The further you go from home, the more exotic things become, whether for holidays, or for an imaginary journey straight down. There are wonders under your feet, much closer than you think.
We’ll start from my house, in an unremarkable suburb of Reading, an almost-city in the Thames Valley, west of London, England.
Put a spade into the ground in my back garden and it sinks in gratefully. This is well-cultivated soil, dug for Britain during the war. The stones within it are mostly flint, occasionally fossils and mostly rounded. Immediately below the soil is a gravel terrace laid down by the River Thames.
A fossil sea-urchin, made of flint, from my garden
This gravel is extremely useful in construction and widely extracted. A map of Reading shows many bodies of water, dug for the gravel. The entire area is covered in these terraces, of various ages. The further up and away from the modern river you go, the older the terraces – they trace how the river has slowly cut its way into the landscape.
My house is not so close to the river, so the gravels are about 250 – 300,000 years old, a time a few ice ages past, when humans had learned to make fire and stone tools. A gravel pit behind my house unearthed some of these tools, now in Reading museum.
We’ve travelled just 6 metres. Below the gravels we get the first ‘solid’ geology – the London Clay and Lambeth Group, layers of sand, silt and mud around 50-60 million years old. Dramatic things were happening at this time – huge volcanoes formed in the west of Scotland, India first collided with Asia to form the mighty Himalayan mountains. In SE England alas, not much was going on, just a bunch of mud and sand being draped over the surface.
What’s most interesting about these rocks is what people did with them. Particular layers are good for making bricks and this has shaped the local area. Reading town centre is full of fine Victorian brick-buildings. The local clay gives red bricks, but mixing in quantities of chalk gave a grey coloured brick. Bricks were made across the Thames Valley, wherever suitable clay was found.
Lidar image of Nettebed. The smooth dry valleys in the top half are chalk, the pitted hill is where the clay was dug
BGS map of the same area. Green is chalk, brown/purple contain clay
Unlike older, more deeply buried sediments, these rocks are still soft and poorly bound together, something that led to the first ever serious railway accident. The Great Western Railway was originally planned to run through the village of Sonning, close to the Thames. However an uncooperative land-owner forced the route to go through a small hill. Any plans to tunnel through were dashed by the low strength of the sediments. Instead a cutting was made – a 17m deep gash through the hill – with the railway running at the bottom of it. On the 24th December 1841, a landslip covered the rails with ‘soil’. A train ran into the slip, causing the deaths of 9 people. These were early days for both railways and engineering geology – the cutting was later widened, making the slopes less steep.
Sonning Cutting
Only 24m below my house, we reach the Chalk. This is a rock-type that is both an icon of Southern England (“white cliffs of Dover”) and deeply weird.
While most sedimentary rocks are made of sand or mud, chalk is almost entirely made up of the remains of living creatures. Sea-levels at the time (95-65 million years ago) were extremely high, meaning much of Britain was underwater. Since little or no sand or mud was being washed into the sea, the only things settling on the sea-bed were the remains of tiny planktonic organisms. These grew tiny beautifully shaped flakes of calcium carbonate that found their way to the sea-bed, building up layer after layer. Other creatures, like sea urchins living in this chalky mud had parts made of silica. These formed into nodules black flint, so prized by our distant ancestors. Rare stones are found in the chalk, thought to have polished in the insides of giant sea monsters, marine reptiles that swam these seas.
Chalk landscapes form the bones of southern England. Slightly harder, they form ridges such as the Chilterns and South Downs. These are strange landscapes, dotted by prehistoric structures and mysterious figures formed by cutting through the thin soils to reveal the pure white chalk beneath.
These chalk hills have sinuous valleys and whale-backed hills. The valleys are dry – chalk is a rock that swallows rivers and is in turn dissolved by water. Near me, above the chalk, a swallow hole recently appeared in a school car park, a reminder of the chasms beneath our feet.
There are creatures beneath my feet. The chalk aquifers are home to creatures called stygobites, ghostly crustacean that eke out a living in wet cracks. Some are thought to have been living in England for up to 19 million years, making them our longest continuous inhabitant
The remains of more ancient life lies deeper still. As we shall see, next time.
