So as I know nuclear bombs are derived from fission reactions: By providing the nucleus with enough power to trigger a chain reaction. If uranium was present why does it take so much to make a nuclear bomb.
Nuclear Physics – Why Does It Take So Long to Build a Nuclear Bomb?
nuclear-engineeringnuclear-physicspower
Related Solutions
A lot can be, and has been, written on the subject, but I'll give you the short and sweet version.
Does nuclear chain reaction start in fuel pellets even before they being installed in reactor? -- No
There are several reasons why this is so.
The number of spontaneous fissions of $^{235}$U is minimal. The branching ratio for that mode of decay is $7 \cdot 10^{-9} \%$, which means that for every billion $^{235}$U atoms that decay, only $7$ of them do so by spontaneous fission. This does not produce enough neutrons to start a chain reaction.
The neutrons released from fission have too much energy to induce many more reactions. The probability of an atomic event is characterized by the associated cross-section. The cross-section for the relevent fissions of $^{235}$U at the fission spectrum average is 1.235 barns. This is not zero, but it isn't very large; compare this to neutrons in the 0.025 eV range where the cross-section is 584 barns.
Fresh fuel rods are not typically enriched very much. The exact enrichment varies depending on a variety of factors, but fresh fuel is typically on the order of 2-5% $^{235}$U; most of the rest of the fuel is $^{238}$U which is significantly less likely to fission due to neutrons in the average fission spectrum.
As to your confusion, yes, the fuel is sufficiently enriched to sustain a chain reaction; that is what it is designed for. It is designed, however, to be inside a reactor when that happens. Inside a reactor, there are other things that start and sustain the chain reaction. The primary of these is a moderator.
A moderator is a substance that slows the neutrons down from the fission energy of around 2 MeV to the average temperature of the moderator, around an eV or so. In all commercial reactors in the United States, this moderator is plain old water. Some reactors, though, use heavy water and others use graphite. Either way, the function is critical for (most) nuclear reactors. There are such things as fast reactors, but I'll let you research that on your own.
Also, for fuel pellets and rods are radioactive, how do we transport them? -- Very carefully.
Fresh fuel rods, as explained above, are not dangerously radioactive and can be handled without a great deal of extra caution. Fresh fuel is made into pellets that go into rods; the rods are assembled into assemblies which are transported to the power plant in transportation casks. These are often times not much more than a wooden box with packaging material. They can be loaded on the back of a semi or onto a train and shipped to the power plant.
Spent fuel rods are typically moved by machine and only then a very short distance into cooling pools. These pools provide sheilding while allowing the removal of excess heat generated by fission products. After many years in a cooling pond, many nuclear power companies have started to move spent fuel to dry cask storage containers. These containers provide the same benefits of the cooling ponds, but require less maintenance.
I don't understand why exactly it leads to a powerful explosion instead of just a burst of ionising radiation.
This radiation, representing most of the initial energy output by a nuclear weapon, is swiftly absorbed by the surrounding matter. The latter in turn heats almost instantly to extremely high temperature, so you have the almost instantaneous creation of a ball of extremely high kinetic energy plasma. This in turn means a prodigious rise in pressure, and it is this pressure that gives rise the blast wave.
The same argument applies to the neutrons and other fission fragments / fusion products immediately produced by the reaction. But it is the initial burst of radiation that overwhelmingly creates the fireball in an atmospheric detonation, and the fireball that expands to produce most of the blast wave.
Best Answer
If you throw a bunch a uranium ore in one blob, nothing happens.
If you chemically purify the ore so that the only element present is uranium, still nothing happens.
The runaway chain reaction needed for a uranium-powered bomb involves U-235, an isotope having three fewer neutrons than the most common natural isotope U-238. According to Wikipedia,
Now U-235 and U-238 have essentially indistinguishable chemistry.1 How do you purify the material to be mostly U-235? The traditional technique is to make the gaseous compound uranium hexafluoride, $\mathrm{UF}_6$, and use some mass-based physical process to separate out the ${}^{235}\mathrm{UF}_6$ from the ${}^{238}\mathrm{UF}_6$.2 This is slow, tedious work. In fact, the Oak Ridge facility built to do this during WWII was by some measures the largest building in the world at the time. It extracted U-235 one tiny bit at a time, and even after months of operation, there was just enough material for a single bomb, with nothing left to spare to even test the device.
For plutonium weapons, there is a similar complication. Bombs are made from Pu-239. Pu-240 is too unstable, and if there is too much of it in your bomb, it leads to a premature reaction (by a small fraction of a second), scattering most of the fuel rather than detonating it. The problem is even more difficult given that plutonium isn't found naturally in large quantities - it is produced entirely as a byproduct in uranium-based reactors.
In the end, it takes a large industrial infrastructure to manufacture either type of fission bomb since you need large amounts of an isotopically pure substance.
1 Even the chemical difference between normal hydrogen, consisting of 1 proton, and deuterium, which is a proton and a neutron, is small. Changing the mass of the nucleus, even by a factor of 2, does very little. Now imagine changing the mass by a factor of only about 1%.
2 Note that there is only one natural isotope of fluorine, F-19, so the only difference in masses for the molecules comes from the uranium.