Condemned prisoners
Empire B has sent a few spies to learn the secret of Cobalt-Thorium G. Unfortunately for them, they have been caught, and await execution. Luckily for them, Empire A has had a change of heart and has allowed them a chance to pore over all of the technical schematics and interview key scientists!
Unluckily for them, they will only get a chance to answer a single Yes/No question about whether the plans work, in the presence of international observers, before their meeting with the intestinal spindle. *(this is via a highly restricted choice of hand signals, to prevent any untoward outburst or covert sign)