Inspired by reading Tim Robinson, I’m very interested in the idea of delving deep into a particular place and all the stories it can tell. Tim Robinson sought to know everything about the places he lived, taking in geology, archaeology, history, language, place names and more. He lived in the west of Ireland for most of his life and produced numerous books. I’ve been on holiday and written a few blog posts – a few ripples in comparison to the great splash of his work – but I hope they resonate harmoniously.
I’ve written about an area just south of Lochinver in the west of Scotland. This first post is about two places and why Assynt is so empty.
Hill of the horses
Cnoc nan Each – “Hill of the horses” is both a hill and a place – shown on the OS maps as Cnocnaeach. Fewer than 2 kilometres from the pie-munching tourists of Lochinver it could not be more different.
View North over Cnocnaneach. Ruined building in the foreground. No trees, no people.
It’s an attractive spot, a flat-bottomed area underlain by sediment and a contrast to the craggy wastes surrounding it. A range of different ruins show that people lived here for thousands of years. A range of structures (well described here and here) attest to countless generations tending livestock, making a living off the land.
All are gone. The gaelic-speaking people – who lived here and named every hill – were “cleared away” in the Nineteenth Century, put onto ships to Canada, all in the name of ‘improvement’. In economic terms, subsistence agriculture made no sense. To the land-owners, removing the people and filling the land with deer and sheep was a rational decision. The people who were cleared saw this as a tragedy, but nobody cared what they thought. Nobody with power, anyway.
Lady Constance Bay
Just 3 kilometres west of Cnoc Nan Each lies a small bay, marked on some OS maps as ‘Lady Constance Bay’. It’s named – I infer – after Lady Constance Gertrude Sutherland-Leveson-Gower, a member of the British aristocracy. At 18 she married the man who became both the Duke of Westminster and the richest man in Britain.
Lady Constance Bay – view from the South
Constance is unlikely to have spent much time in Lochinver, but as the daughter of the 2nd Duke of Sutherland, she does have a strong connection to the place. Her family were descended – via the Scottish and then British aristocracy – from the head of Clan Sutherland. In Medieval times a clan head was a warrior chief, but by the Nineteenth Century the role was one of absentee landlord. Constance’s grandmother – as owner of most of Sutherland – ordered the clearances there in the name of improvement. The interior of Assynt was depopulated in 1812 on her command.
By 1847 the land was largely empty. In that year, Constance’s father – who liked to spend his money on improvements to his many buildings – planted Culag Woods. This attractive piece of woodland just south of Lochinver is clustered around Cnoc na Doire Daraich or “Hill of the oak thicket”. Lady Constance Bay lies just offshore the woods. Constance was 14 at the time; the bay was surely named in her honour.
New beginnings
Culag woods remains. It’s a community woodland and clearly well loved. The name Lady Constance Bay is not shown on the maps of the wood. I don’t know if the omission is deliberate, but it seems appropriate. Constance’s descendants remain some of the richest people in Britain, but they no longer own Assynt.
Change is slowly coming to Scotland. These two places are still on land owned by absentee billionaires, but following a community buyout a large portion of the interior of Assynt is owned by the Assynt Foundation, to be managed on behalf of local people. A fascinating aspect of the Highlands are the debates over how to move on from the ecological and human impacts of the clearances. Rewilding, wind farms and community buy-outs are all things bringing change to the area, but each raise as many questions as answers. As an outsider, it’s not for me to say how things should change – I’m not even clear whether my holidays there bring benefit or harm – but I’ll be watching with fascination.
This post is the final of three articles first published by Xiaoduo Media in “Front Vision”. Front Vision is a Chinese online science magazine for children. My original English text produced with permission.
Nuclear fission was discovered as Europe slid towards war and this meant that research became focused on nuclear weapons. After the war, thoughts returned to dreams of generating clean unlimited power. Dwight Eisenhower, the world war 2 general who became US president talked in 1953 of using ‘atoms for peace’. Using nuclear reactors to generate power was a key goal for many countries.
