Llanelli fought their way back into the match when Matthew Brayley stormed over to give Luke Richards and easy conversion before No 8 Dan Godfrey broke Aberavon hearts when he stormed over in the corner to edge Llanelli in front.
Aberavon winger Richard Carter scored a hat-trick and was only denied a fourth with a cruel bounce of the ball with the Wizards' remaining points coming via the boot of fly half Jamie Davies with a conversion and two penalties. Nick Reynolds, who played as a sub the day before for the Scarlets against the Ospreys, scored Llanell's first try while Richards knocked over a conversion and three penalties in a fiercely competitive game.
A relieved man after the game, Buchanan, said: "It was a very tense match, everything you could hope for in a semi-final. Commiserations to Aberavon who put in a terrific performance but I though we derseved the win in the end.
"Rugby is played over 80 minutes, we hadn't taken all our chances early on and put a few things right in the second half. These are a proud bunch of players and it will be great to take another side to a Cup final at the Millennium Stadium.
"We haven't mentioned it until now but the double is now certainly a possibility. Hopefully this win will give our league form an extra boost - if we do well against Newport and Cardiff in the next seven days we have every chance."
Aberavon head coach Simon King said, 'We are hugely disappointed, we didn't feel we played anyway near our potential, it seems to happen to us in semi-finals. I felt the yellow card to Darryl Thomas was pivotal, especially in such a tight game.'