Over-pressurisation

In Austria, as well as all over Europe, the first and repetition tests of all pressure equipments including steam drums are required for security reasons within fixed time intervals. These repetitive inspections are done differently in the most European countries, but most time these inspections include, according to the European Pressure Equipment Directive" and the specific national law any kind of over-pressurisation (e.g. hydrotest) and visual inside inspection. 

For the repetitive inspections the required hydrotest can only be performed for a limited number of the small cylinders, and even then the drums have to be removed from the line and the cylinders will be supported in defined distances for the weight of the water and the pressurisation. For the new and long cylinders even this is impossible, because they loose due to the additional weight of the water and the over-pressurisation their roundness and balances. Therefore the law in the most countries within and outside of the EU accept as a replacement of the hydrotest an additional application of different NDT methods, which were often done by an ultrasonic measurement of the wall thickness of the cylindrical part and a MT of the flat covers. 

Advantages of the use of atmospheric pressure devices over pressurised vessels are [44] ... 

At this stage, a number of credible maloperations will have been defined that can lead to vessel over-pressurisation. In order to cope with all the credible runaway scenarios, the relief system will need to be sized for the "worst case runaway" reaction that can occur, and this is normally the maloperation that will give rise to the highest rate of temperature and/or pressure rise over the relief range. 

Nitrogen flushing may be carried out if permitted— but do not over pressurise the tanker. 

Less supply of demineralised DM feed water to absorption tower less absorption of NO2 can cause environmental pollution the tower can get overheated and over pressurised Standby feed water pumps and sufficient storage of DM water provide safety valves for pressure release Design and constiuction of the tower as a pressure vessel provide safety valves for pressure release... 

This paper presents a discussion of theoretical and experimental investigations of the efficiency of aerosol transport through narrow structures such as gaps around containment penetrations or cracks in containment caused by over pressurisation. 

Where high pressure systems are required, leak-before-break compliant equipment has been used that allows depressurisation before a high energy ruptures could occur. Additionally, pressure relief is provided to high pressure systems, where possible, to ensure that over-pressurisation does not occur. 

Condition II faults, at worst, result in a reactor trip with the plant being capable of returning to operation. Condition II events are not expected to result in fuel rod failures, reactor coolant system failures, or secondary system over-pressurisation. 

Gas relief valves on the accumulators protect them from over-pressurisation and potential explosion. The system also includes the capability to remotely vent gas from the accumulator, if required. 

The normal residual heat removal system must be protected against spurious and inadvertent over-pressurisation, because, once cormected, the reactor coolant system pressure boimdary is extended to include the entirety of the normal residual heat removal system. That part of the system outside the containment has a design pressure that is lower than the main part of the reactor coolant system. 

The second potential over-pressurisation pathway for the normal residual heat removal system is by way of the discharge branch lines, which each connect to a direct vessel injection line. Each line contains two normally closed check valves, which, as reactor coolant pressure boundary valves, are designed forthe reactor coolant system design pressure. The branch line cormectsto a common header, which penetrates the contaimnent. The header contains two containment isolation valves. 

Concrete Containment Subject to Aircraft Crashes and Seismic Effects and Over Pressurisation... 

Fig. 14.26 Density conditions explored in the liquid at 2000K. Starting from p = 1.84 gcm (over-pressurised liquid at the glass density), lower densities along path (1) were sampled corresponding to typically orri ia/y(/() = 1.49 gcm ) and sf retched (/) = 1.13 gem conditions. The system was then compressed back to the starting density, path (2). Values for the average pressure and fraction of boroxols (/> are indicated. The inset shows the evolution of /) along these paths...

Most of the studies completed to-date identified the areas of particular concerns like Mid-loop operation for PWRs or Cold Over-pressurisation for BWRs. Many of the SLP PSAs found that simple modifications like change in procedures or additional instrumentation could significantly reduce the risk during plant outage. Additionally, many studies highlighted the possibility for other cost effective solutions. 

The risk profile of a typical plant shows the peak at the moment of shutdown of the reactor and again at the start up. The peak at shutdown is related to switch over to the RHR System, and it is due to possible inoperability of the system which cannot be verified before the system s operation is required. This phenomena is well understood and the measures to reduce the risk are mainly related to assuring the diversity in heat removal (like steam generators full with water, etc.). The risk peak at the start-up is often related with the cold over-pressurisation event which is especially dangerous during the water solid conditions, when inadvertent safety injection or malfunction of charging systems can cause major rupture of RCS. Interlocks and procedures were developed and employed to cope with this phenomena. 

With regard to BWR type reactors, the single risk-dominant initiator is the cold over-pressurisation. There is potential for cold over-pressurisation during filling the reactor at the end of a Shutdown sequence. The status and proper actuation of safety and relief valves were found very important for BWR type reactors during shutdown state. 

Original Swedish Barrier Analysis approach was used on two NPPs, Ringhals 4 and Forsmark 2. These were limited scope studies evaluated selected predefined events such as main circulation pump overhaul, cold over-pressurisation, refuelling, control rod drive overhaul, testing and inspections. 

The over-pressurisation events may cause a serious problem at both PWR and BWR reactors. Those events could be dealt with by administrative, hardware and procedural improvements. 

The plant technical specifications are typically defining the pressure/temperature limitation which are applicable during shutdown operation. In addition, some plants have backfitted their technical specifications to prohibit the use of specific pumps (i.e. those with high head delivery) to fill the reactor, or require the operability of adequate relief capacity during periods when the increased risk of over-pressurisation exists. 

Reduction of the probability of over-pressurisation incident is one of the areas where operational improvements are the dominant choice. Specific procedures which were implemented by selected plants covers the equipment to be used for filling the rector (for example limiting the use of high pressure pumps) as well as define the specific sequence of activities to be followed. 

The use of the auxiliary feedwater pumps for filling the reactor above certain level is forbidden. This greatly reduce the possibility of cold over-pressurisation. 

Overhaul, Cold Over-pressurisation, refuelling, Control Rod Drive Overhaul, testing and inspections), and plant removal of the SG manway to the installation of the last nozzle dam. 

HARDWARE - To protect against boron dilution, the suction of CVCS was switched to RWST - To protect against cold over pressurisation, a diverse relief paths has been engineered. - A diverse and more accurate level measurement parameter has been engineered to help control fhe level when going mid-loop... 

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