I came across an obscure 1955 article by two American engineers in a firm long vanished:
In solid-fuel reactors, fuel is held in containers, usually called assemblies, and must be removed by a batch method. Ideally, solid-fuel handling equipment would load and unload the reactor without interrupting neutron production. Actually, many considerations prevent attaining the ideal. The foremost objection to refueling with solid fuel during reactor power operation is the possibility of creating unstable operating conditions by the addition of large amounts of reactivity.
What I find interesting is that at about this time the Canadians are designing a reactor one of whose features is precisely the ability to refuel without shutting down the reactor.
Bolton, J. A. & Calabretta, P. T. 1955. “How to load solid-fuel reactors.” Nucleonics, 13, Jun., pp. 52-55.