If the firm has a given cash flow, the present value of it at WACC (the firm total value) does not change if the capital structure changes.
If this is true, it implies that the WACC will remain constant no matter how the capital structure changes.
This situation happens when no taxes exist. To maintain the equality of the unlevered and levered firms, the return to the equity holder (levered) must change with the amount of leverage (assuming that the cost of debt is constant)