Thermal efficiency of nuclear power plant?

Answer 1

The average thermal efficiency of a nuclear power plant, which accounts the heat produced in the reactor and the electricity produced in the turbo generator, is about 33-34%, varying on the type of the reactor as well as which fluid is used to cool the reactor and transfer the heat.

Answer 2

In any power station using steam turbines there is a lot of heat wasted. This is simply due to the need to condense the low pressure steam at the turbine back end, to return feedwater to the steam raising units. The turbine condenser is run under vacuum to extract as much energy from the steam as possible, the degree of vacuum is essentially set by the temperature of the condenser cooling water, the lower the better. The waste heat is at only a few degrees above the cooling water temperature, which is derived from a lake, river, ocean, or from air cooling towers, so that's the best that can be done for efficiency in that location, but the waste heat being at almost ambient temperature is not useful for any purpose.
This applies whether the source of heat is a nuclear reactor or a fossil fired boiler. The overall efficiency tends to be greater the higher the temperature of the steam at the turbine high pressure end, so fossil fired plants are generally better than nuclear reactors which run at lower outlet temperatures. High temperature gas cooled reactors can be as efficient as modern fossil fired plants, but they have other drawbacks and the ones most used ie PWR and BWR have an overall efficiency of about 33 percent, so 67 percent of the heat produced is wasted and just goes into the condenser cooling water. For a modern coal fired plant the efficiency can reach about 40 percent so 60 percent is rejected to ambient.
Read Wikipedia article 'Rankine Cycle' to understand this better.