Best estimate transition-phase calculations, supported by the results of small-scale experiments, indicate that, after a core-disruptive acci­dent, the fuel would be in the form of a mass of debris dispersed in the primary coolant. The internal structure of the reactor vessel (in the case of a pool reactor, the inner vessel, the primary pumps, the intermediate heat exchangers and the decay-heat rejection heat exchangers) would be damaged and probably inoperable but the ves­sel itself would be intact and would retain the primary coolant. The coolant would continue to serve to remove the decay heat from the fuel and would circulate by natural convection. The coolant itself would lose heat to the emergency cooling equipment, probably the RVACS system (see section 5.2.4). There would be plenty of time to ensure the correct operation of the RVACS (see Figure 5.1).

If the coolant is sodium the fuel debris would fall towards the bottom of the reactor vessel. To eliminate the possibility of it accumu­lating and forming a critical mass it may be necessary to put in place a structure in the form of a tray shaped to catch the fragments in a sub­critical layer. A tray of this type is usually known as a “core-catcher”. It might be made of neutron-absorbing material to reduce the chance of criticality.

Provided it was porous the mass of debris would be cooled by sodium circulating within it. If the fuel were to coagulate in some way so that the sodium could not circulate it might become hot enough to damage the structure on which it rested, so the core-catcher has to be arranged in such a way that the coolant is able to circulate underneath it to keep it cool. The core-catcher would also act to protect the reactor vessel itself from the risk of being damaged and even melted by the hot fuel. In this way the core debris would be retained safely and

ess steel

[1] in the proportions 56:20:15:9. If plutonium of this isotopic com­position is substituted for pure 239Pu in the reference reactor there is very little change in the spectrum but there is a large effect on the enrichment, which goes from 26% to 30%. However this is rather mis­leading because the enrichment is defined as (total Pu)/(Pu+U), and “total Pu” now contains a significant amount of fertile material. The actual ratio fissile/(fissile + fertile) falls to 22%. This is because 241Pu has a higher fission cross-section than 239Pu and a higher value of v.

Thorium. Figure 1.11 compares reactors utilising the uranium and thorium cycles by showing the effect of replacing 239Pu and 238U with