How to design a nuclear power plant
A self-sustaining nuclear reaction produces a lot of energy as heat. This is used to generate steam which turns turbines and generates electricity. This is the goal of any nuclear power plant, but many different types are possible.
Different fuels are used. Uranium and Plutonium are the main fissile elements. Processed Uranium ore can be used, or be enriched to contain more Uranium-235. Mixtures of Uranium and Plutonium may also be used.
Once the fuel is chosen, you need to decide what size of reactor you want, and what output of energy is required. Reactors can generate new fissile elements as well as energy, is this required? Inside the reactor, there needs to be something to extract the heat, usually a liquid that can be pumped through the core of the reactor. A moderator is required, a substance to slow the neutrons so they cause more fission reactions. Sometimes the coolant acts as a moderator.
To control the fission reactions, control rods are used. These are made of a material that absorbs neutrons so that lowering them into a reactor can slow or stop the fission reactions. Safety is important, ensuring that reaction rates are steady and no dangerous highly-radioactive material is released into the environment. Reactors don’t last forever, over time radiation damage weakens them. Modern ones are designed to make it easier to decommission them, removing the most radioactive materials and making them safe.
Devices used in the Manhattan project were based on Enrico Fermi’s first structure, the Chicago Pile-1. These were called a ‘pile’ as they was literally a pile of blocks cooled by air.
X-10 graphite reactor used at Oak Ridge during the Manhattan project. Source
The X-10 reactor was used to produce Plutonium. It was a cube of graphite blocks, 7.3m along each side. This was surrounded by 2.1 metre of concrete to absorb radiation, for the safety of the workers. The graphite acted as a moderator, slowing the neutrons. It contained 1260 holes passing through the blocks. Small cylinders (“slugs”) of natural Uranium were covered by Aluminium and pushed into the holes into the core of the reactor. After a few days, during which fission reactions created Plutonium within them, the slugs were pushed out of the other side of the reactor to drop into water for later processing.
Steel rods mixed with Boron were used for controlling the fission reactions. Boron absorbs neutrons, so pushing these rods through channels into the reactor would stop or slow the fission reaction. These horizontal rods were used to control and stop reactions to allow refuelling. For safety, three 2.4m steel rods coated in neutron-absorbing Cadmium were held vertically above the core. They were held in place by a fail-safe mechanism. If electric power was lost they would automatically drop into the core and completely stop the fission reactions.
To avoid damage to the Aluminum-coated fuel slugs the core had to be kept below 200 °C. To control the temperature, air was driven through the core by three fans and then filtered to remove any radioactive particles. The air was taken from outside, meaning that on cold days the reactor could be run at a higher power level as the air cooled it more effectively.
Later designs placed the fissile material within a closed container to allow liquids to be used for cooling. These containers are called ‘reactors’, a name borrowed from chemical engineering.
Generating electricity
The air-cooled pile designs saw the heat as a problem, to be removed from the core and lost as hot air from a chimney. If the goal is to generate electricity from a nuclear reactor then the heat is the useful output. The fluid used to cool the reactor (the ‘coolant’) is passed into a series of pipes called a ‘heat exchanger’. This removes heat from the coolant by heating water into steam. The steam is then used to drive a turbine, spinning a rod within a magnetic field within a generator that produces electricity.
USA developments: first generation of electricity
After the war the Americans built EBR-1, an experimental ‘breeder reactor’. As America started a programme of building many nuclear weapons, there was concern over quantities of Uranium ore available. This breeder reactor generated energy but also converted Uranium into Plutonium and created fissile material (fuel) as well as consuming it. It was cooled by a liquid metal mixture of Sodium and Potassium. This material reacts to water and catches fire in the air, so it has to be handled carefully. The EBR-1 was the first reactor to produce electricity, but initially only enough for 4 light bulbs. It was the first liquid metal cooled fast breeder reactor.
Sodium cooled fast reactor. Image source: Wikipedia
This design had various differences compared to the X-10. As a fast breeder type of reactor, a moderator was not required to slow the neutrons. The fuel was placed into 0.6cm diameter rods, which were joined as groups of 60 into hexagonal structures called ‘assemblies’. Overall the core size for EBR-1 was less than 6 litres in volume. Even with all the shielding and heat exchanger added, it was only as large as a single room.
