Experimental reactor systems operate with small shut-down margins and with, in many cases, manual refueling. Thus in college-based experimental systems a refueling fault is not unlikely. Accidents have occurred in which an assembly was dropped into a near-critical core or a control rod was withdrawn (16).

However, in large fast power reactors the system is shut down during refueling by as much as 10% ($ 30) and the only way in which the system could be inadvertently brought to criticality would be by removing a number of control rods completely or by misloading a number of enriched assem­blies from an outer enrichment zone into che central region and then remov­ing one or more control rods. Such a procedure would require a large number of consecutive errors on the part of refueling operating personnel.

In addition, a well designed refueling system will have interlocks and

grapple identifying features that allow the operator to positively identify what type of assembly, be it control or fuel, the refueling machine is han­dling at any time. Interlocks can also be included to prevent removing two control rods sequentially, and procedural controls will also prevent sequen­tial errors.

Thus a refueling accident must be a well planned program of events and cannot be considered a credible CDA initiator in a large fast power reactor system.