Between them Australia and Canada have maybe 70% of the easily mined uranium so nobody seems to worry. However Australia has just refused uranium shipments to India and Russia so it can't be taken for granted.

Strategic Petroleum Reserve covers us for 33 days. A strategic uranium reserve could cover us for years. Even without one, reactors don't shut down when uranium is cut off until the next scheduled refueling time for a particular reactor. Thus, power generating capability will taper off at a linear rate over a year+ even if companies don't already have the next set of fuel rods on hand.