A Design Basis Accident in the PWR is the large break Loss-of-Coolant Accident. After the initial Blowdown the reactor core will be at low pressure, steam-filled and heating up due to decay heat. Some fuel clad temperatures will be above the value at which spontaneous quenching can occur. The subsequent refilling of the pressure vessel by liquid from the Emergency Core Cooling System (ECCS) so that the core is cooled and quenched is known as Reflood. The important considerations in reflood are the hydraulic flows in the primary circuit, the heat transfer in the core, and the feedback between the two.
At the end of blowdown the pressure vessel will contain very little liquid but the (passive) accumulators and (active) pumped injection systems of the ECCS will be injecting liquid into the vessel downcomer via the intact cold legs. After about 10 seconds the vessel lower plenum will be full and liquid will reach the bottom of the core fuel rods. Quench fronts will quickly become established on the rods and will move up the core, lagging behind the liquid level as an inverted annular flow regime is established. After a further 10–20 seconds some 25% of the core will be flooded, but at this point the accumulators are exhausted of liquid. Reflood continues at a much lower rate (a few cms per second) due to the continuing pumped injection until the whole core is flooded and quenched. Safety analyses indicate that reflood should be complete some 150–300 seconds after accident initiation. This timescale is too short for any operator intervention, and the reactor safety system operations described here are completely automatic.
Reflood is unlikely to be a steady, continuous process. One major perturbation occurs when the accumulators empty of liquid. As the liquid in the pipework connecting the accumulators to the reactor circuit is replaced by the nitrogen cover gas the volume flow rate increases. Thus there is a surge of liquid into the vessel, followed by a transient pressurizing of the top of the downcomer by nitrogen. This produces a surge of liquid into the core, which in turn produces a burst of heat transfer and quenching. The resulting increase in steam production can pressurize the core region to such an extent that some liquid is driven backwards out of the core, up the downcomer and out of the break. After this short-term transient reflood continues but with a somewhat lower vessel liquid inventory.
Heat transfer in the inverted annular flow regime, especially at the quench front, generates a significant upward steam flow, which carries some of the liquid up through the core in the form of entrained droplets. This dispersed flow produces some heat transfer from the fuel at higher elevations. The peak clad temperatures occur near the top of the core, and continue to rise during the early stages of reflood. Eventually, as the quench front moves closer to the peak temperature positions, the precursory cooling is sufficient to halt the rise. Analysis shows that temperatures remain below the 1200°C safety limit, even when pessimistic assumptions about the availability of the ECCS are made. At some stage during reflood much of the fuel cladding will be above the maximum temperature at which liquid water can exist. The quench front proceeds into such regions by a combination of precursory cooling and axial conduction in the cladding, with precursory cooling probably being the dominant mechanism under the particular conditions of reflood.
The outlet flow from the top of the core will initially be pure steam but later will be a dispersed flow containing liquid droplets. This flow continues around the coolant circuit to the break position via the steam generator (if the break is in the cold leg). The pressure difference which this flow requires, which is enhanced by evaporation of droplets in the steam generator (the pressurized secondary side of which still contains hot fluid at this stage), increases the pressure in the core region and thus resists the inflow of liquid into the core from the downcomer. This phenomenon is known as Steam Binding. The extra pressure at inlet is provided by an accumulation of liquid in the downcomer giving an increased differential between downcomer and core. Thus there is a complicated feedback between the rate of steam generation in the core and the rate of accumulation of ECCS liquid in the downcomer to provide the gravity head necessary to overcome the steam binding. The reflood phase is said to be complete when the fuel is completely quenched. After a further period of cooling the circuit will be in a subcooled state and coolant injection can be manually switched to a recirculating residual heat removal mode, which can be continued indefinitely. (See also Boiling; Rewetting of hot surfaces.)