Two definite milestones in the history of nuclear power were the manufacture of plutonium, the first man-made element, in 1941, and the first sustained nuclear reaction in 1942. Both milestones were modified in 1972 when it was discovered that there had been an operating nuclear power reactor 1.5 billion years ago, and that it had produced 3,300 pounds (1,500 kg) of plutonium. There were actually 16 reactors, which ran for a few hundred thousand years, breaking all run-time records and producing energy at an average rate of 100 kilowatts, in the Oklo uranium mine, in Gabon, Africa.

This remarkable discovery was made at the Pierrelatte Uranium Enrichment Facil­ity in France. Output from uranium mines was routinely analyzed by a mass spec­trometer, to insure that every atom of uranium fuel was accounted for and none were being diverted for weapons production. In May 1972, samples from the Oklo mine showed a curious discrepancy. Particular attention was given to the fissionable isotope U-235. The normal concentration of U-235 in raw uranium is 0.7202 percent. The Oklo samples showed only 0.7171 percent, and the difference was significant. The French Commissariat a I’energie atomique launched an immediate investigation, finding con­centrations of U-235 as low as 0.440 percent in the Oklo uranium.

Clues from a detailed analysis of the mineral content of the mine led to a startling conclusion. On September 25,1972, the Commissariat announced their finding: Self — sustaining nuclear chain reactions had occurred at the Oklo uranium mine about 1.5 billion years ago, producing 12,000 pounds (5,400 kg) of fission waste products and depleting the fissionable uranium in the ore.

The natural reactors formed in uranium-rich mineral deposits when groundwater inundated the ore. The water acted as a neutron moderator, bringing the concentrated uranium deposits to criticality, raising the temperature of the ore to a few hundred degrees Celsius, and boiling the water. As the water boiled away, a natural reactor would shut down, resulting in a pulsed action, over an interval of about 2.5 hours, as water once again collected in the ore, repeating the process for 100,000 years. At the time, more than a billion years ago, the U-235 concentration in the ore was about 3 percent, which is comparable to the fuel used in some power reactors today. Since the U-235 component decays away faster than the remainder of the uranium ore, the concentration of U-235 in natural uranium has dropped to about 0.7 percent since the natural reactors last powered up.

By 1864, the concept of electromagnetism in space was reconsidered by a Scottish mathematician and theoretical physicist named James Clerk Maxwell (1831-79). Faraday’s knowledge of algebra had been weak, and he could not formulate a mathematical argument for his idea, but Maxwell was a genius at calculus and had earned the Second Wrangler of Math­ematics degree at Trinity College, Cambridge. Maxwell was interested in everything scientific. He wrote an original essay in college, “On the Stability of Saturn’s Rings,” in which he concluded that the rings were not completely solid, nor liquid, but were composed of “brickbats.” He did

some important work on color and color blindness and took the world’s first color photograph in 1861, of a Scottish tartan. He studied Faraday’s work on magnetic lines of force, and with that as an inspiration, he for­mulated a set of 20 differential equations, in 20 variables describing the magnetic and electrical fields in both static and dynamic conditions.

The equations were complicated and difficult to fathom, but in these equations was a perfect, mathematical prediction that there exist waves of oscillating electric and magnetic fields that travel through empty space at a predictable speed. The speed predicted happened to be the speed of light, and Maxwell jumped to the conclusion that light is an electromagnetic wave, vibrating in a frequency band that we can detect with our eyes. Maxwell would be proven correct.

The implications of Maxwell’s equations remained an elegant but unap­plied theory until Heinrich Rudolf Hertz (1857-94), a German mathema­tician and physicist, made an accidental discovery in 1887. Hertz earned his Ph. D. in 1880 at the University of Berlin and became a full professor at the University of Karlsruhe in 1885. He had dabbled in the investiga­tion of many subjects, including meteorology and elasticity, but in 1887 he was working with a newly invented piece of high-tech equipment. It was a high-voltage coil, producing sparks a half-inch long, with a buzzer built into the end of the coil to sustain the spark. Hertz was fascinated by the effect of light on the spark. He noticed that the spark seemed to dim when ultraviolet light hit it. The light was apparently knocking electrical charge off the spark gap, and this was an exciting finding.

Of even greater importance than this photoelectric effect was an unex­pected by-product of the high-voltage spark. As Hertz turned off the lights to get a better look at his spark under ultraviolet, he noticed something out of the corner of his eye. There was another spark occurring in the room, in the gap between the ends of a loop of wire that was not connected to the apparatus. To his amazement, the spark produced by his high-voltage coil was somehow perceived and replicated by another spark gap, sitting on another table in the room. This concept of action at a distance seemed profoundly strange. There were no electrical wires connecting the two pieces of equipment, and yet if he threw the switch on his spark coil, a spark would occur on a loop of wire on the other side of the room. He was affecting the loop of wire, the antenna, by generating Maxwell’s electro­magnetic wave. Hertz had discovered radio, and he had confirmed Max­well’s vision of radiating waves.