The liquid metal was very effective as a coolant, transferring out large amounts of heat for power generation, running at a temperature of around 300 degrees C. The radioactive material and coolant were all contained within a closed loop meaning the reactor could run constantly for a long time. Vertical control and safety rods could be dropped down into the core.
Once the EBR-1 had proved the general design, much larger scale reactors were built. EBR-2 had a core of 10 times the volume and operated at higher temperatures making it more suitable for electricity generation
British designs for reactors
The British had merged their nuclear weapons programme into the Manhattan Project and so got to share the secret research it produced. The Americans didn’t share their enriched Uranium, however. Wanting to build their own nuclear weapons, the British had to make their own fissile material.
First in 1950/51 they built two nuclear piles for creation of Plutonium for weapons. Then they had to design a reactor that could run on non-enriched Uranium fuel as building their own enrichment facilities would be too expensive. The resulting Magnox reactor design used graphite as a moderator and CO2 for coolant. Four reactors based on this design became active at Calder Hall in 1956. These were the first in the world to provide large amounts of electricity for public use. However, they were also used for Plutonium generation, at least until 1964.
This Magnox design was used in other countries, but because of it’s dual purpose it was not as efficient as later designs that were focused purely on energy generation.
Soviet Union nuclear research
Research into nuclear weapons in the Soviet Union started in 1943 when they noticed that no articles on nuclear physics were being published in the US. They correctly inferred that was because of a secret weapons development programme and so started their own. Soon they were assisted by Klaus Fuchs. A brilliant German scientist, Fuchs was a communist who had fled to the UK to escape political persecution. He was therefore trusted to work on the Manhattan project as he was an enemy of the German government. However his political beliefs meant he saw no problem in sharing secrets from the Manhattan project with the communist USSR.
The first Soviet bomb test was in August 1949, using a Plutonium device based on the American ‘fat man’ design. The Plutonium in the bomb was created in reactors similar to the America X-10 design, using graphite moderators and cooled by air.
As in other countries reactors were soon designed for electricity generation. By 1954 a dual-use reactor had been built and connected to the electricity grid, at Obninsk. This APS-1 Obninsk design used 5% enriched Uranium fuel, was graphite moderated and cooled by water. It was the basis of the RBMK design used for many reactors built in the Soviet Union, including those built at Chernobyl in modern-day Ukraine. Reactors built by the Soviets followed the same principles to those in other countries but had different designs.
Portable reactors
These new reactors built for power generation were large, forming multi-storey buildings. Much smaller nuclear reactors are possible and can be built small enough to move.
The US Navy started research into nuclear reactors in the late 1940s. A nuclear reactor on a large ship can use steam to power turbines for electricity or to drive the propellers directly Aircraft carriers even use the steam directly to power catapults to help planes take off. The reactors are expensive, but allow the ship to stay at sea without refuelling for years. The UK, Soviet and French navies also have nuclear-powered vessels.
Portable reactors need to be small, but powerful enough to provide all a ship’s energy needs. They also need to be simple and easy to maintain – ideally they should not require refuelling for a very long time. Modern portable reactors can be run for decades without refuelling.
Early on, in the 1950s, the US Navy experimented with a Sodium cooled fast reactor (similar to the EBR-1 design) but this didn’t perform well. They switched to a pressurised water reactor design. This uses water as both a moderator and a coolant, but it’s held under pressure, preventing it from boiling even at high temperatures. The pressurised water is kept in a closed container. Pressurised water is circulated through the core and into a heat exchanger which is used to generate steam. A separate loop of piping takes the steam to drive a turbine and power the ship. A condenser is a device that turns the steam back into water before being pumped back into the heat exchanger. The pressure vessel part of the system may be only a few metres in size and still produce enough energy to power a huge ship.
