Fusion's Hidden Bonus: Batteries, Calm Plasma, and Neutron Practice

A Fusion Reactor Could Also Be a Nuclear Battery Factory

What if the same machine that heats your city could also fill the world's entire supply of a critical medical isotope — as a side effect?

A fusion power plant, if we ever build one, works by smashing hydrogen together. That reaction releases a flood of high-energy neutrons — fast particles that carry a lot of energy and have to go somewhere. Right now, the standard plan is to let them hit a lithium blanket, breed tritium (the hydrogen fuel you need to keep the reaction going), and call it a day. A team using OpenMC neutron-transport simulations — think of it as a detailed recipe calculator for what happens when you bombard different materials with neutrons — asked a different question: what else could you grow in that blanket at the same time? Their answer is striking. Swap in neodymium-148 as the blanket material and those fusion neutrons transform it into promethium-147 — a beta emitter used in nuclear batteries that power deep-sea sensors, spacecraft, and some pacemakers. Their calculations suggest one gigawatt of fusion heat could produce over a tonne of promethium-147 per year, roughly one billion curies. The entire current US inventory of a similar isotope, nickel-63, sits at the curie level. This is many orders of magnitude more. Think of it like a bakery that uses its waste heat to dry herbs in the same oven: the primary product stays the same, but you're not wasting the energy you're already paying for. The catch is significant: this is all simulation, using a simplified flat-slab geometry, not a real reactor blanket. No physical experiment was run. The numbers are promising enough to take seriously, but the path from 'neutron conversion fraction looks good on paper' to 'we are manufacturing isotopes at scale' involves a very long list of engineering problems nobody has solved yet.

Why Plasma Suddenly Stops Being Turbulent — And How We Now Know Why

Turn up the pressure in a garden hose past a certain point and the chaotic spray suddenly snaps into a clean, smooth jet — fusion plasma does something eerily similar, and this paper explains the mechanism.

One of the most important things that happens inside a tokamak — the doughnut-shaped magnetic bottle used in most fusion experiments — is called the L-H transition. L stands for low-confinement, H stands for high-confinement. Plasma starts in L-mode: turbulent, leaky, hard to hold. Then, if you heat it past a threshold, it flips into H-mode: calmer, tighter, far better at keeping its energy in. ITER, the big international machine being built in France, is designed to run in H-mode. The transition is essential. We've known it happens for decades. We haven't fully understood why. This paper, combining analytical calculations with simulations using the VMEC equilibrium code and flux-tube gyrokinetic tools, proposes a clear mechanism. When plasma pressure rises high enough, two things happen to the magnetic field geometry. First, the bootstrap current — a self-generated electrical current that plasma drives on its own — bends the field lines in a way that weakens the magnetic shear (imagine a stack of twisted ribbon; the twist gets gentler). Second, the pressure itself distorts the shape of the magnetic surfaces. Together, these changes push the plasma into what physicists call a second stability region: a zone where the turbulent waves that normally shred confinement simply can't grow. The result is a large, measurable drop in turbulent heat and particle losses when you move from L-mode to H-mode pressure profiles. The catch: the simulations use a simplified circular tokamak geometry, and the full turbulent transition involves non-linear dynamics not captured here. This is a piece of the puzzle, not the whole picture.

Sweden Just Opened a Neutron Shooting Range for Fusion Materials

Before you can trust a material to survive inside a fusion reactor, you need a place to hammer it with the exact type of neutrons a reactor produces — Uppsala University just turned one on.

Here is a problem nobody talks about enough in fusion: the materials. The inside wall of a fusion reactor will be hit, continuously, by 14-megaelectronvolt neutrons — the most energetic neutrons produced in a D-T fusion reaction. That kind of bombardment does strange things to metals: it displaces atoms, creates voids, makes materials swell, crack, and eventually fail. If we don't know how candidate materials hold up, we can't design a reliable reactor. Testing requires a neutron source that matches the real thing. That is what NESSA is — a compact facility at Uppsala University in Sweden, built around a Sodern GENIE 16 sealed-tube generator that produces 14 MeV neutrons by running the same deuterium-tritium reaction as a fusion power plant, just at table-top scale. Think of it as the fusion equivalent of a car crash-test facility: you don't need to build a full highway to study what happens in a collision. The commissioning paper reports that NESSA hit a neutron yield of 4.33 × 10⁸ neutrons per second — within measurement uncertainty of the factory specification of 4.4 × 10⁸. Four independent nuclear simulation codes (FLUKA, Geant4, MCNP, PHITS) all agreed with each other and with the physical measurements to within a few percent. The facility is calibrated, validated, and open to users. The catch: at roughly half a billion neutrons per second, NESSA is thousands of times less intense than a real reactor will be. It is useful for nuclear cross-section measurements and early materials screening — but not for long-term bulk irradiation damage studies. Those will need much larger facilities, and none exist yet at full fusion relevance.

Read these three together and you see fusion research doing something it rarely gets credit for: thinking about what the machine is for beyond electricity. The nuclear battery paper asks whether a fusion plant could be an isotope factory on the side — a genuinely new idea for the economics of fusion. The L-H transition paper chips away at one of the oldest unsolved questions in confinement physics: why does plasma suddenly cooperate? And NESSA is the unglamorous, necessary work of building the test infrastructure that doesn't exist yet. None of these is the big result. All of them are the kind of unglamorous, parallel progress that actually moves a technology forward. The honest picture of fusion in 2026 is not one dramatic breakthrough — it's dozens of groups, in Sweden and in simulation, filling in gaps that the headline machines haven't gotten to yet.

The L-H transition paper is part of a growing body of work using gyrokinetic codes to pin down confinement physics ahead of ITER's first plasma campaign, now expected in the late 2020s — watch for more simulations validating against JET and ASDEX Upgrade data as ITER approaches. On the materials side, NESSA is now open; the first external user experiments will be the real test of whether compact neutron sources can meaningfully accelerate the materials qualification timeline.

• Wikipedia: L-H transition in fusion plasmas
• Wikipedia: Nuclear battery
• ITER official site: why materials matter