Nuclear safety functions

In some cases, assigning no (or limited) nuclear safety function to the balance of plant, so that it could be built to local standards by local constructors using local labour with financing denominated in local currency ... 

Some concepts intend to reduce the number of safety-related functions of the balance of plant, e.g., the SVBR-75/100 (18), or even to release the balance of plant from any nuclear safety function whatsoever, e.g., the STAR-H2 (29). Then, the balance of plant could be preconstructed or constructed in parallel to reactor site assembly by local companies and local labour to local building standards, and can be financed in local currency. 

The balance of plant is assigned no nuclear safety function, and passive load following is achieved via reactivity feedbacks in response to heat demand communicated only by means of the molten salt intermediate loop. These two features could help totally decouple reactor safety performance from equipment failures and plant operator or plant maintenance personnel mistakes in the balance of plant. 

First off, the balance of plant (BOP) would have no nuclear safety function. Moreover, the STAR-H2 heat source reactor is being designed not only for passive safety response to Anticipated transients without scram (ATWS) initiators but also for passive load follow. The only information flow path from the BOP to the reactor would be the fused salt intermediate heat transport loop, which will convey the BOP heat request to the reactor by means of its flow rate and return temperature (see Fig. XXIV-3). In this way, the reactor could passively adjust its power to match heat demand while remaining in a safe operating regime. The safety implication of passive load follow is that the reactor would safety respond to all possible combinations and timing of ATWS initiators taken more than one at a time it would also safety respond to all conceivable human errors of the maintenance crew and the operator. In summary, all faults exterior to the reactor vessel might be safely accommodated on the basis of passive thermo-structural feedbacks. 

The role of the operator is to monitor that the nuclear safety functions are being successfiilly maintained by the various systems, diagnose the cause of the initiating fault, act to prevent any escalation of the situation and decide on the long term requirements, if any. In most cases there should be nothing preventing the return of the reactor to power. 

The reactor internals must fulfil the following nuclear safety functions during normal operation and following Design Basis initiating events ... 

The Level 2 features are the duty systems that can be deployed during operational transients, equipment faults or human errors before a nuclear safety function is seriously challenged. On ihe APIOOO, the following SSCs are in this category ... 

Nuclear safety for the APIOOO is less dependent than current plants on ihe duty systems (IAEA Level 1) and the systems that are deployed to control abnormal operation and detect failures (IAEA Level 2), because of the presence of robust safety measures (IAEA Level 3) that stop ihe loss of nuclear safety function these Level 3 safety measures are robust because they do not require support systems such as ac power, component cooling water and service water. The five IAEA levels of protection are discussed in more detail in Section 8.2.1 of this PCSR. 

The safe operating envelope of the APIOOO is defined by the operational parameters within which it can be safely operated, and by the protective safety measures that must be available in case a duty system fails. The limiting operational parameters are the boundary conditions assumed by the transient analysis for each fault in the design basis fault schedule (see Section 5.2 of this PC SR). The conditions placed on the availability of protective safety measures are based on the required reliability of providing the nuclear safety function, given the postulated frequency and consequences of each fault. 

The identification and substantiation of design requirements on plant systems, structures, and components, which show how nuclear safety functions are maintained throughout the lifecycle. 

The STAR reactor concepts employ an ambient pressure primary and extensive levels of passive safety [7, 13] to be consistent with a worldwide deployment of many thousands of STAR plants to remove all nuclear safety functions from the balance of plant and to facilitate siting near urban centres. 

Abhitt, J. F., 1969 A Quantitative Approach to the Evaluation of the Safety Function of Operators in Nuclear Reactors, Atomic Health and Safety Board, UK. AHSB(s) R 160. 

C icci, R. H. V., 1980, A Methodology for Evaluating the Probability for Fire Loss of Nuclear Power Plant Safety Functions, Ph.D. Thesis at Rensselaer Poly. Inst., Troy, NY. 

The control of the nuclear and chemical reactivity in case of accidents is insured by the emergency shutdown systems. The safety function devoted to the thermal power extraction from the HYPP is directly linked to the control of the chemical reactivity because the kinetics of chemical reactions increases with the temperature. The HYPP must be cooled by emergency systems, water streaming on equipments, spraying systems, and so on. 

