13.9.3.1 Sodium-Cooled Fast ADS. ADS have more ‘inertia’ than the corresponding critical reactor in that they are less sensitive to both positive and negative feedbacks (Bell, 1994). For example, with the source still on, they have lower but wider power peaks than in the critical reactor. In the ADS, the power rises earlier due to the lesser influence of negative feedbacks such as Doppler, axial expansion and structural effects. It falls later due to the lesser influence of fuel dispersion. In the sodium voiding phase, pin failures could occur.

The more sub-critical the ADS, the more the above features are seen. To avoid core meltdown, the source must be switched off, before much sodium voiding occurs. As stated earlier, fuel slumping can lead to re-criticality and power excursion because in a fast system the core is not in its most critical configuration prior to the event (Theofanous and Bell, 1985).

The ADS has some advantages over the critical reactor in that the time constants for the power excursion are longer and rapid power excursions are not possible at least when the ADS is in its original configuration. There is similarly a longer time period to detect sodium boiling or pin failures and hence initiate a beam switch-off.

13.9.3.2 Gas-Cooled Fast ADS. Gas-cooled fast ADS share the similar advantages and disadvantages that gas-cooled fast critical reactors have compared with fast sodium systems. Advantages include the utilisation of a chemically inert gas, and it may be possible to use water for post-accident cooling, e. g. if an in — or ex-vessel core-catcher can be designed and concerns of re-criticalities can be addressed.

Gas-cooled systems can also clearly suffer cooling failure events, but since system pressures are comparatively much higher than the sodium coolant system, LOCA accidents are an additional issue. In all cases, shut-off of the beam is crucial for preventing a core melt. A disadvantage of the gas system is that decay heat removal cannot be achieved solely by natural convection, thus back-up diesel generators are needed to be on stand-by in the event of loss of power to the active circulation pumps.

13.9.3.3 Lead-Cooled Fast ADS. Lead coolant has a number of advantages as a coolant compared with liquid sodium. A lead system would not suffer from positive feedback effects on reactivity in the event of boiling. It is only a weak moderator and, therefore changes in reactivity do not result due to changes in density effects.

It is also relatively chemically inert to air and water. The one disadvantage is the relatively high melting point 327°C, which means electrical heating would be required during start-up and there may be the possibility of freezing and blockage in the event of electrical system failure.

The Rubbia design includes lead as a coolant and it has been analysed against cooling failure transients such as LOF due to pump failure and LOHS due to loss of feedwater. The system has good natural circulation cooling characteristics so LOF is not an issue. For LOHS, meltdown could occur if the beam is not shut-off. This could also occur for slow reactivity insertions under similar conditions. However, the Rubbia system incorporates a specific provision to shut off the beam, based on shielding of the target by a rising liquid lead level under accident conditions. Further provision is also included in the Rubbia design to ensure long-term removal of decay heat by air natural circulation of the guard vessel.

13.9.3.4 Thermal ADS with a Circulating Salt-Fuel Mixture. Thermal systems have generally larger cores than fast systems because power densities are lower. This has the advantage of greater thermal inertia under coolant failure accident conditions allowing more time for accident detection, prevention or mitigation. Without switch-off of the beam though, pressurisation, heat-up and boiling would occur. However, this could be mitigated by spallation target melting and material movement leading to neutronic shut-down.

The time-scale for decay heat-up of the larger core systems is of the order of tens of hours since the fuel is distributed around the core and primary circuit and the system is in natural circulation mode. Some long-term cooling system/procedure though would need to be established, i. e. the salt-fuel mixture may need to be drained into a cooled tank. For smaller systems it may be possible to remove all the heat via natural circulation.

An advantage of a liquid fuel system is that short-lived fission products can be removed to reduce the fission product inventory.

The precipitation of fuel or MA may be a concern in salt-fuel ADS. This phenomenon could lead to flow impedance and loss of cooling in selected areas but also the density variation around the circuit could lead to criticality concerns.

There is also some concern that loss of cooling could lead to power increases due to a positive temperature coefficient in pure salt/Pu/minor actinide mixtures, since no 238U or 232Th with their absorption resonances would be present.

There is also the issue of possible explosive contact between molten salt and water; there may be potential for this event in some designs (Hohmann et al., 1982).

Inspection of components is also difficult in molten salt-fuel systems because pumps and heat exchangers become contaminated with radioactive material. Furthermore leaks would result in contamination of the whole containment.