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Reactor designs like fission-fragment and some others use large masses of neutron reflectors to attain criticality. (For that matter, the very first pile did too, and in a staggered form rather similar to what I'm describing here.) For some of these applications, such as rocket engines, total mass is pretty important; for just about any application, the cost of the reflectors is at least a relevant consideration, even if it's not the most important one. And dusty plasma reactors use radiation exclusively to cool the fuel, leading to a rough upper limit on the size of any one core. So is it practical to put multiple reactors close together, sharing segments of their reflector walls? In the case of the dusty plasma FFRE, the basic reactor design is a longish cylinder, which seems amenable to being turned into a hexagonal cylinder like a beehive cell, which tessellate nicely.

If this is done, could the mass in each such shared wall also be reduced slightly to account for the fissile mass on each side contributing to the cross-section?

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This is basically what the pressure-tube design in the CANDU reactor is.

The K-infinity for a 2D CANDU bundle is (from DRAGON calculations we did in a course run by E. Nichita) about 1.12 - each one of them is critical on its own. All the bundles are surrounded by a common D2O moderator.

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If a neutron escapes from a reactor in the direction of another reactor, it can still induce fission in the other reactor, so it would not make sense to put a neutron reflector in regions between the reactors. In that sense, when considering multiple reactors put close together, the reflectors would need to be put all around them, as in the case of a single reactor. If we want to save mass for the same power, it is then probably more economic to have one single (bigger) reactor.

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  • $\begingroup$ Well, FFREs in particular are limited on the upper end by the size of the dusty plasma cloud that can efficiently cool by radiation (and to a lesser extent, the size of cloud that won't absorb too many fragments), so sticking them all together isn't the ultimate solution. $\endgroup$ Nov 5, 2015 at 18:07

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