Not much. Helium is an inert gas. It does not want to react with anything. Plutonium is highly radioactive, and some of its isotopes are fissile. In any case, the two would not react. Nothing nuclear is going to happen beased on the mixing of the two elements. The only possible problem is if a critical mass of plutonium is gathered together. Then a criticality incident will occur and a radioactive mess will result with the release of a cloud of airborne and highly radioactive nuclides.
At room temperature nothing; after heating above 100 0C plutonium hydride (PuH2) is formed.
Plutonium is a high-density metal, hydrogen a low-density gas.
Iodine crystals
I think I will let SOMEBODY ELSE find out!
If you think to an alloy, this alloy is possible.
Depending on the concentration of the hydrogen peroxide, (if low enough) it will just decompose into water and oxygen gas. If the hydrogen peroxide is high enough in concentration it will just heat up and mix with the water (most likely the first will occur).
You get a mixture of hydrogen and xenon.
water
Sulfur and hydrogen form organic compounds, plutonium not.
No.
Plutonium is a high-density metal, hydrogen a low-density gas.
It gets softer.
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Iodine crystals
Hydrogen, H2, and a salt are formed.
BOOM.
Neptunium react with hydrogen forming hydrides with formulas between NpH2 and NpH3.
You just get a gas mixture of argon and hydrogen. Being less dense, the hydrogen will rise above the argon.