Control Rods Flashcards
Name the materials used for thermal neutron absorption in control rods.
- Boron steel, boron having a high neutron absorption cross section
- hafnium or cadmium, metallic elements which are strong neutron absorbers
- silver; and
various alloys of these metals.
Describe the nuclear properties of active neutron absorber material in the control rod.
- Boron is used in control rods because of its high thermal neutron cross section (sa = 3837 barns at 0.025 eV). Boron also exhibits a large cross section into the epithermal energy region.
- Hafnium (Hf) or silver-indium-cadmium (Ag-In-Cd) rods have large absorption cross sections for thermal (Cd) and/or epithermal (Ag, In, Hf) neutrons. Silver-indium-cadmium rods are excellent neutron absorbers over a large energy range. The Ag-In-Cd rods absorb all neutrons from thermal energy to approximately 50 eV
- When hafnium absorbs a neutron, the resulting stable isotopes still have a high cross section for absorption for thermal neutrons. This leads to longer control rod life.
Predict the direction of change in reactor power for a change in control rod position.
- As the control rod assembly is withdrawn from the core, a neutron absorber is removed.
- As a control rod is inserted into the core, the control rod’s macroscopic cross section for absorption is increased. This means fewer neutrons are available for absorption in the fuel. Since fewer neutrons are available to cause fission, negative reactivity is added to the core. This negative reactivity causes reactor power to decrease.
- Control Rods affect the non-leakage probability and the thermal utilization factor terms the most.
Define Reactor Trip
It is the rapid insertion of all control rods to their fully inserted position.
Define Control Rod Worth
The effectiveness of a specific control rod in absorbing neutrons is called control rod worth (CRW). The reactivity change inserted will be greatest when the tip of the rod (change is at the tip) is moving through a region of the core with a high flux.
Reactivity changes due to rod motion are largest when the tip of the rod moves through regions where the neutrons are relatively important to the chain reaction (core center).
Define Differential Rod Worth
(DRW) is the change in reactivity resulting from a unit change of rod position.
Differential rod worth is proportional
to the square of the local relative flux.
Highest DRW occurs at a rod height below the core midplane.
Define Integral Rod Worth
The reactivity inserted by moving a control rod from a reference position to any other rod height,
The integral rod worth is zero when rods are fully withdrawn and becomes more negative as the rods are inserted.
Describe the effects of control rods on power peaking or hot channel factors.
Flux shaping is performed to minimize power peaking, control rod worth and resultant fuel problems, and to optimize fuel depletion.
It is accomplished by establishing a specific pattern of control rod withdrawal and insertion called a rod sequence.
The rod sequence in a PWR is designed to control the radial power distribution in the core. The rod sequence is established by the grouping of individual control rods into rod banks and withdrawing the rod banks in a specific sequence in order to maintain bank overlap.
Explain the shape of the curve for differential control rod worths versus rod position.
When the control rods are near the bottom, the maximum flux shifts back to the core midplane.
Because of this flux shift, the highest differential rod worth occurs at a rod height below the core midplane. Makes a bell shaped curve.
Explain the shape of the curve for integral control rod worth versus rod position.
As the control rods are withdrawn, positive reactivity is inserted. The integral rod worth is zero at zero steps and increases as the rods are withdrawn.
The integral rod worth is zero when rods are fully withdrawn and becomes more negative as the rods are inserted. The differential rod worth is the slope of the integral rod worth curve.
Explain the direction of change in the magnitude of control rod worth (CRW) for a change in moderator temperature
As the moderator temperature increases, it becomes less dense. At this lower density, neutrons are able to travel a greater distance before interacting with the water molecules. Since the neutrons travel a greater distance, they have a higher probability of reaching a control rod.
Therefore, as moderator temperature increases, control rod worth increases due to the control rod’s increased sphere of influence.
Explain the direction of change in the magnitude of control rod worth (CRW) for a change in boron concentration and fission product poisons.
Boron and Xenon-135 strongly absorb neutrons in the thermal energy ranges, but not much in energy levels above that. High concentrations of Boron or Xenon in the core tend to reduce the neutron flux in the thermal energy range.
This effect, known as spectrum hardening is responsible for the competition effect that boron and xenon have with each other. It also causes rod worth to be slightly less when they are in high concentration.
Over core life, boron concentration decreases and the rods become worth more due to a slight “softening” of the flux.
State the purpose of flux shaping.
Flux shaping is a method by which radial and axial neutron flux distribution is formed in the reactor core.
Flux shaping is performed to minimize power peaking, control rod worth and resultant fuel problems, and to optimize fuel depletion.
State the purpose of rod sequencing and overlap.
The rod sequence in a PWR is designed to control the radial power distribution in the core.
To expedite reactivity changes with minimum rod height movement, the control rods are operated in symmetrically arranged groups to form a bank of control rods.
Bank, or group, overlap provides a more uniform differential control rod worth and a more uniform axial neutron flux distribution during control rod maneuvers.
Discuss rod insertion limits.
During reactor operations, the rods must be maintained above the rod height specified for the given power level. The power defect and MTC temperature drops add positive reactivity on a trip. Insertion limits overcome this with margin to shutdown and keep the reactor shutdown.
The rod insertion limits minimize the consequences of an ejected rod accident, guarantee a sufficient shutdown margin from a given power level, and produce an axial flux distribution which prevents high local peak power levels.
Maintaining the control rods high in the core, at full power conditions, prevents an ejected control rod from inserting too much positive reactivity.
