The SG9 in the latest Virginia class submarines are only 30MW. As of 2001 The Navy has constructed over 235 nuclear reactors, with many more since. An average of >5 per year. For Nuclear reactors that is mass production.
Also because of the way the Navy works their reactors are less bespoke for each ship than traditional land based reactors - and better fit the mass production model. This is something that could scale.