Wilhelm Roentgen (1845-1923), a German physicist, was also fas­cinated by the high-voltage coil and its novel effects. Roentgen had

graduated from the University of Zurich in 1869 with a Ph. D. and was named the physics chair at the University of Wurzburg in 1888. He was studying the effects of a high-voltage coil connected to an evacuated glass tube. Study of the newly discovered cathode rays was popular in Europe in the 1890s, and it seemed as if everyone on the leading edge of science was experimenting with some form of vacuum tube. The cathode ray would soon be identified as a stream of electrons, or small components of atoms stripped off by high voltage, but in 1895 the ray was only known to travel from one end of the tube to another, from the negative to the positive high-voltage electrodes, causing the glass to fluoresce. Roentgen wanted to find out if he could cause the rays to leave the tube and enter the air surrounding it. In the late afternoon of November 8,1895, he tried a special tube, built by a colleague, having a thin, aluminum window on the end. The cathode rays might penetrate the aluminum, and he would use a piece of cardboard painted with barium platinocyanide as a detector.

Being careful, Roentgen devised a cardboard shield to fit over the tube so that no fluorescent light would escape and spoil his measurement, but as he dimmed the lights in the laboratory to test his shield with the tube running at full power, he noticed something out of the corner of his eye. Just as Hertz had noticed his sparks, Roentgen noticed that his piece of cardboard, on a lab bench more than a meter away, was shimmering with yellow-green light. He had hoped to get cathode rays out of the tube, but he knew that they could not have enough energy to bore through the air and hit the barium screen that far away. He had discovered a new type of ray. When the cathode rays hit the aluminum window at the positive electrode end of the tube, they were stopped, and the sudden deceleration produced high-energy rays, invisible and streaming out the end of the tube, just as Maxwell’s equations had predicted. Experiments over the next few days proved that these new rays were more powerful than light and could penetrate solid objects. Needing a quick, temporary name for his discovery, Roentgen called them X-rays.

By 1896, atomic science was progressing rapidly, with physics journals having trouble keeping up with the rate of discovery. Antoine-Henri Bec — querel (1852-1908), a French physicist, was caught up in the excitement and was investigating the work of Wilhelm Roentgen. Although he had studied physics at the bcole Polytechnique, there were practical consider­ations for getting a paying job, so he also studied engineering at the bcole des Pont et Chaussees and became chief engineer in the Department of Bridges and Highways.

Practical work did not keep him from his fascination with Roentgen’s work, which was very successful, with immediate applications in medi­cine, but not completely understood. The composition of cathode rays was unknown. It was known only that something would stream from the negative electrode, or cathode, at one end of a glass tube, with the air removed, to the positive electrode at the other end of a glass tube, when 30,000 volts were applied to the electrodes. When the cathode rays hit the glass at the positive end, they caused the glass to glow, but, aside from that, the cathode rays were invisible in a hard vacuum. Roentgen still did not realize that his X-rays were produced by electrons hitting his big, alu­minum, positive electrode, because the electron had yet to be discovered. Becquerel went to the weekly meeting at the museum national d’Histoire naturelle in Paris on January 20, 1896, to hear a report on Roentgen’s work in Germany. Roentgen was convinced that his powerful X-rays, which

would penetrate light-shielding and fog photographic plates, were pro­duced by the induced fluorescence in the end of the tube.

It occurred to Becquerel that if the weak fluorescent glow at the end of a cathode-ray tube produced X-rays, then he could produce a greater flux of X-rays by using a material that would give a bright, robust fluores­cence under ultraviolet light. He immediately bought all the fluorescent materials he could find and began experimenting, using the ultraviolet component of sunlight to excite fluorescence and using sealed photo­graphic plates to record his X-ray production. Although his experiments were carefully assembled, he was getting no results. In 10 days of experi­menting, he could not fog any film with fluorescence-induced X-rays. On January 30, he read an article on X-rays, and it encouraged him to keep trying.

Becquerel bought some uranium salt, uranyl potassium sulfate, the most strongly fluorescent substance available, sprinkled some atop a sealed photographic plate and exposed it to sunlight for several hours. The experiment was immediately successful, or so he thought. When he devel­oped the plate, he could see the black silhouette of the sprinkled uranium salt on the negative. Obviously, he had found the right fluorescent mate­rial to make X-rays using sunlight. The commercial possibilities of the discovery were wonderful. He could manufacture a simple medical X-ray machine that would require no electricity and no fragile glass tubes and could be used in remote locations.

Just to make sure of the results, on February 26, Becquerel prepared another photographic plate, wrapped in thick, black paper, with a small amount of uranium salt on top. Unfortunately, the weather in Paris had turned cloudy. With no sunlight, he slipped his experiment into a dark drawer in his desk. The next day was cloudy as well. On March 1, for some odd, serendipitous reason, Becquerel decided to go ahead and develop the plate, without any ultraviolet light having excited the fluorescent uranium.

To Becquerel’s amazement, the plate was clouded, as if the light-shield had been defective, but the shape of the dark cloud was a perfect replica of the irregular scattering of uranium salt. Furthermore, the clouding on a plate abandoned in a dark drawer for three days was much darker than he had achieved in sunlight for a few hours. He started putting the evidence together, and he realized that the sunlight and the fluorescence had noth­ing to do with the effect. It was something in the uranium that was cloud­ing the plates. Henri Becquerel had discovered some kind of force that

could cloud a photographic negative, through the light-tight cover, requir­ing no high-voltage tube to produce it. It was something that could not be felt, seen, heard, tasted, or smelled. He gave it a name: Becquerel rays.

In a few years, Becquerel’s important discovery would be given a new designation by Marie Curie (1867-1934), radioactivity.