Role of Slc System in Nuclear Reactor Functioning

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Boiling water nuclear reactor cores are major sources of revenue for power producing utilities. The utility is able to maximize the amount of energy output from the nuclear reactor while minimizing the cost of the reactor operation then the utility will realize an increase in profit. A utility may choose to maximize the energy output from their nuclear reactors in one of three ways. Either the utility may increase the operating cycle length of the reactor thereby increasing the amount of energy per cycle and increasing the amount of time for refueling outages, or the utility may choose to increase the power level of operation thereby increasing the amount of available distributive energy at any given time, or the utility may choose to utilize a combination of both practices.

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The designer has many options to improve SLCS but anyone optioned may endure a list of consequences, some of which may result in extreme economic concern. The designer may request that the reactor cycle length was decreased; obviously, if the utility wishes to increase profit by increasing power output, this option is not acceptable. The designer may request the utility to increase the boron concentration or enrichment of B10 in the boron solution utilized by the standby liquid control system. However, this course of action is limited by increased aggravation caused from the Nuclear Regulatory Commission (NRC) licensing, availability for the utility to plan for the change economically, ample time to complete the concentration increase in time for the next cycle loading, capabilities of the installed accumulator tank to support the concentration increase and the ability to keep the boron soluble in solution. The designer may choose to increase the number of fuel bundles loaded into the cycle and load more fuel bundles with a smaller enrichment.

The purposes of the Standby Liquid Control (SLC) System are to inject enough neutron absorbing poison solution to the reactor vessel to:

  • Shut down the reactor from full power with no control rod motion
  • Maintain the reactor in a subcritical condition as the plant operators cool the plant down to 70ºF.

SLC is a backup system designed to shut down the reactor under the most reactive conditions. SLC provides a means of shutting down the reactor without control rod insertion in the event of an Anticipated Transient Without Scram (ATWS). An ATWS is an operational condition whereby control rods do not fully insert after a scram signal has been processed. ATWS is one of the “worst case” accidents, consideration of which frequently motivates the NRC to take regulatory action.

The SLC System provides the operator with a relatively slow method of achieving reactor shutdown conditions. A successful reactor scram shuts the reactor down in seconds; whereas SLC takes up to 10 minutes to inject the amount of boron required to keep the reactor shutdown under all conditions. The poison is injected just below the core plate and is carried into the core by natural circulation. In addition to its shutdown capabilities, the SLC system may be used as a high-pressure injection source during low water level conditions in the emergency operating procedures. The functional classification of the SLC System is that of a safety-related system.

The Standby Liquid Control system, consists of:

  • A heated storage tank
  • Two 100% capacity positive displacement pumps
  • Two 100% capacity explosively actuated (squib) injection valves
  • The piping necessary to inject the neutron absorber solution into the reactor vessel
  • A test tank with necessary valves and piping to adequately test the system without injecting poison into the reactor vessel

The neutron poison used by SLC is the B^10 boron isotope as contained in a sodium pentaborate decahydrate solution. The B^10 isotope, which occurs as less than 20% of natural boron has a very high neutron microscopic absorption cross-section of 3.84 x 103 barns. B^11 which occurs as over 80% of natural boron has no significant cross section for neutron absorption. Enriching the sodium pentaborate decahydrate solution to 85-90% B^11 concentration in the storage tank dramatically reduces the time to shut down the reactor and increases the shutdown margin maintained with control rods stuck in a withdrawn position.

When a neutron is absorbed by a B^(10 )nucleus, an excited B^11compound nucleus is formed. The neutron absorption imparts a high energy to the compound nucleus. The excited compound B^11nucleus is more likely to result in alpha decay than to form a stable B^11nucleus. A B^11alpha decay forms a helium and lithium nucleus as represented below. As a poison, B10 absorbs a neutron which provides enough energy to cause the compound nucleus to separate into Lithium and Helium atoms.

The NRC ATWS guidelines established in 10CFR50.62 (commonly referred to as the “ATWS Rule”) established a more rapidly achieved shutdown margin by either increasing poison injection rates or enhancing the B^(10 )concentration of the poison. Each BWR is required to have an SLC system with a minimum injection capacity and boron content equivalent to an 86 gpm at 13% by weight of natural sodium pentaborate solution. Many licensees chose the B^(10 )enrichment option, in large measure because it did not require upgrading SLC equipment. Per NUREG-1780, the NRC has concluded that the licensee actions per the ATWS Rule have been effective in mitigating the potential effects of an ATWS. BWRs that have upgraded in allowable power have also had to commensurately increase the minimum SLC injection capacity.

The SLC system capacity is required to supply sufficient poison to provide a negative reactivity worth greater than the combined positive reactivity effects of:

  • All control rods fully are withdrawn
  • The Complete collapse of all voids
  • Doppler (fuel cooling)
  • Complete Xenon decay
  • Temperature (cool down to 70ºF)

If the volume in which the sodium pentaborate decahydrate is distributed becomes larger, then the poison concentration will correspondingly lower. Therefore the conservative volume estimate assumes the following:

  • The reactor water level is at Level 8 (56.5 inches)
  • The RHR system is actively in the Shutdown Cooling (SDC) mode
  • The recirculation system loops are communicating with the reactor pressure vessel

In addition to the above considerations, a -0.05% ΔK/K is included as an additional shutdown margin. In the event of SLC failure, the Reactor Core Isolation Cooling (RCIC) system can be configured as an alternate poison injection system.

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