One fairly obvious failure mode is that it has no checks on the other outputs.
So from my understanding, the AI is optimizing it's actions to produce a machine that outputs electricity and helium. Why does it produce a fusion reactor, not a battery and a leaking balloon?
A fusion reactor will in practice leak some amount of radiation into the environment. This could be a small negligible amount, or a large dangerous amount.
If the human knows about radiation and thinks of this, they can put a max radiation leaked into the goal. But this is pushing the work onto the humans.
From my understanding of your proposal, the AI is only thinking about a small part of the world. Say a warehouse that contains some robotic construction equipment, and that you hope will soon contain a fusion reactor, and that doesn't contain any humans.
The AI isn't predicting the consequences of it's actions over all space and time.
Thus the AI won't care if humans outside the warehouse die of radiation poisoning, because it's not imagining anything outside the warehouse.
So, you included radiation levels in your goal. Did you include toxic chemicals? Waste heat? Electromagnetic effects from those big electromagnets that could mess with all sorts of electronics. Bioweapons leaking out? I mean if it's designing a fusion reactor and any bio-nasties are being made, something has gone wrong. What about nanobots. Self replicating nanotech sure would be useful to construct the fusion reactor. Does the AI care if an odd nanobot slips out and grey goos the world? What about other AI. Does your AI care if it makes a "maximize fusion reactors" AI that fills the universe with fusion reactors.
https://robotics-transformer-x.github.io/
Ok to drill down: the AI is a large transformer architecture control model. It was initially trained by converting human and robotic actions to a common token representation that is perspective independent and robotic actuator interdependent. (Example "soft top grab, bottom insertion to Target" might be a string expansion of the tokens)
You then train via reinforcement learning on a simulation of the task environments for task effectiveness.
What this does is train the machines policy to be similar to "what would a human do", at least for input cases that are similar to any of the inputs. (As usual, you need all the video in the world to do this). The RL "fine tuning" modifies the policy just enough to usually succeed on tasks instead of say grabbing too hard and crushing or dropping the object every time. So the new policy is a local minimum in policy space adjacent to the one learned from humans.
This empirically is a working method that is SOTA.
In order for the machine to "detonate an explosive" either the action a human would have taken from the training dataset involves demolitions (and there are explosives and initiators in reach of the robotic arms which are generally rail mounted) or the simulation environment during the RL stages rewarded such actions.
The reason it saves 10-1000 times the labor is for task domains where the training examples and the simulation environment span the task space of the given assignment. I meant "10-1000 times the labor" in the worldwide labor market sense, for about half of all jobs. Plenty of rare jobs few humans do will not be automated.
For example if the machine has seen, and practiced, oiling and inserting 100 kinds of bolt, a new bolt that is somewhere in properties in between the extreme ends the machine has capabilities on will likely work zero shot.
Or in practical spaces, I was thinking mining, logistics, solar panel deployment, manufacturing are cases where there are billions of total jobs and spanning task spaces that cover almost all tasks.
For supervision you have a simple metric : you query a lockstep sim each frame for the confidence and probability distribution of outcomes expected on the next frame. As errors accumulate (say a bolt is dissolving which the sim doesn't predict) this eventually reaches a threshold to summon a human remote supervisor. There are other metrics but this isn't a difficult technical problem.
You also obviously must at first take precautions: operate in human free environments separated by lexan shields, and well it's industry. A few casualties are normal and humanity can frankly take a few workers killed if the task domain was riskier with humans doing it.
I would expect you would first have proven your robotics platform and stack with hundreds of millions of robots on easier tasks before you can deploy to domains with high vacuum chamber labs. That kind of technical work is very difficult and very few people do it. Human grad students also make the kind of errors you mention, over torque is a common issue.