Primary Load – Definition

The load on the system due to gravitational force, spring force, internal or external fluid pressure, etc are called primary loads. The load on pipe support due to restricted thermal expansion of pipe comes under primary load.

Characteristics

i). The stresses generated due to primary loads are non-self-limiting in nature. It means when the stress exceeds the yield point, the load will not dissipate. As long as the load is applied, the stress will present. The stress will not diminish with time or due to deformations. Hence they are called not self-limiting.

ii). The primary load is force driven type i.e. it is due to external force acting on the system.

iii). The failure due to primary load is relatively sudden, quick and catastrophic once the induced stress is greater than the failure stress.

iv) The loads should necessarily satisfy the simple laws of equilibrium.

Secondary Load

1. The loads that cause self-limiting stresses are called secondary loads. · Eg. Loads due to temperature changes, settlement of foundations etc.
2. The secondary loads are displacement driven.
3. Secondary loads are not catastrophic in nature.
4. Secondary stresses are strain-induced stresses.
5. Secondary loads are self-limiting because the load disappears in the form of local yielding and distortion.

A safe shutdown earthquake is a level of ground motion where the features of the nuclear power plant are required for the safe shutdown of the reactor. The features are

a) Those which assure the integrity of coolant pressure boundary.
b) Those required for the safe removal of decay heat from the reactor.
c) Capability to shut down the reactor.

d) Confining the radioactive exposure to mitigate the offsite exposure.

The PGA of this level of earthquake should not be less than 0.1g. The probability of occurrence of this earthquake once in 10,000 years. In general safety-related components should withstand SSE.