During the initiation phase the geometry of the core is reasonably intact. The prompt-critical excursion takes place so rapidly that there is no time for much movement. It is therefore relatively easy to calcu­late what happens by means of a code (much more complex that the simplified model described earlier) that couples transient neutronics with heat transfer and fluid mechanics, including melting and boiling, in multiple channels.

The subsequent “transition” phase is made more complicated by potential melting of the entire core structure. There are two major concerns: that the fuel might accumulate into a new super critical mass (“recriticality”), or that there might be a violent thermal interaction between the molten fuel and the coolant. The latter is of concern because, under circumstances that are very imperfectly understood, such an interaction might be explosive. (As explained in section 5.1.2, this is analogous to a “steam explosion” caused by contact between molten metal and water. It is often called a “vapour explosion” or a “molten fuel-coolant interaction”, MFCI.)

Transition phase calculations are very complicated because they involve transient three-dimensional multi-phase fluid mechanics and heat transfer coupled with transient neutronics, and the uncertainties in the results are large. In principle it is possible to surmount this difficulty in a safety argument by making pessimistic assumptions at every point of uncertainty in the calculation, but in practice this usually results in predicted releases of mechanical energy capable of breaching any reasonable containment.

It is of course impossible to validate a complete transition phase calculation code experimentally without destroying at least one, prob­ably several, complete reactors. Small-scale experiments can however be used to validate individual steps in the calculation. For example kilogram-scale MFCI tests show that sodium vapour explosions are rare and when they do occur they are mild. Similarly small-scale tests on the motion of molten fuel indicate that recriticality in the core is very unlikely.

Figure 5.14 illustrates some of the phenomena and the difficulties encountered in a transition-phase calculation for a Slow LOF accident in a large sodium-cooled core. It shows the state of the fuel, cladding and coolant as functions of axial position and time in one subassembly. As the coolant flow-rate decreases it gets hotter and eventually starts to boil at the core outlet level. The vapour ejects the liquid coolant from the subassembly, mainly upwards into the hot pool. Liquid from

the hot pool then falls back into the voided subassembly, boils and is ejected again, and this cycle may take place several times in a few seconds. The motion of the coolant is accompanied by large reactivity changes. When the liquid coolant is ejected the fuel is deprived of most of its cooling and within a second or so the cladding melts so that the fuel itself is free to move. If the fuel remains in the core it then melts after a further period of a few seconds.

The possibility of recriticality arises when the cladding melts. When this happens the fuel pellets or fragments of pellets are likely to be carried out of the core region by the coolant (either vapour or a two — phase boiling mixture) or by fission-product gas escaping from the plena in fuel pins, but there is a small probability that liquid reentering the core may carry fuel towards the core centre. If the fuel melts there is a possibility of an MFCI.

If the results of small-scale experiments are used in a “best estim­ate” (as opposed to “pessimistic”) transition-phase calculation the fuel is assumed to be swept out of the core and the resulting energy release is predicted to be mild and containable. Such a conclusion is usu­ally found satisfactory (by nuclear licensing authorities, for example) because the low frequency of the initiating event (the TOP or LOF), coupled with the low probability of failure of the protective system, is sufficient to demonstrate that the frequency of containment failure (and consequent release of radioactive material into the environment) is acceptably low.