13.9.1 Scenarios

It can be seen from the earlier discussion that there are different ADS concepts being considered based on a number on different sub-critical reactor types.

Some of these reactor types have attracted considerable levels of safety research in regard to critical reactor operation. In principle, there are similar categories of accidents that could occur in ADS sub-critical reactors as could occur in critical reactors (Wider, 1997), see, e. g. Table 13.5.

ADS discussed above have included fast systems with solid fuel and liquid metal (lead or sodium) cooling and fast systems with circulating molten salt/MA. Fast reactors with

gas cooling have also attracted some attention previously. Thermal systems have been considered with circulating molten salt/minor actinide/Pu and graphite moderator. Thermal systems have also been considered with molten salt/actinide/Pu or a water/oxide slurry circulating in pipes with heavy water moderator.

In low-pressure fast or thermal reactor systems, various categories of loss of cooling accidents can occur. Typical examples are loss of flow (LoF) due to pump failure, or loss of heat sink (LoHS) due to pump failure in the secondary heat removal loops, or feedwater pump failure. Loss of decay heat removal is another example.

For high-pressure systems, loss of coolant accidents (LoCAs) are an additional possibility, occurring due to a break or leak leading to a sudden depressurisation, e. g. in a gas-cooled fast reactor.

ADS could also be vulnerable to transient overpower (TOP) in the case of fast reactors. Concerns for fast systems include possible reactivity insertions associated with moderator insertion, or a possible positive void coefficient in the case of a sodium — cooled fast reactor. Under more extreme accident conditions and core meltdown, reactivity insertions could result from fuel movement. Reactivity induced accidents (RIAs) are a possible concern in thermal reactors. In all systems, inadvertent withdrawal of control rods or control rod ejection in pressurised systems are possible scenarios although since ADS are sub-critical there may be fewer control rods than in critical reactors. The accumulation of fissile material in circulating liquid fuel systems or due to extreme perturbations, e. g. due to earthquakes could cause reactivity insertion. Finally there is the question on whether there are scenarios leading to a sudden increase in accelerator power.

As for a conventional critical reactor, a high degree of reliability is required for the operation of the key safety functions. In the case of an ADS, the most important requirements are the accelerator shut-off system and the decay heat removal systems.