Reactor power regulating system

All elements of the reactor power regulating system shall be described (design criteria and reliability analysis). All interfaces between the power regulating system and the reactor protection system should be identified and analysed to confirm that they do not lead to degradation of safety. 

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A.813. This section should include a description of the instrumentation systems which are provided in the reactor control room for indicating the status of the protection system, the reactor power regulation system and other important systems. 

The reactor power regulation system is implemented as a software module within the Supervision and Control system. This system, when active, is responsible of regulating reactor power to its setpoint, compensating reactivity changes (temperature effects. Xenon, sample insertion, fuel bumup, etc.) and to perform power changes to new levels upon modification of the power setpoint... 

The Nuclear Instrumentation comprises the Neutron Flux Instrumentation and a Nitrogen-16 Gamma Channel. Neutron flux measurement instrumentation supplies information on physical parameters to the Reactor Protection System and the Reactor Power Regulating System, describing the state of the reactor regarding neutron production and its evolution. 

The Reactor Power Regulating System (RPRS) controls reactor power from source level to 125% of full power level, using neutron and gamma instrumentation ... 

The Start-up channel has both log and linear outputs over its whole operating range. This channel is fed into the Reactor Protection System and also into the Reactor Power Regulating System through a functional and electrical isolated interface that assures absolutely independence of the Reactor Protection System. 

The Channel measures the gamma activity of the primary coolant. with linear output to be used covering the range from 0.1% to 125 % of full power level. It provides the output signal to the Reactor Power Regulating System. 

E. No credit is taken for CEA motion prior to the reactor trip. Normally the Reactor Power Cutback System (RPCS) would automatically effect a partial scram to rapidly reduce reactor power. The Reactor Regulating System (RRS) would also (slowly) insert CEAs to reduce reactor power. 

The passive power regulation system, described above, is on itself capable of shutting down the reactor. In addition, the CHTR has been provided with a secondary shutdown system. Under normal operation this system has a set of seven shut-off rods held on top of the reactor core by individual electro-magnets, which are passively released under abnormal conditions when the temperature of the core goes up. The shut-off rods are lifted up by active means. The requirement for these rods was that, when inserted in the fuel tubes, they should be able to bring the reactor to a subcritical state with necessary margin, even when the initial reactivity balance in the reactor is at its maximum. The maximum (with Keff= 1,111) is reached in an uncontrolled cold state therefore, the required worth of the shut-off rods should be evaluated namely for this state. 

Power Regulating System (RPRS) to allow an automatic control of the reactor power. 

The control rod system provides for automatic control of the required reactor power level and its period reactor startup manual regulation of the power level and distribution to compensate for changes in reactivity due to burn-up and refuelling automatic regulation of the radial-azimuthal power distribution automatic rapid power reduction to predetermined levels when certain plant parameters exceed preset limits automatic and manual emergency shutdown under accident conditions. A special unit selects 24 uniformly distributed rods from the total available in the core as safety rods. These are the first rods to be withdrawn to their upper cut-off limit when the reactor is started up. In the event of a loss of power, the control rods are disconnected from their drives and fall into the core under gravity at a speed of about 0-4 m/s, regulated by water flow resistance. 

Rod withdrawal error/subcritical/low power the reactor control and instrumentation system (RCIS) and automatic power regulator (APR) systems prevent this event from occurring. 

The absence of soluble boron control leads to simplification of the plant process system and to the enhancement of the reactor self-regulation properties. The use of burnable poison reduces the reactivity margin which has to be compensated for by mechanically driven absorber rods. The mechanical system to control the reactivity, consisting of cluster-type absorber rods, fulfills the functions of reactivity compensation, power control and emergency protection. 

Reactivity control 1 Reactor regulating system 2 Shutdown system 1, shut-off rods 3 Shutdown system 2, Gd poison mjection 4 Loss or dilution of DjO moderator 1 A 2 P 3 P 4 P Power manoeuvres mclude ramp setback, stepback and tnp 2 and 3 are engmeered safety systems for tnp 1 can serve m an assisbng role to shutdown sy stems No 1 and 2 In 4 HjO from ECC or leakage or boilmg down of moderator leads to subcnticality... 

