinides in accelerator-driven subcriticals and in stand-alone critical reactors is of contemporary interest as a means to reduce long-term high-level waste radiotoxicity. This topical report is focused on the neutronic design challenges of a pure critical actinide transmuter. The key objectives of the design were set to be (1) the attainment of a high actinide burning rate comparable to that of the ATW and (2) the attainment of plausible reactor physics characteristics so that the reactor safety performance is at least comparable to that of traditional fast breeder reactors. The proposed conceptual design is a Pb-Bi cooled 1800MWth-core with innovative streaming fuel assemblies that exhibits excellent reactivity performance upon coolant voiding, even for local voids in the core center. The core employs metallic, fertile-free fuel where the transuranics are dispersed in a zirconium matrix. The large reactivity excess at BOL is compensated by a system of double-entry control rods. The arrangement of top-entry and bottom-entry control rods in a staggered pattern allows the achievement of a very uniform axial power profile and a small reactivity change from CRD driveline expansion. Excellent void reactivity performance of the proposed design was demonstrated, together with other desirable features such as a very uniform power profile and tight neutronic coupling. A relatively long refueling interval of one and a half years is achieved using a two-batch refueling scheme. In terms of the TRU destruction rate per MWt per full power year the design is comparable to the accelerator-driven systems and other studied pure burner concepts based on sodium-cooled fast reactors. The effective delayed neutron fraction was found to be about 25% less than that of typical oxide-fueled fast reactors, making the requirements on reactor control performance more demanding. The Doppler coefficient was found to be negative with a magnitude appreciably lower than the typical values of oxide fuels in sodium-cooled reactors, but comparable to the values observed in IFR cores with metallic U-Pu-Zr fuels. The fuel thermal expansion coefficient is also negative, having a magnitude approximately equal to the Doppler coefficient. The proposed core can also incinerate long-lived fission products with an efficiency of about 2.6% of the initial Tc99 inventory per FPY – about the same as critical sodium-cooled pure burners under investigation elsewhere, but less than Tc99 incineration efficiency claimed for accelerator driven systems, like ATW. The strategy of mixing Tc99 uniformly in the fuel within the core at the expense of zirconium matrix was found to yield slightly better Tc transmutation efficiency than the use of designated fuel assemblies with zirconium hydride rods at the core periphery. Thermalized fuel assemblies are penalized by low neutron flux because of self shielding; in addition they increase capture to fission ratio in TRU nuclides in the adjacent fuel assemblies, worsening the TRU burning capability. The incineration of Tc99 in fast spectrum in the rods placed on the core periphery appears to be a more promising alternative than transmutation in thermalized fuel assemblies.