Preliminary Design of a PuO2-UO2 Core for Demonstration in the Fermi Reactor

Work has been in progress since early 1966 on the preliminary design and development of a PuO2-UO2 core for demonstration in the Fermi fast reactor. The main aim is to demonstrate the performance and operating characteristics of the mixed oxide fuel in a fast reactor environment. The fuel design would be a close prototype of that projected for larger fast reactors. The present schedule provides for criticality in early 1970 and power operation by the end of that year. The preliminary design uses the nominal capability of the Fermi reactor: 48-in. high, 36-in. diameter core (800 liters core volume) and reactor power of 430 MW(th). The bulk sodium inlet and outlet temperatures will be 600 and 900°F respectively. Each 2.693-in.2 stainless-steel sub-assembly contains 60 fuel pins divided into 48-in. core and 19.5-in. gas plenum length. The reference fuel design provides for pelletized fuel although vipak elements are also under study. A smeared fuel density of about 84% and mechanical mixed powders will be used in all cases. A maximum of 139 such sub-assemblies can be used in the active core zone. Power flattening by varying fissile concentrations is not planned, to simplify fuel handling. It is expected, however, that the fuel management scheme foreseen (initial operation with a smaller core, edge addition of fuel to compensate for burn-up) will eventually lead to full reactor power. A considerable power flattening occurs in the single-zone core because of centre fissile depletion and edge fuel addition as burn-up proceeds. A total plutonium concentration of about 26% is calculated, and gives a critical mass of slightly over 500 kg. If plutonium is in short supply when core fabrication is initiated in mid-1968, an outer core zone where 235U replaces the plutonium may be used, without altering, however, the single-zone power distribution. Some physics features of larger cores are present including a small positive sodium coefficient in the central part of the core and reliance on the doppler power coefficient for the major feedback component. The importance of these coefficients is demonstrated in establishing the safety limits of reactivity insertions in the core. The effects of fuel slumping, sodium voiding, and sub-assembly meltdown are considered in the analysis. The rod runaway and flow failure accidents are treated in detail for various credible and incredible assumptions. In a hypothetical accident the energy release is determined by means of 2D-EX, a two-dimensional excursion code. (author)