"fission" with -zh-?

[geoffrey nunberg]

Listening to some nuclear energy experts talking about Japan on the radio this morning, I was led to wonder whether  "fission" is the only English word in which orthographic [Vssion] can be pronounced with a voiced fricative. When I checked with my graduate students, I was surprised to find that that's the pronunciation used by all but one of them (and the one is from S. Africa). If so, it must be by analogy with "fusion," right?

Geoff

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