First, I would recommend to go through some literature on low noise signal processing, just get well ground in the ideology there:
The questions asked there are usually something like: can the energy below Boltzmann kT/2 be detected? - Yes, it can be.
And commercial charge sensitive...
I think the key point is that the melting caused by heat concucted by radiation (mostly infrared) not air.
At first, the surface of the snow gets dark from smoke in some spots and absords heat quite effectivly at those spots. The spots start to melt through the snow but what is important to...
I think the picture was taken many years after the disaster.
10.000 R/hour activity was observed in first minutes after the accident - it is way lower by now.
Still, one has to note that at least one person from reactor 4 personal who was right at the spot during the explosion still...
Several notes:
1. For PWM it is better to drive gate not via resistor but with special MOSFET buffer IC ( low impedance, high speed, high current driver). This way you reduce transition time between on/off stete and power dissipation.
2. The diode is a must. It should be rated for high enough...