ELi5: Why experimental fusion/fission reactors are not built smaller first?

Question

Wouldn’t it be cheaper and more feasible to build and try experimental reactors 10x smaller for research and development purposes and _then_ built the proper size when they are proven to be efficient?

In: 2

Answer 1

Fission and specially fusion become more efficient and more capable of maintaining a chain reaction when they are bigger. Both require to have at least a minimum size to maintain any sort of chain reaction. What you propose is exactly what scientists are doing, building smaller reactors first for research; but for fusion specifically we haven’t ever built a reactor big enough to keep a chain reaction going, due to mostly lack of funding.