Six other protesters were handed five-month suspended sentences for the October stunt at the plant in Cattenom, near the border with Luxembourg, which was intended to show the facility's vulnerability to attack.
Greenpeace France, represented by local head Jean-Francois Julliard, was also fined 20,000 euros ($24,500) by the court in the northeastern town of Thionville.
"Greenpeace crossed a red line," said prosecutor Christelle Dumont, adding that any debate about nuclear safety "must be in accordance with the law".
Greenpeace lawyer Alexandre Faro said the protesters were fighting for ideals and "they do not deserve hard time in prison".
The prosecution had requested six-month suspended sentences for six of the activists, including Greenpeace France's chief anti-nuclear campaigner Yannick Rousselet.
Non-suspended six-month sentences were requested for the two other activists, who had previously been convicted of breaking into two other French nuclear sites.
The eight activists broke into Cattenom before dawn on October 12, and the state-owned EDF energy company that operates the plant said the protesters were detained before they reached the nuclear zone, and that the plant's safety was not threatened.
Greenpeace said the fireworks were set off at the foot of a spent fuel pool -- where nuclear plants store highly radioactive fuel rods that are removed from reactors after their use.
After Greenpeace activists broke into another nuclear plant in November, the French government opened a parliamentary inquiry into nuclear safety and security.