SB 1.7.28 P The Son of Drona Punished
na hy asyanyatamam kincid
jahy astra-teja unnaddham
astra-jno hy astra-tejasa
na--not; hi--certainly; asya--of it; anyatamam--other; kincit--anything; astram--weapon; prati--counter; avakarsanam--reactionary; jahi--subdue it; astra-tejah--the glare of this weapon; unnaddham--very powerful; astra-jnah--expert in military science; hi--as a matter of fact; astra-tejasa--by the influence of your weapon.
O Arjuna, only another brahmastra can counteract this weapon. Since you are expert in the military science, subdue this weapon's glare with the power of your own weapon.
For the atomic bombs there is no counterweapon to neutralize the effects. But by subtle science the action of a brahmastra can be counteracted, and those who were expert in the military science in those days could counteract the brahmastra. The son of Dronacarya did not know the art of counteracting the weapon, and therefore Arjuna was asked to counteract it by the power of his own weapon.