Yes, if there is no contingent beneficiary named, then the IRA beneficiary would be the decedent's estate when the beneficiary disclaims. You should check with the custodian of the IRA to make sure of the status of the contingent beneficiary and also to learn what options there are for the estate to receive distributions from the IRA. The beneficiary must file a written disclaimer in the Probate Court where the decedent lived within nine months.
The only thing I would add is seek out counsel from a tax attorney or accountant as to the tax ramifications of each of the beneficiary's options! You don't want to create an adverse taxable event!