Scram signal

Matter Scram signal Reactor pressure (MPa) Minimum DNBR... 

Improved detection of pump shaft breakage should he installed for safety circuit action. The Pressure Monitor does provide a degree of protection in this area, hut pressure decay time to the Pressure Monitor may he slow allowing boiling in the downcomer before initiation of a scram signal. The Ball 3X System will not protect for a single broken shaft. Pressure or differential pressure Instrumentation should he applied to... 

Another feature of the MTB control arrangement is the safety circuits which protect against higher-than-normal power levels. Unsafe levels causW a reduction in current to the electromagnets and subsequent release of the control rods. The time required for.release of the rods had been measured as approximately 15 msec before installation in the Mock>Up. During operation the time interval between the "scram signal and shutdown of the reactor was observed to be approximately 30 nsec. 

The heat loss from the secondary side occurs by steam dump to the atmosphere as the turbine-generator combination stops on the scram signal. The condenser is lost if there is a total loss of electric power. The safety and steam dump valves open within seconds of the start of the accident. 

The PRSS is also used as an active reactor shutdown system. While the MCP continues to run, a scram signal can activate a motor driven valve to close the MCP delivery pressure transmission line and thus the hydraulic pressure valves open. In this case the poison is injected by forced circulation due to the MCP delivery pressure. 

In fuel salt flow decrease accident [XXX-23], the primary pumps are locked and control rods do not drop regardless of scram signal. As shown in Fig. XXX-5, the maximum fuel temperature is 900°C, which is below 1500°C temperature limit for fuel (see Table XXX-1). 

One-second time constant between scram signal and 1% rod insertion ... 

Rods selected for manual control (or automatic power setback 0 are operated at either slow (6 feet per minute) speed or intermediate (30 feet per minute) speed by a low-pressure (1500 psig) hydraulic system. The rate of reactivity increase by control rod withdrawal is limited by hydraulic pump capacity. Certain neutron absorbing rods must be reserved for the scram (safety) function. Upon receipt of a scram signal, the safety rods, plus all operational control rods, are driven full-ln at high speed (about... 

The pumps on one side of the reactor are usually supplied with BPA power and the pumps on the other side with local power. However, in case of need both sets can be operated from either power source by manual switching When a scram signal occurs valves on the exit of the scram accumulator open and the hydraulic fluid flows from the accumulator through a check valve into the cylinder At the same time, a valve on the line at the end of the other piston rod opens giving a low-resistance flow path for the oil and bypassing the normal constant-flow orifices. The scram accumulator capacity is sufficient to drive a fully withdrawn control rod to the full in position ... 

ATWS Reactor Shutdown Diversity Scram Signal System ARI ATWS Sequence... 

The scram signal actuates a by-pass between the two arms, so that the poisoned solution is pushed by gravity into the in-core branch. 

About 9000 pcm have to be controlled by the scram system. According to the design specifications, 1.5-2 seconds are allowed. from the scram signal to the full insertion of the poisoned liquid column. These times were satisfactorily met in full scale experiments. 

Tests, inspections, and analyses conducted after the event led to the conclusion that the east SDV was substantially full of water at the time of the event, leaving insufficient room for the discharge water. Accordingly, upon scram actuation, the CRDs rapidly drove the control rods partially into the core but rod motion prematurely ceased when pressure quickly equalized on each side of the pistons. Following each scram actuation, the scram signal was reset by the operator, allowing more water to drain from the SDV and permitting the rods to insert... 

Generally, the scram signal should be released before the emergency core cooling system (ECCS) signal. In consideration of this relationship, the low pressure scram set point, which is 24.0 MPa, is above the ADS/LPCI set point (one of the ECCS set points), which is 23.5 MPa. 

Fig. 6.25. The main coolant flow rate decreases linearly to 50% of the rated flow. Flow rate low level 1 is detected and the scram signal is released at 1 s. Although the trip of the RCP itself would release the scram signal, it is conservatively neglected. The cladding temperature increases until 3.6 s due to the decrease in the flow rate and then decreases due to the decrease in the power. The increase in the hottest cladding temperature is 60°C which is the highest among the abnormal transients. It is sensitive to the coast-down time and the scram delay as shown in Table 6.11.

This is the typical transient where both RCPs trip. However, its sequence is different from a total loss of reactor coolant flow accident as described in Sect. 6.4. In the loss of offsite power, the motor-driven condensate pumps are assumed to trip instantaneously. The turbine control valves are quickly closed due to a turbine trip. The turbine bypass valves open immediately after that. A scram signal and AFS signal are released by detecting the loss of offsite power or turbine control valves quickly closed or condensate pump trip. Both RCPs are assumed to trip at 10 s... 

This is a typical pressure decreasing transient. The maximum turbine control valve opening is assumed and it is 130% of the rated value. The cladding temperature is always below the initial temperature because the main steam flow rate and therefore the core coolant flow rate increase. A scram signal is released when the pressure reaches the low level 1 (24.0 MPa). A depressurization signal is released when the pressure reaches the low level 2 (23.5 MPa). After opening the ADS, the reactor behavior is similar to that shown in Fig. 6.7 [1]. 

This is a typical flow increasing transient. The demand of the main coolant flow rate is assumed to rise stepwise up to 138% of the rated flow as is assumed in the feedwater control system failure of Japanese ABWRs. Since increase in the core coolant flow rate is mild in ABWRs due to the large recirculation flow, the feed-water flow rate is assumed to increase stepwise. This assumption is too conservative for the Super LWR. The main coolant flow rate is gradually increased by the control system in the safety analysis. The calculation results are shown in Fig. 6.31. The reactor power increases with the flow rate due to water density feedback. A scram signal is released when the reactor power reaches 120% of the rated power. The maximum power is 124% while the criterion is 182%. The increase in the pressure is small. The sensitivity analysis is summarized in Table 6.15. 

Shutting down the reactor at power or period scram signal, scram signals of the PIS or failure of the city electricity network. 

Alarm and scram signals on period in the intermediate and power ranges from the NFCS. 

Alarm and scram signals on power in the power range from NFCS. 

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