Chemical treatments module (experimental)
XENON POISONING
Within the core of a nuclear reactor, xenon is produced as a byproduct of nuclear fission. When uranium or plutonium atoms undergo fission during the nuclear reaction, various fission products, including xenon, are released. This inert gas has the ability to absorb neutrons, which can adversely affect the efficiency and stability of the reactor.
Xenon acts as a "poison" for the reactor by absorbing the neutrons necessary to sustain the fission reaction. If xenon accumulates significantly, it can decrease the reaction rate and, in some cases, even halt the chain reaction, known as "xenon poisoning".
To counteract this effect, chemical treatment with boron is employed. Boron is a chemical element that effectively absorbs neutrons. By introducing boron into the reactor coolant, the impact of xenon is mitigated as boron absorbs neutrons before xenon has the chance to do so. This helps maintain the fission reaction at a controlled level, preventing a loss of reactor efficiency.
BORON AND THE PRESSURIZED WATER REACTOR
In a Pressurized Water Reactor (PWR), boron serves several crucial purposes:
• Neutronic Control: Boron acts as a neutron absorber in the reactor. It controls the chain reaction by absorbing neutrons, which is essential for regulating and maintaining the nuclear fission rate within safe limits. This helps prevent reactor overload and ensures that the fission reaction remains at a controlled level.
• Core Reactivity: The boron concentration in the reactor coolant can be adjusted to control the core reactivity. This adjustment is necessary to compensate for changes in the concentration of other neutron absorbers, such as xenon, which can affect reactor efficiency.
• Reactor Shutdown: In emergency situations or during reactor shutdown, the boron concentration in the coolant can be increased to ensure the nuclear reaction comes to a safe and efficient halt.
Boron can also influence the coolant temperature by modifying neutron absorption capacity. This can be used to control the reactor temperature and prevent undesirable conditions.
The normal and typical concentration of boron in parts per million (PPM) in a Pressurized Water Reactor (PWR) is usually in the range of 2,000 to 3,000 PPM.
ADDING BORIC ACID SOLUTION TO THE COOLANT
1. Check Core Vessel Capacity: Verify that there is sufficient capacity within the core vessel to accommodate the additional boron solution.
2. Set Dosage Rate: Set the desired dosage rate per minute. You can refer to the dosing chart to control the concentration effectively. Activation of the dosing pump occurs when specifying the desired dosage amount. Once the operator inputs the required quantity of boron solution to be added to the coolant, the dosing pump is initiated to begin the dosing process. The pump is responsible for delivering the specified amount of boric acid solution into the reactor coolant at the designated rate.
3. To Stop the Dosing Pump: When it's necessary to stop the dosing process, set the dosage amount to 0 (zero) to cease the addition of boron solution to the coolant.
These steps ensure a controlled and measured addition of boric acid to the coolant, allowing for the precise management of boron concentration within the reactor system.
ION ABSORPTION COLUMN
An ion absorption column is a device designed to selectively remove specific ions from a solution. The purpose of such a column is often related to purification or separation processes. In the context of boron in nuclear reactors, a boron absorption column is typically used to control the concentration of boron in the reactor coolant.
In nuclear reactors, boron is introduced into the coolant as boric acid (H₃BO₃). Boron has the ability to absorb neutrons, and by adjusting the concentration of boron in the reactor coolant, the reactivity of the reactor can be controlled. Reactivity control is crucial for maintaining a stable and controlled nuclear reaction within safe operational limits.
The boron absorption column is a key component in this process. It helps regulate the boron concentration in the reactor coolant by selectively removing or adding boron, depending on the reactor's operational requirements. By manipulating the concentration of boron, operators can fine-tune the reactor's reactivity and ensure safe and efficient power generation.
REMOVING BORIC ACID FROM THE COOLANT
1. Verify Manual Safety Valve Status: Ensure that the manual safety valves are open to allow proper flow and circulation within the system.
2. Activate Chemical Treatment Building Circulation Pump: Start the circulation pump in the chemical treatment building to direct the coolant towards the ion absorption column. Refer to the filtration chart to monitor the amount absorbed per minute.
