They are replacing diesel subs with long range UUVs and short range UUVs.
The USN doesn't have any diesel subs... that is why they like to train with smaller countries that do have them...
The energy from fission is transferred to the water.
The fission process creates high speed neutrons and fission products. These will interact with the fuel matrix and the water, slowing down. In the process they will transfer their kinetic energy to the fuel and the water. The fuel will heat up and will transfer its energy to the water through conduction and convection. Hence, the water will heat up
A nuclear reactor is used to boil water to drive turbines to generate electricity... in such a system why would it need to cool the water down? Cold water doens't drive turbines and wont generate electricity. To cool down the reactor you turn it down which lowers the fuel rods into the control matrix and stops the fission reaction which stops generating heat.
Introducing cold water into the system reduces energy... why?
The water in a pressurised water cooled nuclear reactor is a closed circuit... the high pressure means instead of boiling at 100 degrees C it boils at much higher temperatures which makes the steam much more energetic. If they have to cool anything down they could expose the pipes that the high pressure coolant is flowing through to sea water... but they could control the heat of the sea water going in to the sea by changing the amount of water being exposed and the time it is exposed for.... think of a hot bath of water and a tube that is say 20mm thick that pumps water up out of the bath and down beside the bath and through a swimming pool on the ground outside the window. The water in the bath is 80 degrees C, but the water in the swimming pool is only about 3 degrees so the bath water in the pipe is about 80 degrees as it moves through but where the pipe goes in to the swimming pool the temperature of the pipe is lowered by the cold water in the pool. The water in the bath is hot but not hot enough to heat up the water in the swimming pool by very much... but with a constant flow of bath water through the swimming pool the bath water will be quickly cooled, but because of the volume of the water in the swimming pool the swimming pool might only get a few degrees warmer.
Go back to the sub... if there was a reason they needed to cool the water they could use a radiator with a huge surface area that is sealed so no water from the sub leaves the sub... but cool sea water enters the sub and cools the liquid in the radiator and then goes back in to the ocean.
IR is used to detect submarines... BUT ALL SUBMARINES. When any type of submarine moves through the water while submerged it disturbs the water and mixes water in different layers.
Pressure effects the temperature at which water freezes and at which it gets heavier or lighter so normal sea water consists of multiple layers but with depth comes increased pressure which changes density so it is not the case that the surface water is the warmest and the bottom layer is the coldest.
A large object like a submarine can stir water from different layers and send it rippling up to the surface... so you get an IR trail because warmer water from different layers breaks the surface so you can see a warm trail on the surface with a thermal device... of course it doesn't always work efficiently sometimes the water stirred up is mixed temperatures and not easy to see... some times it is because a pod of whales or a large school of fish have disturbed the layers... it is not a reliable way to hunt subs...
Sonars will be improved. What are the other sensors that you are referring to?
The Soviets/Russians had all sorts of different sensors on their subs... many of which are still secret... I don't pretend to know all about them.... needless to say look at this image and tell me what sort of sonar or radar it is...
they have very good countermeasures in form of small torpedo-like devices that imitate the sub signature.
They had three for export in the early 2000s... the MG-74ME is a multipurpose system that can create noise to jam enemy sonar and torpedo homing systems and also attract torpedos by creating a target simulating a sub signature to attract passive homing weapons. It can use high power sonar emissions, or it can mimic a specific sub type and manouver in a way that a normal sub would to evade torpedoes and anti sub formations.
Used for training but can also be used as a countermeasure.
There is also the Berilly-E self propelled multirole automated sonar countermeasures system that can basically do similar things, and then there is also the Akhilles self propelled target/submarine simulator that basically can simulate a ship or sub for the purposes of training but can also be used in combat... all three are 533mm calibre and can be launched from standard sub and ship mounted torpedo tubes of that calibre.
I am sure they have something like this on their newer subs... as I said these three weapons are listed for export in a book released in 2000.