THE ROLE OF PASSIVE SYSTEMS IN ENHANCING SAFETY AND PREVENTING ACCIDENTS IN ADVANCED REACTORS

Transcription

1 THE ROLE OF PASSIVE SYSTEMS IN ENHANCING SAFETY AND PREVENTING ACCIDENTS IN ADVANCED REACTORS M. Aziz Nuclear and radiological regulatory authority Cairo, Egypt Abstract Most of the new reactor designs are introducing inherent and passive safety features that do not depend upon external source of power or human actions for their successful performance. In view of this, a desirable goal for the safety characteristics of an innovative reactor is that its primary defence against any serious accidents is achieved through its design features preventing the occurrence of such accidents without depending either on the operator s action or, the active systems. Passive systems are credited a higher reliability with compared to active systems, because of greater availability due to lower probability of hardware failure or human error. 1. INTRODUCTION Passive system is the system whose functioning does not depend on an external input or human action such as actuation, mechanical movement or supply of power. As a result, passive safety systems are being considered for numerous reactor concepts (including in Generation III and III+ concepts) and are expected to find applications in the Generation-IV reactor concepts [1,2]. Passive systems are credited a higher reliability with respect to active ones, because of a smaller unavailability due to a minimize hardware failure and human error. The use of passive safety systems such as accumulators, condensation and evaporative heat exchangers, and gravity driven safety injection systems reduces the costs associated with the installation, maintenance and operation of active safety systems that require multiple pumps with independent and redundant electric power supplies. In this paper the categorization, types and nature of passive systems are discussed. Typical deployment of passive systems among several advanced reactor are shown. The impact of introduction of passive systems on advanced Reactors are analyzed for advanced pressurized and boiling water reactors for two measures; the core damage frequency (CDF) and the large early release damage frequency (LERF) and compared with typical reactors. The deterministic and probabilistic analyses for passive systems are discussed as well as the challenges facing passive systems. 2. PASSIVE SYSTEMS CLASSIFICATIONS Passive systems are composed entirely of passive components and structures or a system which uses active components in a very limited way to initiate subsequent passive operation. Passive systems can be categorized into four classes A, B, C and D according to the degree of inclusion of passive action in the system. Actions required to operate passive or active system may include: input signal, external power source, moving mechanical parts and moving working fluids [1, 3]: Class A: this category is characterized by no input signal, no external power source, no moving mechanical part and no moving fluids. Examples are: physical barriers against the release of fission products, such as nuclear fuel cladding and pressure boundary systems hardened building structures for the protection of a plant against seismic. Class B: this category is characterized by no input signal, no external power source, no moving mechanical part but with moving fluids. Examples are: reactor shutdown/emergency cooling systems based on injection of borated water produced by the disturbance of a hydrostatic equilibrium between the pressure boundary and an external water pool; containment cooling systems based on natural circulation of air flowing around the containment walls, with intake and exhaust through a stack or in tubes covering the inner walls of silos of underground reactors; Class C: this category is characterized by no signal inputs, no external power sources, with moving mechanical parts, whether or not moving working fluids are also present. Examples are : accumulators and or storage tanks and

2 discharge lines equipped with check valves; overpressure protection and/or emergency cooling devices of pressure boundary systems based on fluid release through relief valves. Class D: This category is characterized by signal inputs of intelligence to initiate the passive process, energy to initiate the process must be from stored sources such as batteries or elevated fluids, active components are limited to controls, instrumentation and valves to initiate the passive system manual initiation is excluded. Examples are emergency core cooling and injection systems based on gravity that are initiated by battery-powered electric or electro-pneumatic valves, Core makeup tank, Elevated gravity drain tank, passive cooled steam generator natural circulation, passive residual heat removal heat exchangers and isolation condensers. 3. PASSIVE SYSTEMS IN TYPICAL NUCLEAR POWER PLANTS Passive systems and components are included in all advanced Nuclear power plants of GEN-III. Typical passive cooling systems can be demonstrated in several reactors such as ABWR-II, APWR+,AP-1000, WWER- 1000,and ESBWR 3.1. Advanced Pressurized Water Reactors (APWR+) The APWR+ is a four loop type PWR with 1750 MW(e) output, which is being developed as the successor of APWR and conventional PWRs, aiming at more enhancements in economy, safety,reliability, reduction of the operators workload, and harmony with the environment as shown in Fig. 1. APWR+ employs the following concepts for its safety system including passive features [2]: - Passive core cooling system using steam generator - Advanced Accumulators - Advanced Boric Acid Injection Tank FIG. 1 Passive systems in APWR+ 3.2 AP Reactor ( Advanced Passive ) The AP600 and AP1000 are pressurized light water reactors designed by the Westinghouse Electric Corporation to produce 600 MW and 1100 MW of electric power, respectively[1,4]. Both designs employ passive safety systems that rely on gravity, compressed gas, natural circulation, and evaporation to provide for long term cooling in the event of an accident. The reactor employs the following features as shown in Fig. 2: - An in-containment refueling water storage tank (IRWST) - A passive residual heat removal (PRHR) system - Two core make-up tanks (CMTs)