3. Turn Off Circulation Pump: When the desired reduction in boron concentration is achieved, deactivate the circulation pump to stop the flow of coolant through the ion absorption column.
These steps outline a controlled process for either adding or removing boric acid from the coolant. By carefully managing the circulation and absorption rates, operators can maintain the boron concentration within the desired levels, contributing to the safe and efficient operation of the nuclear reactor.
ABSORPTION COLUMN MAINTENANCE
The removed boron accumulates in the ion absorption columns over time, diminishing their effectiveness. It is essential to perform regular maintenance and cleaning procedures to address this issue.
As boric acid is absorbed in the ion absorption columns, the accumulated boron can form deposits or coatings on the column surfaces. This buildup can hinder the columns' ability to efficiently absorb boron from the coolant, resulting in a decrease in their overall effectiveness. Over time, the reduced absorption capacity can compromise the system's ability to control boron concentration accurately.
Regular maintenance and cleaning become necessary to restore and maintain the optimal performance of the ion absorption columns.
PROCEDURES FOR CLEANING THE ION ABSORPTION COLUMN
1. Verify Manual Safety Valve Closure: Ensure that manual safety valves are closed. If the cleaning solution enters the cooling circuit, accelerated corrosion of metals may occur.
2. Open NaOH Tank Valve and Close Water Valve: Open the manual valve of the NaOH tank and confirm that the water valve is closed.
3. Activate Chemical Treatment Building Circulation Pump: Start the circulation pump in the chemical treatment building to circulate the NaOH solution through the ion absorption columns. Utilize the absorption capacity chart to monitor the cleaning procedure.
4. Close NaOH Valve After Boron Removal: Once the accumulated boron is eliminated, close the NaOH valve and open the water valve for the cleaning phase.
5. Turn Off Circulation Pump and Close Water Valve: After removing any remaining NaOH residues, turn off the circulation pump, and close the water tank valve for column cleaning.
These steps outline a methodical process for cleaning the ion absorption columns using a sodium hydroxide (NaOH) solution. Following these procedures helps maintain the columns' efficiency by removing accumulated boron and ensuring the long-term effectiveness of the boron control system.
PH OF THE COOLANT
The pH of the coolant in a nuclear reactor is a critical factor that can affect both the effectiveness of boron and the integrity of pipes due to the risk of corrosion.
Effectiveness of Boron:
• Low pH (Acidic): A low pH of the coolant (acidic environment) can affect the solubility of boric acid. At a low pH, boric acid may precipitate and become less effective in controlling reactor reactivity. This could compromise the ability to adjust reactivity precisely, which is crucial for reactor safety and efficient operation.
• High pH (Basic): A high pH can also influence the solubility of boric acid. At very high pH levels, borates may form, which are less efficient in neutron absorption. Therefore, maintaining a proper balance in pH is crucial to optimize the effectiveness of boron in regulating reactivity.
Pipe Corrosion:
• Low pH: Acidic pH increases the risk of corrosion in pipes and components of the cooling system. Acidic corrosion can damage pipes and other metallic materials, potentially compromising structural integrity and system safety.
• High pH: High pH can also contribute to corrosion, especially in the presence of certain contaminants and specific conditions. This type of corrosion is known as pitting or stress corrosion.
The pH of the coolant is recommended to be between 7 and 9.
IMPACT ON MODERATOR AND PUMP OPERATION
The oxide produced from the corrosion process can affect the moderation capability of the coolant. Oxide deposits may hinder the coolant's ability to moderate neutrons, disrupting the nuclear reaction. Additionally, the presence of oxide can interfere with the smooth operation of circulation pumps, reducing their efficiency and potentially leading to pump failures.
pH Effects and Feedback: The oxide formed due to corrosion can affect the pH of the coolant. This change in pH can create a damaging feedback loop within the circulation system. As the pH deviates from the desired range, it can further accelerate corrosive processes and exacerbate the deterioration of pipes and components. This feedback loop poses a significant threat to the overall functionality and safety of the reactor's cooling system.