CERMET ALLOYS FOR HYBRID FISSION-FUSION NUCLEAR REACTOR

CERMET ALLOYS FOR HYBRID FISSION-FUSION NUCLEAR REACTOR Juana L. Gervasoni1, 2, Jorge García Gallardo1 and Mauricio Petaccia1 1Bariloche Atomic Center, CNEA (National Atomic Energy Commission), Bariloche 8400, Argentina 1,2 CONICET (National Council of Scientific and Technical Research), Argentina Among the innovative reactors for nuclear energy application, it is observed a worldwide interest in the concept fission-fusion hybrid reactors (FFHR), so there is a diversity of designs thereof. In essence, an FFHR consists of three parts [1]: a fusion device that acts as a source of neutrons, a system where the fissionable fuel is placed, and a tritium generating blanket that feeds the required fuel to the fusion reactor. Usually, all the system is also fitted with a neutron reflector. These parts are arranged concentrically forming a three-layer system to optimize the use of neutrons. Achieving a reasonable neutron yield, in order to drive an FFHR, is difficult with current fusion devices, which is why the use of so-called multiplier cascades has been proposed. These cascades consist of concentric shells where the fissile material is placed, separated by a very large empty space. The dimensions, shape, and fuel of the shells and the size of the empty space between them, determine the multiplying capacity of the system. Figure 1 shows a model of FFHR following these principles, where two shells of 8% enriched Uranium are placed as fuel (this fuel has a metallic behavior), a Lithium silicate is used as TBB, and a Tungsten layer plays the role of reflector and shielding. Figure 1: Scheme of a FFHR reactor As we can see, each zone and interface described have different nuclear properties causing changes in the neutron flux spectra as it moves outwards. The energy spectra of transmitted neutrons in Tungsten for the incident 14 MeV present reactions lead to a transmutation cycle which gives a net conversion of Tungsten into Osmium and Rhenium and traces of other metals, even Platinum and Gold. All these metals do not represent a problem for the structural properties of the material as was previously seen under fission spectra, as we show in ref. [2] To improve the yield and retention of fission products, there is another model of hybrid reactor, based in the concept of concentric shells, forming with an specific alloy of W, the so-called Cermet, (Ceramic+Metal) allowing to reach near to 3000° K of temperature. CerMets were developed to build reactors for air and space propulsion, as is described in [3] With this conceptual reactor, composed of a nuclear fusion device that generates 14 MeV neutrons to propel a subcritical assembly, and using CerMet W/[U(50%)O2] in the two shells, we obtain its neutron spectra, shown in Figure 2. We reproduce a fast neutron spectrum equal to the fast spectra of fission reactors. Notice that for fast reactors, we cannot put water to cool them.