Fundamentally, safe operation of any nuclear plant requires that the design has:

• trip systems, which respond to conditions outside the safe operating envelope and which require shut down of the reactor

• shut-down systems

• cooling to remove residual heat from the reactors after shutdown

The AGR safety assessment considers a whole range of faults on the plant. Included in these are:

• loss of reactor coolant through a breach in the pressure circuit

• water ingress as a consequence of a boiler tube leak

• faults involving damage to the fuel stringer

• internal hazards such as fire

• external hazards such as seismic events

Each reactor fault is taken to its end point, which is the termination of the fault and to which a radiological consequence, which is the radiation exposure received by the most exposed member of the public, is allocated. This consequence is determined generally by calculation based on an assessment of:

• the damage to the fuel. If the fuel pin clad is breached in any way, there will be a release of radioactivity into the coolant

• the transient undergone by the fuel. Increased fuel temperatures will lead to additional releases from the fuel. Fuel oxidation, due to exposure to reactor coolant for example, will enhance releases from the fuel and could produce fine U3O8 particulate material

• retention within any containment. This is a significant mechanism for reducing releases to the environment

• releases to the environment in any leakage or other loss of coolant

• potential radiation doses to the public based on external exposure from the radioactive plume and any radioactivity deposited on the ground, inhalation of radioactivity and ingestion of contaminated foods

The radiological consequences of fuel route faults are addressed in the same way but the event trees are not as complicated. Waste treatment plant faults are generally of lower significance than reactor and fuel route faults because of the smaller source terms.

After all end point exposures are assessed, including faults on the fuel route and the waste treatment plant, the risk from the plant is determined. The risk is determined from each end point exposure and the probability, as determined from plant reliability assessments, of that sequence. The acceptability of the plant is determined by assessment of this risk against criteria developed within the operator’s own organisation. The NII have their own assessment criteria (the Safety Assessment Principles or SAPs).

Operator risks are also determined from the radiation doses received during both normal operation and as a consequence of faults.