Collins Russian Dictionary 2nd Edition © HarperCollins Publishers 2000, 1997:
simmer [ˈsɪmər] vi (CULIN) кипе́ть*(impf) на ме́дленном огне́
simmer down vi (fig) (inf)остыва́ть (осты́ть*perf)
* is used to mark translations which have irregular inflections. The Russian-English side of the dictionary gives inflectional information.