REACTOR REGULATING SYSTEM CSA N290 4 Provides for control of the reactor power and neutron fla distnbution dunng plant operabon It also has the capability to shut down the reactor for anUcipated operabonal occurreoces... 

The fundamental design requirement of the reactor regulating system is to control the reactor power at a specified level and, when required, to maneuver the reactor power level between set limits at specified rates. The reactor regulating system combines hie reactor s neutron flux and thermal power measurements by means of reactivity control devices and a set of computer programs to perform three main functions ... 

The function of fhe zone control system is to maintain a specified amount of reactivity in the reactor, this amount being determined by the deviation from the specified reactor power set point. If the zone control system is imable to provide the necessary correction, the program in the reactor regulating system draws on other reactivity control devices. Positive reactivity can be added by withdrawal of absorbers. Negative reactivity can be induced by insertion of mechanical control absorbers or by automatic addition of poison to the moderator. 

The majority of pressure tube HWRs have protective functions in the reactor regulating system—a normally operating process system—that reduce reactor power when required to maintain process conditions in a safe operating range and provide protection of components and equipment. The power reduction can be gradual (power setback) or rapid due to absorber rod drop into the core (stepback). 

A very small LOCA (or leak) is defined as having a break discharge flow rate that can be handled by the heavy water makeup system without the need for any safety system intervention. A small break LOCA is defined as a pipe break that carmot be compensated by the heavy water makeup system and extends multiple feeder pipe ruptures such that the reactor regulating system, without credit for stepback action, is capable of limiting any power excursion. A large break LOCA is defined as a pipe break beyond the range of breaks in multiple feeder pipes which give rise to uncompensated coolant void reactivity and a resultant power excursion. 

Negative temperature reactivity coefficients and negative coolant void reactivity effect, provided by an appropriate selection of the design parameters the incorporation of negative temperature reactivity coefficients facilitates realization of passive safety features [XV-2] and also simplifies the power control system so that only feed water control in the power circuit can regulate reactor power ... 

Simplified emergency power supply system since emergency cooldown of modules is performed only by passive systems based operation, sodium natural circulation in the reactor and self-regulated natural circulation of atmospheric air, removing heat from... 

Control Passive power regulation and reactor shut-down systems... 

Since normal action of the reactor regulating system (RRS) can compensate for the slow increase in reactivity due to coolant void, and delay the trip on high power, two cases are considered that with the RRS inactive and that with the RRS operating normally. 

It would even be possible to "close the loop" and let the PCs assume some control functions. For example, the analog fe bau k loop connectirig reactor power and period to the regulating rod (usualfy put on line to automatically maintain reactor power) could be implemented in srrflware If the PCs were allowed to control regulaung rod position. However, this step would go beyond passive cormection to the Instrumentation system ruid could not be taken lightly. 

Since the Super LWR does not use saturated steam, the main steam temperature changes with the power to flow rate ratio in the core. It needs to be kept constant in order to avoid too much thermal stress or thermal fatigue on the structures. Since the Super LWR has no superheaters that are utilized to control the main steam temperature as in FPPs, another method is needed. The analysis results described in Sect. 4.3.2 show that the main steam temperature is sensitive to the feedwater flow rate. Thus, the main steam temperature is controlled by regulating the feedwater flow rate. It is also suitable from the viewpoint of the safety principle of the Super LWR, i.e., keeping the core coolant flow rate (described in Sect. 6.2) because the feedwater flow rate indirectly follows the reactor power in this control method. The plant control system employed for the Super LWR is shown in Fig. 4.16. The plant control strategies of the Super LWR, PWRs, BWRs, and FPPs are compared in Table 4.3. 

The reactor power is not sensitive to the flow rate because the Super FR is a fast reactor with small reactivity feedback from coolant density. The reactor power is mainly regulated by the CRs. Therefore, the responses of the reactor power do not significantly differ with the four control systems including the reference one. Since the responses of the core and main steam pressures are very fast and determined by only the turbine control valves, they are almost the same with the four control systems. The changes in the main steam temperature obtained by the plant stability analyses are summarized in Table 7.37 [31]. The advantages and the issues of each control system are discussed below. 

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