From this 2004 interview:

SG: But that is something on which in history there is still a question mark and let’s not wait for your book. Is it true that we came close to testing but we did not under American pressure?

PV: Oh, this is something which I have answered several times. This secret will perish along with me. It will never come out of my mouth. It’s for you and lots of books have been written. All off the mark. Some a little less off, some very much off.

SG: So, when will be the right time for history to know what exactly happened?

PV: No, they will never know. It’s from me and it will not come from you. You can go on approximating.

SG: But why must you not make it part of the record of history? Lots of time has gone by now.

PV: I am under an oath. An oath for me is something very sacred. It’s not like cabinet papers being circulated in advance.