No, that is not really correct. A two-stage thermonuclear weapon works by setting off a primary stage, which then sets of the secondary stage. Both the primary and secondary stages use both fission and fusion, and most of the energy in each stage comes from fission reactions. In the primary, a fusion-boosted fission reaction is used (this involves filling the hollow core of the fissible pit with deuterium and tritium, which then fuses when the pit is imploded, releasing lots of energetic neutrons and so improving the yield of the pit fission). The secondary uses three distinct nuclear reactions: first is a fission of the internal sparkplug, followed by fusion of the DT fuel, and finally fission of the DU pusher (the majority of the energy for the entire 2-stage bomb comes from the final pusher fission).
Originally Posted by AlphaNumeric
So, there are 2 distinct fusion reactions and 3 distinct fission reactions in a modern thermonuclear bomb design, and the bulk of the yield comes from the final fission reaction. If we're going for a reductive description, it would be that a fission primary sets off a fusion reaction which then sets off a really big fission reaction using depleted uranium.
Meanwhile, I suspect very much that you did not, yourself, actually check your knowledge, but are simply repeating popular misconceptions. You'll find that everything I said is corroborated by the relevant Wikipedia page, which happens to include explicit sections on popular misconceptions about how thermonuclear bombs work (bolds mine):
Two-stage thermonuclear weapons are essentially a chain of fission-boosted fusion weapons (not to be confused with the previously mentioned fusion-boosted fission weapons), usually with only two stages in the chain. The second stage, called the "secondary," is imploded by x-ray energy from the first stage, called the "primary." This radiation implosion is much more effective than the high-explosive implosion of the primary. Consequently, the secondary can be many times more powerful than the primary, without being bigger. The secondary can be designed to maximize fusion energy release, but in most designs fusion is employed only to drive or enhance fission, as it is in the primary.
For the secondary to be imploded by the hot, radiation-induced plasma surrounding it, it must remain cool for the first microsecond, i.e., it must be encased in a massive radiation (heat) shield. The shield's massiveness allows it to double as a tamper, adding momentum and duration to the implosion. No material is better suited for both of these jobs than ordinary, cheap uranium-238, which also happens to undergo fission when struck by the neutrons produced by D-T fusion. This casing, called the pusher, thus has three jobs: to keep the secondary cool, to hold it, inertially, in a highly compressed state, and, finally, to serve as the chief energy source for the entire bomb. The consumable pusher makes the bomb more a uranium fission bomb than a hydrogen fusion bomb. It is noteworthy that insiders never used the term hydrogen bomb.
Finally, the heat for fusion ignition comes not from the primary but from a second fission bomb called the spark plug, embedded in the heart of the secondary. The implosion of the secondary implodes this spark plug, detonating it and igniting fusion in the material around it, but the spark plug then continues to fission in the neutron-rich environment until it is fully consumed, adding significantly to the yield.
Also note my earlier qualifer: "modern designs." As indicated above, it is indeed possible to design thermonuclear weapons for maximum fusion yield. But this is not done in any modern, deployed designs. The main reason to do that would be to cut down on radioactive by-products (since the fusion reactions are producing helium mostly) and so make a "clean bomb." But, fusion fuel is expensive and you get way, way, way more yield per dollar by including a DU pusher and fissing that. Plus you still have to set off a fission bomb in the primary regardless, so the resulting system is never going to be all that "clean."