Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Of Lost Neutrons Within Reactors

  1. Aug 27, 2015 #1
    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 absorb slower neutrons if these particles have left the fissile area? Whether the moderator is water, heavy water or liquid sodium can lost neutrons penetrate the moderated area or building material? If so, isn't this hazardous?

    Also, what degree of electromagnetic radiation does the moderator stop? If not all, is the remaining degree consistently released capable of harming workers or penetrating the building material and harming outsiders?
  2. jcsd
  3. Aug 28, 2015 #2


    User Avatar
    Education Advisor
    Gold Member

    Hello Phaeous. Welcome to the forum.

    I assume the title is supposed to be included in your text.

    The arrangement of fissile materials, moderator, and structural materials in a reactor is chosen most carefully. You need the reactor to be mechanically strong enough to safely support the various forces such as the weight of the various reactor components, the pressure and motion of the coolant, motion of reactivity devices, etc.

    But primarily, the material is arranged such that neutrons are slowed to thermal speeds and then arrive at fissile material with the fewest lost that can be achieved.

    If a neutron starts at typical energy from a fission, and is travelling through water, it will scatter some few times, probably about 12 times, before reaching thermal energy. During this travel it will move some distance, maybe as much as a meter or two. Probably quite a bit less. I would have to look up the average. And it will depend on the details of the reactor.

    But that gives you some idea of what is most efficient. The usual reactor design is fissile fuel arranged in rods in a background of moderator. Other designs are pellets of fuel, plates of fuel, etc. But the usual pattern is an array of small-ish chunks of fuel in a background of moderator.


    So the neutrons bounce around in the moderator. They slow down. And enough of them find the fissile material in the fuel rods to keep the reaction going.

    Neutrons can find other things besides fuel and get absorbed. For example, iron is a neutron sucker. Thermal neutrons that find iron parts of a reactor tend very strongly to get absorbed. This is a problem because the absorption releases a gamma (a photon) that heats the surrounding material. So reactor design tries to keep the iron parts to as little as possible. Some other things that happen when a neutron hits structure components include such things as making it brittle. Or causing the molecular bonds to become dislocated so the structure stretches under force. Pressurized components tend to balloon under neutron radiation.

    As well, as you mentioned, neutrons are radiation. And they induce radioactivity in materials that absorb them. And reactors produce other kinds of radiation besides neutrons. All of these are a challenge. They must be accounted for and designed for. This is a challenge, but it is well understood how to do it.


    The picture shows the NRU reactor in Chalk River. It typically operates with 130MW of power. I have stood on the upper deck plate in the picture while it is operating. Quite an experience. I was there to help with data analysis of an experiment, and the technician invited me to observe while he made a measurement. And I was standing there watching. And suddenly I realized, my feet were unpleasantly warm. And the tech turned to me and said "You're feet are hot, aren't they?" And indeed they were since the upper deck plate is 70C during operation. But the radiation at the deck is very little, such that they used to take school children on tours. And they stopped because of 911 paranoia, not the very small radiation.
  4. Aug 28, 2015 #3


    User Avatar
    2017 Award

    Staff: Mentor

    The neutrons get scattered in random directions, the chance that they get back to the reactor fuel at some point is quite large. Most designs try to mix fuel and moderator somehow to improve the efficiency.
    Containing the neutrons is certainly important for reactors, and the vessels get radioactive over time.
    No because the reactors are shielded.

    Edit: Didn't see DEvens post.
  5. Aug 28, 2015 #4
    Thank you for the replies; they were quite valuable.

    Farewell :-)
  6. Aug 29, 2015 #5


    User Avatar
    Staff Emeritus
    Science Advisor

    Reactor cores tend to be heterogeneous systems in which the fissile material (fuel) is physically separated from the moderator. Neutrons are produced in each fission event, and at least one of two or three fission neutrons must survive to cause another (subsequent) fission in order to maintain a steady-state (critical) process.

    Fission neutrons have a spectrum of energies in the MeV range. In thermal reactors, the moderator (usually water) slows the neutrons to eV range. A substantial portion of the population of neutrons are retained in the core, where they are absorbed by the fuel, burnable absorbers, structural materials or moderator. A small amount of neutrons will 'leak out' of the core. These are absorbed by the core baffle/barrel, which functionally separates the incoming coolant from the coolant in the core and the coolant leaving the core. The baffle/barrel provides some shielding. The core and baffle are surrounded by a pressure vessel which provides a structural boundary for the coolant. The volume between the core barrel and pressure vessel form the so-called downcomer (for vertically oriented reactors/cores). Both the coolant in the downcomer and reactor pressure vessel (RPV) provide shielding from neutrons and gamma rays.

    The reactor and its pressure vessel are enclosed in a reinforced concrete 'containment' structure which also provides shielding from gamma radiation. Personnel do not enter containment while the reactor is operator. Besides the radiation, the temperature is quite high. The primary system of a PWR operates between about 285 C (core inlet) and 330 C (core outlet), depending on the reactor design, while a BWR operates between 220 C (feedwater temperature) and 286 C (core outlet).


    Reactor systems and core configurations are designed to minimize the leakage of neutrons from the core, primarily to reduce the activation and neutron damage to the surrounding structural components. This also reduces the radiation leaking out of the core and pressure vessel.
    Last edited: Aug 29, 2015
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook