Page 10 of Daikon
“Yes, that’s exactly right, Sagara-chūsa.
If there is a greater concentration of U-235 atoms in the sample, the neutrons released in the first reaction will have a greater chance of encountering other U-235 atoms and causing them to split too.
One atom splits to begin the process, the released neutrons cause two other atoms to split, then four, then eight, then sixteen, thirty-two, sixty-four, and so on. ”
Kan stopped, embarrassed by the realization that he was now talking so quickly. Colonel Sagara motioned for him to continue.
“This is called a chain reaction,” said Kan.
“The process, and the release of energy, increases exponentially, and it happens very quickly. In a fraction of a second, trillions and quadrillions of atoms are fissioning, and the explosive release of energy is immense. It would exceed any bomb ever created.”
“?‘A weapon powerful enough to destroy a whole city,’?” murmured Colonel Sagara, thoughtfully nodding.
Kan recognized the line from the Project Ni-Go report he had helped write. “Yes, Sagara-chūsa. Exactly right.”
“So, to create a uranium bomb, the proportion of these 235 atoms in uranium has to be increased.” The colonel glanced down at the open file under his hand. “This Separation Team you headed, that was your task? To separate uranium-235 atoms? To create the material to make a uranium bomb?”
“Yes, Sagara-chūsa. The process is called enrichment.”
“I see. And how much of this uranium-235 did you enrich?”
Kan sought a new position on his chair as he brought to mind the vast effort of building a thermal diffusion separator and trying to get it to enrich uranium hexafluoride.
In theory it should have worked, neutrons being pulled away from some of the uranium-238 atoms as the hex gas rose through the separator so that the minute quantity of green sludge collected at the top contained an increased proportion of U-235.
Even a small degree of enrichment would have been an achievement. But there had not been even that.
“None, Sagara-chūsa,” he said. “None of the samples showed any increase in the proportion of U-235 when they were tested in the cyclotron. Or there was so little enrichment it couldn’t be measured.
It could have been due to leakage. The uranium hexafluoride gas we were using was corrosive and would eat through the tubing.
But I believe we overcame that. I don’t think that was the reason.
I think the gap the gas passed through was the problem, the gap between the hot and cold surfaces that should have drawn the neutrons away.
It had to be two millimeters wide, two millimeters precisely the entire length of the apparatus.
And we couldn’t achieve that degree of precision. Even the slightest warp in the—”
Sagara held up his hand to stop him. “So no uranium was enriched. Not even a gram.”
“No, Colonel. It was an extremely difficult process. We didn’t have enough time.”
The office door opened and the colonel’s aide, Captain Onda, quietly entered.
He placed a file on Sagara’s desk and took a position standing at his side.
Sagara picked up the file and leaned back in his chair to peruse it, pausing twice to direct penetrating glances at Kan.
The scientist, with mounting apprehension, had the distinct feeling that the pages being read were about him.
The colonel finished with the file and set it aside.
“Yes, well, let’s talk about the amount of U-235 that would be necessary to make a uranium bomb.
” He referred again to the open Project Ni-Go file on his desk.
“In his report of March 1943, Dr. Nishina estimated that it would take ten kilograms of enriched uranium to build an atomic bomb. Ten kilos with the U-235 content enriched to ten percent. What are your thoughts about that?”
“I… I would support Dr. Nishina’s estimates on that matter,” Kan cautiously ventured.
Sagara flicked the end of his cigarette into the ashtray. “I want your honest assessment. Speak candidly, please.”
“I was not directly involved in those calculations. The theory work was done by Dr. Nishina and the Theory Team. Tamaki’s team. I was involved in the—”
Sagara brought his hand down hard on the desk, making Kan jump. “Speak candidly, please!”
Kan clasped his hands tightly and tensed his stomach, trying to stem the onset of renewed shaking. He did not succeed. Colonel Sagara, with a look of annoyance, leaned back in his chair and gave the scientist a moment to master himself.
Kan tried again. “I… I believe Dr. Nishina’s estimate was correct for a theoretical explosion.
He is a better mathematician than me and I can’t fault his calculations.
But…” He paused for another breath. “But they were based on the assumption of an efficient explosion. An explosion in which most of the U-235 atoms would fission. In a real bomb, an explosion would not be so efficient.”
“Explain that,” demanded the colonel.
