The Harbinger

Dear Ask the Harbinger Experts,

How can I heat things up with my old lady in the bedroom, IYKWIM?

Love,

The Men of MucheDumbre

For this question, I didn’t feel we had anyone on staff that could properly respond. In fact, I fear it may have come for one or more of our staff members. So, I asked the Chairman of Hott himself, Mao. I think he has some pretty valid points.


Up to this point, it has been assumed that the reactor core consists of a homogenous mixture of fissile material, coolant and if the reactor is thermal, moderator, whereas in most reactors, the fuel is actually contained in fuel rods of one sort or another. Such nonhomogenous reactors are divided into two classes. If the neutron mean free path at all neutron energies is large compared with the thickness of the fuel rods, then is unlikely that a neutron will have more than one successive collision within any one fuel rod. In this case, the reactor core is homogenous as far as the neutrons are concerned and the reactor is called quasi-homogenous. However, if the neutron mean path at some energy is comparable or smaller than the fuel rod, then the neutrons may undergo several collisions within a fuel rod, the fuel moderator must be treated as separate regions and the reactor is said to be heterogeneous. Reactors with fuel rods in the form of thin plates are often of the quasi-homogenous type, especially if the fuel is highly enriched and therefore present in the rod at low concentration. Most modern (thermal) power reactors, however are fueled with only slightly enriched (a few weight percent) uranium, the fuel rods are not thin, and these reactors are heterogeneous.
From the standpoint of the neutrons, a quasi-homogenous reactor is in fact homogenous. Thus, the formulas developed for homogenous systems can be used with quasi-homogenous reactors. It is merely necessary in the computation to use the atom densities for the equivalent homogenous reactors.
How would the CANDU (CANada Deuterium Uranium) reactor be classified since it uses heavy water as coolant/moderator and natural uranium (0.75% - 1% uranium 235) as fuel as opposed to enriched uranium? Since it uses natural uranium and the absorption of deuterium (D = 2H) for thermal neutrons is very small- much smaller, for example, than the cross-section of ordinary hydrogen (H = 1H). However, since the deuterium in D2O is twice as heavy as hydrogen in H2O, D2O is not as effective as moderating neutrons as H2O. Neutrons on average lose less energy per collision in D2O than they do in H2O, and they require more collisions and travel greater distances before reaching thermal energies in H2O. Because of the greater number of successful neutron collisions, the deuterium moderator can be considered a separate region, even though the use of natural uranium the CANDU can be classified as a heterogeneous reactor from a neutron standpoint and not the quasi-homogenous reactor standpoint.
As a side note, the thermodynamic efficiency of the CANDU is about 30% and it can be refueled when the reactor is in operation.

md is
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