Today’s nuclear plants are expensive and produce waste that remains radioactive for thousands of years. Some people consider them dangerous but that, Chernobyl excepted, is more potential than real. Like the original atom bomb, the energy is from fission – breaking apart large atoms such as uranium. Since the 1940s, the hope has been to produce clean energy the opposite way, namely by fusing isotopes of the smallest atom, hydrogen, to produce helium and large amounts of electricity which do not depend on the wind blowing or the sun shining. This is how the sun and all other stars produce light and heat. The trouble is that the hydrogen isotopes, deuterium and tritium, have positive nuclei and are extremely reluctant to fuse. Stars use their massive gravitational forces to overcome this
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Today’s nuclear plants are expensive and produce waste that remains radioactive for thousands of years. Some people consider them dangerous but that, Chernobyl excepted, is more potential than real. Like the original atom bomb, the energy is from fission – breaking apart large atoms such as uranium. Since the 1940s, the hope has been to produce clean energy the opposite way, namely by fusing isotopes of the smallest atom, hydrogen, to produce helium and large amounts of electricity which do not depend on the wind blowing or the sun shining. This is how the sun and all other stars produce light and heat. The trouble is that the hydrogen isotopes, deuterium and tritium, have positive nuclei and are extremely reluctant to fuse. Stars use their massive gravitational forces to overcome this reluctance but we have no means of replicating anything like those forces on earth.
Never ones to let such obstacles get in the way, researchers claim they can deliver commercial fusion power, if not by 2050, then at least by the end of the century.The research budgets have grown: in October 2019, the UK committed “£220M to the conceptual design of a fusion power station – the Spherical Tokamak for Energy Production (STEP).” “A ‘tokamak’ is a machine that confines a plasma using magnetic fields in a donut shape that scientists call a torus. [It can also be spherical.] Fusion energy scientists believe that tokamaks are the leading plasma confinement concept for future fusion power plants.” The Russians invented this technology around 1958 and the word “tokamak” is an acronym of its description in Russian. The plasma process is explained later.
The US Department of Energy’s Office of Fusion Science spends “approximately $700M per year.” On the 16th June, the UK Atomic Energy Authority announced an agreement with the Canadian start-up company, General Fusion, to build a £400M demonstration plant at the authority’s Culham campus. Note that this will not actually produce any electricity, it will just show how that might be done. Of course, we would be considering far larger sums for the real thing; the budget for ITER, the European fusion project, is US $22bn. The US Department of Energy thinks it will cost $65bn.
We need to look at these mutually repulsive isotopes a little closer. Hydrogen, as most people know, is a single proton. Deuterium has the same proton with a neutron attached. Tritium is an isotope of hydrogen with a half-life of 12 years with the same old proton but now with two neutrons. It is produced as a by-product in nuclear fission reactors, and would also be formed in fusion reactors by using liquid lithium as the primary coolant.
When deuterium and tritium nuclei fuse, they form helium kinetic energy. There are many other candidate atoms that could emit energy and power but as the lighter they are, the (relatively) easier it is to produce energy by fusing them, these two isotopes of hydrogen seem the way to go. We have heard a lot about hydrogen as the clean fuel of the future but it has, as 1930s airship makers found out, its dangers. If large scale fusion energy worked, the r helium output might be available as fuel in place of the combustible hydrogen. Apparently, whilst it is rare on earth, there is plenty of helium-3 (helium-3 is regular helium with each atom short of a neutron) on the moon and 25 tons of the stuff would power the USA for a year. But this is speculation.
In terms of energy per unit mass, the yield of fusion is much greater than that of the fission of heavy elements like uranium. This is why nuclear weapons rapidly evolved from fission bombs to hydrogen (fusion) bombs. That leads to the thought that the hybrid fission/fusion model used by hydrogen bombs might be controlled and used for electricity generation in place of the purely fusion models now being pursued. Unfortunately nuclear fission is only hot and dense enough in a critical mass of several kilogrammes of uranium or plutonium. Then there is a massive nuclear explosion, which would not be popular with the locals. The temperatures that can be safely reached in a controlled nuclear fission device, i.e. a reactor, are way too low for fusion to occur in a deuterium-tritium or deuterium-deuterium mixture.
Returning to the fundamental problem of getting the two repulsive isotopes to coalesce, one must first collect minute quantities of deuterium from the sea, or wherever, and tritium from those fission reactors you are trying to get rid of. Whilst deuterium is rare on earth, there is plenty in the wider reaches of the solar system, notably on Jupiter, which may be of comfort for the longer term. Sourcing deuterium, otherwise known as “heavy hydrogen”, in small quantities on this planet for pilot fusion plants is not a major problem; every million atoms of hydrogen taken from the sea yields about 156 atoms of deuterium. Doing that on a commercial scale, however, might prove more challenging.
Next, the two isotopes in the form of gases have to be mixed and made into a very, very hot plasma heated by an ionising electric current. Note that it takes a lot of energy to get the hydrogen, and then the deuterium, and then heat the mixed gas to, say, 100M Kelvin - roughly the same as degrees Celsius and six times hotter than the sun’s core. At these temperatures, the gas becomes a plasma, i.e. a high energy state of matter in which the electrons are stripped away and move freely about. It is the high temperature that gives the isotopes enough energy to overcome their mutual repulsion. If you are wondering how I know all this, this blog was based on emails from a respected physicist who prefers not to be named as he is not a specialist fusion scientist.
We are not done yet. Tritium is radioactive and may leak from reactors. There may well be substantial environmental radioactivity releases. The plasma vessel can only be handled remotely for the year after use. Lithium is used as the buffer material but availability of that too, thanks to heavy use for batteries, is threatened. It is even claimed we will run out in 2025. There are plenty more engineering and production issues to resolve, and differing methodologies to select, but we need not consider those now. They all add up to a colossal energy requirement to fuel and operate a nuclear fusion plant. So far no one has managed to make a fusion device produce more energy out than in, for more than several seconds. If we do, will we have to trawl the solar system to gather the raw materials?
It does not look like anyone yet knows the answers to all of the above issues. We will hear of “break-throughs” and some will be real and some mythical, “cold fusion” for example. Nevertheless, it seems certain that nuclear fusion will make little or no contribution to zero carbon by 2050.
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