Udet said, “We tried to shut down. But the moderator was too far out of the core to have any immediate effect. The hydrogen in the core and the jacket boiled quickly and started to expand…”

“And now you’ve got a runaway,” Muldoon continued. “Because the reactor was designed with a positive temperature coefficient.”

Michaels sighed and locked his hands behind his head. “Just pretend I don’t know what you’re talking about.”

Muldoon grinned tightly. “I know. It took me a while to figure this stuff out. Look: suppose the temperature of your core rises. And suppose that the core is designed so that when it heats up, the reactivity drops — that is, the reaction rate automatically falls. That’s what’s meant by a ‘negative temperature coefficient.’ In that case you have a negative feedback loop, and your reaction falls off, and the temperature is damped down.”

“Okay. It’s kind of self-correcting.”

“That’s right; the whole thing is stable. That’s how they design civilian reactors. But in the case of NERVA, that coefficient was positive, at least for some of the temperature range. So when the temperature went up, the reactivity went up, too—”

“And the rate of fission increased, leading to a further temperature rise.”

“And so on. Yes.”

Michaels glared at Udet. “I can see the fucking headlines now, Hans. Why the hell did we fly an unstable reactor?”

Udet sat forward, his face pale, a muscle in his neck rope-taut with anger. “You must understand that we are not building a reactor to supply domestic electricity, here. We are not heating coffeepots. NERVA 2 is a high-performance booster, a semiexperimental flight model. Stability is not always the condition we require.”

Michaels frowned. And you just hate having to answer these asshole questions, don’t you, Hans? “Why do we need instability? What do you mean?”

Seger put in, “It’s like a high-performance aircraft, Fred. A ship that’s too stable will wallow like a sow. So you might design for instability. If a bird’s unstable, it can flip quickly from one mode to another; if you can control that, you’ve gained a lot of maneuverability”

“But that’s a big if, Bert. And evidently, when it got to the wire, we couldn’t control it. Hans, why didn’t you beef up the control system to cover for this?”

Udet punctuated his words by thumping the edge of his hand on Michaels’s desk. “Because — of — unacceptable — weight — penalties.”

Michaels dreaded having to put this man in front of the commission. “Let’s move on. What next?”