13.9.4.1 TOP Accident in a Sodium-Cooled ADS. Since the sodium-cooled ADS is initially sub-critical, fast or medium ramp rates can be accommodated for sizeable reactivity insertions. For example, calculations (Wider, 1997) of $170 per second for a total insertion of $2.65 at a sub-criticality of —$3 and $6 per second for a total insertion of up to $3 for a sub-criticality of —$5 showed no significant power excursion. For a critical reactor, the power would be 2000 times the nominal power.

For slower ramp rates, e. g. at $0.1$ per second, for a similar total insertion of about $3 at a sub-criticality of —$3 per second, a failure of a single channel occurred. At a sub­criticality of —$5, failure also occurred but later, and it did not occur at all for a — $10 sub-criticality. This compares with the critical reactor case in which pin failures in 1 out of 10 channels occur.

Thus for fast ramp rates without scram, the ADS behaves in a benign manner, for slow ramp rates there may be limited core damage at a later stage for insufficient sub-criticality or failure to scram the beam.

13.9.4.2 TOP and RIA Accidents in a Gas-Cooled ADS. Regarding fast and medium ramps, the gas-cooled fast ADS would behave similarly to the sodium-cooled ADS, i. e. benignly. For slower ramp rates, there could be rather more pin failures, because fuel dispersal may be less and, therefore provide less negative feedback.

13.9.4.3 TOP and RIA Accidents in a Thermal Molten Salt-Fuel Mixture. A thermal ADS would also act benignly under fast or medium ramp insertions. This system would also probably show an advantage for slower insertions compared with the fast systems above, because with a fluid system, the pins would probably not fail.