Jamie Edwards, a pupil at Penwortham Priory Academy, in Lancashire created the reactor from scratch with help from his school, breaking the record of US student Taylor Wilson, who was 14 when he created nuclear fusion in 2008.
Jamie, who started work in October in an under-used school science laboratory, recreated a process known as 'inertial electrostatic confinement' which dates back to the 1960s.
"One day, I was looking on the internet for radiation or other aspects of nuclear energy and I came across Taylor Wilson," said the junior scientist who faced a race against time to complete the project before his 14th birthday this week.
His application for funds was rejected by various nuclear laboratories and universities.
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"I was a bit stunned and I have to say a little nervous when Jamie suggested this but he reassured me he wouldn't blow the school up," said Priory head Hourigan, who agreed to give 2,000 pounds to the project.
Jamie ordered parts and equipment from Lithuania, the US and UK, working on the project every break and lunchtime as well as after school.
"It's magnificent really. I can't quite believe it - even though all my friends think I am mad," Jamie said.