3.1. SEVERE ACCIDENT MITIGATION EQUIPMENT
The equipment needed for monitoring and mitigating severe accidents is defined in SAMGs. Reliably performing mitigation equipment can reduce the consequences of a severe accident.
The mitigation equipment installed in the containment typically includes: containment isolation valves, motor or air operated valves on emergency core cooling injection lines, power cables and penetrations. This equipment needs to reliably perform its intended safety function during and after exposure to severe accident environmental conditions. There is also mitigation equipment that may be indirectly exposed to the consequences of severe accidents (e.g. elevated temperature and radiation values).
Depending on the plant design and SAMG mitigation strategies, the reliable performance of the following systems may be needed:
Systems ensuring the containment integrity including containment shell, penetrations, isolations valves, hatches, airlocks seals etc.;
Reactor coolant system (RCS) depressurization;
Hydrogen mitigation (monitoring and recombination);
Containment heat removal system;
Accident monitoring system.
3.2. SEVERE ACCIDENT MONITORING INSTRUMENTATION
Reference [6] states that electrical and I&C equipment needed for severe accident management has to perform reliably under severe accident conditions.
The main function of the accident instrumentation is to provide reliable and unambiguous information even during the extreme conditions of a severe accident. The main parameter for determining inadequate core cooling is typically the core outlet temperature for PWRs, and the reactor coolant level for PHWRs and BWRs. The main parameter for determining the containment integrity is the pressure inside the containment and radiation releases outside the containment. Other parameters indicating potential degradation of the containment fission product barrier include temperature, reactor pressure vessel water level (RPVL), containment sump water level, combustible gas concentration, and radiation level.
Accident measurement channels consisting of sensor/transducer, associated cables, connections, terminal boxes and containment penetrations are typically qualified for DBA conditions. After transition to severe accident conditions, the aforementioned equipment is exposed to conditions above their design limits which could result in loss of the associated measurements channel. According to Ref. [1], this might require extension of the capability of this equipment. Alternatively this can be achieved by limitation of the consequences of the severe accident at the installation positions (shielding).
Accident monitoring instruments designed for DBA conditions may not be able to ensure measurement accuracy over wide ranges of parameters when subjected to severe accident conditions.
This is acceptable because the trending of these parameters is more important than obtaining precise values of a specific quantity.
3.2.1.Instrumentation for indicating the status of fission product barrier integrity
The experience during the last forty years has shown that the determination of the integrity of the fission product barriers needs reliable performance of instrumentation and equipment. The indications obtained from monitoring instrumentation during and after severe accident allow the operator to determine when to implement specific mitigating strategies and measures as well as to determine the effectiveness of such strategies and measures. Instrumentation may indicate:
The possible re-criticality of the reactor;
The indication of a reactor pressure vessel melt through;
The location of the core debris/corium;
The success and effectiveness of water injection (i.e. level and flow rate) into the reactor and/or the containment;
The success of cooling the core debris/corium and the containment heat removal;
Factors possibly jeopardizing containment integrity, e.g. flammable concentration of hydrogen, steam explosion, molten core concrete interaction or reaching the containment design pressure;
Temperature levels that would jeopardize steam generator tube integrity.
Some additional information serves to monitor and estimate the progression of the accident:
Neutron flux measurements (existing measurements can be used as long as the core is within the pressure vessel);
Trend of containment pressure and temperature;
Reactor vessel pressure and temperature;
Water levels at relevant locations;
Temperatures in the cooling chains, flow rates in cooling systems;
Gas concentration in different locations of the containment (hydrogen, carbon monoxide);
Dose rates inside/outside containment;
Activity measurements in release paths;
Positions of isolation valves and actuators.
Reference [7] identifies a set of SAMG accident instrumentation that might be useable to provide information in the event that portions or all of the normal monitoring system fails as well as non-instrumented information sources that may be used to gather needed information. However, monitoring during severe accidents needs to be accomplished by using systems that are designated for severe accident use.
3.2.2.Spent fuel pool instrumentation
Reliable indication of spent fuel pool (SFP) water level is necessary to ensure: (i) water level is adequate to support operation of the normal fuel pool cooling system, (ii) water level is adequate to provide substantial radiation shielding, and (iii) operating personnel are aware of a decrease in water level to the point where actions to implement addition of makeup water are needed. In addition, reliable indication of the SFP water temperature is necessary for determining whether adequate cooling for the spent fuel is being achieved.
3.3. CONSIDERATION OF MATERIAL PROPERTIES
The material properties of the equipment need to be known in order to understand the potential impact on capability of the equipment to perform reliably and to assess the limitations of the equipment. For some materials the direct exposure to radiation or humidity has to be avoided (e.g. PTFE Polytetrafluoroethylene (PTFE) Teflon®, Polyimide). Other materials that could degrade under severe accident environmental conditions need to be carefully evaluated.
For example, PTFE insulated seals and cables are susceptible to exposure to ionizing radiation (alpha and β). These polymeric materials lose their mechanical properties. Consequently, parts of the equipment made of these materials may degrade to the point where the equipment cannot perform reliably. PTFE insulated seals in containment penetrations, when exposed to high γ doses, may result in the loss of containment integrity. In contrast, the use of some materials, e.g. metals, glass, ceramics, high performance polymers which have good resistance to high radiation, is recommended.
3.4. SUPPORTING AND AUXILIARY SYSTEMS
Support systems such as power supplies, air supplies, sampling system piping, component cooling with connection points are necessary to enable the mitigating system to perform reliably. Such vital support systems (excluding I&C and electrical power supply) include:
Compressed air;
Heating, ventilation and air conditioning (HVAC) for equipment and personnel;
Emergency lighting in the plant buildings;
Communication and security systems.
Some components or parts of supporting and auxiliary systems are located in the auxiliary building, areas adjacent to the containment and are likely to be exposed to elevated radiation and temperature levels. In order to ensure their functionality in long term, either qualification or an assessment of reliable performance is needed.
4. ESTIMATION OF ENVIRONMENTAL PARAMETERS