The most serious design basis accident (or DBA) that can occur in a nuclear power plant is a loss-of-coolant accident or LOCA. The probability of most LOCAs occurring is relatively small (see Chapter 33), and since the 1980s, not a single LOCA has occurred in a commercial nuclear power plant that has led to significant damage to the core. The accident at Three Mile Island (TMI) in 1979 involved a stuck pressure relief valve on the pressurizer (which controls the pressure in the primary loop), but if the sticky valve had been diagnosed properly, even this LOCA would not have occurred. In the United States, numerous design limits are imposed by regulatory authorities to ensure that hypothetical LOCAs are properly anticipated, categorized, and contained. These limits include

Thermal design limits

Structural design limits

Oxidation or chemical design limits.