artis said:
Well a neutron goes in,capture happens the nucleus splits into 2 alphas +2n which are free to fly off is what I understand but it seems that is not the only reaction that can happen with Be9+n so one would need to look at reaction rates for each of them.
Yes, one would have to look at the other reactions and determine how each would contribute to the neutron population, or not. Consider that
235U releases on average slightly more than 2 neutrons per neutron absorbed, and more like 2.2 to 2.3 neutrons (on average, so usually either 2 or 3), and that would be more preferable than 2 neutrons per neutron absorbed.
artis said:
I guess from what you confirm is that Be9 doesn't really make that much of a difference in a LWR core in terms of producing more neutrons, but I suppose it does come in handy for a fast neutron prompt uncontrolled chain reaction
Be has been used in neutron sources in commercial nuclear reactors. Early primary sources would contain an alpha emitter mixed with Be, whereby an (α,n) produces a neutron, while secondary sources would use
123Sb mixed with Be, in which
123Sb + n =>
124Sb =>
124Te + β
- + γ (1.64 MeV), which induces a (γ,n) reaction in Be. Primary sources are now
252Cf.
https://www.frontier-cf252.com/antimony-beryllium/
https://www.osti.gov/servlets/purl/4275578
I will not comment on nuclear weapons technology.
artis said:
Maybe not directly on topic but I've always wondered why UO2 fuel is ceramic? It's main ingredient is still Uranium which is a metal, or do they classify metals as ceramics whenever there is a certain metal to oxygen ratio in the oxide, because in nature many metals oxidize naturally in contact with oxygen like the surface of Aluminum and yet we don't say the surface of Aluminum has become ceramic?
Ceramic usually refers to a compound of a metal, or metals, and non-metal, usually C, N, or O. For example, U ceramics are uranium carbide (UC), uranium nitride (UN) and uranium dioxide (UO
2). One can substitute Pu or Th for U in these compounds. We also find other carbides, nitrides and oxides. In the case of U, the ideal ceramic is UO
2, but there are forms such as U
4O
9, U
3O
7, U
3O
8, and UO
3, which are undesirable for fuel since the U density is lower and the thermal conductivity is lower. UN and UC have lower melting points than UO
2, but they have much greater thermal conductivity. There is also the concern about chemical compatibility with water in the event the cladding is breached which has occurred with some regularity in commercial LWRs, even with attempts to reduce failure rates to zero. We also have U(C,N), a carbonitride, and UCO, a carboxide. Except for special test/experimental fuel rods, commercial nuclear fuel contains UO
2.
There are other forms for U, since U
3Si or U
2Si
3, which are kind of an intermetallic, and U-10%Mo and U-Zr alloy or intermetallic forms. On has to consider fission product dispositions with each system, in addition to coolant compatibility.
With respect to oxides of aluminum, we would refer to Al
2O
3, or alumina. And one may find alumina pellets in some fuel designs, or HfO
2 (hafnia), or ZrO
2 (zirconia). With respect to an oxide layer on a metal, we might say the surface is passivated, depending on the environment. In the case of metal like Al, Ti, Zr, Hf, Cr (in stainless steels), very thin submicron layers (or nanometers) of oxide form that protect the underlying metal for further oxidation/corrosion, unless the layer is disturbed/compromised.
In some fuel, one may find neutron-absorbing burnable poisons, or burnable absorbers, e.g., Gd
2O
3 (gadolinia) or Er
2O
3 (erbia) blended into the UO
2, or ZrB
2 coated on the outer (circumferential) surface of the UO
2 fuel pellets.
artis said:
This does happen in any LWR fuel irrespective of core design right?
I could say in any U238/235 fuel mass capable of a chain reaction?
Yes, in a thermal reactor, or epithermal reactor,
238U will absorb fast and epithermal neutrons, to become
239U, which undergoes beta decay to
239Np, which undergoes beta decay to
239Pu. However, some of the 239 isotopes of each element can also absorb neutrons to become 240 and 241, which also undergo beta or alpha decay, or in some cases fission. As burnup increases, more and more
239Pu,
240Pu,
241Pu,
242Pu, are produced as well as isotopes of Am and Cm, with quite a lot on the outer surface of the fuel pellets.
With respect to
235U, some of the neutron captures do not result in fission, but instead a gamma ray is emitted which leads to a more stable
236U, which can absorb a neutron to become
237U. Those nuclides undergo beta decay forming
236Np and
237Np, respectively, and some of those nuclides can absorb a neutron, emit a beta particle, and form Pu isotopes, or emit an alpha become a lower mass nuclide.
Edit/update: There are also compounds UCl
3 and UF
3 usually mixed with other chlorides or fluorides, which are considered salts.