Shutdown margin

The shutdown of SMART can be achieved by a function of one of two independent systems. The primary shutdown system is 32 shutdown banks of CEA of which absorbing material is B4C. The control banks are dropped into the reactor core by the gravity force and immediately stops die neutron chain reactions. These control banks have sufficient shutdown margin to bring the reactor fi-om hot full power to hot shutdown, even with a most reactive bank stuck out of the core. For the case of failure of the primary shutdown system, the emergency boron injection system is provided as a backup system and consists of two tanks, 6m ... 

Total Reactivity Uncertainties Affecting Cold Shutdown Margin... 

Control of the fission energy generation rate during reactor power operation and assurance of adequate shutdown margin is provided by movable control rods. The mechanisms which drive the control rods are located in penetrations in the reactor vessel top head. 

Reactor shutdown is maintained by the insertion of sufficient poison material to meet the shutdown margin, and by transferring the decay heat to the primary coolant. 

The six control rods located in the central reflector are not "safety-related" and are inserted only from hot-shutdown or low-power conditions to achieve a cold shutdown. Boronated graphite pellets housed in hoppers above the core provide a reserve shutdown capability. Upon actuation, these pellets drop into channels in selected columns of the active core to provide reactor shutdown in the event that the control rods are inoperable, or if necessary, to provide additional shutdown margin over what may be provided by the control rods located in the hexagonal side reflector. 

In the shutdown mode, the reactor vessel is fully pressurized or, at different times, in various stages of depressurization. Afterheat from fission product decay is generated at rates of up to about 7 percent of the core power level prior to shutdown, depending on the time interval since shutdown. The core decay heat is removed by the HTS. When the HTS is not available, the heat is removed by the Shutdown Cooling System (SCS). The outer control rods are normally fully Inserted during shutdown, and meet the required shutdown margin, with due allowances for uncertainties, even if the maximvim reactivity worth rod remains fully withdrawn. For cold shutdown, the control rods in the inner reflector are also Inserted and for this case, the maximum reactivity worth control rod is in the inner reflector. The neutron flux level is continuously monitored by the source range detectors. 

Provide sufficient neutron absorbing material to ensure that the core shutdown margin is met... 

The RCSS has several design limits. These include limits on fast neutron fluence to the graphite components, peak graphite stress limits, fuel particle packing fraction limits, metallic component temperature limits, including control rods, and core shutdown margin limits. These limits are ... 

The control rods are used for reactivity control during operation, as well as ensuring that a minimum shutdown margin of 0.01 A p is met ( A p is... 

Random Reactivity Uncertainties Affecting Shutdown Margins Uncertainties in calculations, input data, measurements, fuel loadings, basic constants, etc., must be taken into account in any estimate of core reactivity and shutdown margin calculations to ensure that the minimum criteria are met. Two types of uncertainties are considered, i.e., random uncertainties and systematic errors. The reactivity effects of random uncertainties, such as fuel loading tolerances, can be combined in a root mean square (RMS) fashion while the reactivity effects of systematic errors, such as core impurities, must be summed. 

From the results given in Table 4.2-10 it is seen that the inclusion of the nine random uncertainties results in combined uncertainties of typically +1.7 to +2.0 percent Ap for EOC conditions. In the worst case of the maximum worth stuck rod, the shutdown margins, including uncertainties, range from 2.56 percent Ap to 8.65 percent Ap depending on time in cycle. 

TOTAL REACTIVITY UNCERTAINTIES AFFECTING COLD SHUTDOWN MARGIN... 

Studies for the selection of the reference design con-figuratiem consisted of determining the effects of lumped poison rods on Y rod worth and the effects ni various fuel and lumped poison rod arrays on the shutdown margin for stuck rod conditions and total reactivity. 

This criterion is used with fire UK Monte Carlo code MONK. kN, ON ok computed values of keff and its standard deviation for the new case. EpD and E M are allowances for errors due to program, data, specification, and modeling of the system. Er allows for the possible existence of situations more rewtive than that mod d, and L is a limit < unity which cim ipve additional pessimism, or impose a particulat shutdown margin. 

The SBLC system is adequate to bring the reactor from the hot operating condition to cold shutdown and to hold the reactor shutdown with an adequate margin when considering temperature, voids, Doppler effect, equilibrium, xenon, and shutdown margin. It is assumed that the core is operating at normal xenon level when injection of liquid control chemical is needed. 

To increase the heat removal rate, an auxiliary fan located in the exhaust stack may be activated. The fan is also started if augmented decay heat removal capacity is required to reduce system temperatures for maintenance activities or to benefit shutdown margin. 

Sub-critical multiplication/shutdown margin measurements with approach to critical. 

Maximum Nxmiber of Rods Under One Actuator Minimum Shutdown Margin-operating Minimum Shutdown Margin-accident ... 

It is necessary to load 31 fuel assemblies with guide tubes for control rods (GT assemblies) to gain a satisfactory shutdown margin under cold zero power shutdown conditions. The Reactor Protection System comprises two independent fast acting shutdown systems. Shutdown System-1 (SDS-1) is based on mechanical shutdown rods with boron carbide based absorbers in 31 fuel assemblies it provides sufficient negative reactivity with all rods inserted, with one maximum worth rod not available, in the cold shut down condition. Shutdown system-2 (SDS-2) is based on liquid poison injection into the moderator. 

Fully automated reactor start-up can be achieved by the LRM, yet another passive device incorporated in the RAPID concept. Figure XVII-5 shows the LRM basic concept. LRM is similar to LIM however, Li is reserved in the active core part prior to reactor start-up. The LRM is placed in the active core region where the local coolant void worth is positive, as is also the case with LEMs and LIMs. The RAPID is equipped with an LRM bundle in which 9 LRMs and an additional B4C rod are assembled. The reactivity worth of the LRM bundle is +3.45, once each LRM includes a 95% enriched Li enclosed in a 20mm-diameter envelope. A B4C rod is used to ensure the shutdown margin (-0.5 ). An automated reactor start-up can be achieved by gradually increasing the primary coolant temperature with the primary pump circulation. The freeze seals of LRMs melt at the hot standby temperature (380°C), and Li is released from the lower level (active core level) to the upper level to achieve positive reactivity addition. An almost constant reactivity insertion rate is ensured by the LRMs because the liquid poison, driven by the gas pressure in the bottom chamber, flows through a very small orifice. It would take almost 14 hours for the liquid poison to move into the top chamber completely. A Sn-Bi-Pb alloy is used as the freeze seal material to ensure the reactor start-up at 380°C. 

Soluble neutron absorber material - the neutron absorbing material dissolved in the moderator to compensate for fuel bumup or to maintain the required reactor shutdown margin. During reactor operation, the soluble neutron absorber is gradually removed by diluting the moderator. Data providers should choose the appropriate option from the... 

The shutdown margins for both the horizontal rod and Ball 3X systems for a number of reactivity slates are given in Tables 7 -l aod 7 5 2. 

It is seen that either safety system is able to maintain the reactor sub-critical under all credible conditions. A simultaneous cold water accident and optimum flooding incident resulting in a five per cent Increase in reactivity (which is considered incredible) would result in a reactivity state outside of the strength of the horizontal rods. However, assuming a Just-critical state prior to the postulated accident, the shutdown margin of the Ball 33C system would be positive even for this accident. 

Reactor Condition Excess Reactivity Minimum Probable Ball 3X Strength ( ) Shutdown Margin ... 

New operating procedures have been written on boron dilution, as well as on new surveillance procedures for calculation of shutdown margin and sub-criticality. 

One plant has introduced the "variable shutdown margin" into the technical specifications, another one... 

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