PLANT ANTICIPATED OPERATIONAL OCCURRENCES

The plant anticipated operational occurrences (AOOs) are described in detail in Section 11.6. For the AOOs in which the outer control rods are tripped, the trip occurs quite early in the event, before there can be a significant temperature transient or environmental change affecting the equipment. For AOO-5 the trip may occur long after the event is initiated and there is no significantly detrimental transient Imposed on the CRDs prior to the time when they are signalled to release the rods. 

The plant anticipated operational occurrences (AOOs) are described in Section 11.6. Those which affect the reactor Internals are discussed below. 

In the United States, commercial nuclear power plant operators are required to monitor and report any detectable quantities of radioactive materials released to the environment (NRC 1996). Table 6-1 summarizes releases of radiostrontium isotopes with half-lives >8 hours to the atmosphere and water for 1993 from PWR and BWR nuclear power plants. Nearly all of the radioactive material reported as released in effluents are from planned releases from normal plant operation or anticipated operational occurrences. The latter includes unplanned releases of radioactive materials from miscellaneous actions such as equipment failure, operator error, or procedure error, and are not of such consequence as to be considered an accident (NRC 1993b). 

Anticipated operational occurrences are off-normal events, usually plant transients, which can be coped with by the plant protection systems and normal plant systems but which could have the potential to damage the reactor if some additional malfunction should happen. Their typical frequency of occurrence may be more than 10 year Some of the anticipated occurrences (PIEs - postulated initiating events) are due to the increase of reactor heat removal (as might occur for an inadvertent opening of a steam relief valve, malfunctions in control systems, etc.). Some are due to the decrease of reactor heat removal (such as for feed-water pumps tripping, loss of condenser vacuum and control systems malfunctions). Some are due to a decrease in reactor coolant system flow rate, as in the case of a trip of one or more coolant pumps. Some are connected with reactivity and power distribution anomalies, such as for an inadvertent control rod withdrawal or unwanted boron dilution due to a malfunction of the volume control system for a PWR. Events entailing the increase or decrease of the reactor coolant inventory may also happen, due to malfunctions of the volume control system or small leaks. Finally, releases of radioactive substances from components may occur. 

Criterion 64 - Monitoring radioactivity releases. Means shall be provided for monitoring the reactor containment atmosphere, spaces containing components for recirculation of loss-of-coolant accident fluids, effluent discharge paths, and the plant environs for radioactivity that may be released from normal operations, including anticipated operational occurrences, and from postulated accidents. 

A system important to safety, i.e., provided to ensure the safe shutdown of the equipment or otherwise control an operation or to limit the consequences of anticipated operational occurrences and design basis incidents. Safety systems consist of the protection system, the safety actuation systems, and the safety system support features. Components of safety systems may be provided solely to perform safety functions, or may perform safety functions in some plant operational states and non-safety functions in other operational states. Safety system support features are considered the collection of equipment that provides services such as cooling, lubrication, and energy supply required by the protection system and the safety actuation systems. 

Design basis 10-Uio-i Anticipated during nuclear power plant service life Anticipated operational occurrence... 

The radiation related risk resulting from normal nuclear power plant operation and anticipated operational occurrences is also an important element of plant safety performance. Relevant indicators include data on... 

Assess readily the general state of the plant in normal operation, anticipated operational occurrences and accident conditions ... 

The plant conditions considered in the design basis analysis include anticipated operational occurrences and design basis accidents (DBAs). The division is based on the frequency of the occurrence. 

Anticipated operational occurrences are those events that are more complex than the manoeuvres which are carried out during normal operation and that have the potential to challenge the safety of the reactor. These occurrences might be expected to occur at least once during the lifetime of the plant. Generally they have a frequency of occurrence greater than 10 per reactor-year. 

For many PIEs the control systems wiU compensate for the effects of the event without a reactor trip or other demand being place on the safety systems (Level 2 of defence in depth). However, the anticipated operational occurrences category should include all the PIEs which might be expected to occur during the lifetime of the plant and for which operation can resume after rectification of the fault. 

Typical examples of PIEs leading to anticipated operational occurrences could include those given below. This list is broadly indicative. The actual list will depend on the type of reactor and the actual design of the plant systems ... 

