How are dangerous to build things studied?
Such as, nuclear reactors, jet engines, sawing machines.
How do engineers discover, how they ought to be built, while avoiding getting or getting others hurt?
Is there perhaps a component of "anticipating to avoid risk based on earlier experience"? But how does this allow designing potentially dangerous things?
It is hard to give a general answer, a lot will be highly domain specific. It's also hard to answer as engineers don't work in a vacuum. Depending on wether one designs a product to be mass-produced or a one off industrial plant, safety relevant specifications may arrive at the engineers desk from the client, from an official ordinance, their professional conscience or even from their marketing department. The designing engineers may then use a few tools to arrive at a reasonably safe design, these include industry codes and norms, simulations, testing and risk assessments.
Codes:
For a lot of machines and components, you have codes that specify how the device is to be constructed. For example pipe class ratings tell you what wall thicknesses and flange types to use for many temperature and pressure ranges, a comparativly simple example.
Tests
Certain safety critical behaviors can be tested, a classical example would be a crash test dummy in a vehicle. This is not always viable or economical, AFAIK testing is common for complex, mass produced goods like cars where the costs of the test are ameliorated over many produced units.
Simulations
Tests can sometimes be replaced or aided by complex simulations, this is again common in car design.
Risk assessments
Depending where you live, every machine sold will have a CE marking, which (among other things) means the manufacturers did a risk assessment. Industrial plants, too, need one to operate legally (in theory). This means someone or many someones made a long list of all liekly hazards like burning on open flames, on hot surfaces, electrocution, cutting on sharp edges, injuries due to fast moving parts etc. For each risk, they will evaluate how often it will ocur, how many persons will be in danger, how serious the danger is, and if - in sum - the risk is deemd unacceptable, the design goes back to the drawing board to install further safeguards or remove the risk some other way.
The difficulty in answering this uestion is that the answer depends highly on the field you work in. You can crash a car into a wall and examine the test dummy, you can't test a nuclear plant the same way (too heavy) and to do it with a sawing machine would be plain dumb. Every domain has it's own standards.
Is there perhaps a component of "anticipating to avoid risk based on earlier experience"?
Previous experience is (hopefully) baked into the codes and norms the engineers work with (often very implicitly so, it must be said).
Last not least, there's many overlapping concerns regarding safety. The area I know best, plant design, you have the safety of the individual machines, you need to consider hazardous incidents (release of dangerous substances) as well as workplace safety, explosion protection, fire protection. The last three usually have their own dedicated expert who, at several points, looks over the design and it's execution.
That depends on the degree and extent of the hazard involved. Analysis of all the individual components with precision and specificity is highly imperative, but no amount of numericals can exactly potray or define the real world outcome of an event. Destructive testing is one way engineers simulate a catastrophic outcome of a system, and it done in heavy protection/containment. If you're talking about something comparable to a nuclear meltdown, engineers prefer making conservative estimate which in the end is worth the risk. But in the case of cutting tools, I believe dummies tend to suffer the blow until it's tweaked to an allowable safety level.
Building models, computer simulations, and making the real thing and going away helps.
Sometimes they screw up despite that, either by not knowing everything about the subject, or simply not thinking about rare/unusual events.
The most well known example is probably Castle Bravo, where a nuclear test went wrong at the Bikini Atoll. They evacuated everyone close to the blast, but they didn't now the bomb's outer shell would react and add 9 megatons to the original 6 MT yield. They didn't know everything about the subject (U238), so they didn't bring everyone far enough. Marie Curie suffered from radiation poisoning, because she didn't know enough at the time about the heath effects. Lake Karachay became polluted because soviet scientists didn't know enough (or simply ignored) about handling radioactive materials.
Rockets are tested before the first real flight without people/expensive equipment. Using a cheap concrete block (or an outdated electric car) is better if something goes wrong. Because things tend to go wrong the first time. New airplanes are tested by simulations, then without engines (like a towed glider), possibly with remote control. Rocket or jet engines are first fixed to something, and only cameras and sensors are close to be safe.
Of course, you need to know what you're dealing with. When something goes wrong with a sawing machine, you might want to be behind a thick wall. If something goes wrong with a nuclear reactor, you want to be on a different planet. :)
