What impact does the coolant temperature limit (t_crit) have on PWR thermal efficiency?

The coolant temperature limit, commonly referred to as the critical temperature (t_crit), plays a significant role in the thermal efficiency of a Pressurized Water Reactor (PWR). In nuclear reactors, thermal efficiency is largely influenced by the thermal cycle of energy conversion, which can be represented through the Rankine cycle.

Higher coolant temperatures generally lead to improved thermal efficiency since they increase the average temperature difference between the hot and cold reservoirs. However, there are limits to how high these coolant temperatures can go due to materials constraints and safety considerations. When t_crit is maintained at lower temperatures, the thermodynamic cycles are less favorable, meaning that a portion of the thermal energy generated does not get converted into useful work as efficiently.

Since lowering the coolant temperature limit restricts the effective thermal cycle and reduces the overall efficiency with which thermal energy can be converted to electrical energy, this results in decreased thermal efficiency. Thus, establishing a lower critical coolant temperature can result in reduced performance in terms of energy output relative to input heat, hence affecting the plant's overall efficiency negatively.