I'm talking about "The Hunt for Red October" novel by Tom Clancy*, where the sinking of the fictional Soviet submarine "Politovskij" is described.
It found interesting to read how the imminent cause for the nuclear reactor explosion (in the writer's tale) was the breakdown and drifting of a butterfly valve part in a refrigeration circuit...!
The various steps leading to the incident are described in details: well, I feel that is just... techno-babble**, more than a realistic scenario; but I would like to know the opinion of the other Members who have read the book...
I don't have any Itallian, but I've read the book (actually several times) and found the reactor failure you mention to be credible in the original.  I don't know why anyone would use a butterfly valve in critical on/off service, but I know that sometimes they do.  As I recall the valve was in the main cooling loop which might be called a "refridgeration" loop in a translation, but I've never heard it called that for primary cooling of a reactor (usually I see the term used for loops that include phase change).
Maybe someone with direct experience in Soviet submarine design will address your more specific issues, but with experience in U.S. submarine power cycles I found it credible.

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