In this vessel, which also features an adjustable opening, the particles reflect off the walls and undergo collisions with one another.
Evolution of the particle number: For every particle that leaves the vessel, two new particles are generated inside. This results in a net gain of one particle; however, with a sufficiently large population and opening size, the total number increases rapidly. An adjustable upper limit disables particle production once it is reached.
If too many particles accumulate inside the vessel, a core meltdown may occur!