"Reactor" (リアクター Riakutā) is an archetype of Machine monsters from Yu-Gi-Oh! 5D's. They are used by Greiger in the anime. Their effects inflict 800 damage to the opponent when they play a particular type of card. All Reactors except "Summon Reactor・SK" destroy the designated card, but none of the "Reactors" negate the effect of the card.
The lettered suffixes of "Summon Reactor・SK", "Trap Reactor・Y FI", and "Spell Reactor・RE" spell out "SKY FIRE". Together they combine to Summon "Flying Fortress SKY FIRE". In the Japanese version, the Reactors are called "Summon Reactor・AI", "Trap Reactor・RR", and "Magic Reactor・AID". Their suffixes spell "AIRRAID", while "Flying Fortress SKY FIRE" is "Giant Bomber Airraid".
Visually, they are all based off of WWII-Style aircraft, specifically bombardment-type or fighter/bomber aircraft. They were released in the Pack, Crimson Crisis, and are solely supported by the OCG/TCG effect of "Dark Flattop".
The use of "Reactors" can force an opponent into a difficult situation, especially when multiple "Reactors" are on the field. Simply playing cards will cause the opponent to take damage, and the effects of the "Reactors" destroying them will make playing Equip, Continuous and Field Cards very difficult. This can be especially detrimental to Different Dimension, "Uria" and "Hamon", "Volcanic" and other Decks that rely on such cards. The main weakness of "Reactors" is that the lower-Level monsters have a survivability issue (which is partially mitigated by "Fake Explosion"), so to be effective a Deck involving "Reactors" must rely upon cards that can block or stall an opponent's attacks until "Flying Fortress SKY FIRE" can be Summoned. Both can be Summoned with "Mystic Tomato", being DARK monsters.
All of the "Reactors" are DARK, so cards such as "Dark Armed Dragon" may be very useful in tight situations.
"Dark Flattop" provides an easier method to Special Summon "Flying Fortress SKY FIRE" after it is destroyed.
Since the "Reactors" have good field control, they would be good in field control based Decks. However they would take up most of the card spaces, so your main purpose of the "Reactors" should be getting "SKY FIRE" out.
Dark Reactor Deck
The Dark Reactor Deck takes the advantage that all the three "Reactors" are DARK. The monster can be easily Summoned using "The Dark Creator" or replaced by means of "Phantom of Chaos". Because in this approach it is necessary to bring them into the Graveyard we can make use of "Foolish Burial", "Armageddon Knight" and "Dark Grepher". "Allure of Darkness", "Trade-In" and "Reinforcement of the Army" can easily speed up the Deck.