Many US ports already X-ray ship containers to see if they contain hidden nuclear material. But X-ray machines may fail to spot hidden uranium or plutonium if it is hidden behind lead shielding.
So Westinghouse Electric proposes generating a powerful controlled beam of neutrons by propelling deuterons into tritium. These will then cut through lead shielding and excite any fissile material, creating an "echo" of neutrons caused by a "fast fission" effect.
The echoes only last a microsecond or so and are quickly damped out, so there is no risk of a triggering a chain reaction. But they can be detected by a silicon carbide semiconductor sensor, which is switched on and off in between the trigger pulses. The sensor then gives instant and unambiguous warning of hidden nuclear cargo."
Speculation: It seems that if one did remotely detect a bunch of material in an enemy country then a prolonged and higher volume of neutrons could trigger a chain reaction. A way to blow up fissile material remotely.