Portable pressurised water reactors have a few differences to larger ones used on land. To ensure a high power output from a small reactor, enriched Uranium is often used. Substances that act as a ‘burnable neutron poison’ are added to the core of the reactor. These absorb neutrons, to help balance the fission reactions. Over time they decay helping to maintain this balance over decades even as the contents of the reactor change as the Uranium fuel is consumed.
The most common use for portable reactors is in submarines. Previously submarines were powered by batteries that were recharged by diesel engines. This recharging had to be done at the surface (to get air) which made the submarines more vulnerable to detection and attack.
Nuclear powered submarines can remain submerged for weeks or even months. As the Cold War between the US and USSR developed, these submarines were built as vehicles to launch nuclear missiles. Both sides feared the other would launch a sudden devastating nuclear attack called a first strike. To prevent this they wanted the ability to retaliate with their own nuclear weapons and so act as a deterrence. Storing the weapons safely in submarines hidden deep under the sea created a ‘balance of terror’ as any country launching a nuclear first strike could be in turn attacked. Some believe this has helped ensure nuclear weapons have never been used since 1945.
The oddest portable nuclear reactor was used actually inside the Greenland Icecap. Camp Century was an American military and scientific research base. It was secretly intended to see if nuclear missiles could be stored for use beneath the ice, but also for other scientific research. People lived and worked in covered trenches dug into the ice. They had plenty of power from a small nuclear reactor installed for that purpose.
As we’ve seen early uses of controlled fission were mostly linked with military goals. They did however prove that nuclear power was a viable source of electricity. Later designs and goals were more focused on civilian uses.
This post is the second of three articles first published by Xiaoduo Media in “Front Vision”. Front Vision is a Chinese online science magazine for children. My original English text produced with permission.
War and fascism – everything changes
The research described in the previous chapter involved scientists from across Europe, all sharing information and working towards a common goal. Through scientific papers, they shared their ideas equally, allowing people in many countries to contribute. But In 1933 Adolf Hitler took power in Germany. His fascist ideology, based on lies and hatred, blamed Jewish people and other countries for holding back Germany. Between 1933 and 1939 he and other fascist leaders started to discriminate against Jews and it was clear to all that even more terrible things were coming. Many Jewish scientists left Europe for America, to escape persecution. For example, Otto Frisch and Lise Meitner were refugees in Sweden fleeing the Nazi takeover of Austria when they identified nuclear fission.
Another Jewish refugee was Leo Szilard. In 1933 crossing the street in London he had a vision. If slow neutrons could induce nuclear reactions in elements, what if such a reaction created two neutrons? And each of those neutrons in turn made another reaction? 1-2-4-8-16-32-64-128 .. It grows and grows – a nuclear chain reaction. In 1939 (now safe in the USA) he heard of the discovery of fission in Uranium and realised the possibility of creating a fission chain reaction. Nuclear fission, theoretical calculations suggested, released unusually large amounts of energy. A nuclear chain reaction involving fission would suddenly release massive amounts of energy – it would be a bomb of enormous destructive power.
Szilard wasn’t the only person to realise the potential of nuclear fission chain reactions. In the UK – from September 1939 at war with Germany – James Chadwick (the discoverer of the Neutron and student of Rutherford), led serious investigations into the practicalities of making a nuclear bomb. At the same time, the German government started it’s own investigations, led by Werner Heisenberg.
In the USA in 1941, researchers created Plutonium from Uranium. They kept their discovery secret. Theoretical studies suggested that Plutonium, like Uranium, was fissile (could undergo nuclear fission) and could be produced in large quantities from Uranium.
How to make an atomic bomb
By 1941, the basic theory of how to make an atomic bomb was known to many physicists. What’s required is to generate a nuclear chain reaction involving nuclear fission. Two materials were known that did this, Plutonium and Uranium. However only one of the isotopes of Uranium was suitable, Uranium-235. Natural Uranium contained a mix of Uranium-235 and Uranium-238 and nobody knew an easy way to separate them out.
If you somehow managed to get hold of enough of either material, you needed to create a ‘critical mass’ of it. This is where a large enough volume material is tightly packed together such that not too many neutrons escape out, but instead enough hit other fissile atoms and create yet more neutrons and so sustain a chain reaction.
