The World’s First Nuclear Reactor

by Nicholas Mee on November 22, 2020

In 1972, technicians at the Tricastin nuclear plant at Pierrelatte in France made an alarming discovery. Routine tests appeared to show that a batch of uranium had been tampered with.

A test tube containing processed uranium ore known as yellow cake. Credit: Weirdmeister – Wikimedia.

Natural uranium consists of two isotopes. It is 99.28% uranium-238 and 0.72% uranium-235. But the French technicians found that some of the uranium-235 in their batch was missing. It was just 0.6% uranium-235 rather than 0.72%.

Had some of the precious and potentially deadly isotope been extracted elsewhere? Was this a sign of a clandestine nuclear weapons programme? An explanation was required and an investigation was launched by the French authorities.

Nuclear Reactors

Around a fifth of the electricity in the United States is generated by nuclear reactors. The fraction in the UK is similar, while in France three quarters is nuclear in origin. Currently, 440 civilian reactors generate electricity in forty-five countries.

Nuclear reactors produce energy in a chain reaction, which operates as follows. A uranium-235 nucleus fissions after absorbing an uncharged nuclear particle known as a neutron. This is because when the neutron binds to the uranium-235 nucleus sufficient energy is liberated to split the nucleus into two smaller fragments. Crucially, two or three neutrons are also released, along with a substantial amount of energy, and these neutrons may then go on to induce the fission of further uranium-235 nuclei. Each fission releases more neutrons and these neutrons may cause further fissions. In a nuclear reactor this process is controlled so that on average just one neutron from each fission event leads to a subsequent fission so the chain reaction continues at a steady rate.

A uranium-235 nucleus absorbs a neutron, which causes it to fission and release further neutrons.

Two ingredients are required to sustain a chain reaction—nuclear fuel and a moderator. The fuel in most nuclear reactors is enriched uranium which contains an artificially enhanced proportion of the readily fissionable isotope uranium-235. Typically, the fuel is enriched to between 3% and 5% uranium-235.

The moderator is necessary to slow down the neutrons, which greatly increases the chance that they will be captured by a uranium-235 nucleus. A good moderator is one that contains nuclei with a low mass, as this reduces the number of collisions needed to slow down the neutrons. (When a neutron hits a heavy nucleus it behaves like a rubber ball hitting a wall—it bounces off without losing much energy.) Another important feature of a good moderator is that its nuclei do not capture too many neutrons, which would dampen down the chain reaction. The most suitable materials are water, heavy water and ultra-pure graphite. The choice between these options is one of the most important factors in the design of a reactor.

Fermi’s first nuclear reactor known as Chicago Pile 1. It used natural uranium with blocks of graphite acting as the moderator, as can be seen in this drawing.

The nuclear reactor was developed in the United States during the Second World War, originally to manufacture plutonium for nuclear weapons. Enrico Fermi constructed the prototype in a disused squash court at the University of Chicago and it went critical for the first time on 2 December 1942. But was this really the world’s first nuclear reactor?

The Oklo Uranium Mine

The investigations by the French authorities in 1972 led to a uranium mine in the Central African country of Gabon, then a French overseas territory. The reason for the missing uranium-235 turned out to be quite incredible. Fermi’s Chicago nuclear reactor had been foreshadowed at least 1.7 billion years earlier by a natural nuclear reactor in a uranium deposit at Oklo near Franceville. Detailed analysis of the unusual nuclear isotopes found in the deposit has enabled physicists to construct a picture of what happened there long long ago.

Gabon, Central Africa.

In an exceptionally rich seam of uranium ore the groundwater acted as a moderator and sustained a nuclear chain reaction. The ore now contains isotopes of neodymium, ruthenium and other elements whose presence can only be explained by assuming these nuclei were produced by the fission of uranium-235. This also explains why the proportion of uranium-235 nuclei in uranium extracted from the ore is lower than elsewhere on Earth. In some areas its concentration is as low as 0.44% due to the fission of much of the isotope in the ancient natural reactor.

Through the analysis of xenon isotopes within the rock, it is estimated that the reactor would go critical for about half an hour generating around 100 kilowatts of power and boiling away the groundwater that was acting as a moderator for the nuclear chain reaction. With the loss of moderator the reactor would then shut down for about two and a half hours until it had cooled sufficiently for liquid water to seep back in and restart the reactor. This three-hour cycle is believed to have continued for hundreds of thousands of years.

