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Contents
 
Graphite Behaviour under Irradiation

1. Microscopic Behaviour and Wigner Energy
2. Macroscopic Behaviour and Stress Reversal
3. Radiolytic Corrosion
4. Irradiation Hardening
 

1. Microscopic Behaviour and Wigner Energy

 

β and γ radiation causes electron excitation i.e. the energy level of the electrons is increased and as a result heat is produced. In graphite, the energy is generally rapidly dissipated with no permanent damage to the molecular structure.

 

However the fast neutrons produced during fission impact directly with the carbon atoms in the graphite lattice. The energy required to displace a carbon atom from the lattice is generally much lower (25-60 eV) than the thermal energy of the neutrons (14 MeV) so many such displacements occur, note also due to secondary collisions between recoiling carbon atoms.  In typical AGRs each atom is displaced an average of 20 times during reactor life.

 

The displaced carbon atoms create vacancies in the graphite lattice. Some of these re-combine, others are trapped within the lattice, and some aggregate to produce more complex defects. The deformations in the lattice are energetic (stored as strain), and cause the basal planes to contract.

 

Two temperature regimes, relevant to reactor operation, can be distinguished.  At below 300°C major strains are induced in the graphite lattice due to displaced atoms gathering at the interstitial planes. This causes the lattice to collapse and store large amounts of strain energy. Overall the graphite expands longitudinally and shrinks in the transverse direction as described above. The strain energy stored in the lattice is known as the Wigner Energy. The Wigner energy decreases sharply around 300°C where fewer higher strain defects are formed.

 

When graphite irradiated at low temperatures is subsequently heated up, the high-strain defects are annealed and their Wigner energy is released as heat. The amount of energy can be equivalent to a temperature rise of approximately 300°C. It was the over-rapid release of this energy during an operation designed to anneal the Wigner strain that caused the Windscale fire in 1957.

 

At above 300°C (more relevant to AGRs) the interstitials are sufficient mobile to aggregate not in small clusters but to large areas of new graphite sheet (see Figure 42), interweaved between the original layers. In effect, the irradiation merely redistributes mass within the lattice, contracting the basal planes and expanding in the perpendicular direction – however the dimensional change is much small than that of the lower temperature regime.

 

Fig 42. Effect of Irradiation Damage on a Graphite Lattice

 

2. Macroscopic Behaviour and Stress Reversal

 

Having looked at the microscopic (i.e. crystalline) behaviour under irradiation this section explains the macroscopic behaviour of graphite as a block (i.e. polycrystalline), and more specifically the phenomenon of stress reversal.

 

Under irradiation there is a major difference between crystalline and polycrystalline graphite: the polycrystalline graphite first shrinks in both directions, even though the individual crystallines are deforming without significant volume change, shrinking in the basal planes and expanding across it as described in “Microscopic Behaviour and Wigner Energy” above.

 

The apparent contradiction arises because the crystallines’ expansion mode takes place into empty pores. When this can no longer take place, at stress reversal, the overall growth takes over as the distorting crystallines begin to jack the structure apart. This illustrated in Figure 43. The importance of stress reversal in graphite core ageing is discussed in later.

 

3. Radiolytic Corrosion

 

Carbon dioxide was selected as a coolant due to its overall stability under irradiation. In fact, at the molecular level it breaks down under irradiation unto carbon monoxide plus various other oxidising ions and free radials (‘Ox’). In free gas conditions these rapidly recombine and there is no overall net change.

 

However due to carbon’s high affinity for oxygen, if these reactive oxidising species impinge on a graphite surface they gasify it to carbon monoxide. This process is known as radiolytic corrosion.

 

Under irradiation carbon dioxide breaks down into oxidising ions and free radicals (1)

CO2     à       CO       +      Ox                                                                          (1)

 

In free gas conditions the free radicals recombine with the oxidising ions to reform carbon dioxide (2), however in the presence of graphite the free radicals impinge on the graphite surface and gasify it to form carbon monoxide (3) – this is known as radiolytic corrosion. 

Ox      +        CO        à     CO2                                                                          (2)

Ox      +        graphite  à    CO                                                                          (3)

 

Methane can be used as sacrificial protection to reduce wear to the graphite (4)

Ox      +        CH4     à       products (e.g. hydrocarbon radicals)                             (4)

 

Radiolytic corrosion occurs very quickly regardless of reactor temperature, and occurs as fast on the external surface of the graphite as on the internal pores. It can be slowed down by adding inhibitors to the carbon dioxide to encourage the free radicals and carbon monoxide to recombine rather than attack the graphite. This can be achieved by either adding additional carbon monoxide or methane.

 

Methane mops up the oxidising species in the gas phase similar to carbon monoxide however has some additional reactions that are beneficial to the core. Methane adsorbs on the graphite surface and reacts preferentially with incoming oxidising species, thus providing powerful sacrificial protection for the underlying graphite.

 

The corrosion caused by radiolytic oxidation has two major consequences. Firstly it reduces both the strength and Young’s Modulus (see Figure 43 ) of the graphite, and secondly because it attacks the pore walls it delays or prevents the pore closure involved in stress reversal (addressed in “Macroscopic Behaviour and Stress Reversal”). As well as prolonging shrinkage, this may also postpone the eventual drop in strength due to the jacking apart of the graphite structure.

 

Fig 43. Reduction in graphite strength due to corrosion
Fig 44. Dimensional Change in PGA and Gilsonite graphite
 

 

 

 

 

4. Irradiation Hardening

 

Other engineering properties are also radiation sensitive – the most important being strength and Young’s Modulus. The re-structuring of the lattice caused by radiation as previously described introduces pin dislocations within the graphite matrix. These block deformations and thus increase strength and Young’s Modulus. These fall towards and then below unirradiated values only beyond stress reversal, where the structure begins to degrade.