Gli scienziati infrangono il record per la quantità di energia prodotta durante una reazione di fusione controllata e sostenuta

I reattori a fusione magnetica contengono plasma super caldo in un contenitore a forma di ciambella chiamato tokamak.

La fusione nucleare ha raggiunto una pietra miliare grazie a migliori pareti del reattore: questo progresso ingegneristico si sta sviluppando verso i reattori del futuro.

Gli scienziati di un laboratorio in Inghilterra hanno infranto il record per la quantità di energia prodotta durante una reazione di fusione controllata e sostenuta. La produzione di 59 megajoule di energia in cinque secondi durante l’esperimento Joint European Torus – o JET – in Inghilterra è stata definita “una svolta” da alcune testate giornalistiche e ha suscitato parecchia eccitazione tra i fisici. Ma una linea comune per quanto riguarda la produzione di elettricità da fusione è che è “sempre a 20 anni di distanza”.

Siamo un fisico nucleare e un ingegnere nucleare che studiamo come sviluppare la fusione nucleare controllata allo scopo di generare elettricità.

Il risultato JET dimostra notevoli progressi nella comprensione della fisica della fusione. Ma altrettanto importante, mostra che i nuovi materiali utilizzati per costruire le pareti interne del reattore a fusione hanno funzionato come previsto. Il fatto che la nuova costruzione del muro abbia funzionato così come ha fatto è ciò che separa questi risultati dalle pietre miliari precedenti ed eleva la fusione magnetica da un sogno a una realtà.

Diagramma di fusione deuterio-trizio

I reattori a fusione rompono insieme due forme di idrogeno (in alto) in modo che si fondano, producendo elio e un elettrone ad alta energia (in basso).

fondendo insieme le particelle

La fusione nucleare è la fusione di due nuclei atomici in un nucleo composto. Questo nucleo si rompe quindi e rilascia energia sotto forma di nuovi atomi e particelle che si allontanano dalla reazione. Una centrale a fusione catturerebbe le particelle in fuga e utilizzerà la loro energia per generare elettricità.

Esistono diversi modi per controllare in sicurezza la fusione sulla Terra. La nostra ricerca si concentra sull’approccio adottato da JET: utilizzare potenti campi magnetici per confinare gli atomi fino a quando non vengono riscaldati a una temperatura sufficientemente alta da poterli fondere.

Il combustibile per i reattori attuali e futuri sono due diversi isotopi dell’idrogeno – il che significa che hanno un protone, ma un numero diverso di neutroni – chiamati deuterio e trizio. L’idrogeno normale ha un protone e nessun neutrone nel suo nucleo. Il deuterio ha un protone e un neutrone mentre il trizio ha un protone e due neutroni.

Affinché una reazione di fusione abbia successo, gli atomi di combustibile devono prima diventare così caldi che gli elettroni si liberano dai nuclei. Questo crea[{” attribute=””>plasma – a collection of positive ions and electrons. You then need to keep heating that plasma until it reaches a temperature over 200 million degrees Fahrenheit (100 million Celsius). This plasma must then be kept in a confined space at high densities for a long enough period of time for the fuel atoms to collide into each other and fuse together.

To control fusion on Earth, researchers developed donut-shaped devices – called tokamaks – which use magnetic fields to contain the plasma. Magnetic field lines wrapping around the inside of the donut act like train tracks that the ions and electrons follow. By injecting energy into the plasma and heating it up, it is possible to accelerate the fuel particles to such high speeds that when they collide, instead of bouncing off each other, the fuel nuclei fuse together. When this happens, they release energy, primarily in the form of fast-moving neutrons.

During the fusion process, fuel particles gradually drift away from the hot, dense core and eventually collide with the inner wall of the fusion vessel. To prevent the walls from degrading due to these collisions – which in turn also contaminates the fusion fuel – reactors are built so that they channel the wayward particles toward a heavily armored chamber called the divertor. This pumps out the diverted particles and removes any excess heat to protect the tokamak.

JET Magnetic Fusion Experiment

The JET magnetic fusion experiment is the largest tokamak in the world. Credit: EFDA JET

The walls are important

A major limitation of past reactors has been the fact that divertors can’t survive the constant particle bombardment for more than a few seconds. To make fusion power work commercially, engineers need to build a tokamak vessel that will survive for years of use under the conditions necessary for fusion.

The divertor wall is the first consideration. Though the fuel particles are much cooler when they reach the divertor, they still have enough energy to knock atoms loose from the wall material of the divertor when they collide with it. Previously, JET’s divertor had a wall made of graphite, but graphite absorbs and traps too much of the fuel for practical use.

Around 2011, engineers at JET upgraded the divertor and inner vessel walls to tungsten. Tungsten was chosen in part because it has the highest melting point of any metal – an extremely important trait when the divertor is likely to experience heat loads nearly 10 times higher than the nose cone of a space shuttle reentering the Earth’s atmosphere. The inner vessel wall of the tokamak was upgraded from graphite to beryllium. Beryllium has excellent thermal and mechanical properties for a fusion reactor – it absorbs less fuel than graphite but can still withstand the high temperatures.

The energy JET produced was what made the headlines, but we’d argue it is in fact the use of the new wall materials which make the experiment truly impressive because future devices will need these more robust walls to operate at high power for even longer periods of time. JET is a successful proof of concept for how to build the next generation of fusion reactors.

ITER Fusion Reactor Diagram

The ITER fusion reactor, seen here in a diagram, is going to incorporate the lessons of JET, but at a much bigger and more powerful scale. Credit: Oak Ridge National Laboratory, ITER Tokamak and Plant Systems

The next fusion reactors

The JET tokamak is the largest and most advanced magnetic fusion reactor currently operating. But the next generation of reactors is already in the works, most notably the ITER experiment, set to begin operations in 2027. ITER – which is Latin for “the way” – is under construction in France and funded and directed by an international organization that includes the U.S.

ITER is going to put to use many of the material advances JET showed to be viable. But there are also some key differences. First, ITER is massive. The fusion chamber is 37 feet (11.4 meters) tall and 63 feet (19.4 meters) around – more than eight times larger than JET. In addition, ITER will utilize superconducting magnets capable of producing stronger magnetic fields for longer periods of time compared to JET’s magnets. With these upgrades, ITER is expected to smash JET’s fusion records – both for energy output and how long the reaction will run.

ITER is also expected to do something central to the idea of a fusion powerplant: produce more energy than it takes to heat the fuel. Models predict that ITER will produce around 500 megawatts of power continuously for 400 seconds while only consuming 50 MW of energy to heat the fuel. This mean the reactor produced 10 times more energy than it consumed – a huge improvement over JET, which required roughly three times more energy to heat the fuel than it produced for its recent 59 megajoule record.

JET’s recent record has shown that years of research in plasma physics and materials science have paid off and brought scientists to the doorstep of harnessing fusion for power generation. ITER will provide an enormous leap forward toward the goal of industrial scale fusion power plants.

Written by:

  • David Donovan – Associate Professor of Nuclear Engineering, University of Tennessee
  • Livia Casali – Assistant Professor of Nuclear Engineering, Zinkle Faculty Fellow, University of Tennessee

This article was first published in The Conversation.The Conversation

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