Sixteen natural nuclear reactors were found in the Oklo area, as well as a seventeenth discovered at Bangombé, thirty kilometres to the southeast. This remarkable phenomenon is not known anywhere else in the world. It has been celebrated on stamps issued by the Republic of Gabon.

Could It Happen Today?

Although there may be other ancient natural nuclear reactors awaiting discovery, such a reactor is impossible today. The uranium-235 content of natural uranium is no longer sufficient to sustain a chain reaction with water acting as the moderator. In the distant past, uranium-235 formed a much greater component of uranium and this is what made the natural reactors possible. When the Oklo reactors went critical 1.7 billion years ago uranium-235 composed about 3.1% of the uranium in the ore, a similar concentration to the enriched uranium used in today’s commercial reactors. Uranium-235 undergoes alpha decay with a half-life of 704 million years, so it decays faster than uranium-238, which has a half-life of 4.5 billion years. As time passes the proportion of uranium-235 gradually decreases, so the era of natural fission reactors is long gone. 

Oklo fossil reactor 15. The yellow mineral is uranium oxide (yellow cake). Credit: Robert D. Loss (Curtin University, Perth.)

Unfortunately the natural reactor deposits in Oklo no longer exist as they have now been completely mined out. French scientists are campaigning to preserve the last remaining natural reactor site at Bangombé, so that further research can be carried out in the future.



Further Information

There is further information about natural nuclear reactors in this article: Nature’s Nuclear Reactors: The 2-Billion-Year-Old Natural Fission Reactors in Gabon, Western Africa.

Twenty Thousand Leagues Under the Seas

by Nicholas Mee on November 21, 2020

In 1870 the French science fiction writer Jules Verne published his epic adventure Twenty Thousand Leagues Under the Seas—the story of a modern day Odysseus named Captain Nemo. Nemo is commander of a submarine The Nautilus constructed at a secret location to his own design. Its lavish interior includes a library and an elegant dining room furnished with valuable artworks and a grand pipe organ that Nemo plays while cruising the ocean deeps.

The Nautilus

The Nautilus is a cigar-shaped vessel, seventy metres long and eight metres wide. It is propelled by electricity from sodium-mercury batteries, and has a top speed of eighty kilometres an hour. The crew has plentiful supplies of water produced by distilling sea water, but there is no way to replenish the air, so the ship has a limit of five days beneath the waves.

The technology of the fictional Nautilus was beyond anything feasible in the nineteenth century. The reality was rather different. Although numerous enterprising individuals and several navies had experimented with underwater craft on and off for centuries, there was a long history of disappointment. Optimism was regularly followed by the swift abandonment of each project as the underwater craft’s impracticalities became apparent. The main issue was the source of propulsion. In Verne’s day power was usually supplied by a steam engine fuelled by coal. Unfortunately, burning coal would rapidly exhaust the onboard oxygen supply.

By the end of the First World War submarines were becoming more practical. Diesel or kerosene engines would propel the boat on the surface whilst replenishing the batteries that powered the vessel when submerged. Even with very large batteries, however, the underwater range and speed of such craft was limited.

The Coffin Service

Hyman G. Rickover (1900-1986) joined the U.S. Navy in 1918 and trained as a marine engineer. In 1929 he volunteered for submarine duty and commanded the submarines S-9 and S-48. It was a dangerous assignment. The submarine corps was referred to as the coffin service—accidents were frequent and casualties were not uncommon. From 1933 Rickover served on surface vessels, but his first-hand experience of life beneath the waves in a vulnerable tin can remained at the forefront of his mind. He was determined to improve the welfare of submariners.

Rickover rose through the ranks during the Second World War and in late 1945 he was appointed Inspector General of the 19th Fleet on the west coast of the United States. This was the dawn of the nuclear age and the new technology seemed to offer unlimited possibilities. Rickover was tasked with supervising a nuclear propulsion system that General Electric were developing for naval destroyers. But he could see a much better application for the nuclear reactor—submarine propulsion.

