Coolant Accident

Perfect Strangers (Team 4)
M4.6 Brainstorming Report

Shift Technical Advisor Reactor Operator
Shift Manager

Based on the observations of the simulation the team found that there was a Loss-of-Coolant Accident, (LOCA). Early indications at the 1-minute mark showed a slow but steady decrease in pressurizer levels, rising radioactive airborne levels in containment, and an increase in coolant charging flow rate. At the 11-minute mark there was a rapid decrease in pressurizer levels. This triggered a reactor and turbine trip at around the 13-minute mark. This was followed by an increase in charging flow with the starting of Coolant Charging Pumps, CCP, A and B and Safety Injection was initiated.
There are two main issues that could have occurred during the scenario. The LOCA could have led to the fuel becoming uncovered potentially causing the fuel to melt due to the loss of cooling. The second is a radioactive release to the environment. With radioactive particulates in containment, indicated by the rising airborne radioactivity levels, there was potential for a release if containment failed.
There were several scientific principles associated with the scenario. Fluid dynamics, including pump laws are crucial for designing and operating the pumps within the Reactor Coolant System, RCS. They help us understand how efficiently pumps move coolant throughout the system and ensure adequate flow to remove heat from the core. This leads us to thermodynamics. Understanding how heat moves through the RCS is imperative to harnessing nuclear power. Heat generated by the reactor core needs to be transferred to the coolant, to the steam generators, in the case of PWR, to

the turbines, then condensers, and finally to the heat sink. Radioactivity is also another scientific principle that is important to the scenario. Understanding that fission creates heat and radioactive particulate and gasses is important to understand what is happening in the fuel and RCS. This understanding also allows us to determine if there is a radioactive release into the environment by detecting specific nuclides outside of the containment structure.
The scenario involved various interconnected systems working together to ensure reactor safety. The RCS, where heat generated by the reactor is transferred to the coolant. Steam generators which is the primary heat sink for the RCS. Emergency Core Cooling System which contains a critical safety system which inject borated water into the reactor vessel to ensure the core stays cool. Containment Spray and Isolation which reduces pressure within containment by condensing steam and keeping radioactivity isolated inside containment. There are also monitoring systems such as temperature, pressure, and radiation that allow operators to track the condition of the reactor. Overall, the interconnected operation of these systems is important for mitigating the consequences of a LOCA.
Numerous enhancements can be made to the design of the RCS and auxiliary systems. However, these are limited due to permissible thresholds related to leakage and other RCS parameters. The crux lies in identifying the failure and averting automatic activation to maintain control over the plant. It is essential to employ procedures that facilitate the attainment of flow balance and enable a comparison of the leak with the makeup capacity. An investigation into the circumstances leading to the leakage is necessary to ascertain if it’s a pervasive issue. A design flaw could impact

other operational plants, leading to more shutdowns for repairs. It is also important to consider operator training and expertise. By maintaining a high level of knowledge operators will be able to identify issues earlier and respond faster.
The Shift Manager will be firstly concerned with placing the plant in a safe condition. They will want to continue operations because providing power is their mission. This will happen by working with operators and shift technical advisor, STA, to identify and combat the leakage. When the leakage overcomes the makeup capacity of the plant then they will have to make the call to shut down. When the leak becomes an accident they will initiate the emergency response system activating procedures to get additional personnel and resources to the plant to combat the LOCA. Until the emergency response teams are in place they will be managing personnel to ensure the safety of the plant and personnel.
In the scenario the Reactor Operator will be constantly monitoring plant parameters and initiating emergency procedures as needed to keep the reactor safe. Their main focus is ensuring the core stays covered, potentially isolating the leak, and effectively communicating with the team. Following procedures and reporting observations is important for mitigating any problems. When the scenario becomes a LOCA then the Reactor Operator will primarily be doing the same thing but also ensuring the reactor is shut down.
The Shift Technical Advisor (STA) observing this transient on the plant is concerned with looking into the structural integrity of the RCS system as well as the containment system and its operations. Typically, whenever you talk about a LOCA it flashes to steam which can have an adverse effect on the electrical components and

their operations. With the high energy systems being housed in the containment boundary it is not a direct concern for operator safety as much as it is plant stability and continued operations. The STA has a big picture view of the plant and ensures that the core is safe. The STA does not need to get focused on one thing but instead needs to make sure that all of the fundamentals that are required for reactor safety are being addressed and that there are no casualties being missed because of the disruption. The flow balance to determine the leak rate is important for the plant staff’s decision-making. If the leak rate is outside the technical specifications, the plant must be shut down and placed in specific plant conditions within a certain time period. Decreasing the temperature and pressure of the system will reduce the impact of the failure.
The ethical dimension of this scenario pertains to how it shapes perceptions of the power plant’s operations and safety measures. In essence, any event occurring at a single plant, for whatever reason, has repercussions for the entire industry. In this scenario the first ethical dilemma is whether or not to shut down. Providing power is the mission for a nuclear power plant and any time offline can have severe consequences. The choice between maintaining power supply to the grid or opting for a conservative shutdown for repairs, which could result in financial loss for the company is challenging. Once there were indications of a LOCA then reactor safety and preventing a radioactive release to the environment are the priority. This creates an ethical dilemma on deploying operators and personnel to investigate which can expose them to radiation and physical harm. Determining if the risk of exposure to the few is worth the potential risk of exposure to the many can be difficult. Any incident can cast a shadow over the entire industry, so learning from them and addressing root causes (equipment failure,

maintenance issues, human error) is crucial. The nuclear industry faces significant ethical challenges during accidents. Maintaining a balance between protecting personnel, public safety, transparency, and learning from events is important for maintaining trust and ensuring safe operations for nuclear power.