A hydrogen bomb or thermonuclear bomb is a nuclear weapon that obtains most of its energy from the fusion of hydrogen nuclei into helium nuclei. This fusion typically involves deuterium and tritium nuclei, the heavy isotopes of hydrogen. Deuterium is a stable, naturally occurring isotope with one proton and one neutron in its nucleus, and can be extracted from normal water. Tritium is an artificial, radioactive isotope with one proton and two neutrons in its nucleus, and can be formed in nuclear reactors or, during a nuclear explosion, by the exposure of lithium nuclei to the neutrons formed in that explosion.
Since hydrogen nuclei are positively charged, they repel one another. To get these heavy hydrogen nuclei close enough together to fuse into helium nuclei, the hydrogen nuclei must be heated to fantastic temperatures. This heating is done with a fission bomb—a uranium or plutonium bomb. When the fission bomb explodes, its heat is enough to trigger the hydrogen bomb.
Once the bomb has assembled a super-critical mass of fissionable material, each chain reaction that occurs will grow exponentially with time and lead to a catastrophic release of energy. But you're right in wondering just what starts those chain reactions. The answer is natural radioactivity from a trigger material. While the nuclear fuel's own radioactivity could provide those first few neutrons, it's generally not reliable enough. To make sure that the chain reactions get started properly, most nuclear weapons introduce a highly radioactive neutron-emitting trigger material into the nuclear fuel assembly.
Apart from obtaining fissionable material, this is the biggest technical problem with building a nuclear weapon. Although a fission bomb's nuclear fuel begins to heat up and explode almost from the instant it reaches critical mass, just reaching critical mass isn't good enough. To use its fuel efficiently—to shatter most of its nuclei before the fuel rips itself apart—the bomb must achieve a significantly super-critical mass. It needs the explosive chain reactions that occur when each fission induces an average of far more than one subsequent fission.
There are two classic techniques for reaching super-critical mass. The technique used in the uranium bomb dropped over Hiroshima in WWII involved a collision between two objects. A small cannon fired a piece of uranium 235 into a nearly complete sphere of uranium 235. The uranium projectile entered the incomplete sphere at enormous speed and made the overall structure a super-critical mass. But despite the rapid mechanical assembly, the bomb still wasn't able to use its nuclei very efficiently. It wasn't sufficiently super-critical for an efficient explosion.
The technique used in the two plutonium bombs, the Gadget tested in New Mexico and the Fat Man dropped over Nagasaki, involved implosions. In each bomb, high explosives crushed a solid sphere of plutonium 239 so that its density roughly doubled. With its nuclei packed more tightly together, this fuel surged through critical mass and went well into the super-critical regime. It consumed a much larger fraction of its nuclei than the uranium bomb and was thus a more efficient device. However, its design was so complicated and technically demanding that its builders weren't sure it would work. That's why they tested it once on the sands of New Mexico. The builders of the uranium bomb were confident enough of its design and too worried about wasting precious uranium to test it.
Critical mass is something of a misnomer because in addition to mass, it also depends on shape, density, and even the objects surrounding the nuclear fuel. Anything that makes the nuclear fuel more efficient at using its neutrons to induce fissions helps that fuel approach critical mass. The characteristics of the materials also play a role. For example, fissioning plutonium 239 nuclei release more neutrons on average than fissioning uranium 235 nuclei. As a result, plutonium 239 is better at sustaining a chain reaction than uranium 235 and critical masses of plutonium 239 are typically smaller than for uranium 235.
Almost the instant the nuclear fuel reaches critical mass, it begins to release heat and explode. If this fuel overheats and rips itself apart before most its nuclei have undergone fission, only a small fraction of the fuel's nuclear energy will have been released in the explosion. There are at least two possible causes for such a "fizzle": slow assembly of the super-critical mass needed for explosive chain reactions and poor containment of the exploding fuel. A well designed fission bomb assembles its super-critical mass astonishingly quickly and it shrouds that mass in an envelope that prevents it from exploding until most of the nuclei have had time to shatter.
Critical, sub-critical, and super-critical mass all refer to the chain reactions that occur in fissionable material—a material in which nuclei can shatter or "fission" when struck by a passing neutron. When this nuclear fuel is at critical mass, each nucleus that fissions directly induces an average of one subsequent fission. This situation leads to a steady chain reaction in the fuel: the first fission causes a second fission, which causes a third fission, and so on. Steady chain reactions of this sort are used in nuclear reactors.
When the fuel is below critical mass, there aren't quite enough nuclei around to keep the chain reactions proceeding steadily and each chain gradually dies away. While such a sub-critical mass of fuel continues to experience chain reactions, they aren't self-sustaining and depend on natural radioactive decay to restart them.
When the fuel is above critical mass, there are more than enough nuclei around to sustain the chain reactions. In fact, each chain reaction grows exponentially in size with the passage of time. Since each fission directly induces more than one subsequent fission, it takes only a few generations of fissions before there are astronomical numbers of nuclei fissioning in the fuel. Explosive chain reactions of this sort occur in nuclear weapons.
While your book's claim is well intended, it's actually incorrect. The author is trying to point out that atoms aren't created or destroyed during the reaction and that all the reactant atoms are still present in the products. But equating the conservation of atoms with the conservation of mass overlooks any mass loss associated with changes in the chemical bonds between atoms. While bond masses are extremely small compared to the masses of atoms, they do change as the results of chemical reactions. However even the most energy-releasing or "exothermic" reactions only produce overall mass losses of about one part in a billion and no one has yet succeeded in weighing matter precisely enough to detect such tiny changes.
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