# Nuclear reactor

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 text books?

K^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.

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.

QuantumPion
Gold Member
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 text books?

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.

Astronuc
Staff Emeritus
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 text books?
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.

QuantumPion
Gold Member
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

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:

Astronuc
Staff Emeritus