Supercritical-pressure water cooled thermal reactor with uranium–plutonium–thorium fuel cycle and bidirectional coolant flow articles

Abstract

Computational results obtained for fuel assemblies and a core with uni-and bidirectional coolant flow are presented for a water-cooled reactor with a thermal neutron spectrum. It is shown that a bidirectional scheme for cooling fuel assemblies has advantages over a unidirectional scheme and holds promise for Gen IV water-cooled reactors with supercritical coolant pressure, which make it possible to perfect the technology for closing and drawing thorium into the fuel cycle.The Gen IV water-cooled reactors with supercritical-pressure coolant are considered to be promising and should come on line in approximately 2030. Reactors with a thermal neutron spectrum are being developed abroad as well as in our country as a solution to the near-term problem of replacing light-water reactors and will be followed later on by reactors with a closely spaced fuel-element lattice and a fast neutron spectrum [1,2]. Unidirectional cooling schemes where the coolant is heated as it moves in the core from bottom to top have been adopted for such designs. Since the heating amounts to 230°C in thermal and 250°C in fast reactors, even small variations of the energy-release distribution of the fuel elements result in large differences of the coolant temperature at the exit and the cladding temperature of the fuel elements. In a reactor with a thermal neutron spectrum, the cold moderator is confined inside tubes -"water elements" between which the fuel elements, cooled by the ascending coolant, are arranged in a closely spaced lattice. A difficulty is smoothing the energy-release field, since the moderator is substantially heated and its density varies along the height of the core.For reactors with a fast neutron spectrum, it is suggested that a bidirectional cooling scheme be used to smooth the energy release over the core height [3,4].In the present article, we present the results of a physical calculation, performed using different program systems, and a comparative analysis of a reactor with a thermal neutron spectrum with uni-and bidirectional cooling schemes. Uranium-plutonium-thorium fuel loads are considered.Fuel Assemblies with Cylindrical Water Elements (unidirectional cooling scheme). It has been suggested that a cylindrical channel with two coaxially arranged shells separated by a gap filled with water ( Fig. 1) be used to decrease the heating of the moderator over the core height [5]. Zirconium can be used for the inner tube of the water elements and corrosion-resistance steel for the outer tube. A variant where the outer tube consists of zirconium up to 180 cm from the bottom and steel above this level is also under consideration.The working model consists of a full-scale description of the reactor in the radial and vertical directions taking the end and radial reflectors into account. The VVER-1500 core was used as a prototype for the core, the main difference being the design of the fuel assemblies and the power. The basic characteristics of the core and fuel-assembly variants are as follows: