Marc Grey
In two previous articles nuclear power adoption challenges, new designs and failure modes were discussed. The actual mechanics of nuclear fission and related ancillary concepts were not considered. In this article we will describe basic atomic and nuclear concepts, building up to a more detailed description of how fission works.
Starting at the beginning, matter is made up of atoms. Atoms are made up of a positively charged nucleus surrounded by a cloud of negatively charged electrons (a bit like a mini Solar System). The nucleus is composed of positively charged protons and uncharged neutrons. For the purposes of this discussion, the electrons are irrelevant; we are concerned with nuclear chemistry!
The number of protons determines what element the atom is: e.g. 1 proton means it is hydrogen, 2 means it is helium and 92 means it is uranium. This number is called the atomic number.
As the number of protons increases neutrons are needed to stabilise the nucleus (protons are positively charged, so repel each other electrically, so another force is needed to hold them together – the nuclear force, which the neutrons help with). It is possible to have atoms for the same element with different numbers of neutrons in the nucleus. These are called isotopes: e.g. uranium has a large number of isotopes, the two best known having 146 and 143 neutrons. The sum of proton and neutron numbers is called the mass number, so in the uranium example, those two isotopes are uranium 238 and 235. This is the usual way isotopes are named. In fact, uranium has a very large number of isotopes: 232, 233, 234, 235, 236, 237, 238, 239. However, the two most common are 238 (99.274%) and 235 (0.720%).
The nucleus of elements with larger atomic numbers tend to be unstable, and will spontaneously emit various sub-atomic particles. This process is called radioactive decay (more lazily referred to as the element being radioactive). There are a number of types of decay but the most common are:
- Alpha, the emission of two protons and 2 neutrons (a helium nucleus)
- Beta, the emission of an electron or a positron (positively charged electron)
In alpha decay, the original element loses 2 protons, so becomes a different one. This is referred to as transmutation, e.g. uranium 238 (atomic number 92) becomes thorium 234 (atomic number 90) via alpha decay. Uranium 235 also does alpha decay: it becomes thorium 231. This is often depicted in equation-like form (uranium 235 case):
U235 → Th231 + He4
Beta decay is a bit more subtle, a neutron emits an electron and an anti-neutrino and becomes a proton. This also results in a new element, e.g. thorium 231 becomes proactinium 231 (atomic number 91) via beta decay. Displaying this in equation like form:
Th231 → Pa231 + e– + aν
Both types of decay impart kinetic energy (a few MeV) to the newly transmuted element. This can be harnessed via the thermoelectric effect to make a compact electrical low power source.
The nuclei of elements with larger atomic numbers may be capable of fission, splitting into (usually) two smaller nuclei. This is triggered by neutron impact, but can occur spontaneously in rare cases. The actual process has two steps:
- Neutron absorbed by the nucleus (this temporarily creates a new isotope)
- Nucleus splits into two fragment nuclei (which fly off with significant energy), plus one or more neutrons and one or more photons (gamma rays).
Consider the case of uranium. The 235 isotope is fissionable. The reaction looks like :
n + U235 → U236 → Kr98 + Ba141 + 3 n + γ
The neutron(s) needed to start the fission can either come from the fuel itself (i.e. spontaneous fission), or from cosmic ray interactions, or from a specialised element that has a high spontaneous fission rate (e.g. californium 252). To precisely control reaction startup the last method is the most commonly used.
The amount of energy released is 202 MeV, of which the majority is kinetic energy imparted to the two fission product nuclei: barium (atomic number 56) and krypton (atomic number 36) at 169 MeV. This means that the fission products are hot! Compare this to 4.6 MeV for radioactive alpha decay (of uranium 235); fission is much more energetic. By way of contrast, combustion of TNT releases a few eV per event, so fission releases hundreds of millions of times more energy!
The next consideration is can fission be continued? If one fission event produces >1 neutron, if they go on to produce more fission events then you have an exponentially increasing number of them. This is called a chain reaction. Not all heavy elements will behave this way: for instance, uranium 238 can undergo fission, but tends to scatter the resulting neutrons to inhibit a chain reaction. Uranium 235 on the other hand does not, and a sustained chain reaction is possible.
Examining uranium 235 chain reaction longevity raises the following questions:
- Is there sufficient purity (i.e. compared to uranium 238) for neutrons to hit enough uranium 235 nuclei (this is about enrichment)
- Is there enough uranium 235 so that the 3 neutrons keep hitting fresh uranium 235 nuclei (this is called critical mass)
- Is the shape of the piece of uranium conducive to the neutrons hitting nuclei (called critical shape)
Naturally mined uranium is 99.284 uranium 238, and uranium 235 is only 0.72%. So a method to extract more uranium 235 (or conversely remove more uranium 238) will result in higher concentrations of uranium 235. This is called enrichment.
For a near 100% uranium 235 sphere, it needs to be 52 Kg (17 cm diameter) to achieve critical mass. This leads to the idea of (ideal) critical shape (a sphere) – so it is possible to make an object of critical mass, but of a different shape (e.g. a rod) and transport this safely but arrange for it to form critical shape by means of implosive compression when desired (this idea is used in some nuclear weapons).
Neutrons are vital to keeping a chain reaction going. To complicate matters a little there are two types of neutron that can be involved: prompt and delayed. A prompt neutron is one of those emitted directly by the fission reaction. A delayed neutron is one emitted by one of the fission product nuclei a few milliseconds (or more) later – usually as it undergoes a beta decay.
The distinction is important in different applications! For instance, for a nuclear weapon, the reaction needs to be dominated by prompt neutrons so that the chain reaction fissions as much of the fuel material as quickly as possible (otherwise it just destroys itself without releasing much energy). Conversely, for a nuclear reactor, having the reaction dominated by decayed neutrons allows the fuel to be fissioned at a controlled rate and also varied by neutron absorbing control rods to provide whatever output power is desired.
Finally the speed/energy of neutrons affects the probability of absorption by the receiving nucleus. Typically, neutrons emitted as prompt or delayed are fast (i.e. high energy) and are less likely to be absorbed by a nucleus (this is often referred to as nuclei having a small cross section for fast neutrons). However, neutrons can be slowed by using a moderating medium (something that slows neutrons to close to ambient thermal energy) and this can increase the probability of neutron absorption (i.e. make a larger cross-section).
This is mainly relevant to nuclear reactors and means that using a moderator (e.g. heavy water, water, graphite) enables using less fuel and less-enriched fuel for a viable chain reaction. But this principle does have impact on nuclear weapons too. For instance, the weapons to be used against Japan in 1945 had to be transported with fuel elements disassembled; otherwise if the transporting aircraft crashed into water its moderating effect would result in a prompt critical chain reaction!
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