The interest rate at which the net present value of a project is zero. If the net cash flow on a project starts off negative, and once it becomes positive it remains so, the IRR is unique; but if cash flow at the end becomes negative, as can happen if there are costs of making a plant safe after its useful life is over, the IRR may not be unique. A project is worth investing in if its IRR exceeds the sum of the risk-free rate of interest and an appropriate risk premium.