Nuclear power could solve the submarine’s biggest problems. It was a reliable power source that did not consume oxygen and would run for long periods on very small amounts of fuel. For several years Rickover struggled to convince the naval bureaucracy of the huge potential of nuclear submarines. The Navy wasn’t interested, responding rather like a battleship whose course had been set. But Rickover was persistent and eventually the battleship was turned around. In July 1951 Congress approved construction of the world’s first nuclear powered submarine. Like a twentieth century Nemo, Captain Rickover insisted that it be called the USS Nautilus.

The Pressurized Water Reactor

Many designs for nuclear reactors had been dreamt up by physicists such as Enrico Fermi and Leo Szilard in the years since Fermi’s first reactor. These ranged from those using heavy water or graphite as the moderator to Szilard’s much more exotic fast reactor cooled with liquid sodium. But Rickover needed a safe and reliable design that was compact and easy to operate. It would have to function for long periods without refuelling or maintenance and without the attention of highly-trained technicians.

Rickover settled on a design that matched all his requirements. It is known as a pressurized water reactor (PWR). In these reactors water acts as both the moderator and the coolant, and the water is pressurized to 150 atmospheres so it remains liquid within the reactor core at temperatures of around 300° C. In Rickover’s submarine the fuel would be highly enriched weapons-grade uranium composed of 93% uranium-235. This would dramatically extend the periods between refuelling and reduce the reactor’s physical size. 

Water has a dual role in a PWR, acting as both moderator and coolant. This means that a loss of coolant, due to a leak or the formation of vapour bubbles also implies a loss of moderator, which dampens down the nuclear chain reaction. More generally, if the temperature in the core increases the water density decreases, which reduces the effect of the moderation, so the nuclear reaction slows, bringing the temperature back down. This gives the reactor an inbuilt stability. These inherent safety features have led to the widespread commercial deployment of PWRs around the world.

Rickover’s nuclear reactor for the Nautilus produced 10 MW (megawatts) of electricity, which was sufficient to provide the submarine’s propulsion and power all its vital life support systems such as maintaining air quality, regulating the temperature and distilling fresh water from sea water.

The Mightiest Motion Picture of Them All!

Disney’s Deep Sea Adventure Story.

Walt Disney played his part in selling Rickover’s revolutionary submarine to the American public. In 1954 Disney released its steampunk blockbuster 20,000 Leagues Under the Sea based on the Jules Verne masterpiece. It was the most expensive Hollywood film up to that date and starred James Mason as the dark anti-hero Captain Nemo. Although the film is set in the nineteenth century, it suggests that Nautilus is powered by a new and mysterious source of energy discovered by Nemo. The nature of this energy would be all too obvious to the cinema-goers of the 1950s who were very familiar with the nuclear ambitions of the United States.

The film was released on 23 December 1954, and less than a month later on 17 January, nuclear reality emulated science fiction with the launch of the USS Nautilus. At almost one hundred metres in length, she was somewhat bigger than her fictional namesake. Prior to her launch, all submarines might more accurately be regarded as submersibles. Most of their time was actually spent on the surface, with their dives restricted to relatively short periods. But Nautilus suffered from no such limits. By February 1957 she had logged 60,000 nautical miles, or twenty thousand leagues, under the sea. 

The USS Nautilus.

Her most famous mission came the following year. On 3 August 1958 Nautilus cruised beneath the polar icecap and became the first vessel to reach the North Pole. This was a big boost to American prestige, and ratcheted up the Cold War arms race another notch. The United States was still recovering from the shock of the previous October when the Soviet Union had launched Sputnik, the world’s first artificial satellite. Now the implied threat of Intercontinental Ballistic Missiles (ICBM) was countered by the threat of Submarine Launched Ballistic Missiles (SLBM). The nuclear-powered submarine armed with nuclear missiles would be the ultimate Cold War deterrent. Able to operate throughout the world’s oceans submerged for months at a time and essentially untraceable, it would be capable of launching a devastating nuclear strike even after its homeland had been wiped off the map.

Nuclear Proliferation

The American monopoly on seaborne nuclear propulsion did not last. Nuclear submarines are now found in the fleets of six nations: the United States, Russia, China, the United Kingdom, France and India. There are currently about 200 nuclear powered ships in the world. Most are submarines but they also include aircraft carriers and ice breakers. The reactors in these vessels typically generate around 100 MW of electricity and run on highly enriched uranium containing 20% to 93% uranium-235, which enables them to operate for decades without refuelling. Diesel generators are used as a back-up system in case of a reactor shut-down.