Kan held up his hands shoulder width apart.
“When the bomb is detonated, two pieces of enriched uranium would be brought together…”—he brought his hands together—“the mass would become supercritical, and a chain reaction would begin among the U-235 atoms. Then the mass would blow apart”—he jerked his hands apart—“ending the supercritical condition before all the U-235 atoms had time to split.”
Colonel Sagara’s eyes were wide. “Wait a moment, wait a moment. These two pieces of uranium—how would they be brought together?”
“We made only a few theoretical sketches, Sagara-chūsa, but the general idea was to use a gun apparatus inside a bomb casing. To fire one mass of U-235 into another to initiate the process.”
“?‘A metal tube like a gun barrel,’?” Sagara murmured. He shot a glance at his aide, Captain Onda.
“All right,” he said, returning to Kan. “So you believe that an amount of uranium larger than ten kilos would be needed to build a uranium bomb. Or an enrichment of greater than ten percent?”
“Both, Sagara-chūsa. More uranium and greater enrichment, since only a portion of the material would have a chance to fission.”
“How much more?”
“I could only guess, Colonel. I would say at least double the estimate in the initial report.”
“Twenty kilos at twenty percent enrichment?”
“Possibly. Perhaps more. There is no way of really knowing without experimentation with enriched uranium. And we didn’t produce any. It was an extremely difficult process. We didn’t have enough time.”
“How much time would you have needed?”
“To perfect the enrichment process,” Kan replied, carefully choosing his words, “and amass enough material for one weapon—that would take years.”
“How many years? Two years? Two hundred years? Give me a number.”
This was curious. Did the colonel envision launching a new atomic research project? Such an endeavor, at this late date, would be hopeless.
“Even ten years would be too optimistic,” Kan ventured. “Twenty years, perhaps, assuming there was adequate funding, materials, and personnel. But realistically, I would have to say that it would take even longer than that. When you consider the current war situation—”
Dangerous ground. He stopped himself in mid-sentence.
Colonel Sagara did not seem to notice. “And what about the Americans? You know they have been doing their own research. Could they have mastered the enrichment process and created a bomb?”
Kan had no doubt of the answer. “No. Even for the Americans, it will take years.”
The colonel fell silent and sat regarding Kan with his deep-set eyes. He referred again to the file that Captain Onda had brought him, then returned to his silent perusal of Kan, until the scientist was scarcely able to sit still on his seat.
The colonel finally nodded, evidently having made a decision. He retrieved a paper from beneath the Project Ni-Go report and pushed it across the desk to Kan.
“This came in last night,” he said. “Tell me what you make of it.”
It was a copy of a military cable. Keizo Kan read it, then read it again:
Possible new-type bomb recovered from B-29 crash site, Hikari, Yamaguchi Prefecture. Length: 3.04 m. Diameter: 71 cm. Weight: 4,300 kg. Metal tube like gun barrel, diameter 16 cm, runs through center. Metal rings removed from inside, 25 kg, identified as uran.
He started to shake his head as he stared at the cable and absorbed its full meaning.
“Well?” said Sagara.
“No. Even the Americans couldn’t do it,” said Kan. “This must be something else.”
“But it contains uranium,” said the colonel. “And look: ‘Metal tube like gun barrel.’ We know the Americans have a uranium bomb project. We know they acquired a number of important Jewish scientists from Europe. And we know that this device was flown to Japan in a special plane.”
Kan considered this for a moment. The biggest piece of uranium metal he had ever seen in his life had been the size of three coins placed one on top of the other. Twenty-five kilos of natural metallic uranium were therefore riches beyond imagination. And twenty-five kilos of enriched uranium…
“That’s impossible,” he said. “It can’t be uranium. It must be a mistake.”
Sagara stabbed his finger at the cable. “But it says uran . The Americans have the Shinkolobwe mine in the Belgian Congo. They have all the uranium they need. And we know they have been working on an atomic bomb project. Extremely large scale.”
Kan sat with his head bowed, trying to think. It was unimaginable that even the Americans could have mastered all the problems so quickly and built an actual weapon. The number of separators alone, assuming they worked, would have to number in the thousands.
“If it is uranium in this bomb,” he said, forestalling the rising impatience in Colonel Sagara’s face, “then it must be natural uranium. U-238. Although I don’t know what would be the purpose. A warning, perhaps.”