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Contents
 
Graphite in the Advanced Gas Reactor Fleet

1. Current Design
2. Brick Cracking
3. Reserve Strength Factor (RSF)
4. Corrosion
 
 
 

1. Current Design

 

AGR cores contain 332 fuelled columns, each containing eleven layers of radially keyed bricks. Square interstitials carry control rods. The core periphery is connected via a restraint system to a strong steel tank – this is particularly important in resisting seismic disturbance. The core therefore expands as steel, requiring the keys to slide in the keyways as aforementioned.

 

To ensure optimum dimensional stability, graphite temperatures are kept below 500°C throughout reactor life, despite the radiation-induced drop in thermal conductivity. Shrinkage does nevertheless occur, and because dose rates are not uniform across the core (they are highest nearest to the fuel) graphite components shrink differently and so distort. Individual bricks bow, while their keyway slots tend to close into dovetail shapes faster than the keys themselves shrink. The whole core becomes dished on its top surface. All these effects are allowed for in designing key clearances – too much slop would allow core distortion, too little would cause jamming with consequent risk of damage.

 

It must be ensured that throughout the life of the core the keying system will sustain the loads imposed on it due to brick distortion, gas pressure differences, vibration and differential thermal expansion. The stress pattern due to these external loads is concentrated around the keyway root, and this is where a crack would start.

 

2. Brick Cracking

 

The core assembly relies on each and every brick’s individual strength to maintain its structural integrity and satisfy the key safety functions of the graphite cores. Cracked bricks weaken the lattice allowing the core to become increasingly more susceptible to distortion under normal and operating conditions. There are two types of brick cracking – these are described below.

 

Bore cracking occurs in the early life of the graphite bricks when a crack propagates from the inside wall of the brick to the outside along the full length and thickness of the brick, with the outside wall still intact. This occurs as the inside of the brick bores shrinks faster than the outside, whereby tensile stresses at the bore and compressive stresses at the outside of the brick create potential for bore cracking. The most severely affected bricks have two bore cracks (multiple cracking), one on each side, signifying that the brick is effectively two pieces only joined by the thin sections of the outside wall of the brick.

 

Keyway root cracking is predicted to occur in the later life of the graphite bricks and is caused by stress reversal where material at the outer region is shrinking faster than the brick bore. This induces compressive stresses at the bore and tensile stresses at the outside the brick (inversely to bore cracking).  Stress concentration at the key way route provides a crack initiation site. Cracks that rapidly propagate are likely to cause a second key-way crack on the opposite side of the brick almost instantly – splitting the brick in two. 

 

Keyway root cracking is the most serious of brick cracking cases as it leads to degradation of the keying system.

 

Fig 46. Illustrative example of brick cracking
Fig 47. Comparison of internal stress patterns before and after stress reversal
 
 

 

 

 

 

 

3. Reserve Strength Factor (RSF)

 

To assess the effect of internal stress, the concept of a Reserve Strength Factor (RSF) is used: this is the ratio of residual strength (allowing for irradiation hardening and corrosion weakening) minus the internal stress, to the stress due to the external loads.

 

 

 

 

RSF equation

 

Typically at the end of AGR life:

  • Strength                 =        Original Strength       ×       2.0 (irradiation)

                                                                            ×       0.5 (corrosion)

  • Internal Stress         =        Original Strength       ×       0.2 (rising fast)
  • Load Stress             =        Original Strength       ×       0.08 (normal operation)

                                                                            ×       0.25 (seismic)

        Thus RSF                = 10 (normal operation)

                                    ~ 3 (seismic)

 

The core is designed so that, even in the worse case, the RSF exceeds a defined, adequately safe, minimum level which depends on the situation – a higher factor is obviously required for normal operation than for when allowance is made for seismic loading.

 

4. Corrosion

 

When the AGR was first conceived it was thought that accelerated corrosion could be counteracted by increasing the CO levels to 5-10%. However it is now know that such mixtures are thermodynamically unstable at low coolant temperatures, prompting concern that carbon might deposit on the boiler surfaces. There is also the worry that radiolytic polymerisation of CO might produce deposits on the fuel.

 

The picture was transformed by the accidental discovery of powerful inhibition by methane. The AGR cores were therefore designed to make best use of whatever methane could be allowed. The problem is that methane is destroyed in providing protection for the graphite. Deep within the moderator bricks its concentration is seriously depleted, and corrosion correspondingly increases.

 

The first response, used in some of the AGRs, was to drill vertical methane holes in the bricks as shown in Figure 48. This was done to allow coolant to pass more freely through the holes than the pores, and keep up the methane levels in the graphite. It has also been noted that these methane holes introduce stress concentrations in the brick allowing cracking to propagate more readily.

 

Fig 48. Brick design showing methane holes