Mireia Piera

Fusion burning waves in proton-boron-11 plasmas

Abstract A method is proposed to exploit the aneutronic proton- 11 B fusion reaction by means of igniting a heat detonation wave that expands across the fuel from a small heated region. The ignition process is triggered by a particle beam (or a couple of beams) impinging on an inertially compressed target. We determine conditions for ignition and burn propagation. Although the requirements on the igniting beam current are very high, the method is a clear hint how to produce the cleanest energy from nuclear reactions.

Volume ignition targets for heavy-ion inertial fusion

Inertial confinement fusion (ICF) targets can be imploded by heavy-ion beams (HIBs) in order to obtain a highly compressed fuel microsphere. The hydrodynamic efficiency of the compression can be optimized by tuning the ablation process in order to produce the total evaporation of the pusher material by the end of the implosion. Such pusherless compressions produce very highly compressed targets for relatively short confinement times. However, these times are long enough for a fusion burst to take place, and burnup fractions of 30% and higher can be obtained if the volume ignition requirements are met. Numerical simulations demonstrate that targets of 1-mg DT driven by a few MJ can yield energy gains of over 70. Although direct drive is used in these simulations, the main conclusions about volume ignition are also applicable to indirect drive.

DEUTERIUM-TRITIUM FUSION REACTORS WITHOUT EXTERNAL TRITIUM BREEDING

Abstract An inherent property of deuterium fusion burn-up equations is presented, due to which deuterium-tritium reactions can be exploited without needing tritium breeding in external blankets. A small amount of tritium is added to the deuterium plasma in order to trigger ignition at less than 10 keV, and the same amount of tritium is found in the debris of the burnt-up plasma if the burning temperature is higher than 200 keV. Plasma parameters to exploit this property are very similar to those of inertial fusion confinement. Tritium inventory in a reactor would thus be reduced to a minimum value, because the initial composition of the fuel would be of the type DT x , with x ≈ 0.02, and tritium would immediately be reprocessed to fabricate new fuel.

Inertial fusion features in degenerate plasmas

Very high plasma densities can be obtained at the end of the implosion phase in inertial fusion targets, particularly in the so-called fast-ignition scheme ( Tabak et al. , 1994 ; Mulser & Bauer, 2004 ), where a central hot spark is not needed at all. By properly tailoring the fuel compression stage, degenerate states can be reached ( Azechi et al. , 1991 ; Nakai et al. , 1991 ; McCory, 1998 ). In that case, most of the relevant energy transfer mechanisms involving electrons are affected ( Honrubia & Tikhonchuk, 2004 ; Bibi & Matte, 2004 ; Bibi et al. , 2004 ). For instance, bremsstrahlung emission is highly suppressed ( Eliezer et al. , 2003 ). In fact, a low ignition-temperature regime appears at very high plasma densities, due to radiation leakage reduction ( Leon et al. , 2001 ). Stopping power and ion-electron coulomb collisions are also changed in this case, which are important mechanisms to trigger ignition by the incoming fast jet, and to launch the fusion wave from the igniting region into the colder, degenerate plasma. All these points are reviewed in this paper. Although degenerate states would not be easy to obtain by target implosion, they present a very interesting upper limit that deserves more attention in order to complete the understanding on the different domains for inertial confinement fusion.

Fusion-burning waves ignited by cumulation jets

AbstractA totally new target scheme to exploit fusion reactions is presented. It is based on the propagation of a heat-detonation wave across the fuel that reaches fusion temperatures before expanding. The wave is launched from a small region of the target where fusion ignition temperatures are reached by the crash of cumulation jets. These jets are produced by a couple of hollow-charge conical liners placed close to the target. The collapse of each conical liner creates a dispersive supersonic jet with a specific kinetic energy high enough to ignite the small region of the target where the fusion wave is created. The energy gain can be very high, although it depends on the maximum fusion yield allowable in the reactor chamber.

Heavy-ion-driven targets for small-scale inertial confinement fusion experiments

Two regimes of hydrodynamic evolution are found in the analysis of the performance of small-scale heavy-ion-driven targets. One leads to high density and high compression with moderate temperatures ([approximately]1 keV) for driving energies of 100 kJ for 0.1-mg deuterium-tritium targets. Ignition can then be triggered by a second ion pulse ([approximately]50 kJ). Breakeven could be obtained if a burnup fraction as small as 1% is obtained. The second regime leads to very high temperatures in the central part of the fuel, while the rest of the fuel remains at moderate temperatures (<1 keV), and the density is very low everywhere. Propagated ignition cannot occur in this case because of the small optical thickness of the compressed fuel (<0.1 g/cm[sup 2]). 36 refs., 8 figs., 1 tab.

A General Approach to Nuclear Fission Sustainability and the Need for Specific Solutions. A Case Study on a New Coolant

Extensive exploitation of nuclear raw materials requires the use of “nuclear breeding”, which is a phenomenon that can be attained in fast reactors. However, those reactors have had a complex history with some drawbacks and some important nuclear-policy attacks, as the INFCE initiative launched inside IAEA in 1978. Two points were very relevant in that context: the extensive use of plutonium recycling and an inherent property of fast reactors that could induce positive feedback between reactivity and thermal-hydraulics. In fact, a partial or total loss of coolant could convey a tremendous injection of reactivity, which could produce a catastrophe. An alternative to breeding in critical fast reactors is presented by hybrids, which are subcritical reactors which need an external neutron source for keeping their neutron population alive. Besides that, design and natural responses of the reactor systems against accidental initiating events have to be considered for arriving to the concept of Residual Safety beyond Design Limits. Such a final safety level will depend quite a lot on the type of coolant and the way the fuel is conformed into elements of a given geometry.

Conical targets and pinch confinement for inertial fusion

Conical microducts and minithrottles can be used to accelerate micropellets of fusionable fuel up to very high speeds (∼10 8 cm/s). The central collision of two pellets flying in opposite directions can produce a hot plasma where fusion reactions are triggered. The main drawback of this scheme is the short confinement time provided by the external guide tube (throttle). To obtain high yield, an extra force of confinement is advisable. In this paper, the performance of fuel implosions within conical targets and the effect of ultrashort magnetic fields and pinch forces are analyzed. Although very high currents are needed to stretch the confinement time, modern technologies based on pulse-power machines and fast discharges induced by ultrashort lasers can provide a solution to this problem.