Abstract The Canadian Nuclear Safety Commission (CNSC) used the finite element code FRACON to perform blind predictions of the FEBEX heater experiment. The FRACON code numerically solves the extended equations of Biot s poro-elasticity. The rock was assumed to be linearly elastic, however, the poro-elastic coefficients of variably saturated bentonite were expressed as functions of net stress and void ratio using the state surface equation obtained from suction-controlled oedometer tests. In this paper, we will summarize our approach and predictive results for the Thermo-Hydro-Mechanical response of the bentonite. It is shown that the model correctly predicts drying of the bentonite near the heaters and re-saturation near the rock interface. The evolution of temperature and the heater thermal output were reasonably well predicted by the model. The trends in the total stresses developed in the bentonite were also correctly predicted, however the absolute values were underestimated probably due to the neglect of pore pressure build-up in the rock mass. 

Nuclear safety I C systems have to meet demanding functional and non-functional objectives. They need high reliability and quality of components as well as good properties of architectures such as deterministic behavior, fail-safe and fault tolerant features, functional diversity, and separation. Furthermore, these systems should avoid unnecessary complexity and prevent when possible, operator and maintenance errors. In addition, safety I C systems shall meet the other customer expectations such as modularity, scalability, flexibility, ease of operation. 

Sharing of Structures. Systems, and Components. Safety class structures, systems, and components shall not be shared among nuclear facilities unless it can be shown that such sharing will not impair their ability to perform their safety functions, including, in the event of an accident in one nuclear facility, an orderly shutdown and cooldown of the remaining nuclear reactor fac i I i t i es. 

DISCUSSI ON. The reactor design should be fundamentally safe to ensure that the reactor is capable of being shutdown safely and adequately cooled following postulated accidents. In addition, the reactor facility should be designed to provide defense-in-depth needed to prevent or mitigate the consequences of accidents that could result in uncontrolled release of radioactive materials to the environment. The nuclear safety design criteria ensure that the reactor and the associated safety class SSCs perform their intended safety functions. 

Structures and equipment required to fulfil level F2 safety functions during or after an earthquake shall be identified, on a case-by-case basis, to establish the need for seismic qualification or other means of ensuring its capability to withstand earthquake-induced effects to the extent required by its contribution to nuclear safety. Such equipment shall be seismic category I. 

DOE Order 420.1, Facility Safety, requires the detailed application of that order s requirements to be guided by safety analyses that establish the identification and functions of safety (safety class and safety significant) structures, systems, and components (SSCs) for a facility and establish the significance of safety functions performed by those SSCs. It specifies that nuclear facilities shall be designed with the objective of providing multiple layers of protection to prevent or mitigate the unintended release of radioactive materials to the environment. The safety analyses must consider facility hazards, natural phenomena hazards, and external man-induced hazards. Paragraph 4.4.1 requires safety analyses for hazardous facilities to include the ability of SSCs and personnel to perform their intended safety functions under the effects of natural phenomena. DOE O 420.1 (DOE 1995) incorporates requirements from the cancelled DOE Orders 5480.28, 5480.7A, and 6430.1A(DOE 1993). 

The Nuclear Safety Support function is responsible for maintaining the HCF safety basis (including the Safety Analysis Report (SAR) and TSRs) and performing and/or directing analysis necessary to maintain the safety basis. 

The Paks Nuclear Power Plant (NPP) experience with respect to development of symptom-based emergency operating procedures (EOPs) was reported. These symptom based EOPs improve the performance of the plant in the event of an incident or accident because they provide a framework within which all critical safety functions can be monitored and appropriate actions taken. They provide a complement to event-based procedures because it isn t possible to anticipate all plant events, particularly combinations of individual events. The Paks NPP staff have integrated the existing event-based procedures with symptom-based EOPs to provide a comprehensive framework to appropriately respond to all abnormal and emergency conditions. 

In the Operational Safety research activities of the Community, up to now and especially under FP-4 the emphasis was put on the T component, i.e. the identification and solution of technological problems. Traditionally the technological problems of nuclear reactor safety are related to the 3 basic safety functions, namely controlling the power, cooling the fuel and confining the radioactive material. Their solution lies in the standard 3-levels defence-indepth approach against accidental radioactivity releases, i.e. ... 

The objective of the event-independent part of the Emergency Operating Procedures (EOPs) is to provide means to evaluate and restore the plant nuclear safety. The concept is based on the premise that radiation release to the environment can be minimised if the barriers to activity release are protected (barriers of defence in depth). In order to accomplish this goal, a set of functions has been defined which are critical from the plant nuclear safety point of view. These are the Critical Safety Functions. To be able to evaluate the status of these functions. Status Trees have been designed, one per CSF. Once the state of the CSF is evaluated, based on their state and the rules of priority one can designate a Function Restoration Guideline to be implemented for restoring CSF (see Appendix 3). 

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