On the delivery of any refunding bonds issued pursuant to this chapter, the proceeds of the bonds, less the proceeds of that portion of any refunding bonds issued pursuant to Chapter 17 of Title 6 for improvements, must be deposited with a corporate trustee and held by it under a written trust agreement and in a special trust account, except that the accrued interest, if any, must be used to discharge in part the first interest to become due on the refunding bonds. It is the duty of the corporate trustee to keep the proceeds invested and reinvested to the extent practical in obligations of the United States or any agency of the United States and to apply the principal and interest of the trust so established in the manner prescribed in the trust agreement.