I agree to many points mentioned in the previous post but the answer is:
"Yes, the weight(mass) would change."
My reasons are simple. You state that an unbreakable container is being used, so that the products of the explosion will be contained inside.
However unbreakable this container may be it will not be able to contain all of the radiation produced in the blast. Alpha and beta radiation are likely to be contained but not so the gamma rays and perhaps other form of radiation. If the container gets hot, it will emit a broad spectrum of electromagnetic waves, infra red, and perhaps visible light (it might be glowing red hot).
As all these kinds of radiation carry energy away, the mass (and weight) will change according to the mentioned mass-energy equivalence E=m*c^2.
Another loss would be neutrinos produced during the radioactive decay. Neutrinos are small elementary particles that only weekly with other matter. No container can contain them. Every second huge numbers of neutrinos produced by the sun flow through the earth, flow through every person on the planet, and only a tiny percentage of them interacts with the cores of atoms in their path.
Those neutrinos have a tiny mass and would change the weight.
Your container would not only have to be unbreakable, but also impenetrable to radiation and neutrinos and on top of that, it should not conduct any heat (which is practically impossible).
In this improbable case the "weight" would not change.
Good luck with it. ;-)