As we saw in the previous section, there are a number of circumstances in which beds of fuel debris may be formed, initially submerged in a pool of coolant. If such beds can be effectively cooled, remelting is avoided and dam­age to the vessel or the cavity contained in the bed may be prevented. In recent years, and particularly since the accident at Three Mile Island, much attention has been given to the coolability of such beds.

The cooling of beds of debris is a highly complex process and is strongly af­fected by such variables as the bed particle size, the means of access of the coolant to the bed, the bed depth, and the system pressure. Some mechanisms for debris bed cooling, illustrated in Figure 6.4, are as follows:

1. Once-through flow through the bed. Here it is assumed that the liquid is able to reach the bottom of the bed and is then induced to flow into the bed under the action of natural or forced circulation. Natural circulation would be caused by the difference in density of the coolant inside the bed and outside the bed. This is the same kind of circulation that occurs in some forms of steam-generating boilers. Alternatively, the debris bed may be in a region of the reactor over which a pressure drop occurs in the circulation liquid, and this pressure drop would force liquid through the bed. As illustrated in Fig­ure 6.4a, the first phase is for the heat generated in the bed (from decay heat of the fission products trapped in the bed) to heat the liquid to its boiling point. Then, as the flow passes through the bed, the liquid is evaporated and ultimately converted totally to vapor. From this point on, the temperature rises rapidly with distance up the bed, and if the circulation is too low or the bed too deep, the particles may reach a temperature at which they begin to fuse together. This clearly represents a limit to this form of cooling.

2. Cooling of closed deep beds. Here, as illustrated in Figure 6.4b, the liquid can only enter from the top of the bed. The liquid trickles into the bed, cooling it and generating vapor, which must escape in the direction opposite to that of the liquid flowing in. This causes a flooding phenomenon of the type we discussed in Chapter 2, with the vapor resisting and limiting the entry of liq­uid at the top of the bed. This may mean that only the upper part of the bed is cooled and the lower part may become overheated. This limitation is more severe the smaller the particle size in the bed. Again, drying out and fusing of the lower part of the bed is the limit on cooling in this situation.

3. Shallow-bed cooling. If there is a shallow bed of particulate material on the bottom of the containment and this is covered by a liquid layer, then “chim­neys” may be formed in the layer (Figure 6.4c) through which the vapor may escape, the liquid passing into the bed by capillary action through the par­ticulate layer between the chimneys. This is an efficient way of cooling but can only be applied over a limited range of conditions.

Experiments and calculations show that in case 2, for a 1-m-deep bed, a heat dissipation rate of 750 kW/m3 may be achieved if the particles are 4 mm in di­ameter in a pool of water at 1 bar (atmospheric pressure). However, the maxi­mum dissipation rate before dryout and fusion of a bed composed of particles of 0.1 mm diameter would be only 20 kW/m3. Thus, the effectiveness of the de­bris bed cooling can be estimated accurately only if the particle size of the bed is known. Although a better understanding of the mechanisms of debris bed cooling is now beginnng to emerge, the main difficulty of predicting the parti­cle size that might result in different phases of the accident is still to be re­solved. A typical debris bed might have a power generation rate (from fission product decay) of 1000 kW/m3 some 3 h after initiation of the accident—about the time at which one might expect meltdown in a P’^TC. This Dower could be

(a)

dissipated in a bed 0.5 m thick provided the particle size was greater than 2 mm. These calculations are for a P^WR but a similar picture is obtained for the fast reactor, since its increased fuel rating (and hence fission product decay heating) is offset by the increase in latent heat of vaporization of sodium com­pared with that of water.