What are the Temperature levels in nuclear power plants?

Answer 1

Temperature of the UO2 fuel must be kept below melting point of 2800 deg C. UO2 is not a good conductor of heat and the design parameters of max heat rate and fuel pin diameter have to be optimised to give a satisfactory operating condition.

Answer 2

• Most power reactors 3% to 5% enriched uranium-235.
• CANDU power reactors natural 0.7% uranium-235.
• Breeder power reactors 20% to 95% enriched weapons grade uranium-235. Eventually transitioning to reactor grade plutonium or even mixed transuranics fuel.
• Research reactors various enrichments of uranium-235 or even plutonium. Uranium-233 breed from Thorium is also sometimes used.

Answer 3

There is considerable after heat generated by fission products in the spent fuel removed, so the temperature depends on how it is stored. As it is stored in water filled chambers (in the UK we call them 'cooling ponds', not sure about in the US), the temperature will be little above the water temperature. In the US I believe all fuel has been stored up to now on the power plant sites. In the UK the power stations all have their own cooling ponds, but after a period in these it has been taken to Sellafield in water filled flasks, and transferred there into larger cooling ponds, from which it can then be handled chemically if required to separate into different elements.

Answer 4

In water reactors, PWR and BWR, the fuel itself is in the form of tubes about 10mm diameter, filled with UO2 fuel and clad in zircaloy sheathing. It is important to avoid melting of the sheath, which would release fission products into the coolant, and this has to extend to fault conditions as well as normal steady operation. The melting temperature for zircaloy is about 1800 degC, but as it is in contact with water at only about 320 degC in normal conditions, there is a good margin. The UO2 is a poor thermal conductor so there is an appreciable temperature gradient from the outer radius of the fuel pin to its centre. The melting temperature for UO2 is about 2800 degC. The actual fuel temperature varies both axially and radially in the core, due to the distribution of neutron flux. The maximum fuel temperature is about 650 degC with standard deviation 120degC for PWR's and 800degC with standard deviation of 173degC for BWR's, deduced from a sample of six PWR's and 2 BWR's. A wider survey might give different results. (Survey by Williams and Mueller, Oak Ridge National Lab)

Thus during normal operation there is a good margin to fuel failure levels, the limits to operation depend mainly on behaviour in faults, particularly loss of coolant accidents (LOCA).

Conditions in gas cooled reactors using stainless steel sheaths are somewhat different, but I assume you are interested in water reactors.

Answer 5

In a theoretical unregulated plant, there would be no temperature limit. Without cooling, a nuclear power plant's thermal energy (the energy not collected as electricity or released in other forms) would have nowhere to go, and builds up in the reactor itself. When the reactor reaches the melting point of its material, a nuclear meltdown occurs. Note that this theoretical plant does not exist. Even Chernobyl had extensive cooling systems, but they were not sufficent to handle the operating levels being tested at the time of the disaster.

Coolant systems are typically designed to maintain the temperature of a reactor at safe levels. Outflow water typically stabilizes at around 40 degrees Celsius in cooling ponds, so reactor temperature is likely no more than 60 degrees Celsius at operating levels. If you *need* more specific levels, you likely have the resources necessary to find them - either a professor if you are a student, or more experienced personnel if you actually work at a nuclear plant and are concerned (if so, please stop reading and go talk to your supervisor about it now, just in case).

The coolant system of a nuclear power plant is actually the most complex part of the plant itself. The reactor is typically cooled by materials that have very high specific heats (thus absorbing a lot of energy per increased unit of temperature). Deuterium oxide, or heavy water, is the traditional fluid used (Deuterium being an isotope of hydrogen). This coolant is relatively valuable, and is typically retained, and must be cooled through the use of normal water. When released, this water is still too hot to release into the environment, and is thus retained in cooling ponds for quite some time such that it returns to ambient temperature before returning to the wild.

Since you have also selected nuclear weapons as a category, the fireball produced by "Little Boy," the bomb dropped on Hiroshima at the end of World War 2, was 3980 degrees Celsius, or 7200 degrees Fahrenheit. This is hot enough to ensure total destruction at ground zero of the blast. Larger yield nuclear weapons which were developed and tested throughout the Cold War had even higher temperatures. (The temperatures are clearly an estimate, as any monitoring equipment at ground zero of a thermonuclear blast, along with any operators thereof, would also be destroyed.