So says the Framatome/Afnor [1989] 'Réacteurs à eau pressurisé - Ilots nucléaires - Lexique français-anglais, anglais-français':

"power block (conventional island + nuclear island, less all ancillary buildings): bloc usine
bloc usine: power block (conventional island + nuclear island, less all ancillary buildings)"

However, this is not corroborated by this EDF text where the bloc usine does not comprise the nuclear island :

"La superficie d’un CNPE varie entre 55 et 415 hectares environ suivant le nombre d’unités de production et le mode de refroidissement utilisé. Dans cet espace clôturé, chaque unité de production est constituée :
§ d’un îlot nucléaire (bâtiment du réacteur, bâtiment des auxiliaires nucléaires, ateliers chauds, laboratoires chauds…) à l’origine de la production d’effluents notamment radioactifs liquides et gazeux ;
§ d’un bloc usine (salle des machines, aéroréfrigérants le cas échéant, station de production d’eau déminéralisée, ateliers, laboratoires…) donnant lieu à des rejets chimiques et thermiques (cf. figure 1). Ces deux ensembles forment l’installation nucléaire de base (INB) dont le périmètre est précisé dans le dossier de demande d’autorisation de création (DAC), ce périmètre pouvant également inclure des bâtiments non industriels (selon les sites : restaurant d’entreprise, parking…)."
https://www.edf.fr/sites/groupe/files/contrib/groupe-edf/pro...

I'll try to get a casting vote from my wife's cousin (once removed) who is Ingénieur Sûreté chez EDF somewhere on the Loire, but that might not be decisive, for I suspect that the term is used loosely in your text. Is there mention anywhere of the nuclear island or the conventional island?

And of course if, given that EPRs are a French thing, your English text has origins in French, it is impossible to know from our end which of the meanings indicated above (the INB or the ilôt conventionnel) is meant.


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Note added at 9 hrs (2024-01-24 00:12:35 GMT)
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Warning, incoming ...

In this translation (presumably) of a Russian document, what they call the power block appears to be the bloc usine, since the reactor building is part of the 'power block':
" An artificial substructure was formed under the 2nd power block’s reactor building to achieve the soil condition set forth in the design and regulatory technical documentation."
https://www.rosenergoatom.ru/en/for-journalists/news/31061/

The IAEA refers to 'power block buildings such as containment building, fuel building, primary auxiliary building, secondary auxiliary building, and turbine building', suggesting that the power block is the conventional island alone.
https://www-pub.iaea.org/MTCD/Publications/PDF/te_1284_prn.p...

Things are complicated here - https://www.tandfonline.com/doi/full/10.1080/00295450.2021.1... - where the power block is separate from the nuclear island and while it is similar to the conventional island, it is not exactly the same.

We have a similar situation here: https://www.energypolicy.columbia.edu/sites/default/files/en...

In the images below, the power block referred to seems to be the entire Unit 1 of the NPP in one case, possibly the entire NPP in the other.

Here, a power block is several small modular reactors: "Several modular reactors would be installed together to make up a power block with a single control room, under most concepts."
https://crsreports.congress.gov/product/pdf/RL/RL33558/34


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Note added at 9 hrs (2024-01-24 00:13:28 GMT)
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Note added at 1 day 7 hrs (2024-01-24 21:58:21 GMT)
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Still waiting to get the friendly family Ingénieur Sûreté's phone number ...


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Note added at 1 day 23 hrs (2024-01-25 14:37:47 GMT)
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I spoke to the nuclear engineer who says that bloc usine is a concept unknown to him and consequently, as far as he knows, is unknown in French (Framatome/Areva) nuclear thinking, as the definition in the Framatome/AFNOR dictionary implies, to my mind, but that he would understand it to mean the turbine hall etc., i.e. the conventional island as described in the EDF article I quote above (which at that point I had not mentioned to him). 'Power block' probably does mean tranche as you suggest, but given that in some of the examples of its use 'power block' refers to the entire power plant, the 'non-standard' (?) bloc puissance might be your safest bet in that it gives readers a degree of latitude in their understanding of what it means. That said, I am not familiar with bloc puissance. I suspect it is a convenient translation (rather than adoption of a real French equivalent) of what I suspect is Westinghouse terminology, one that does not correspond exactly to the French approach in the matter. And you would be in UN company (whether that is good or bad I'll let you decide) in using bloc puissance for 'power block'.

"Dans les conclusions du chapitre «Impact sur l’environnement de l’activité opérationnelle du bloc puissance» du rapport sur l’examen périodique de la sûreté des blocs puissance nos 1 et 2 de la centrale nucléaire de Rivne, il est précisé que les indicateurs de radioactivité affichent des données stables au cours des dernières décennies et traduisent une tendance à la réduction de l’impact."
https://unece.org/DAM/env/documents/2014/EIA/IC/ece.mp.eia.i...

"In the conclusions of the chapter “Environmental impact from the operational activity of the power block” of the Report on the periodic reassessment of the security of the power blocks No. 1 and No. 2 Rivne Nuclear Power Plant specify that indicators of the levels of radioactivity for the last decades is constant and has the tendency to the impact reduction."
https://unece.org/DAM/env/documents/2014/EIA/IC/ece.mp.eia.i...