Nuclear Reactions: Basics for Beginners

[brispuss]

I'm fairly new to nuclear engineering.

Basically two questions regarding nuclear fission.

1) Neutrons hit atomic nuclei of fuel in reactor which in turn causes atoms to decay until a stable atomic isotope is ultimately reached. However, surely the "stable" isotopes (and also those unstable isotopes which are now decaying) are themselves bombarded with more neutrons causing further decay and so on until there is nothing left but energy??

Or is it dependent on the (kinetic) energy of neutrons being large enough to overcome the binding energies of nuclei to cause fission? In other words, if the neutron energies are not sufficiently strong enough, then the isotopes (stable and unstable) will not decay (further) as the atomic nuclei binding energies are greater than the neutron (kinetic) energies. Is this so?

The cladding surrounding nuclear fuel (usually zirconium), and also the liquid metal coolant (for liquid metal reactors and either sodium or lead being used) presumably do not decay (due to neutron bombardment) because their respective atomic nuclei binding energies are greater than the energy of the neutrons? Is that so?

2) Presumably isotopes that do decay, decay into almost every possible combination of isotope down the line of decay so that there could be literally hundreds of reactions occurring at anyone moment? If so, how is the total energy of all these reactions calculated to determine the outputs of reactors?

 

[mfb]

1) Neutrons hit atomic nuclei of fuel in reactor which in turn causes atoms to decay until a stable atomic isotope is ultimately reached.

That is missing a step.
You are mixing decays and induced fission. They are completely different processes.

There are a few very heavy isotopes that can split into two (rarely three) lighter nuclei if they are hit by a neutron. The fuel in reactors is made out these isotopes. The lighter fission products are often radioactive and decay to other nuclei later. They cannot be split like the fuel..
They can absorb neutrons, and for some isotopes that makes them radioactive again. Neutrons absorbed by fission products are unwanted because these neutrons are then missing in the chain reaction.

Unrelated to that: In every reaction in a reactor, the total number of protons and neutrons together is conserved. You cannot end up with "only energy", no matter which reaction can happen, because you always have protons and neutrons around. You can only change how they are arranged.

Or is it dependent on the (kinetic) energy of neutrons being large enough to overcome the binding energies of nuclei to cause fission? In other words, if the neutron energies are not sufficiently strong enough, then the isotopes (stable and unstable) will not decay (further) as the atomic nuclei binding energies are greater than the neutron (kinetic) energies. Is this so?

No, and low-energetic neutrons are more efficient in splitting uranium.

The cladding surrounding nuclear fuel (usually zirconium), and also the liquid metal coolant (for liquid metal reactors and either sodium or lead being used) presumably do not decay (due to neutron bombardment) because their respective atomic nuclei binding energies are greater than the energy of the neutrons? Is that so?

They cannot fission. They can absorb neutrons, and the material degrades over time if it absorbs too many neutrons.

2) Presumably isotopes that do decay, decay into almost every possible combination of isotope down the line of decay so that there could be literally hundreds of reactions occurring at anyone moment? If so, how is the total energy of all these reactions calculated to determine the outputs of reactors?

Individual isotopes rarely have more than two relevant decay modes, and often just a single one (beta decay). All the decay energies and lifetimes have been measured. It is not hard to calculate the average power coming from all these decays (the main energy release comes from fission anyway).

 

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Noted, thank you.

However, there are further questions.

What criteria determines whether an atom is fissionable? I would have thought that any atom was capable of fission if the right conditions were present?

When an atom "absorbs" neutrons (instead of being split, per fission reaction), this means that the extra neutrons are added to the nucleus so making the atom mass increase by the number of neutrons "absorbed"? Yes? But in most (if not all?) cases, the extra neutrons will make the atom unstable and therefore the atom will decay and emit whatever particles and radiation in order to decay into a stable isotope.

 

[mfb]

What criteria determines whether an atom is fissionable? I would have thought that any atom was capable of fission if the right conditions were present?

Quantum mechanics. The nucleus has to be heavy to make the process possible in terms of binding energy, and some nuclides work better than others for reasons beyond the [B] level. For nuclear chain reactions you want something with an odd number of neutrons because that releases more neutrons when it fissions.

I would have thought that any atom was capable of fission if the right conditions were present?

You can split every nucleus (apart from individual protons where there is nothing to split) if you bombard it with high-energetic particles, but that is typically not called fission. For fission of heavy nuclei, the neutron doesn't have to be high-energetic.

When an atom "absorbs" neutrons (instead of being split, per fission reaction), this means that the extra neutrons are added to the nucleus so making the atom mass increase by the number of neutrons "absorbed"?

Right.

But in most (if not all?) cases, the extra neutrons will make the atom unstable and therefore the atom will decay and emit whatever particles and radiation in order to decay into a stable isotope.

There are many possible cases.
- If the nucleus had few neutrons before, it can go from unstable to stable - rare in nuclear reactors, e. g. Fe-55 to Fe-56.
- You can have a stable nucleus staying stable, e. g. Fe-56 to Fe-57 and Fe-58.
- You can have a stable nucleus becoming an unstable nucleus, e.g. Fe-58 to Fe-59. Fe-59 decays to Co-59 on a timescale of months.