Undermoderated reactor

The ENDF/B-IV data set has been evaluated for use in highly undermoderated reactor cores. The critical experiments analyzed are those Originially used in the design of the Power Burst Facility (PBF) reactor core. 

An ioportant feature of the undermoderated reactor, quite apart from its better over-all neutron economy, is the fact that the lattice is fall-safe igpon loss of coolant. Water, which acts as both a moderator and a neutron poison. 

An important point for the safety of reactors is the influence of the core temperature on fc, i.e., on the reactivity. Water-moderated reactors can be built to have a negative temperature coefficient an increase in temperature/power will lead to steam bubbles near the fuel elements and to a decrease in moderation. If the reactor has been designed a little undermoderated, a drop in moderation will bring about a drop in reactivity (fc). Such a reactor will be naturally stable against undesired changes in power. One may recall the discussion of the Oklo reactor at the beginning of the chapter. 

For subsequent MSR work at ORNL and elsewhere, more conservative assumptions have been made on the neutron fluence for any structural Hastelloy elements. The level of neutron flux reaching the vessel wall can be limited by many means especially as thermal neutrons dominate He production and thermal neutrons can be more easily shielded, by boron carbide for example. In general as well for some MSR designs, combinations of graphite reflectors and/or undermoderated outer zones can be employed, greatly limiting neutron flux impingement on the reactor vessel wall. 

Reflectors and/or undermoderated zones to greatly lower neutron flux reaching vessel walls Much longer prompt neutron lifetime aids reactor control... 

As a consequence of the undermoderated lattice of thie N-Beactor, the fast to-themal flux ratio Is quite large relative to that of existing, overmodera. ted Bisnford reactors As an lUustratlon of this difference, the ratio of fast flux to thermal flux Is estimated to be 2.50 at N (compared to 1 08 at a K reactor). 

Conversely, at 550 F, a

negative reactivity addition. Reactor cores are designed to operate slightly undermoderated. An increase in power results in negative reactivity which tends to make the reactor seif regulating. 

Undermoderation means that too little or less than optimum water exists between adjacent fuel rods or plates in a reactor core. Overmoderation means too much water exists between adjacent fuel rods or plates. The optimum width of the moderator gap would be such that fission neutrons produced in one fuel rod would be thermalized in the water gap and produce a peak thermal flux in an adjacent rod. This explanation is idealized because coupling exists between a source fuel rod which is producing neutrons and more than one coupled rod in the vicinity, but the principle behind it is correct. 

Because the fuel is undermoderated, the thermal flux peaks beyond the coupled fuel rod. The reaction rate which sustains reactor power is R = temperature increases to Tg, less... 

Undermoderation is the normal condition for most reactors. Moderator to fuel ratio is less than optimum primarily because of resonance absorption. An increase in moderator temperature or voids inserts negative reactivity. 

Because the reactor core will be highly undermoderated (to achieve a high conversion ratio), the possible ingress of any moderation into the core must be considered as a part of the prototypic design process. 

The moderator at an accelerator-based neutron source is used to slow the neutrons down so that they have suitable wavelengths for neutron diffraction, in the same way as for a reactor neutron sovuce. However, in order that the moderation process does not broaden the pulsed time structure of the neutron flux too much, the moderator must be relatively small. (Also note that, unlike a reactor, the process of moderation plays no role in the production of neutrons at an accelerator-based sovuce.) This has the consequence that the neutrons produced by an accelerator-based source are undermoderated and there are many more epithermal neutrons than for a reactor source. Figure 1 also shows the neutron flux for a moderator at the world s most intense pulsed neutron source, the ISIS spallation neutron source at the Rutherford Appleton Laboratory, UK (see Figure 3). 

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