The concept of critical mass can be understood by thinking of a forest fire. If a tree standing alone in a field is hit by lightning and catches fire it releases heat, but this is lost to the atmosphere and soon the fire stops. This is like fission in a small mass of Uranium, a reaction starts but the neutrons don’t hit more fuel and so fission stops. If the first tree is closely surrounded by dead wood and other trees, the heat it produces heats this wood and causes them to start burning – it starts more chemical reactions – and so the fire spreads growing hotter and bigger. This is like a fission reaction starting within a critical mass of Uranium, most neutrons hit more fuel, creating ever more reactions. Nuclear fuel releases much more energy than wood, so things move much more quickly. This would release huge quantities of energy and so create an extraordinary explosion.
Physics is not engineering. Creating enough fissile material and turning it into a working bomb were huge challenges.
Manhattan project
In December 1941, Japan attacked a US naval base called Pearl Harbor and the USA entered the Second World War. Japan and Germany were aligned, so the USA declared war on both Germany and Japan.
Alerted by a letter from Leo Szilard and Einstein, the US government had already started research into nuclear weapons. Knowing of German expertise in nuclear physics, the goal was to create a bomb before Germany did. Now at war, a new US project was created and given the code name of the Manhattan Project. It had enormous resources available to it.
The project called on the best scientists in the world. The best physicists in American universities, like Philip Oppenheimer and the young Richard Feynman were called up in secret. Leo Szilard and Enrico Fermi and others had already fled fascism to reach the USA, and the British project team was merged in as the two countries became allies. In time Niels Bohr was involved, having fled his native Denmark in secret to escape Nazi persecution.
The project brought together different cultures. The many scientists who had fled to the USA from Hungary were called ‘Martians’. This was a joke name as – like aliens from another planet – they spoke an incomprehensible language and were incredibly intelligent. University scientists didn’t always get along with Army engineers. In his memoirs, General Groves – the leader of the Manhattan project – talks of a meeting where scientists gave him their estimate for how much Plutonium was required to make a bomb. Like an engineer, he assumed an estimate was one with maybe 25% or 50% uncertainty. However to physicists a reasonable estimate was one within a factor of ten.
Those on the project were completely focused on one goal, and solving the many technical challenges that faced them. Knowing that there were two types of bomb, based on Uranium-235 or Plutonium and not knowing which would be best, they decided to build both at the same time.
Little Boy, the Uranium weapon
The Manhattan project managed to secure large supplies of Uranium ore, but this contained a mix of Uranium-238 and the more fissile Uranium-235. These different isotopes are chemically identical, making separating them (a process called Uranium enrichment) a huge job. A huge secret site was built called Oak Ridge to focus on Uranium enrichment.
Various different approaches were considered. While chemically identical, U-238 is slightly heavier (because each atom contains more neutrons) so each method finds ways to use the weight difference to separate them.
The first method is to spin the Uranium in centrifuges. These spinning containers separate out the lighter U-235. Attempts by the Manhattan project to build machines failed, but this technique is now the preferred method.
The electromagnetic technique spun Uranium gas through a magnetic field to separate the isotopes. It required huge quantities of copper which was hard to find during wartime. In the end 13 tonnes of silver kept for use as money was used instead. This technique was inefficient, but used known technology and so was used in case the other techniques failed.
The Gaseous diffusion technique relies on the fact that lighter gas diffuses more quickly through a membrane. Huge buildings, four stories high and 0.8km long were built and the technique was very successful. The gas Uranium Hexaflouride was pumped under high pressure into a tube surrounded by a barrier, a membrane that contained thousands of tiny holes. Molecules made of the slightly lighter Uranium-235 (shown as orange dots) could more easily pass through these holes leaving behind gas contained more Uranium-238 (blue dots). This process separated the gas into a enriched stream and a depleted one. The enriched stream would still contain lots of Uranium-238, but it could be passed through into another set of pipes and through another barrier again and again, becoming each time more and more enriched. The depleted steam could be discarded – it was the Uranium-235 that was needed.
