[T]he report’s executive summary certainly gets to the heart of their findings.
“The rhetoric from small modular reactor (SMR) advocates is loud and persistent: This time will be different because the cost overruns and schedule delays that have plagued large reactor construction projects will not be repeated with the new designs,” says the report. “But the few SMRs that have been built (or have been started) paint a different picture – one that looks startlingly similar to the past. Significant construction delays are still the norm and costs have continued to climb.”
Because it is actually not that simple, especially on the “cleaner” and “safer” parts.
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Theoretically the main advantage of the thorium is precisely because its safer and cleaner. When removed from its neutron source thorium quickly ceases fission and decay.
“Theoretically” is worth very little. It is pretty much the same for every concept NPP, that once construction starts on an actual practical plant, ugly problems start coming up all over the place that were not considered or thought of in the concept stage. Corrosion is one of the biggest ones.
See also the Rickover memo.