What is Moderator: Definition and 22 Discussions

In nuclear engineering, a neutron moderator is a medium that reduces the speed of fast neutrons, ideally without capturing any, leaving them as thermal neutrons with only minimal (thermal) kinetic energy. These thermal neutrons are immensely more susceptible than fast neutrons to propagate a nuclear chain reaction of uranium-235 or other fissile isotope by colliding with their atomic nucleus.
Water (sometimes called "light water" in this context) is the most commonly used moderator (roughly 75% of the world's reactors). Solid graphite (20% of reactors) and heavy water (5% of reactors) are the main alternatives.Beryllium has also been used in some experimental types, and hydrocarbons have been suggested as another possibility.

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  1. S

    Is arguing with a moderator a valid reason to ban users?

    Is arguing with a moderator a valid reason to ban users? My old account has just been banned. I called another user stupid because he started ridiculing me. For that I received a warning. Fair enough, 4 points, that's adequate. I pointed at a double standard and argued that the moderator wasnt...
  2. A

    Can a Fission Fragment rocket be designed without a moderator?

    a dusty plasma design : https://www.nasa.gov/pdf/718391main_Werka_2011_PhI_FFRE.pdf has nuclear fuel held in place by electromagnetic fields. It uses a very massive moderator around the fuel to slow and reflect neutrons back to the fuel, to enable continued fission. But, there is one kind of...
  3. A

    Fast reactors: How can they work without a moderator?

    Pardon if this is easy to find but I tried google with little success, so how can a fast reactor work without a moderator? Now I understand that a moderator is commonly used within thermal reactor which is why they are thermal in the first place as the neutrons get slowed down and their energy...
  4. O

    Graphite as a moderator in MSR and VHTR

    Hi, In Gen 4 reactors, graphite is used as a moderator instead of water. Is it because of water's absorption cross section is higher than that of graphite? But as far as I know, you would need more graphite to thermalize neutrons. That is because water is commonly used in nuclear reactors. Is...
  5. Phaeous

    Of Lost Neutrons Within Reactors

    How is a continuous chain reaction maintained within a moderator if it is not fissile material? Are the fissile materials mixed with the moderator or are they coalesced at the center of the surrounding moderator? If it is the latter, how would the surrounding moderator allow fissile material to...
  6. Stephanus

    Closing Threads on Physics Forums - Steven's Inquiry

    Dear staff, I just read an interesting thread: https://www.physicsforums.com/threads/can-you-check-my-equation.827896/ The thread was created yesterday 16 August 2015. But now, 17 August 2015 is already closed. I see no offending content, no protest of the mentors, but it's closed anyway. Can...
  7. parazit

    What Are Effective Neutron Moderators in Nuclear Reactors?

    Hello everyone... I've been trying to make a list about moderator materials used in nuclear reactors. So far I've found some but still wondering if there exists more than I learned and also wondering if they are correct. Please see the list below and feel free to correct my mistakes or to add...
  8. Reza FMZ

    D2O Moderator/Coolant: Is Solidification Possible?

    Hello all, From those guys who worked with different materials especially reactor moderators and coolants, is it practicality possible to compress D2O between two metallic parts until it remains solid in room temperature? In otherwords wants to have D2O in solid at room temperature. Thanks in...
  9. S

    Flux in Moderator vs Flux in fuel

    I was reading about different flux shapes, and it depicted the the shape of the fission neutron spectrum to be in the shape of y=e^-x*sinh(sqrt(x)), where y is the spectrum and x is the energy in MeV. note: put that equation into wolfram for easy visualization Then it depicted the slowing down...
  10. I

    Neutron Flux Profile in a Spherical Moderator

    Hello People I need help with the following assignment: It states: Consider an ideal moderator with zero absorption cross section, Ʃa = 0, and a diffusion coefficient, D, which has a spherical shape with an extrapolated radius, R. If neutron sources emitting S neutrons/cm3sec are distributed...
  11. S

    Helium as Moderator: Neutron Absorption & Travel Distance

    Helium has the only stable nucleus which does not absorb neutrons. At practical reactor conditions, say 300 atmosphere pressure and +300 Celsius temperature, how far would a fission spectrum neutron at say 5 MeV average, travel until it is thermalized? Under the same conditions, how far...
  12. J

    Graphite Moderator Explained: Carbon's Atomic Structure & Neutron Moderation

    Can someone explain to me, in layman's terms, why carbon (especially graphite) has a high scattering cross section? What is it about carbon's atomic structure that makes it a good neutron moderator?
  13. A

    Which substance is the best moderator for fission reactors?

    Which substance is the best moderator for fission reactors, if there is even a definition for "best"? My current physics book says heavy water. Do you agree? Please explain. :smile:
  14. Ivan Seeking

    Goodbye & Farewell from a Moderator

    I wanted to make a last post as moderator to let you all know that after almost eight years I’ve decided to retire from the staff. Business is booming and I was argued out long ago. I could probably tolerate a few more debates, just barely :biggrin:, but the biggest issue is the time. Even a...
  15. U

    Moderator to fuel ratio and under-moderation in different lattices

    Hi all. I need help on an issue I can't figure out. What is the difference between 15x15 and 17x17 fuel when it comes to moderation properties? Standard fuel with these lattices have dimensions 15x15: pitch: 14.3 mm, d_rod: 10.77 mm 17x17: pitch: 12.6 mm, d_rod: 9.5 mm These geometries...
  16. Q

    MCNP - Measuring Neutron Absorption in a Moderator

    Hello all. I'm am a first time poster but a long time visitor. I am having a little trouble that I was hoping someone far wiser and more knowledgeable than myself might be able to help with. I've been using MCNP to investigate criticality in a simple geometry consisting of a central natural...
  17. Ivan Seeking

    Who Is the Temporary Moderator Replacing Victor Hugo?

    Please note that I need to take some time off. In the mean time, Berkeman, one of our Engineering Mentors, being the brave-hearted and daring man that he is, has kindly agreed to moderate the forum in my absence. Please direct all concerns and comments to him. Thanks Berkeman. :smile:
  18. A

    A question on Beryllium as a neutron moderator

    Homework Statement I have an assignment question that asks the me to look up the partial mass attenuation coefficients for Beryllium for Rayleigh Scattering, Compton Scattering, the Photoelectric Effect, and Pair Production in both the nuclear and electron fields on the NIST database for...
  19. M

    Moderator Vacancy Announcement: sci.physics.plasma

    Moderator Vacancy Announcement (MVA) sci.physics.plasma This is a formal Moderator Vacancy Announcement (MVA), begun because the previous moderator of the moderated newsgroup sci.physics.plasma has recently resigned. The goal of this process is to find a new moderator for the group.PROCEDURE...
  20. L

    New Flux in a Sphere of moderator of Radius R

    I have a problem where I must show that neutron flux in the sphere, for a sphere of moderator of radius R. This is one of the problems (5.14.a) in chapter 5 of "introduction to Nuclear Engineering" by J. R. Lamarsh. I would like to include the equation in this request but I'm unable to paste it...
  21. O

    Moderator moves what he dislikes

    Moderator has been moving the themes he dislikes even if you write specific scientific information. He's making fun or gives himself the right to say ancient knowledge means "nothing" and while astronomy or math books or even Michio Kaku's references mess around with religion and philosophy...
  22. russ_watters

    Meet the New Moderator: Who is Greg and What Are His Qualifications?

    Figured this forum was as good as any for making a formal introduction since I'm a mechanical engineer with some aerospace background and a computer science/engineering hobby. I'm happy Greg gave me the opportunity to moderate this forum. Its something I've never done before but always had...