(Disclaimer: I am not a physicist. Any needful corrections will be duly incorporated.)
The other explanations are good, but for those who want something a bit more at the layman's level I offer this:
If you are concerned that somehow an experiment will result in a runaway reaction that winds up destroying a lot of stuff, this should help you understand why this is not a concern.
The fusion reactions that go on in the center of stars (such as our sun) require immense temperatures and pressures, and these pressures have to be sustained long enough that the reaction takes place and that the energy from it is contained so that it can cause the more reactants nearby to react as well.
In the center of a star (such as our sun), this is not a problem. The temperatures and pressures are present, and are contained by the weight of all the stellar material that surrounds it. The energy release by the reaction is also contained to a high degree (some of it, after some thousands of years, escapes and becomes the light that shines on us). Also, there is plenty of unreacted material (free protons and electrons), which take part in the continuing reaction. It should also be understood that in the center of a star there is really no runaway reaction; things proceed at a very steady pace that is surprisingly slow. It will take billions of years for the Sun to get to the point where this reaction ceases.
In a hydrogen bomb--which is a runaway reaction--the process starts with a smaller nuclear explosion (from a fission device), which creates the initial temperature and pressure required for fusion, and there is a large quantity of fusion material (hydrogen and lithium, if I understand these things correctly), which is heated and pressurized by the fission bomb to the point that the fusion reactants can fuse. 473
This is contained only by the inertia of the fusion material, but that does the job for as long as it needs to. Boom.
In the CERN reactor, the initial energy is enough to raise only a small quantity of material to the necessary temperatures, so there is less original energy to start with.
On top of this, whatever fusible material there is in the target is present in far less quantity. This means that there is less total fusion possible, and also far less momentary containment from the inertia of the target materials.
There is no other source of containment that will preserve the conditions required for the reaction to continue. Instead of being held in place to help the reaction continue, a lot of the release energy escapes. In fact, the chief reason we do not have nuclear fusion as a feasible power source is because we have no such means of containment.
As a consequence, instead of a whole lot of fusible material suddenly getting hot enough to fuse, and releasing a huge amount of energy from this, a small amount of material heats up, undergoes whatever reaction is possible and then explodes in a very tiny burst, and that's the end of the reaction. Unless I am mistaken, the firecrackers you can buy at many roadsides are more powerful. Its effects do not escape that environment. 435