[Physics] Can antimatter be used as fuel for nuclear reactors

Question

I completely understand the difficulties of making and storing antimatter, so I am not talking about the mechanism or the way of doing it here, I am just talking about the concept.

As far as I know, nuclear power plants use the heat from the nuclear fission reaction to heat water and use the steam through turbines and generators to generate electricity. So, if we could somehow use the annihilation of matter-antimatter inside a reactor, would it still be a viable way of generating heat and thus electricity ? or is there something special for nuclear fission that is not available for matter-antimatter annihilation ?

Answer 1

Short answer: Yes, it can. Although for near-future application the utility of antimatter would be not as a fuel per se but as a catalyst of nuclear reaction.

The energy density of proton antiproton annihilation is $1.8\times 10^{14}\text{J}/\text{g}$ of antiproton is hundreds times that of fusion or fission reactions.

One field where antimatter could be of use is space where enormous cost of its production is offset by the small mass of the product and relative small size of devices utilizing this energy. Therefore most of concepts for utilizing such energy (at least in the context of near future technology) is for propulsion purposes.

The reaction $\bar{p} + p$ produces mainly $\pi^{+}$, $\pi^{-}$, $\pi^{0}$ mesons, so about 2/3 of reaction energy is available as charged light energetic particles which could rapidly heat up matter or/and initiate other nuclear reactions (both fission and fusion). This would allow to derive most of the energy from such reaction thus reducing antimatter requirements and at the same time maintaining small size (usually much smaller than full scale conventional fusion or fission reactors). Some of the concepts mentioned in this review:

Antimatter-Catalyzed Micro-Fission/Fusion (ACMF):

Here, a pellet of D-T and U-238 is compressed with particle beams and irradiated with a low intensity beam of antiprotons. The antiprotons are readily absorbed by the U-238 and initiate a hyper-neutronic fission process that rapidly heats and ignites the D-T core. The heated fission and fusion products expand to produce thrust ... Gaidos et al. 7 have shown that the interaction between the antiproton beam and target exhibits extremely high-gain yielding a ratio of fusion energy to antimatter rest mass energy $\beta$ of $ 1.6 \times 10^7$ ... Assuming a 3-order of magnitude improvement in the efficiency of producing antiprotons over current values, the net energy gain is 640.

Antimatter-Initiated Microfusion (AIM)

Here, an antiproton plasma within a special Penning trap is repetitively compressed via combined electric and magnetic fields. Droplets containing D-T or D-He3 mixed with a small concentration of a metal, such as Pb-208 or U-238, are synchronously injected into the plasma. The main mechanism for heating the liquid droplet is antimatter-induced fission fragments which have a range of 45 microns ($\mu$m) in the droplet. The power density released by the fission fragments into the D-T or D-He3 is about $5 \times 10^{13}$ W/cm$^3$, which is enough to completely ionize and heat the fuel atoms to fusion ignition. The heated products are directed out magnetic field lines to produce thrust. The $I_{sp}$ and energy efficiency for this concept are higher than ACMF ($I_{sp} \sim 67,000$ sec and $\eta_e \sim 84\% $ with D-He3, and $I_{sp} \sim 61,000$ sec and $\eta_e \sim 69\% $ with D-T). The gains $\beta$ are $10^5$ for D-He3 and $2.2 \times 10^4$ for D-T. Again assuming a 3-order of magnitude improvement in antiproton production efficiency, these gains are near breakeven in terms of net energy flow.

The requirements of antimatter is thus dramatically reduced and, for instance, ACMF propulsion for manned flight to Jupiter (100 tonnes payload) would require only 10$\mu$g of antiprotons (see here (pdf))