Finally, the thermal diffusion technique placed hot Uranium gas in 15m tall tubes. The lighter Uranium-235 gathered more at the top – this was another effective way of separating out the isotopes by weight.
The three techniques were used in sequence, to create more and more concentrated Uranium-235. Eventually about 50 kilograms of uranium enriched to 89% uranium-235 was delivered to Los Alamos by July 1945 where it was turned into a bomb.
The design was simple, a little like a gun. A long tube contained two pieces of enriched Uranium. Each piece alone was too small to sustain nuclear fission reactions. Conventional explosives fired one piece down the tube into the other. This joined mass was big enough to become critical and create a nuclear fission chain reaction and so an explosion. The bomb had the codename “little boy”.
Fat man – Plutonium weapon
Despite being only discovered the year before, Plutonium was seen as another potential fuel for a bomb. Plutonium could be produced from un-enriched Uranium, by generating a controlled nuclear fission reaction. Nuclear fission reactions would produce neutrons, slowed by graphite blocks. The material that slows the neutrons is called the moderator, as it moderates (slows) the speed of the neutrons. Some of these neutrons sustain a steady rate of reactions, others turn Uranium into Plutonium. The rate of reaction was controlled by adding rods that absorbed neutrons to slow or even stop the reactions. The designs ensured there was no ever-increasing chain reaction as was wanted in a bomb, just a steady rate. The reactions produced heat, but the real purpose was creating Plutonium. Importantly, the Uranium fuel could be unprocessed ore – there was no need to enrich the Uranium-235 first.
Enrico Fermi, piled up Uranium oxide and graphite blocks in a squash court in a sports ground at Chicago University and so in 1942 created the first (man-made) self-sustaining nuclear reaction in Chicago Pile-1, the ancestor of all nuclear reactors. Soon a small secret town called Hanford was built to house a number of bigger nuclear reactors all dedicated to producing Plutonium. Once Plutonium had been produced, it could be chemically separated from the Uranium material.
The design of the weapons was done at a site called Los Alamos. This required intense work from many of the world’s best physicists. To know if the bomb would work, they had to understand how it would work. To start with they calculated the energy released by a single fission reaction. This required knowledge of the exact mass of the material before and after, to calculate the missing mass converted to energy according to the Einstein equation.
That was the easy part. To create a sustained chain reaction, a large amount of fissile material needs to be suddenly pressed together to make a critical mass. A problem with the Plutonium was that it contained a mix of different isotopes, meaning the fission reactions acted in a different way. The material produced within the reactors contained more plutonium-240 (a very reactive isotope) than expected. This would start fission reactions much sooner than the plutonium-239. If a gun design was used, like in Little Boy, then the plutonium-240 would fission early and push the Plutonium apart before most of the material – the plutonium-239 – could fission. This would cause the bomb to fail to explode properly.
So a different and more complicated bomb design was required, one that involved taking a sphere of Plutonium and surrounding it with conventional explosives. The physicists had to write equations to model the behaviour of the Plutonium pressed together by explosives, then track the fission reactions as a critical mass was formed. Imagine trying to understand in detail what happens in the heart of a bomb, and then add on the complexity of understanding the fission reactions. Many equations and long difficult calculations were required and because Plutonium behaves different from Uranium, all the calculations were different from those required for the Uranium bomb design. The concept was simple but the design was complicated. The explosives were made with a complicated three-dimensional shapes to focus the shock wave through them and down onto the ball of Plutonium. The yellow detonator, started different conventional explosions with the outer layer of explosives. The curved shape at the based helped join the different shock waves into a single spherical wave that was then increased by two further layers of conventional explosives. This then compressed three different layers, of Aluminium, plastic and then Uranium, each with a different purpose.
Diagram showing the focused shock waves generated by the dark brown explosives. The then compress the nuclear material in the centre (red). Image from Wikipedia
Inside all this, the red Plutonium contained a polonium-beryllium initiator, a device that when compressed provided a good source of neutrons to initiate the chain reaction. The size and round shape of the bomb compared to ‘little boy’ led to it being called ‘fat man’.