The only nuclear-powered submarine ever to fire on an enemy ship is the British HMS Conqueror which sunk the Argentine cruiser the General Belgrano with two torpedoes during the Falklands War in 1982. Of the almost one thousand on board the Belgrano, 323 were killed.

Atoms for Peace

The PWR was first developed for Rickover’s nuclear submarine programme. It is now the most widely used source of commercial nuclear energy. The first, and to date only, commercial PWR nuclear power station in the UK is Sizewell B on the Suffolk coast in southern England. It came into service in 1995 and generates 1.2 GW (gigawatts) of electricity.

Sizewell B, the UK’s first PWR nuclear power station.

Out of the 441 commercial nuclear reactors around the world, 299 are PWRs. They are capable of generating a combined total of 284 GW of electricity.


The Age of Rock

October 31, 2020

Rutherford suggested that the amount of helium trapped within a rock would reveal how much uranium has decayed since the rock formed. And by comparing the amount that has decayed to the amount that remains the age of the rock could be determined. For instance, if half the uranium has decayed then the rock’s age equals the half-life of uranium, which can be measured in the laboratory. 

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Pregnant Camels Often Sit Down Carefully

October 30, 2020

After several months of arduous work Holmes could assign an age to the rock—370 million years. This is considered to be the first accurate radiometric dating. (It agrees with modern results for the Devonian, according to which this period lasted from 419-359 million years ago.) Not bad for an undergraduate project! Holmes was just twenty years old and would graduate later that year. His project must rank as one of the most remarkable ever completed by an undergraduate.

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Why Does Antimatter Matter?

October 17, 2020

There is a very important modern technology that makes use of antimatter. It is known as PET (Positron Emission Tomography) and it is a non-invasive medical imaging technique.

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Rutherford’s Protons and Prout’s Protyles

October 12, 2020

Aston’s Whole Number Rule resurrected Prout’s hypothesis of a century earlier. But now the picture was looking decidedly clearer. In 1917 Rutherford had discovered one of the constituents of the atomic nucleus, a positively charged particle identical to the nucleus of a hydrogen atom. In 1920, and partly as a tribute to Prout, he named this particle the proton, taking the stem from Prout’s fundamental unit—the protyle. Rutherford speculated that the nucleus might have a second electrically neutral constituent with a similar mass to the proton. He named it the neutron.

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Drink a Toast to James Prescott Joule!

October 6, 2020

Joule soon became interested in the fundamental principles of physics rather than the practical considerations of brewing beer. Throughout his career he exchanged ideas with his renowned contemporary John Thomson, better known as Lord Kelvin. Together they worked to understand the connections between energy, work, heat and temperature, a subject known as thermodynamics. Joule’s investigations led to one of the most important principles in physics— the conservation of energy.

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Hell Creek Apocalypse

September 27, 2020

Four decades of detective work by geologists, nuclear physicists, palaeontologists and geochronologists has given us an incredibly detailed and compelling picture of what happened one dramatic day just over sixty-six million years ago—the day that sealed the fate of the dinosaurs.

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A Goddess Spurned and the Fate of the Sun

September 25, 2020

So how will the Sun evolve and what is its ultimate fate?
Within the Sun’s core hydrogen nuclei are gradually fusing to form helium nuclei, releasing a steady stream of energy. The flow of thermal energy from the nuclear furnace supports the Sun against its tendency to collapse under gravity. For as long as the nuclear fuel holds out this balance will be maintained. In the case of the Sun this is around ten billion years. Currently, the Sun is nearly half way through its allotted time span.

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An Exceptionally Cold Case

September 19, 2020

Much has been learnt about Ötzi the Iceman. High levels of copper and arsenic were found in his body, which suggests that he may have been involved in the smelting of copper. He was about 45 years old and he suffered from a number of ailments. He had parasitic worms in his gut, gallstones, arthritic joints, hardened arteries and rotting teeth. He also had numerous tattoos produced by rubbing charcoal into lesions in the skin. From the contents of Ötzi’s stomach it seems that his last meal included fatty ibex meat and einkorn grain. The radiocarbon dating of Ötzi’s remains reveal that he lived some time around 5300 years ago.

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