Understanding Nuclear Reactors: Clarifications Needed

In summary: The average fuel temperature for a PWR is around 1100 F. The surface temperature of the fuel rods is lower due to the poor thermal conductivity of the fuel pellets. In a PWR, the water does not boil because it is at 2250 PSI, where the saturation temperature is ~650 F. The critical mass and size depends on whether it is bare or reflected, and also depends on the temperature and density of the material. That may be why you find different figures. Regarding nuclear reactor I need some clarifiications;1. During fission of U-235 it is said that the temperature raises to several million degree celcius (
  • #1
ananthu
106
1
Regarding nuclear reactor I need some clarifiications;

1. During fisssioin of U-235 it is said that the temperature raises to several million degree celcius (in uncontrolled chain reation). But what will be temperature produced when a controlled chain reaction takes place inside a nuclear reactor? In the case of a coolant such as a pressurised heavy water, its boiling point is only around 300 degree celcicus. Even if the temperature inside a reactors will not be to such an extent of several million degrees as in the case of atom bomb, definitely it will touch a few thousands in the reactor. Then how is that the water remains without boiling in the primary loop and conducts heat to the heat exchanger? What wiil be the temperature range inside a reactor? How is achieved?

2. What is the exact critical mass and size for U-235? Different values are given in different textbooks?
 
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  • #2
The temperature in the reactor is carefully controlled. Depending on the reactor, the temperatures range from few hundred to few thousand degrees. The rate of the reaction determines the rate at which heat is produced. If all heat produced is carried away by the coolant, the temperature doesn't increase. And the rate of the reaction can be adjusted by adjusting positions of fuel and moderators.
 
  • #3
Well, hope you know that the end function of a standard controlled nuclear reactor plant IS to produce steam from water to drive turbines connected to a generator.
 
  • #4
ananthu said:
Regarding nuclear reactor I need some clarifiications;

1. During fisssioin of U-235 it is said that the temperature raises to several million degree celcius (in uncontrolled chain reation). But what will be temperature produced when a controlled chain reaction takes place inside a nuclear reactor? In the case of a coolant such as a pressurised heavy water, its boiling point is only around 300 degree celcicus. Even if the temperature inside a reactors will not be to such an extent of several million degrees as in the case of atom bomb, definitely it will touch a few thousands in the reactor. Then how is that the water remains without boiling in the primary loop and conducts heat to the heat exchanger? What wiil be the temperature range inside a reactor? How is achieved?

2. What is the exact critical mass and size for U-235? Different values are given in different textbooks?

The average fuel temperature for a PWR is around 1100 F. The surface temperature of the fuel rods is lower due to the poor thermal conductivity of the fuel pellets. In a PWR, the water does not boil because it is at 2250 PSI, where the saturation temperature is ~650 F.

The critical mass and size depends on whether it is bare or reflected, and also depends on the temperature and density of the material. That may be why you find different figures.
 
  • #5
ananthu said:
Regarding nuclear reactor I need some clarifiications;

1. During fission of U-235 it is said that the temperature raises to several million degree celcius (in uncontrolled chain reation). But what will be temperature produced when a controlled chain reaction takes place inside a nuclear reactor? In the case of a coolant such as a pressurised heavy water, its boiling point is only around 300 degree celcicus. Even if the temperature inside a reactors will not be to such an extent of several million degrees as in the case of atom bomb, definitely it will touch a few thousands in the reactor. Then how is that the water remains without boiling in the primary loop and conducts heat to the heat exchanger? What wiil be the temperature range inside a reactor? How is achieved?
Nuclear reactors are 'controlled' - by law and by the physics of the process. Uncontrolled reactors are forbidden, and highly undesirable.

This fission itself produces temperatures on the order of 100 million K, but there are only ~1013 fissions going on in 1022 atoms, so the energy of fission is diluted in the surrounding fuel matrix. The power level is controlled and balanced against the flow rate of the coolant to keep the coolant within a constant temperature range.

Nuclear boiling does occur in some of the hottest channels in a PWR core, but bulk boiling does not occur (as is the case in a BWR). The heat is conducted through the ceramic fuel, then through the metal cladding, which keeps the fuel and fission products isolated from the coolant, and then conducted into the coolant via convective heat transfer (and conduction). Different assemblies have different power levels, so some run a bit cooler than others, and the assemblies on the periphery of the core run at about 30-50% of core average power depending on various factors.

2. What is the exact critical mass and size for U-235? Different values are given in different textbooks?
That depends on the enrichment and geometry (including whether or not it is reflected), and neutron energy spectrum. The simplest (and smallest) would be a sphere of pure U-235.
 
  • #6
Astronuc said:
Nuclear reactors are 'controlled' - by law and by the physics of the process. Uncontrolled reactors are forbidden, and highly undesirable.

This fission itself produces temperatures on the order of 100 million K, but there are only ~1013 fissions going on in 1022 atoms, so the energy of fission is diluted in the surrounding fuel matrix. The power level is controlled and balanced against the flow rate of the coolant to keep the coolant within a constant temperature range.

Nuclear boiling does occur in some of the hottest channels in a PWR core, but bulk boiling does not occur (as is the case in a BWR). The heat is conducted through the ceramic fuel, then through the metal cladding, which keeps the fuel and fission products isolated from the coolant, and then conducted into the coolant via convective heat transfer (and conduction). Different assemblies have different power levels, so some run a bit cooler than others, and the assemblies on the periphery of the core run at about 30-50% of core average power depending on various factors.

That depends on the enrichment and geometry (including whether or not it is reflected), and neutron energy spectrum. The simplest (and smallest) would be a sphere of pure U-235.

Actually I think the smallest possible reactor would be a sphere of pure Cf-251 reflected with beryllium :biggrin:

Although on second thought, a 4 kg sphere of Cf-251 would probably produce so much heat from decay that it would melt itself. :tongue2:
 
  • #7
Just imagine how much 4kg of Cf-251 would cost.

And someone didn't specify whether or not they want a prompt critical system or if they were thinking about delayed neutrons or moderated neutrons.

webelements said:
Californium is a radioactive rare Earth metal named after the state of California and the University of California (USA). Californium-252 is a strong neutron emitter and one microgram emits 170 million neutrons per minute, making it a biological hazard. It has a few specialised uses but only a few of its compounds are known.
Actually, Cf-252 is used a primary neutron source in commercial nuclear reactors.
 

1. What is a nuclear reactor?

A nuclear reactor is a device that creates and controls a sustained nuclear chain reaction. This reaction can produce a large amount of thermal energy, which is used to generate electricity or power various industrial processes.

2. How does a nuclear reactor work?

Nuclear reactors use the process of nuclear fission, in which the nuclei of atoms are split apart, releasing energy. In a reactor, this energy is harnessed to heat water and create steam, which drives turbines to generate electricity.

3. Is nuclear energy safe?

Nuclear energy is generally considered safe, but there are risks involved. The main safety concern is the potential for accidents, such as a meltdown or release of radiation. However, modern reactors have advanced safety features and strict regulations to minimize these risks.

4. What are the benefits of nuclear energy?

Nuclear energy has several benefits, including producing large amounts of electricity without emitting greenhouse gases, helping to reduce reliance on fossil fuels, and providing a reliable source of energy that is not affected by weather conditions.

5. What are the different types of nuclear reactors?

There are several types of nuclear reactors, including pressurized water reactors, boiling water reactors, and advanced gas-cooled reactors. Each type uses a different method to control the nuclear chain reaction and generate energy. The type of reactor used depends on factors such as location, cost, and desired energy output.

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