Is expiation obligatory for someone who swore not to repeat a prayer, then discovered the presence of a barrier preventing water from reaching the skin during ghusl?

The intention specifies and restricts oaths, as does the reason behind them. So, if the questioner repeats the prayer due to a compelling reason, not out of doubt, then this does not fall under her oath, nor does it constitute a breach, and no expiation is required. However, if she repeats it due to doubt, then she has broken her oath, and an expiation for an oath becomes obligatory upon her.