I must confess that I am not a mathematician nor a nuclear physicist, but I did some research into the uranium decay processes and the information and numbers that I came up with are somewhat alarming. I certainly hope that my speculation, information, and calculations are all wrong. Perhaps someone can point out the errors that I have made and also point to some facts that have been derived from actual measurements.

First, to recap the idea that I have been toying with for quite some time. It is a known fact that a fluid expands when it is heated. Also it is a fact that the linear velocity of molecules can be greater in less dense fluid. Earth's atmosphere is a fluid that decreases in density further away from the surface. The Earth's atmospheric temperature has been in equilibrium with the incoming energy and the outgoing energy for a long time, that is; the energy from the sun and the nuclear processes in the earth have heated and expanded the atmosphere to a certain average density level and that level has remained fairly constant for most of the time that the planet has existed. Perhaps in the very early Earth, the atmospheric temperature was higher, which would have created an atmosphere that would have extended further away from the surface. The higher temperature could have been caused by a higher concentration of radioactivity in the atmosphere, this higher atmospheric temperature would have created a situation in the 'thermosphere'* that would have had a high number of molecules escaping from the Earth's gravitational influence. Such escaping molecules would have taken not only the molecules but also their 'heat energy' with them, leaving the Earth's atmosphere that much cooler. After millions of years, the radioactivity in the atmosphere would have decreased and the atmospheric temperature would have cooled and settled to an average temperature and density that would have lasted until our present 'nuclear age'.
Assuming then that the nuclear processes in the atmosphere were responsible for some of the heat of the Earth's atmosphere, then the realization that; by reintroducing radioactivity into the atmosphere, the radioactivity may cause an increase in the average temperature of the atmosphere, should be valid.

Some numbers.

One metric ton = million grams
One mole is the same as Avogadro's number which equals (almost) 6*10^23 atoms.

Uranium atomic weight is about 238.03 moles per gram
Lead atomic weight is about 207.2 moles per gram
Difference in atomic weights per mole 30.83
When one gram of Uranium 238 decays into lead, about 31 moles of matter has been converted to energy.

1000000/238.03= about 4,201.17 moles of uranium atoms per ton.
4,201.17 / 30.83 = 136.27

There is about 136 grams of material missing from a ton of U 238 after conversion to lead.
According to Einstein's formula of e=mc^2, we can do the following calculation.
C=3000000000 m/s
C^2 = 9.e18. 9.e18*136 = 1.224e21
One ton of U238 converted to Lead will release 1.224e21 Joules of (kinetic) energy.

According to this web site--http://web.ead.anl.gov/uranium/guide/facts/index.cfm --US has uranium hexafluoride stored, amounting to 704000 metric tons of which 476000 metric tons is uranium and the rest if fluorine.

If one ton of uranium can release 1.224e21 Joules of energy then 476000 tons can release 5.82624e26 Joules of energy.

From a previous post by R.L. I quote the following information; "energy needed to heat atmosphere 1 degree is 5*10^21 Joules".

Now; the R.L.s figure must be multiplied by 10,000 in order to get close to the energy level that the amount of uranium that the US is storing (as uranium hexafluoride) is able to release. This does not take into account the radioactivity from tailings that come from all the mining operations and the uranium use in power reactors of various countries or storage of radioactive waste all over the world. All this radioactivity adds to the possibility of increased kinetic energy imparted to the Earth's atmospheric molecules, if the molecules move faster than seven miles per second they can escape into space. (Provided of course that they do not collide with other molecules.)

It is a good thing that hot air rises 'and perhaps escapes to space' because if the air was heated to ten thousand degrees at the surface of the Earth, there surely would not be life on this planet.

The reason that we are not crispy critters 'yet' is mainly because of two factors, one is mentioned above and the other is that, besides the quantity of input and output of energy, 'time' must be taken into consideration when calculating the heating effects.

Uranium 238 has a half life which is very long, somewhere about four billion years, which means that only half of the available energy is released within that time period.
5.82624e26 / 2 = 2.91312e26 That is still a very large sum, but when that sum is divided by the number of years, namely 4*10^9, the yearly energy generation is only 7.2828e16 Joules, which is large, but far below the energy needed to heat the atmosphere by one degree.
If uranium 238 is bombarded by neutrons it changes to plutonium 239, that isotope has a half life of about 20000 years. By the time one ton of plutonium 239 isotope has changed to lead, it has lost about 190 grams of mass. That amount of mass represents 190* 9.e18 = 1.7e21 Joules. Half of that is about 8.56e20 Joules divide that by 20,000 = 4.28e16 Joules per year from one ton of plutonium. Now if there was 400000 metric tons of plutonium 239 created from uranium, then the yearly energy release from that amount would be about 4.28e16 * 400000 = 1.7e22 Joules.
That quantity represents about a ¼ of the energy needed to raise the Earth's atmospheric temperature by one degree. Add to these numbers all the energy from other existing radioactive isotopes, and the energy release into the atmosphere seems alarming.
If I knew the quantity and the type of radioactive isotopes that exist in the Earth's atmosphere, I could do the calculations necessary to show what the temperature rise might be in a year, but if I did that, then I would need to figure out what quantity of molecules and heat is escaping from our planet. I would rather leave those calculations to someone that is an expert in mathematics, thermodynamics and the field of nuclear isotopes and their decay energies.

I am satisfied by the knowledge that radioactive isotopes in large enough quantity CAN impart enough kinetic energy to the atmospheric molecules so that they can escape from the Earth's gravitational field.

There are many questions still unanswered about the radioactivity in the atmosphere, such as; if the average temperature of the atmosphere were to heat up by one degree, how would it affect the global climate or weather patterns? *Why is there a 'thermosphere' around the planet Earth? Is there a thermosphere around other planets in the solar system?
If the thermosphere is created by the energy from the sun alone, then the planet Venus should have a thermosphere that is much 'hotter' than that of the Earth's.

To Sam, I would like to say that it is not necessary to look for a planet X that might destroy the Earth, since we humans have the ability and stupidity to destroy the Earth all by our selves.

Johannes

Doubt
It is the human condition to question one god after another, one appearance after another, or better, one apparition after another, always pursuing the truth of the imagination, which is not the same as the truth of appearance.
Alain [Émile-Auguste Chartier] (1868–1951), French philosopher. The Gods, Introduction (1934; tr. 1988).