While זרע means seed, גרעין means pit as in the pit of a peach, the core of the fruit.
Hebrew borrows the physical גרעין and applies it to the core of something quite difficult to grasp even if still material: the nucleus of an atom. Thus we have גרעיני – nuclear, as in:
אוי ואבוי לעולם אם איראן תשיג נשק גרעיני.
Woe to the world if Iran obtains nuclear weapons.
גרעיני applies not only to weapons of mass destruction, but to anything else nuclear, such as family – משפחה גרעינית:
לא הזמנו את כולם, רק את המשפחה הגרעינית וכמה חברים.
We didn’t invite everyone, only the nuclear family and a few friends.
