"Relevante" really has two meanings for translation purposes. It is what you might call an "unreliable friend". Sometimes it means "relevant": that it, germane, pertinent, applicable: worthy of consideration in relation to what is being discussed or considered, but sometimes -- more often, in fact -- it simply means important, significant, major: worthy of consideration in its own right, without any specific sense of relevance to a particular context. The DRAE definitions are:

"1. adj. Sobresaliente, destacado.
2. adj. Importante, significativo."

So does it mean "relevant" or "significant" here? In principle, both seem possible. Merely looking for each phrase on the Internet does not, in itself, decide the question.

The passages you have quoted come from Argentina's Ley Nacional de la Actividad Nuclear (Law 24,804, passed 2/4/97, published 24/4/97). The phrase occurs several times, but the most relevant (sorry!) part is Article/Section 30 (Chapter III), "Definiciones", which includes the following:

"e) Instalación nuclear, concepto entendido en los términos definidos en el artículo 1°, inciso j) de la Convención de Viena de Responsabilidad Civil por Daños Nucleares aprobada por ley 17.048;
d) Instalación nuclear relevante, incluye reactor nuclear, instalación crítica, instalación radiactiva relevante y acelerador relevante, de acuerdo a las definiciones establecidas o a establecer por la Autoridad Regulatoria Nuclear; [...]"
http://www.ambiente.gov.ar/?aplicacion=normativa&IdNorma=109...

Now, in this context, "relevant nuclear facility" (I'd use this rather than "installation") would presumably mean one to which the law or certain provisions of the law is/are applicable, whereas "significant/major nuclear facility" would mean one of an appreciable size.

There is a very helpful document in English, the Third National Report of Argentina's Joint Convention on the Safety of Spent Fuel Management and on the Safety of Radioactive Waste Management. It's a translation, and is not faultless, but it is apparently official. It contains several references to "significant nuclear facilities":

"F.6.4 Planning of Dismantling and Decommissioning of significant nuclear facilities".

So perhaps that's what it means in the law. However, the same document contains what is apparently an official translation of the law itself, and it uses the term "relevant nuclear facility". Here is the same part of Article/Section 30, quoted in Spanish above:

"c) Nuclear facility: Concept understood in the terms defined by Article 1, item j, of the Vienna Convention on Civil Liability for Nuclear Damage approved by Law Nº 17048.
d) Relevant nuclear facility: It includes nuclear reactor, critical facility, relevant radioactive facility and relevant accelerator, as defined or to be defined by the Nuclear Regulatory Authority."
http://www.cnea.gov.ar/pdfs/seguridad/THIRD NATIONAL REPORT....

I am still not entirely sure whether "relevant" is a correct translation here or a calque of "relevante", but I would be inclined to follow this version, given that it comes from an official source. I think that would be a safe option.

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Note added at 15 hrs (2013-08-27 12:01:32 GMT)
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Hi Richard,
Having slept on this, I'm much less sure that "relevant" is the right word to use here. I'm inclined to think it is a mistranslation and that "significant" or "major" is the sense. In fact I think I would modify my general statement on the translation of "relevante". Because it looks like "relevant", we're inclined to translate it that way when "relevant" makes sense, but it doesn't really mean that at all. See the DRAE synonyms, "sobresaliente", "destacado", "importante", "significativo": no mention at all of "pertinente" or "aplicable" or anything like that. It really doesn't mean "related to the matter at hand". It just means "important" or "outstanding".

"Relevant" can be OK for "relevante" to the extent that if something is important it is worth considering and therefore relevant, and conversely if it is not "relevante" it is trivial and therefore not pertinent. But I really think that in this case the official translator of the Argentine law has probably been led astray by the resemblance between "relevante" and "relevant", and that the latter is not the right word. I think I would now recommend "significant" or "major".

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Note added at 1 day19 hrs (2013-08-28 15:53:11 GMT) Post-grading
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Thank you, Richard. My pleasure.
I think we should leave the glossary entry for the moment and see what answer you get. In the absence of any indication to the contrary, I'd go with "significant".