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Contents
 
Safety Requirements of an Advanced Gas Reactor
 
1. Reactor Core Design Features and Safety Provisions
2. Fuel
3. Pressure Vessel
4. Diagrid, Gas Baffle and Restraint Tank
 
 
1. Reactor Core Design Features and Safety Provisions
The design of the reactor core complies with the general geometrical and physical requirements of the core as a moderator and a structure. The graphite brick and key geometry is optimized to give the best possible component and interconnecting strength, which is derived from tests conducted on slices of graphite bricks. Tests are also conducted to determine the ultimate capacity of restrain system components.

The sources of stresses on the reactor core have been identified as the following:

  • Pressure induced loads in graphite columns, keys and the restraint system;
  • Instability loading generated by brick bowing resulting from irradiation induced distortion;
  • Vibration loading, including seismic loading;
  • Self weights and dropped channel components;
  • Any other interacting loads from in-channel components;
  • Transient loads arising from differential response to power changes or faults;
  • Irradiation induced stresses in the graphite;
  • Thermal stresses in restraint and support structures.

There are significant safety provisions incorporated into the design of the reactor core to address the above stresses.

Redundancy within the core structure is provided by horizontally by the inter-brick keying system and vertically by the inter-brick shear connections. The result is that any incidence of high loading leading component failure does not result in gross channel distortion or failure of surrounding components. Inter-brick seals are provided to serve as features to provide alignment and concentricity between brick ends. The presence of seals reduces the increase in coolant flow into an empty channel that normally occurs and this guards against flow starvation into neighboring channels. 

Keyway clearances are kept to a minimum to prevent brick-to-brick jamming and to reduce cumulative core moments. This is provided in accordance with the maximum allowable clearance required for channel straightness and the minimum clearance dictated by graphite distortion. From the viewpoint of distortion, the keying length in the nine middle layers extends over approximately one third of each layer, while in the top and bottom layers, this length can be increased: it is in these two layers that the maximum loads are transmitted.

Within the interconnecting restraint beams connections between adjacent restraint beams provide a high degree of redundancy in the restraint system. The connections that are capable of transferring shear loading between beams and still allowing circumferential movement and a limited amount of rotation. In the event of a Warwick link failure, the loading applied to a beam can be transferred to its immediate neighbour without seriously affecting core behaviour.

 

2. Fuel
The fuel constitutes the principal source of radioactivity in the reactor and as such presents the main potential hazard of radioactivity. To ensure that the release of activity from the fuel pins is strictly limited, it is essential that all the fuel components are of high integrity under all operating and fault conditions. As such that are a number of safety related objectives pursued at the design stage of the reactor together with measures to secure them.

Measurements of the channel gas outlet temperature, fission product level in the coolant and coolant composition are used to assess and, if necessary, to control stringer performance and integrity.

Failure Modes
Design Safeguards
Pin failure during reactor operation.

Post Irradiation Examination (PIE) of components.

Continuing design development of fuel components.

Provision of adequate cooling under normal and fault conditions.

Control of reactor operation within limits predicted to be acceptable to the fuel.

Provision of an environment compatible with coolant corrosion, deposition, flow-induced vibration and acoustic fields.

Design of a pin structure to withstand pressure differentials and temperature transients associated with credible reactor faults.
 
Mechanical damage; in particular to prevent dropping during refuelling and other handling operations.

Design development of lifting, supporting and handling components and the development of handling techniques.

Provision of a compatible in-reactor fuel route geometry and gas flow conditions.

Provision of environmental conditions for the fuel stringer tie bar compatible with coolant temperature and corrosion, flow-induced vibration and acoustic fields.
Radioactive contamination of the gas circuit due to spalled oxide.

Incorporation of central hoists inertial collectors to collect active dust.

Development of fuel pin coatings to reduce oxide spalling.
 

 

Table 1: Failure modes and design safeguards of nuclear fuel

 

 

3. Pressure Vessel

The safety design requirements for the pressure vessel are incredibly high to provide very high reliability against failure. Potential failure modes mostly relate to overloading or deterioration of the tensioning system or severe local stressing due to thermal effects. Potential failure modes are listed in the table below, together with brief comments on the design and operational features provided to afford protection against them.

 

 

 

Failure Modes
Design Safeguards
Over-pressurization.
Large margin between plant operating conditions and ultimate failure conditions. High reliability of safety relief valve systems to limit pressure under all plant operating conditions and fault conditions.
Reduction in tendon load due to concrete creep or tendon relaxation.
Periodic measurement and adjustment, if necessary, of tendon prestress. Tendon operation within the elastic range. Periodic monitoring of strain gauges and top slab deflections and thermocouples.
Tendon corrosion or material property changes.
Programme of inspection.
Overheating of the pressure vessel causing thermal expansion and relaxation of tendons.
Twin pressure vessel cooling circuits and high thermal capacity of concrete with temperature monitoring of concrete and cooling water to make undetected heating of concrete incredible. Acceptable temperature transients following loss of cooling circuits and reactor trips.
Unacceptable temperature stresses in top or bottom slabs due to faulty operation or failure of cooling circuits.
Individual control on cooling water flow to standpipes. Acceptable temperature transients following loss of cooling circuits and reactor trip. High reliability fuelling machine emergency cooling systems.
Disablement of tendons from heating due to linear leakage.
Large redundancy of tendons and linear leakage monitoring.

 

 

Table 2: Failure modes and design safeguards of the pressure vessel

 

 

4. Diagrid, Gas Baffle and Restraint Tank

The structures mentioned above are key components to the reactor coolant circuit, the purpose being to determine the flow path of the coolant through the moderator and fuel channels. Serious breach of the structure could cause a reduction in coolant flow over the fuel, which in turn, could lead to fuel failures.

Component
Failure Modes and Design Safeguards
Baffle

Credible failures of weld or parent material must not exceed sub-critical defects.

Credible failures must not cause disruption of the boilers (which the baffle supports).

Credible failures must not cause movement of the circulator casings that are connected to the baffle skirt.

Credible failures must not cause sufficient movement of the dome to prevent control rod entry.

The baffle material properties are such that any initial defect escaping inspection during manufacture will not grow to critical size during reactor life or before it is detected (unless shown to be inconsequential).

Failure of a man-access penetration.
Diagrid

The structural redundancy of the diagrid ensures that significant disruption of the reactor core is not possible as a result of a credible growth of defects in structural welds or parent material.

The design is such as to make instability failures, for example, weld buckling, impossible.

The growth rate of possible defects will be low and observable by in-service inspection by remote TV cameras.
Restraint Tank

Low stress levels and in-service inspection facilities ensure that defects in weld or parent metal will not grow undetected during reactor life to critical lengths.

The integrity of the restraint cylinder connections to the core restraint system and the gas baffle will ensure that significant disturbance of the fuel and control rod channel alignment will not occur.
 

 

 

Table 3: Failure modes and design safeguards of the diagrid, gas baffle and restraint tank.

 
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