Her reaction told him that the information from Frunze was probably sound. She had immediately understood the significance of what he’d said. ‘Please answer the question,’ Volodya said sternly. ‘Even though we’re friends, this is official business.’

‘Okay. Do you know what an isotope is?’

‘No.’

‘Some elements exist in slightly different forms. Carbon atoms, for example, always have six protons, but some have six neutrons and others have seven or eight. The different types are isotopes, called carbon-12, carbon-13 and carbon-14.’

‘Simple enough, even for a student of languages,’ Volodya said. ‘Why is it important?’

‘Uranium has two isotopes, U-235 and U-238. In natural uranium the two are mixed up. But only U-235 is explosive.’

‘So we need to separate them.’

‘Gaseous diffusion would be one way, theoretically. When a gas is diffused through a membrane, the lighter molecules pass through faster, so the emerging gas is richer in the lower isotope. Of course I’ve never seen it done.’

Frunze’s report said that the British were building a gaseous diffusion plant in Wales, in the west of the United Kingdom. The Americans were also building one. ‘Would there be any other purpose for such a plant?’

She shook her head. ‘Figure the odds,’ she said. ‘Anyone who prioritizes this kind of process in wartime is either going crazy or building a weapon.’

Volodya saw a car approach the barricade and begin to negotiate the zigzag passage. It was a KIM-10, a small two-door car designed for affluent families. It had a top speed of sixty miles per hour, but this one was so overloaded it probably would not do forty.

A man in his sixties was at the wheel, wearing a hat and a Western-style cloth coat. Beside him was a young woman in a fur hat. The back seat of the car was piled with cardboard boxes. There was a piano strapped precariously to the roof.

This was clearly a senior member of the ruling elite trying to get out of town with his wife, or mistress, and as many of his valuables as he could take; the kind of person Zoya assumed Volodya to be, which was perhaps why she had declined to go out with him. He wondered if she might be revising her opinion of him.

One of the barricade volunteers moved a hedgehog in front of the KIM-10, and Volodya saw that there was going to be trouble.

The car inched forward until its bumper touched the hedgehog. Perhaps the driver thought he could nudge it out of the way. Several more women came closer to watch. The device was designed to resist being pushed out of the way. Its legs dug into the ground, jamming, and it stuck fast. There was a sound of bending metal as the car’s front bumper deformed. The driver put it in reverse and backed off.

He stuck his head out of the window and yelled: ‘Move that thing, right now!’ He sounded as if he was used to being obeyed.