A synonym for the phrase "organic test reactor" could be "pilot plant" or "experimental facility." Another possible synonym could be "prototype reactor" or "research reactor," which both convey the idea that the facility is used for scientific investigation and development. A more technical synonym could be "neutron generator," which refers to a device that produces neutrons for use in nuclear research. Other synonyms could include "test bed" or "experimental reactor," both of which evoke the idea of a facility used for testing and evaluation purposes. Ultimately, the choice of synonym will depend on the context and specific usage of the phrase.