They ran tests of the explosives (without any Plutonium) and used X-rays to see what happened inside, but still any big mistake in these calculations and the bomb wouldn’t work.
German efforts
Scientists who before the war had cooperated now found themselves on opposite sides of a vicious war. Non-Jewish German scientists like Werner Heisenberg might not be supporters of Hitler’s fascism, but most chose to stay and support their country as it fought a war.
Heisenberg was asked to lead efforts in Germany finding uses for nuclear fission. He understood as well as anyone the possibility of making a bomb, but focused mostly on the potential for energy generation, rather than weapons. Some people speculate that he did this as he didn’t want to give Hitler such a powerful weapon, but no-one knows for sure.
The Manhattan Project was started precisely to get a bomb before Germany, but nobody knew that Heisenberg would fail to deliver. To slow down any German efforts, the UK army, working with locals, attacked a site in Norway that was a source of heavy water, useful for nuclear research. In 1944 Heisenberg gave a talk in Switzerland, a neutral country. A US agent was sent with instructions to kill him if the talk suggested that Germany was close to having a bomb, but of course there was no need.
Testing and use in war
By 1945 the Manhattan project had achieved so many things. It had produced enough Uranium-235 for a single bomb and larger quantities of Plutonium. We’ve seen how complicated the calculations required for bomb design were – what was needed now was a test, to see if nuclear weapons actually worked.
A version of the fat man weapon was built and a site deep in the desert chosen. At 05:29 on the 16th July the device was exploded at the Trinity Test site. A huge flash of light lit up the desert, with colour changing from yellow, red to purple. A ball of fire slowly rose leaving behind a column of smoke, forming a characteristic mushroom shape. After about 40 seconds the blast wave reached the observers. Enrico Fermi dropped a series of pieces of paper and saw how far they were blown by the wind. He had already made a rough calculation and from the distance the paper was blown he estimated the size of the explosion to be the same as that created by exploding 10,000 tonnes of TNT.
The bomb contained 6.2kg of Plutonium of which 1kg was transformed by the fission reactions. Of that mass, about a gram was converted directly into energy. Enough energy to turn the desert sand beneath into glass.
The initial reaction of the scientists watching the test was joy at their success, but slowly they became solemn as they realised how terrifying a weapon they’d created. The blast wave spread outwards and was heard over a hundred kilometres away. A fake cover story was invented to explain it as the new bomb was still secret. The political implications spread around the world.
Nazi Germany had been defeated a few months earlier, but the war against Japan continued. The Japanese armed forces had been mostly defeated outside of Japan but refused to surrender. Faced with the prospect of having to invade Japan the US Army estimated that this would mean half a million US soldiers and 5 million Japanese people would be killed. The American president turned to his new atomic weapons as an alternative. On 6 August 1945 the Little Boy Uranium-235 weapon was dropped on the Japanese city of Hiroshima. Three days later a Plutonium fat man weapon was exploded over Nagasaki and six days after that, Japan surrendered and the war was over. Many of the scientists who worked on the Manhattan project argued the bombs should not have been used and were appalled that 10,000s of civilians had been killed by what they had built. The alternative argument is that they ended the war early and so saved more lives that way. Probably people will never agree which interpretation is correct.
Mushroom clouds over Hiroshima (L) and Nagasaki (R). Image from Wikipedia
Aftermath and impact on science and beyond
Nuclear physics moved in only a few years from experiments on tables in laboratories to a huge engineering project. Theoretical physicists would produce equations that within days were being used to design new machines and industrial processes.
Some of those who worked on the Manhattan project moved back into basic research. Having demonstrated it’s practical value, physicists had little difficulty finding the funding for building the bigger and bigger machines required to explore deeper inside the atom. The search began to find out what smaller particles neutrons and protons were built up from.
Some others stayed in weapons research. The process of fusing small atoms together – nuclear fusion – produces even more energy than fission and within 10 years a fusion weapon – the hydrogen bomb – had been made and tested. Others focused on more peaceful uses of nuclear fission. This included fields like medicine but many thought of the heat produced by the Manhattan reactors and saw it as a source of cheap energy.