The computer codes for the safety analysis of DBAs/anticipated operational occurrences and should draw on the operating experience that can be derived from similar nuclear power plants and relevant experimental data. Since anticipated operational occurrences are expected to occur once or more during the lifetime of a plant, there is some accumulated basis of operating experience and data for these transients. 

An event should not generate a subsequent more serious plant condition without the occurrence of a further independent failure. Thus an anticipated operational occurrence by itself should not generate a DBA, and such an accident by itself should not generate a beyond design basis accident. 

Expected in the life of the plant) Expected Anticipated operational occurrences Anticipated transients, transients, frequent faults, incidents of moderate frequency, upset conditions, abnormal conditions No additional fuel damage... 

The word emergency is used in many different contexts and different areas of human activity. In Ret [4] emergency is defined in part as A nonroutine situation or event that necessitates prompt action primarily to mitigate a hazard or adverse consequences for human health and safety, quality of life, property or the environment . In IAEA safety assessment publications the following terms have been established for identification of different operational states and accident conditions of a nuclear power plant normal operation, anticipated operational occurrences (AOOs) (abnormal operation), design basis accidents (DBAs), beyond design basis accidents (BDBAs) and severe accidents. 

A safety parameter display system (SPDS) is part of the information system important to safety which provides information for the safe operation of the plant during normal operation, anticipated operational occurrences, and accidents. 

The solid waste management system is designed to collect and accumulate spent ion exchange resins and deep bed filtration media, spent filter cartridges, dry active wastes, and mixed wastes generated as a result of normal plant operation, including anticipated operational occurrences. The system is located in the auxiliary and radwaste buildings. 

The eontrol system is capable of manoeuvring the plant through eertain transients. This manoeuvring is done without the need for manual intervention and without violating plant proteetion or component limits. The PLS provides high reliability during these anticipated operational occurrences and meets the following objectives (see subsection 7.7.2 of Reference 6.1) ... 

The numerical collective dose target for occupational radiation exposure is 1000 man-mSv/year from all anticipated operational occurrences. Section 12.4 of the EDCD (Reference 12.1) estimates the collective operational dose, including reactor operations and surveillance, routine inspection and maintenance, in-service inspection, special maintenance, waste processing, and refuelling at 219 man-mSv. This dose represents less than 25% of the collective dose target and is a significant improvement on the current UK nuclear power plant average (discussed in Reference Error Reference source not found.). 

The waste acceptance capability of the radioactive waste incineration facility shall enable the facility to process the waste volumes generated during normal operation and the anticipated operational occurrences of the nuclear power plant or facility. 

According to INSAG-10 [3], defence in depth consists of a hierarchical deployment of different levels of equipment and procedures in order to maintain the effectiveness of physical barriers placed between radioactive material and workers, the public or the environment, during normal operation, anticipated operational occurrences (AOOs) and, for some barriers, accidents at the plant. 

Examples of anticipated operational occurrences are loss of normal electric power, maliunction of individual items of a normally mnning plant and failure to function of individual items of control equipment. 

This section should provide relevant information on the instrumentation and control systems as described in paras 3.65-3.70. The reactor instrumentation senses the various reactor parameters and transmits appropriate signals to the control systems during normal operation and to the reactor trip systems and engineered safety features and systems during anticipated operational occurrences and in accident conditions. The information provided in this section should emphasize those instruments and their associated equipment that constitute the protection systems and those systems rehed upon by operators to monitor plant conditions and to shut the plant down and maintain it in a safe shutdown state after a design basis accident. Information should also be provided on the non-safety-related instrumentation and control systems used to control the plant in normal... 

This section should provide the estimated annual occupancy of the plant s radiation areas during normal operation and in anticipated operational occurrences. In order to reduce radiation doses to workers, the necessity of their presence in certain plant areas where radiation levels are high should be investigated (in order to limit working hours in those areas). 

The plant shall be designed to operate safely within a defined range of parameters (for example, of pressure, temperature, power), and a minimirm set of specified support features for safety systems (for example, auxiliary feedwater capacity and an emergency electrical power supply) shall be assumed to be available. The design shall be such that the response of the plant to a wide range of anticipated operational occurrences will allow safe operation or shutdown, if necessary, without the necessity of invoking provisions beyond the first, or at the most the second, level of defence in depth. 

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