That phrase "thermonuclear explosion" probably manifests as, or feels like, being thrown into internal chaos for a period of time. If can always be rationalized as something other than a transition to 4D, especially if it occurs during a period of external chaos and the person experiencing it has no awareness of any such transition.
Ra (Law of One) speaks on this:
Session 63
July 18, 1981
QUESTIONER: In that case, at some future time the fourth-density sphere will be fully activated. What is the difference between the full and partial activation of this sphere?
RA: I am Ra. At this time, cosmic influxes are conducive to the formation of basic particles of true green color and, therefore, to the formation of material of that nature. However, there is a blending of the yellow and green ray environments at this time that requires the birth of transient energy distortions of the mind/body/spirit complex.
When the full activation of the true green love density occurs, the planetary sphere will be solid and habitable as such, and the birth that is in progress will have been transformed, through the process of time, shall we say, into the type of vehicle appropriate for the full appreciation of the fourth-density planetary environment. At the present confluence, the green ray environment exists to a much greater extent in time/space than in space/time.
QUESTIONER: Can you describe the difference you mention regarding time/space and space/time?
RA: I am Ra. For your understanding, we will use the working definition of the inner planes. There is great subtlety in this vibratory sound complex, but in and of itself, it may satisfy your current need.
QUESTIONER: I am going to make a statement for your correction if necessary. What is happening is that as our planet becomes encompassed in the spiraling movement of the entire great galaxy and our planetary system spirals into its new position, the fourth-density vibrations become increasingly pronounced. These atomic nucleus vibrations begin to create, with increasing intensity, the fourth-density sphere and the fourth-density body complexes that must inhabit it, is that correct?
RA: I am Ra. That is partly correct. The concept of the creation of green ray density body complexes must be corrected.
This creation will be gradual and will begin with your own type of third-density physical vehicle; then, through sexual reproduction and evolutionary processes, it will give rise to the fourth-density body complex.
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Many will not like this because they expect something else.
For me the process explained is very logical.