I would not recommend reheating food in Styrofoam (generic name polystyrene) or clamshell type food containers in a microwave oven. Next time heat the soup in a glass or ceramic container or microwave safe cookware. Only use microwave approved containers to cook or reheat food. Plastic storage containers not intended for use in a microwave oven can warp or melt allowing dangerous chemicals to leak into food.
Here is some information from the FDA Plastics and the Microwave.