What does the scram device detect?

Answer 1

A reactor scram or trip is initiated automatically if certain reactor operating conditions depart from their normal range of values, which could mean that if not acted on, reactor fuel damage could result. The reason for the scram could be to protect against release of activity from the reactor, or it could be to protect the investment against damage. The actual parameters selected depend on the type of reactor, and obviously water reactors and gas-cooled reactors will have different operating envelopes, but I expect you are interested mainly in PWR and BWR designs.

Parameters chosen for scram purposes are neutron flux (ie reactor neutron power), reactor cooling sytem temperature, reactor cooling system pressure, reactor cooling system flow, steam pressure, and comparison of feedwater flow and steam flow.

Instruments measuring these parameters feed into safety circuits which when actuated cause the control rods to be inserted, and maybe other actions. Since shutting down a large reactor involves a large loss of income for the owners, and it can take a couple of days to get the plant back up to full power, unnecessary scrams are to be avoided, and the safety systems are designed so that no single instrument or component failure will cause a trip. This is done by having a voting system, with 3 or 4 identical instruments for each parameter, and only acting if 2 out of 3, or 2 out of 4, give the same indication.

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