The first set of plutonium production reactors at Hanford were water-cooled, but even the Americans admitted that this scheme, while elegant and impressively complicated, invited a disastrous steam explosion. Let a water pump fail, and the coolant stalled in the reactor would flash into steam, sending the works skyward. Besides all that, unlike the Yanks, the Brits had no mighty river like the Columbia to waste as an open-loop cooling system. Using seawater was a possibility, but there would be destructive corrosion. Penney’s design team decided to do it the easy way, just like X-10, blowing air through the loose-fitting fuel channels and up a smokestack, sending the generated heat and whatever else managed to break loose up into the sky and out over the Irish Sea. Each reactor would be equipped with eight very large blowers, two auxiliary fans, and four fans dedicated to shutdown cooling.[121] Each main blower was a monster, driven by a 2,300-horsepower electric motor running on 11,000 volts. The exhaust stacks were designed to be an imposing 410 feet high, made of steel-reinforced concrete.
Such a reactor could be made using heavy water as the neutron moderator, as the Canadians were demonstrating, but it was cheaper and easier to use graphite, just like the X-10. It would take 2,000 tons of precision-machined graphite blocks to make one reactor.
Graphite is a strange and wonderful material. It is a crystalline form of carbon. So is diamond, and a diamond is the hardest material on Earth, hard enough to saw through a sapphire. Graphite is completely different, being soft enough to cut with a butter knife and be used to lubricate automobile window channels. In recorded history, an enormous deposit of natural graphite was first discovered on the approach to Grey Knotts from the town of Seathwaite in Borrowdale parish, Cumbria, England, about 13 miles northeast of Windscale. Locals found it exceedingly useful for marking sheep, and news of this proprietary livestock branding method leaked out in 1565 after decades of use. Expanding on this theme, entrepreneurs in Cumbria proceeded to invent the lead pencil, and from this wonderful application the mineral acquired its official name from the Greek word grapho, meaning to write or draw.
As happens to many such discoveries, a military use was soon found and the English government clamped down on the source. It had been determined that lining the molds for making cast-iron cannonballs with graphite resulted in smooth, well-thrown projectiles, and the military-industrial complex of 16th century England was well pleased. Afterward, the manufacturing enterprise found myriad uses for the fine, pure English graphite from Cumbria, from arc-light electrodes to the throw-out bearings for MGB sports cars. British industry thus had a long and complete history of working with graphite. They may not have known much about working with uranium in a controlled-fission environment, but they knew their black, greasy mineral like the bottom of a Guinness glass.
The United Kingdom, still aching from the post-war abandonment, was trying to impress the United States nuclear establishment with their rapid ascent to atomic power status and thus reestablish a badly needed intimate connection to the material and industrial strength of its former colony. By showing that they were on equal footing and had expertise to contribute, they wished to demonstrate that an Anglo-American nuclear alliance could be a strong bulwark against the Red Menace that was having its way with Eastern Europe. The Windscale construction job was one of the largest, most time-pressed in English history, employing nearly 5,000 workers, including more than 300 surveyors, architects, and building engineers. Everything was in short supply except determination.
The Americans, irritatingly miles ahead of the Brits in these matters, were hard to impress, but they were concerned with what they had heard about the British plutonium production initiative. In 1948, a team from Los Alamos slipped into the headquarters at Harwell to give some advice. There were a couple of days of classified seminars with Bill Penney’s top scientists. All the talk came down to this: Whatever you think you know about graphite is wrong. The conversation between the American scientific delegation and their British counterparts might have gone like this:
“For example,” offered the Yank, “there’s the Wigner Growth.”
The Brit lowered his teacup slowly, seeming to twitch slightly. “Say what?”
“The Wigner Growth. If graphite is exposed to fast neutron flux, as it will be in your production pile design, the recoil action of neutrons colliding with carbon atoms will displace these atoms in the graphite crystal. Over time, changing the distance between atoms changes the physical size of your graphite block. It seems to grow.”
“But how do you …,” Nigel sputtered.
“Control it? You don’t. You have to allow for it in the design. The only thing you can control is the direction in which it grows. If you make your blocks by extruding them, then the blocks will lengthen only at right angles to the extrusion axis.”
“Well, dash it all.”
It was best to learn of this wrinkle early on in the construction. Heroic redesign of the core allowed each block to expand horizontally, at right angles to the axis of extrusion, while being held in place with graphite slats. By March 1949, word had arrived from hastily convened experiments at Harwell, trying to replicate the effect claimed by the Americans. British graphite, so wonderful for making pencils, was completely different from the synthetic graphite they used in the U.S.A. It expanded in all directions. That was bad. The prediction was that after running just 2.5 years, the pile would be warped and unusable. The fuel channels would close up and trap the aluminum canisters in the core. The Canadians came through with a modified prediction. Yes, British graphite would expand on all axes, but only at one fifth the length of the American product. That was good. Under that condition, the pile might operate for as long as 35 years, if the internal stresses did not crumble it like a tea biscuit. Other potential problems were revealed too late to be designed out so easily.
Later in 1948, Cockcroft, head of the entire British nuclear enterprise, came back from a fact-finding tour of the X-10 at Oak Ridge with some more disturbing news. They would have to filter the pile-cooling air before releasing it to the general atmosphere. Of all the 70,000 fuel canisters in one pile, if the delicate aluminum case around just one of them were to break open, then its highly radioactive contents would go straight up the chimney and rain down on the dairy farms of Cumbria. That could be a problem. When building their improved version of the X-10 up on Long Island, New York, the Americans had put filters between the atomic pile and the air exhaust stack, just to prevent a fission product spread if the fuel broke apart or caught fire. “We shall do the same,” pronounced Cockcroft.
121
No published plan diagram of the Windscale reactors shows the blowers. That is because the blowers were located in two separate buildings per reactor, one left and one right, connected by large concrete tunnels. The Windscale reactors are still there and can be seen on Google Earth. Windscale Unit 1 is at latitude 54.423796°, longitude -3.496658°. The stack on Unit 1 has been torn down and the base is filled with concrete. Unit 2, to the left of Unit 1, looks complete, but the west-side blower building has been torn down and made into a parking lot.