[thedoc (OP)]

Tom asked the Naked Scientists:
   
My son Frank would like to know how why hydrogen bombs create radioactive elements.
What do you think?

[Colin2B]

Tom asked the Naked Scientists:
   
My son Frank would like to know how why hydrogen bombs create radioactive elements.
What do you think?

How old is Frank? How much physics does he know?
If old enough there are plenty of descriptions of the physics on the net eg Wiki.
If he is sub teens you might try an analogy:
In the same way a bonfire creates hot things eg sparks, flames etc an A or H bomb is like a nuclear bonfire, with the reactions creating lots of radioactive elements. Both convert material from one form to another. It is possible to think of radioactive materials as being higher energy and giving this up (as rays or particles) as they decay - similar to sparks(short life) or embers (longer life) giving up heat as they cool down.
Not a perfect answer, but maybe it will help.
Above all, do encourage him to read as much as possible and ask teachers when he finds things he can't understand. An enquiring mind is the key and if he is asking these questions he is on the right track.
Hopefully we will see him posting questions here soon!

[PmbPhy]

Tom asked the Naked Scientists:
   
My son Frank would like to know how why hydrogen bombs create radioactive elements.
What do you think?

Hydrogen bombs, aka thermonuclear bombs, use fission bombs to create the fusion of hydrogen. It's those fission bombs that have all the radioactive elements. See: http://en.wikipedia.org/wiki/Thermonuclear_weapon

Fusion, unlike fission, is relatively "clean"—it releases energy but no harmful radioactive products or large amounts of nuclear fallout. The fission reactions though, especially the last fission reaction, release a tremendous amount of fission products and fallout.

[Colin2B]

Fusion, unlike fission, is relatively "clean"—it releases energy but no harmful radioactive products or large amounts of nuclear fallout. The fission reactions though, especially the last fission reaction, release a tremendous amount of fission products and fallout.

Interesting how our minds make assumptions when reading! When I read the question my mind automatically inserted "fission bomb to trigger fusion=H bomb". However, your reply clarifies and makes me think the question is really about cleanliness of fusion, Frank not realising the fission trigger. Perhaps Frank is older (and somewhat wiser) if he questions this apparent anomaly.
Only thing I would add is that my mind model classifies neutrons as radioactivity, which I suspect is what you mean by relatively clean.
Thanks for the reply.

[evan_au]

Both uranium/plutonium bombs (fission) and hydrogen bombs (fusion) are built with radioactive materials. Fission bombs use radioactive tritium (hydrogen) as part of the trigger mechanism. Fusion bombs use uranium or plutonium as part of the trigger. Some of these raw materials remain after the explosion.

Most of the radioactive materials are changed into other elements during the nuclear explosion, and the "daughter nuclei" from the breakdown of uranium and plutonium are quite radioactive. They pose a danger to humans and the nearby ecosystem.

Both hydrogen bombs (fusion) and uranium/plutonium bombs (fission) release a burst of neutrons when they explode. This turns surrounding matter into radioactive isotopes - the bomb materials, the bomb casing, the air and the ground.

Anything caught up in the fireball will be launched high into the atmosphere, and could be carried long distances. Once the fireball cools, materials with high melting points solidify, and come back to earth as radioactive fallout.

Nuclear weapons were realized to be a suicidal dead-end once more than one party had them (which was inevitable). Despite several attempts to create nuclear weapons that could be used in warfare, the bulk of the victims will inevitably be civilians.

Controlled nuclear fusion holds out the tantalizing possibility of abundant "clean" energy, but it has been a very long time coming. There is radiation released during hydrogen fusion, but it is contained within a reactor building. Over time, the materials of the reactor shell will become radioactive, and will eventually have to be safely stored in carefully controlled radioactive waste dumps. 

[alancalverd]

The question was about creating, not containing, radioactive elements.

Two basic sources:

One is the fission fragments of the initiating fission bomb. When a uranium or plutonium nucleus undergoes fission it breaks into several smaller chunks, most of which are "overloaded" with neutrons (the fewer protons in a nucleus, the fewer neutrons it can hold, and the ratio is nonlinear) and thus unstable. 

The other is the activation of the material in the target and surrounding area by the radiation emitted from the bomb. The principal actions are neutron capture (which tends to produce alpha-emitting nuclides) and gamma-nucleon interactions (producing mostly beta-emitters). 

Despite several attempts to create nuclear weapons that could be used in warfare, the bulk of the victims will inevitably be civilians.

Not "will be" so much as "have been" - the only demonstrations of their effectiveness. There are some nuclear artillery shells and depth charges, but their use, even on a limited strike against armour, would almost certainly invoke nuclear retaliation against a soft target such as Birmingham.
 

[UltimateTheory]

Tom asked the Naked Scientists:
   
My son Frank would like to know how why hydrogen bombs create radioactive elements.
What do you think?

Analyze for instance fusion between Deuterium and Tritium:

D+ + T+ -> He-4 + n0 + 17.6 MeV

Notice that reaction of fusion can create free and highly accelerated electric neutral neutron. Which can join with any other atom (because no Coulomb's barrier). Then from stable isotope after capturing free neutron we can receive unstable isotope. And perhaps chain reaction (if unstable are also isotopes to which it'll decay).

Read about neutron radiation
http://en.wikipedia.org/wiki/Neutron_radiation

and neutron activation
http://en.wikipedia.org/wiki/Neutron_activation

[ClaesN]

We are protected! Go out and ask the questions and don’t give in for answers you don’t like, even though those who answer are certain and might get angry. Try to get finance to new projects. Look at what others have done in history. The crazier the scientists were the better, like Tesla for example.  When obvious things become clearer don’t point fingers and tell people bad names. Don’t worry, they feel the shame.

Regards
Claes N