Reactor reactivity control

The control rods perform dual fimctions of power distribution shaping and reactivity control. Power distribution in the core is controlled during operation of the reactor by manipulation of selected patterns of rods. The rods, which enter from the bottom of the near-cylindrical reactor, are positioned in such a manner to counterbalance steam voids in the top of the core and affect significant power flattening. These groups of control elements, used for power flattening, experience a somewhat higher duty cycle and neutron exposure than the other rods. 

Control rods are cooled by the core leakage (bypass) flow. The core leakage flow is made up of recirculation flow that leaks through several leakage flow paths  

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Reactivity control during and after the external event allowing, either automatically or through operator action, the power of the research reactor to be reduced to a sufficiently low level to maintain a suitable margin to deal with later events or an evolution in the emergency. Redundancy and diversity in the reactor reactivity control system should be demonstrated. 

Slider motion (direct reactor reactivity control), Brayton unit speed (reactor coolant flow rate), and HRS flow all represent independent methods of affecting reactivity and are controlled by separate control systems on the spacecraft. To avoid power swings and control oscillations caused by interactions from the respective controllers, only one factor affecting reactivity may be changed at a time. Communication between the respective controllers is imperative. 

The low-power-density, low enrichment reactor core uses soluble boron and burnable poisons for shutdown and fuel bumup reactivity control. Low worth grey rods provide load following. A heavy uranium flywheel extends the pump coastdown to allow for emergency action during loss-of-flow transients. 

Boric acid [B(OH)3] is employed in primary coolant systems as a soluble, core reactivity controlling agent (moderator). It has a high capture cross-section for neutrons and is typically present to the extent of perhaps 300 to 1,000 ppm (down from perhaps 500 to 2,500 ppm 25 years ago), depending on nuclear reactor plant design and the equilibrium concentration reached with lithium hydroxide. However, boric acid may be present to a maximum extent of 1,200 ppm product in hot power nuclear operations. 

Reactivity Control. The movable boron-carbide control rods are sufficient to provide reactivity control from the cold shutdown condition to the full-load condition. Supplementary reactivity control in the form of solid burnable poison is used only to provide reactivity compensation for fuel burnup or depletion effects. The movable control rod system is capable of bringing the reactor to the subcritieal when the reactor is an ambient temperature (cold), zero power, zero xenon, and with the strongest control rod fully withdrawn from the core. In order to provide greater assurance that this condition can be met in the operating reactor, the core is designed to obtain a reactivity of less than 0.99, or a 1% margin on the stuck rod condition. See Fig. 7. 

One is the secondary- coolant reduction test by partial secondary loss of coolant flow in order to simulate the load change of the nuclear heat utilization system. This test will demonstrate that the both of negative reactivity feedback effect and the reactor power control system brings the reactor power safely to a stable level without a reactor scram, and that the temperature transient of the reactor core is slow in a decrease of the secondary coolant flow rate. The test will be perfonned at a rated operation and parallel-loaded operation mode. The maximum reactor power during the test will limit within 30 MW (100%). In this test, the rotation rate of the secondary helium circulator will be changed to simulate a temperature transient of the heat utilisation system in addition to cutting off the reactor-inlet temperature control system. This test will be performed under anticipated transients without reactor scram (ATWS). 

Although relatively little tritium is produced from natural lithium contaminant in thermal reactors by reactions (8.48) and (8.49), the li source of tritium is also produced by the (n, a) reaction with boron used for reactivity control ... 

The water chemistry of CANDU reactors embraces control of corrosion and corrosion-product transport in the coolant system, control of radiolytic decomposition of the moderator (51) and control of the concentration of soluble neutron absorbers used to adjust reactivity and control of boiler-water chemistry to minimize tube corrosion (52). The major chemical engineering effort has dealt with coolant technology and I will confine this review to that aspect of water chemistry. 

We observe that almost two thirds of the anomalies involve the reactivity control function (command of control rod positions), the containment (notably dome isolation and reactor building ventilation) and standby supplies (diesel generator sets, batteries and switchboards that they supply). The anomalies concerning sodium only involve eight events... 

The 4S FR concept was proposed and developed by CRIEPI — Toshiba (Japan). Tlie main features of reactor design are a tall thin reactor core (an equivalent diameter 90 cm, length 4 m) and axially moveable radial reflector for compensation of bumup reactivity. Its reference design of 50 MW(e) was for 10 years of electric power output without refuelling and without the use of the safety rod for bum up reactivity control. Now, the CRIEPI — Toshiba team has proposed a new design variant for 24 years of full power operation without refuelling. It uses part of the reactivity worth of the safety-rod in addition to the radial reflector segments for bumup reactivity control. 

Resulting from irradiation of reactor internals increase in forces to be applied for movement of reactivity control members was observed by the end of a specified lifetime. 

CORE AND REACTOR SYSTEM 3.1. Core and Reactivity Control System... 

The Reactor Core Subsystem (RCSS) consists of fuel elements, hexagonal graphite reflector elements, plenum elements, startup sources, and reactivity control material, all located inside a reactor pressure vessel. The RCSS, together with graphite components of the Reactor Internals Subsystem, constitutes a graphite assembly which is supported on a graphite support structure and restrained by a core lateral restraint structure. (See Figures 4.1-1 and 4.1-2). 

The annular reactor core consists of fuel elements, graphite reflector elements, plenum elements, reactivity control material, and neutron startup sources. Each of these components is described below. 

Additionally, the reactivity control system(s) shall be designed, fabricated, and operated such that during insertion of reactivity the reactor thermal power will not exceed acceptable values. 

Each well contains three neutron detectors. Two neutron detectors provide neutron flux signals to the PPIS for use in the reactor trip circuitry. The third neutron detector provides a neutron flux signal for use by the NCS and Rod Control Systems for reactivity control during plant operation. The detectors used are fission chambers. The ranges covered are shown in Figure 4.3-11. 

With this failure, the block elements in the core column above could drop a short distance and became jammed on the damaged post. The functions of the post would continue to be met since the core column flow would be maintained and it would be possible to insert reactivity control material. Safe shutdown of the reactor would not be affected, nor would the flow mixing or shielding functions of the posts. 

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