3 - A four stage automatic depressurization system (ADS) - Two accumulator tanks (ACC) - A lower containment sump (CS) - Passive containment cooling system (PCS) FIG. 2 Passive Safety Systems used in AP 1000 Designs 3.3 VVER Reactor WWER employ the following passive features as shown in Fig. 3[1, 2]: - Passive quick boron supply system, - Passive subsystem for reactor flooding HA-1 (hydro accumulators of first stage), - Passive subsystem for reactor flooding HA-2 (hydro accumulators of second stage), - Passive residual heat removal system via steam generator (PHRS), - Passive core catcher. FIG. 3 Passive Safety systems in WWER-1000 Reactor 3.4 Economic Simplified Boiling Water Reactor (ESBWR ) 4500 MWth ESBWR employs the following safety features in the design [2]: - Gravity driven cooling system (GDCS), - Automatic depressurization system (ADS), which consists of the depressurization valve (DPV) and safety relief valve (SRV),

4 - Isolation condenser system (ICS), - Standby liquid control system (SLCS), - Passive containment cooling system (PCCS), and - Suppression pool (SP). FIG.4 Passive Safety systems in ESBWR 4. ROLE OF PASSIVE SYSTEMS TO STRENGTHEN SAFETY AND MITIGATE ACCIDENTS Passive systems do not require human intervention or input signal to start, so they eliminate or minimize both errors due to human intervention and failure of both on and off site power supply, Accident frequency ( failure probability per reactor per year) is reduced and safety is enhanced. In PSA reactor accidents can be measured by Core Damage Frequency (CDF) and large early release Frequency (LERF). So Safety goals for advanced reactors are to reduce CDF and LERF as compared to the exiting reactors and reduce release of radioactive material to the environment. Table 1 illustrates CDF and LERE for small and medium reactor. Table 2 illustrates CDF and LERE for some advanced reactors as compared to existing NPP. From the tables we see that inclusion of Passive systems reduce accidents (CDF and LERE) by two fold. TABLE 1. C O RE DAMAGE FREQ UENCY AND LARGE EARLY RELEASE ACCIDENT FO R SMR [1, 2] Reactor type SMART IRIS CDF/reactor.year 8.56x10-7 2x10-8 LERE/reactor.year < x10-10 TABLE 2 CORE DAMAGE FREQUENCY AND LARGE EARLY RELEASE ACCIDENT FOR LARGE REACTORS [1, 2] Reactor type Existing reactors ABWR (1700 MWe) APWR+ (1750 MWe ) AP-1000 (1000 MWe) VVER-1000 (1000 MWe) ESBWR ( 1500 MWe) Core Damage Frequency /reactor.year x10-7 < x10-7 < 10-5 ~ 10-8 Large Early Release Frequency/reactor.year < x10-8 < 10-7 < DETERMINISTIC AND PROBABILISTIC ANALYSIS Deterministic analysis for passive systems model system design and phenomena which occurs during operation, focus on accident types, consequences and releases without considering the probabilities of different event

5 sequences. Thermal Hydraulic computer codes are usually used for deterministic analysis of passive systems such as RELAP and ATHLET computer codes [5]. Probabilistic analysis (PSA) for passive systems evaluate the failure rate (or frequency per unit time ) for the components and systems,and the probability for certain accident sequence to occur ( such as core damage Frequency ). PSA constructs both event tree and fault tree for certain accident scenarios. Three levels of PSA are generally recognized. Level 1 comprises the assessment of plant failures leading to determination of the frequency of core damage. Level 2 includes the assessment of containment response, leading, together with Level 1 results, to the determination of frequencies of failure of the containment and release to the environment of a given percentage of the reactor core s inventory of radio nuclides. Level 3 includes the assessment of off-site consequences, leading, together with the results of Level 2 analysis, to estimates of public risks [6, 7, and 8]. 6. CHALALLENGES FACE PASSIVE SYSTEMS The following challenges and problems still face passive systems [4, 5, and 6]: - Passive systems have little operating experience and their driving force is small, which can be changed even with small disturbance or change in operating parameters. - Physical behavior of passive systems should be studied carefully especially in case of reactor transients. - Aging of passive systems must be considered for long plant life. - Flow instability which include density wave, flow pattern transition instability should be analyzed carefully. - Phenomena such as Thermal stratification in large pools and effect of non condensable gases on condensation should be studied. 7. CONCLUSION - The reliability of the passive systems are high because it can continue to work in severe conditions such as loss of electricity and station blackout. - Passive systems strength improves safety of the reactor and it should be used in combination with active systems to prevent accidents. REFERENCES 1. Passive Safety Systems and Natural Circulation in Water Cooled Nuclear Power Plants. TECDOC-1624, IAEA, November, Status of advanced light water reactor designs IAEA TECDOC-1391, Vienna (2004). 3. A.K. Nayak and R.K. Sinha, Role of Passive Systems in Advanced Reactors., Progress in Nuclear Energy 49(2007) Natural Circulation Phenomena and Modeling for Advanced Water Cooled Reactors. IAEA, T ECDOC-1677, Vienna, Passive safety Systems in Advanced water cooled Reactors, Case Studies, and IAEA TECDOC 1705, Vienna (2013). 6. Progress in Methodologies for the Assessment of Passive Safety System Reliability in Advanced Reactors. IAEA TECDOC- 1752, Vienna (2014). 7. Technical Feasibility and Reliability of Passive safety systems for Nuclear Power Plants, IAEA, TECDOC-920, proceeding of an advisory group meeting held in Julich, Germany November A.K. Nayak.etal., Passive system reliability analysis using APSARA methodology. Nuc. Eng. and Design. 238 (2008)