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Name: Yaal
Status: student
Grade: 9-12
Location: IL
Country: USA
Date: Spring 2014


Question:
What is the neutron flux of a light water reactor?

Replies:
Hi Yaal,

Neutron flux is defined as the number of neutrons passing a unit area per second. It is equal to the neutron traveling velocity times the neutron density and has a unit of [neutrons/cm2-s]. In a light water reactor, neutron flux is a function of positions inside the reactor. Typical average neutron flux level is on the order of 1.0E13 neutrons/cm2-s. Please feel free to contact me if there is anything confusing to you.

Best wishes, Tingzhou Fei Reactor & Fuel Cycle Analysis Nuclear Engineering Division Argonne National Laboratory


Hi Yaal,

Good question! We will leave the derivation to the reader, you may wish to look at :http://link.springer.com/article/10.1007%2FBF01832114#page-1

First several assumptions must be made: Neutron flux is determined in a cylinder; the water is boiling; neutron density, scattering, capture length and energies are well established so that we may make a zero order Bessel Function(Jo) of those assumptions. ar is 2.4/effective radius and r is the radius.

Nr = Jo(ar, r)

Selection of your reactor, its size and the radius from the core at 100C are important to arrive a good estimation.

Hoping this helps, Peter E. Hughes, Ph.D., Milford, NH


Hi Yaal

Neutrons are the bullets that split the Uranium atoms in a light water nuclear reactor. When the Uranium atoms get hit by a neutron, the Uranium atoms break down into two smaller pieces and produce heat. That heat is transferred by the water in the reactor to a heat exchanger that isolates the reactor's radioactive water from the water that converts to the steam that spins turbines for generating electricity or turning the propellers of a nuclear submarine.

http://www.youtube.com/watch?v=igf96TS3Els

"Neutron flux", is a measure of how many neutrons are bouncing around in the reactor AND how many Uranium atoms are getting split and producing heat. The number of neutrons bouncing around inside the reactor splitting Uranium atoms is controlled by "control rods" that are made of substances that absorb neutrons. "Control rods" are the "gas pedal" of the nuclear reactor by controlling the number of neutrons and therefore the amount of heat being produced in the reactor.

Control rods are pulled OUT of the reactor to INCREASE the amount of heat being produced in the reactor and pushed IN the reactor to absorb more neutrons and REDUCE the amount of heat produced in the reactor.

Here are some on-line articles that go into more detail on the subject:

http://en.wikipedia.org/wiki/Nuclear_reactor

http://www.atomicarchive.com/Fission/Fission1.shtml

http://www.nrc.gov/reading-rm/basic-ref/glossary/neutron-flux.html

Sincere regards, Mike Stewart


Hi Yaal,

Good question! We will leave the derivation to the reader, you may wish to look at : http://link.springer.com/article/10.1007%2FBF01832114#page-1

First several assumptions must be made: Neutron flux is determined in a cylinder; the water is boiling; neutron density, scattering, capture length and energies are well established so that we may make a zero order Bessel Function(Jo) of those assumptions. ar is 2.4/effective radius and r is the radius.

Nr = Jo(ar, r)

Selection of your reactor, its size and the radius from the core at 100C are important to arrive a good estimation.

Hoping this helps, Peter E. Hughes, Ph.D., Milford, NH


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