Reactor no. 546
Wash THE reactor with purified water. Perform THE washing through THE pump. Upon washing completion, empty THE reactor.
In the example you have given, the sentence already sounds odd to English ears.
"Wash the reactor with purified water" sounds perfectly idiomatic.
I chose "the" because it is the definite article – we are talking about a particular reactor. You need an article and in this case it makes most sense with the definite article. Purified water does not need an article: it is a mass noun. However, if the purified water was supplied as part of a washing kit, and was known to us, it would be possible to talk of "the purified water [that you have in front of you]"
"Perform the washing through the pump" sounds odd and it is not clear what is meant. Does it mean that the pump should also be washed, or that the use of a pump is necessary to complete the washing process? I am assuming that the pump is a particular pump that is nearby; if pumps are available in a store room somewhere and you would be well advised to go and fetch one, then you could also say "Perform the washing through A pump [which could be one of many in the store]".
"Upon washing completion, empty THE reactor."
It sounds strange (but not necessarily wrong) to say "Upon washing completion". A more idiomatic construction would be "Upon completion of the washing process" or "When [the] washing is complete…". Again, it is "THE reactor" that gets emptied because it is the reactor we have in front of us and have been discussing (definite article), not any old reactor (indefinite article).
I hope that helps and my explanation isn't too complicated or muddled!
Student or Learner