1st Prius Failure... but no runaway...

The CHRS canister holds around 3 quarts of fluid.

If the pump didn't work, the fluid in the canister would become cooler than it otherwise might be. The position of the valve that controls flow of coolant to that canister depends upon the extent to which coolant is required to heat the cabin heater core. Hot engine coolant may continue to flow into the canister while the engine is running. The fluid in the canister will be hotter than ambient temp but cooler than if the pump functioned upon engine shutdown.

I understand that the motivation for the CHRS system was to reduce emissions during the initial warmup period and to help 2G Prius qualify for the AT-PZEV designation. I doubt that it has much effect on mpg.

 

Just to clarify. The water is in the engine block and the "thermos" at the same time. The only thing the pump does is to exchange the colder water in the engine block with the "thermos" warmer water when first starting up after the car has been sitting awhile. What was in the engine block goes into the thermos and what was in the thermos goes into the engine block. If you run sufficiently long enough then the circulation valve is set so that water is circulating through the engine block and thermos (and radiator) at the same time with the water being at the same temperature throughout. If that is the case it will just shut the valve to the thermos when you turn off the car and not even do any pumping. There are sensors that determine whether the engine block or thermos water is hotter, upon which it bases its decision on how to circulate the water with the pump.