Non-Zero
The probability of failure in the viva is not 0. We know this because despite the many thousands of candidates who pass their viva every year, a handful of people don’t. A couple of universities have told me the odds are less than one in a thousand.
So, statistically, the probability is non-zero.
It could happen, but why should it happen to you? For any candidate thinking, “It could be me!” let me ask, “Why?! Why would it be you?”
The chances of succeeding are far, far, far greater than the probability of failing.
Your success isn’t “chance” of course. Your success in the viva is another victory, the latest, the greatest since you started on the path to being a PhD.