Each type of scram has a combined delay which depends on the delays in obtaining signals from the system and delays in acting upon these signals.

(a) System physical transport delays are usually small except for the transmission of temperature in the reactor, plenums, and heat exchangers. The transport of fission products from failed fuel to fission product and delayed neutron detectors elsewhere in the system also incurs a delay.

(b) Instrument delays vary with the instruments in question.

(1) Reactivity instrumentation, flux, and period meters incur a delay due to noise filtering; the latter incur the more serious delays as they need more filtering.

(2) Thermocouples have varying delays depending on whether the couple is sheathed or not. A |-in. ungrounded thin sheath thermocouple in a LMFBR might have a 1000 msec delay with a further 10 msec for its amplifier.

(3) Flux/flow ratio measurement delays depend on the fact that separate measurements are divided. Electronic dividers may have no delay, whereas electromechanical servos might add 100 msec and thermal dividers could add 500 msec.

(4) Tachometer measurements are rapid; a dc tachometer filter delay would be about 5-10 msec whereas an ac induction tachometer can have filter delays from 5 to 50 msec.

(5) Flowmeters can have delays of 10 msec or less.

(6) Pressure meters incur a delay through the diaphragm and isola­tion device which may be between 250 and 500 msec, depending on the capillary tubing used. There may be an added 100 msec sensor delay.

(7) Under — or overvoltage relays would have to be protected from spurious operation due to surges and false alarms by time lags which could amount to 500 msec.

(c) Logic delays following the reception of the signal would typically be 20 msec.

(d) Once the signal has been received and analyzed and dispatched to the shut-down mechanism, a further rod release delay including the scram breaker dropout would add a further 120 msec.

2.6.3.1 Total Scram Delays

From these various component delays we obtain the total typical delays in a LMFBR system shown in the accompanying tabulation.

The temperature signals are noticeably slower and have additional phys­ical delays which are not included in the tabulated figures. These transport delays range from 500 msec in the IHX measurement to 1000 msec at the outlet plenum or even longer at low flow levels.

Scrams using such combined delays as these are used in the safety evalua­tion representations.