Nuclear reactor coolant circuits normally operate at pressures well above atmospheric. Blowdown is the depressurization following the opening of a breach in a circuit, either by accident or by design.
This arises as part of an important class of safety problems known as LOCAs (Loss-Of-Coolant Accidents). The PWR primary circuit, shown diagrammatically in Figure 1, operates in single phase at a pressure of about 15.5 MN/m2 and a maximum temperature of 320°C. Upon a rupture of the circuit, the important thermal hydraulic considerations are the timescale of the depressurization, the amount of coolant lost from the circuit and the magnitude of the heat transfer in the reactor core. The effect of the forces generated on the reactor structures is also important, but this is not discussed further here.
For safety purposes, the maximum breach size deemed to be credible is the double-ended offset shear of a hot or cold leg (DECLB). In a modern 4-loop design, this gives a discharge area of about 2×0.75 m2 in a circuit containing some 250,000 kg of coolant. Following such a breach, the pressure falls very quickly (< 1 sec) to about 11 MN/m2, the saturation pressure corresponding to the highest temperature points in the circuit (core outlet and hot legs). After this point, two-phase conditions appear and the rate of depressurization is determined by two-phase critical flow at the breach. After about 25 seconds, the pressure has fallen to the containment equalization value of 0.2–0.3 MN/m2 and the blowdown phase is over. Smaller breach sizes can occur almost anywhere in the circuit and are much more probable than the DECLB. In safety studies, breaches leading to blowdown taking hundreds to tens of thousands of seconds are considered. For a given size of smaller breach, the location of the breach becomes important. Breaches at the bottom of a component tend to discharge liquid and give rise to relatively slow rates of depressurization. Breaches at the top tend to discharge steam and give rise to relatively higher rates of depressurization, but less loss of coolant.
Depressurization of the hot coolant to near-atmospheric conditions leaves some 70% of the coolant in the liquid phase, sufficient to keep the reactor core completely covered if all the liquid remains in the circuit. In the worst case, however, depressurization occurs so rapidly that flashing occurs throughout the circuit, and the high steam flows thus generated carry much of the liquid around the circuit and out of the breach. At the end of blowdown, very little liquid remains in the circuit and the reactor core is completely uncovered. It is the function of the Emergency Core Cooling System (ECCS) to refill the circuit and recover the core. One component of the ECCS consists of large tanks, known as accumulators, which contain water at a pressure of about 4 MN/m2, and which automatically inject into the cold legs when the circuit pressure falls below this value. This infection thus begins before the blowdown phase is over, and some of the injected water is carried out of the breach by the blowdown flows in the phenomenon known as Downcomer Bypass.
In the DECLB, a large reverse flow is very quickly established at the vessel inlet nozzle, causing the whole core flow to reverse within a fraction of a second. Within a second, the core is voided and the fuel dries out and heats up rapidly. The initial rise in clad temperature, see Figure 2, is due to radial temperature equalization across the fuel and clad, which has a time-constant of a few seconds. In the absence of any significant heat transfer, the clad temperature would continue to rise by a few tens of degrees per second due to the decay heat even though the fission reactions in the core stop almost instantaneously in this accident. The temperatures would thus reach the 1200°C limit, beyond which a runaway oxidation would occur, before the end of the blowdown period. In fact, analysis shows that the core flows — two-phase and single-phase steam — during blowdown give rise to heat transfer which is sufficient to halt the rise in clad temperature well before the 1200°C limit is reached. Over the first few seconds of the transient, the breach flow rapidly reduces from its initial peak as the pressure falls and conditions become two-phase. However, the circulating pumps in the intact legs continue to rotate under inertia even if the electrical supplies have been lost. The result is that a net forward flow into the vessel is reestablish, and some liquid flows back into the bottom of the core. This leads to partial rewetting of that region and some heat removal throughout the core. Later in the transient, the core flow again reverses, and some residual liquid may drain back from the steam generators and hot legs to cause partial quenching at the top of the core. Even in the high power regions which stay in dryout, some 50% of the heat is removed by blowdown cooling, and temperatures at the end of blowdown are well below danger levels.
At the end of blowdown in this worst case scenario, the core is in dryout and heating up due to decay heat. The pressure vessel may contain very little water. But the ECCS will be injecting liquid into the intact cold legs. This will take the transient into the Refill and Reflood (q.v.) phases.
Collier, J. G. and Hewitt, G. F. (1989) Introduction to nuclear power. Hemispher Publishing Corporation.