Reactor Physics Flashcards
(14 cards)
Where leakage would be higher? Sphere or Slab?
Neutron Flux Distribution: In a sphere, neutron flux is more evenly distributed, reducing leakage. In a slab, neutrons are more likely to reach the boundaries and escape.
How is the neutron balance controlled?
- Production:
- Determined by the total fissile content of the core
- Initial fuel loading
- Conversion of fertile nuclides (breeding)
- Absorption:
Control rods to change power level - Safety roads to provide shut-down
- Soluble boron in reactor coolant
- Burnable absorbers
- Leakage:
Core size and shape - Reflection of neutrons back into the core
- Density of core materials
- Sometimes temperature dependent
- Moderation
During operation, there is no way to measure the local power at every location within the core Typically, only total core power is known how do we know that?
- Heat Balance Method – The reactor’s thermal power is determined by calculating the total heat removed from the system. This is done by measuring:
- The temperature difference between the reactor coolant inlet and outlet.
- The coolant flow rate through the reactor
How can operators know that they are not melting fuel at steady state or during
transients (or accident conditions)?
- Temperature and Power Monitoring – Sensors continuously measure fuel temperature, coolant temperature, and neutron flux to ensure safe operating conditions.
- Reactor Protection Systems – Automatic shutdown mechanisms, such as control rods and emergency cooling systems, activate if temperatures exceed safe limits.
What are the criticality values?
Sub-critical (k<1), critical (k=1), super-critical (k>1).
Methods of removing decay heat
Utilising the primary cooling system
* Steam generators can be used they dissipate heat
directly to the main condenser or the atmospheric steam dump system.
* In some reactors are equipped with a residual heat removal system (RHR)
* Utilising an open system
* Decay heat is moved by the feed-and-bleed method, necessary method in some pressurized water
reactors
The feed-and-bleed technique involves passing hot coolant out of the primary system through a relief valve
Explain why the decay heat is dependent on the reactor operation time.
The amount of decay heat relates to the emission of gamma and beta radiation
from unstable isotopes, which are generated through while the reactor operate.
* Therefore, the amount of decay heat depend on the atomic density of those
isotopes
Pros and cons of boron
pro: concentrtion is easily controlled, uniform absorption across the core.
cons: makes coolant slightly acidic, can deposit as crud.
Why do we use Zr as structure material?
Low neutron absorption, high corrosion resistance, withstand extreme temperatures, mechanical strength and durability.
- What sets the distance between the fuel rods?
- Why the fuel rods are small
Neutron Moderation & Reactivity Control – Proper spacing ensures efficient neutron moderation and prevents excessive reactivity.
- Heat Transfer & Cooling – Fuel rods generate intense heat, so spacing must allow sufficient coolant flow to prevent overheating
- Structural Integrity
- Minimizing Radiation Damage
- Efficient Heat Transfer – Smaller fuel rods allow for better heat dissipation to the surrounding coolant, preventing overheating and ensuring stable reactor operation.
- Why the control rods organised in a “spider” lattice?
- Uniform Reactivity Control – The spider-like arrangement ensures that control rods are evenly distributed across the reactor core, allowing for consistent neutron absorption and precise reactivity control.
- Efficient Shutdown Mechanism – In emergency situations, the spider lattice allows for rapid insertion of control rods, ensuring a quick and effective shutdown of the reactor.
what is a pressuriser
Pressuriser ensures that:
* There is controlled over-pressure in the core to suppress
bulk boiling, and
* Primary pressure is controlled and vessels are protected
during expansion of the coolant in the RPV and the rest of
the primary circuit.
Decrease the inlet coolant temperature and
the end of cycle power. (Why??)
- As fuel burns over time, its ability to sustain a chain reaction decreases. Lowering the coolant temperature increases the density of the moderator, which enhances neutron moderation and adds positive reactivity to the core.
the peripheral positions are loaded with the fuel with the highest fuel burnup
* Can you think why?
- Lower Reactivity – High-burnup fuel has already undergone significant fission, meaning it has a lower multiplication factor (( k_{\text{inf}} )). Placing it at the periphery helps prevent excessive power peaking in those regions.
- Minimizing Neutron Leakage – The outer regions of the core are more prone to neutron leakage. Since high-burnup fuel produces fewer neutrons, placing it at the periphery reduces neutron loss, improving overall neutron economy.
