Criticality

29 11 2010

“We’ve had some serious problems with the purity of the graphite,” Faraday said.

Clarendon exchanged a quick glance with the other members of the Presidium. “How serious?”

“Well,” Faraday replied, “at first we were measuring cross sections of between fifty and five hundred by ten to the negative twenty-seven square centimetres. Unacceptably high. Using graphite of such unrefined quality would only lead to a runaway reaction and a meltdown.”

“And now?”

“After some to-and-fro with our colleagues over at Alpha-4, we’ve managed to get the average cross section down to four point oh to four point seven by ten to the negative twenty seven square centimetres.”

Faraday took a stylus to his tablet and wrote: 4.0 – 4.7 x 10-27 cm2.

“For a uranium and graphite reactor to operate at all, we need a cross section for slow neutron absorption of no more than this.” Beneath his scrawl, Faraday scribbled another figure: 4 – 5 x 10-27 cm2. “It turned out after some testing that our first batches were contaminated with traces of boron and other rare elements.”

“So it – the graphite – it is safe to use?”

Faraday smiled. “We would not be here today if it were not.”

Faraday went on to explain to the delegation that, in the six months since the First Congress, the two mining sites had assembled a stockpile of six tons of uranium and almost four hundred tons of graphite. Not all of this graphite had been mined in that half-year; much had been mined before the Congress, and, after refinement, was put to use as the moderator in the nuclear reactor.

Through the transparent blast shield, the group looked down on a channel dug into the regolith and bedrock. The channel sloped downwards through a shallow gradient over fifty meters, at the end of which was the reactor. The reactor was dug into a pit a further ten meters into the ground. The regolith and fractured bedrock were the primary shielding for the reactor. Between the control room and the reactor were blocks of basalt lined with concrete and paraffin, and lead walls filled with boric acid.

The pile itself was a cubic lattice of uranium metal rods suspended within a sphere of graphite. This sphere was embedded within polished slabs of basalt, so that the entire reactor appeared as a black-grey monolith set into the red-brown crater wall. Steel cables ran from the top of this sphere along suspended pulleys back to the control room. These cables were attached to the control rods: six bars of cadmium that hung above the centre of the pit.

“The cadmium rods control the reaction.” Faraday explained, as he gestured to the point where the cables disappeared into the pit. “They can be lowered or raised into the reactor. Cadmium is a strong absorber of neutrons; dropping these rods into the reactor prevents a chain reaction from developing.”

Clarendon asked, “And, presumably, these rods can be dropped down in an emergency to prevent a runaway reaction?”

“Indeed. We call that a ‘scram.’ ”

The reactor had been constructed in assemblies. Graphite was laid in layers and into this mass of graphite were drilled the holes for uranium slugs. There was room between each of the uranium slugs for neutrons from one slug to bounce off carbon atoms in the graphite before entering another slug. This action slowed the neutrons and allowed them to better resist absorption by U238 nuclei and instead be absorbed by U235.

“This is a rather modest design,” Faraday said. “Necessarily so because of our limited resources.”

Clarendon stepped forward and brushed his fingertips along the control panel. “Mmm. Once we have amassed more experience from this reactor, we may build others. Larger, more efficient. But the colony does not yet have an urgent need for trans-uranic elements.”

Faraday brought up a display on the blast shield, and tapped away at it with his stylus. Graphs slipped from the heads-up display into the margins until the central display was flanked by a half-dozen graphs and tables. Through these coloured images, Clarendon and Faraday watched a group of figures retreat from the generator several hundred meters away.

Faraday turned to Clarendon and the other delegates. “Shall we?”

The Presidium members nodded, and Faraday worked the controls and began to withdraw the cadmium control rods from the reactor. Immediately, the neutron counter began to click away, and a line graph on the right spiked. Faraday pointed out a number of graphs – gamma-ray and neutron counters, reactor power levels, galvanometers – and commented on their significance. Faraday pointed at one graph in particular.

“That’s a boron trifluoride counter, buried under the reactor. It’s showing neutrons have penetrated the basalt shield around the pile.”

Faraday withdrew more of the control rods, but kept two of the emergency rods within the pile. Neutron levels multiplied. There was linear growth of reactor power, a steady but shallow rise on a line graph updated in real-time. The clicking noise grew rapid. One of Faraday’s colleagues was calling out data from counters in decimals of one.

“Point seven five. Point eight. Point eight five. Here it comes.”

When the neutron intensity reached one, Faraday locked both the control and emergency rods in place. He turned to the information that his colleague was examining. After a few moments of altering the filters and examining the data, he turned to the Presidium members.

“We have achieved criticality.” Faraday smiled. “We have a self-sustaining chain reaction.”

“Mmm,” Clarendon replied. “That’s it?”

“That’s it. There are no bright lights or loud noises to announce criticality, but with nuclear reactors, it is generally better that way.”

Clarendon did not return Faraday’s grin. “And to introduce lithium to the reaction? To breed tritium?”

Faraday explained that the design of the reactor included a channel, like one of the holes drilled for the uranium slugs, into which materials could be remotely introduced.

“We’ll need to boost the power of this reactor before we do that, however,” he continued. “Reactors produce one gram of plutonium per day at a thermal power level of 500-1500 kilowatts. This pile could be powered up to around three thousand kilowatts, but not for any substantial period of time.”

“And it is now running at…?”

“About 500 watts.”

“Ah.”

“As more uranium is refined at the diffusion plant, we’ll add more assemblies to the pile and slowly boost its power. Nonetheless, we should appreciate the first nuclear reaction generated using materials not of Sol.”

Now Clarendon smiled. “With the exception of the stars themselves, Dr. Faraday.”

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