The reactors were normally mechanically stirred but reactor 4 had operated without a working stirrer since November 1973; free phase water could have settled out in unstirred reactor 4 and the bottom of reactor 4 would reach operating temperature more slowly than the stirred reactors. It was postulated that there had been bulk water in reactor 4 and a disruptive boiling event had occurred when the interface between it and the reaction mixture reached operating temperature. Abnormal pressures and liquor displacement resulting from this (it was argued) could have triggered failure of the 20-inch bypass.