5 U.S. Code § 8451 - Disability retirement
An employee who completes at least 18 months of civilian service creditable under section 8411 and has become disabled shall be retired on the employee’s own application or on application by the employee’s agency.
(A) Notwithstanding paragraph (1), an employee shall not be eligible for disability retirement under this section if the employee has declined a reasonable offer of reassignment to a vacant position in the employee’s agency for which the employee is qualified if the position—
is at the same grade (or pay level) as the employee’s most recent grade (or pay level) or higher;
An employee who is applying for disability retirement under this subchapter shall be considered for reassignment by the employee’s agency to a vacant position described in subparagraph (A) in accordance with such procedures as the Office shall by regulation prescribe.
An employee is entitled to appeal to the Merit Systems Protection Board under section 7701 any determination that the employee is not unable, because of disease or injury, to render useful and efficient service in a position to which the employee has declined reassignment under this section.
For purposes of subparagraph (A), an employee of the United States Postal Service shall not be considered qualified for a position if such position is in a different craft or if reassignment to such position would be inconsistent with the terms of a collective-bargaining agreement covering the employee.
A Member who completes at least 18 months of service as a Member and is found by the Office to be disabled for useful and efficient service as a Member because of disease or injury shall be retired on the Member’s own application.
An employee or Member retiring under this section is entitled to an annuity computed under section 8452.