THE ROLE OF PASSIVE SYSTEMS IN ENHANCING SAFETY AND PREVENTING ACCIDENTS IN ADVANCED REACTORS Moustafa Aziz Nuclear and Radiological Regulatory Authority (ENRRA) , Cairo , Egypt moustafaaai@yahoo.com International Conference on Topical Issues in Nuclear Installation Safety: “ Safety Demonstration of Advanced Water Cooled Nuclear Power Plants ”  IAEA Headquarters in Vienna , Austria, (6 – 9) June 2017

Contents INTRODUCTION PASSIVE SYSTEMS CLASSIFICATIONS REVIEW on PASSIVE SYSTEMS IN TYPICAL NUCLEAR POWER PLANTS ROLE OF PASSIVE SYSTEMS TO STRENGTH SAFETY AND MITIGATE ACCIDENTS DETERMINISTIC AND PROBABILISTIC ANALYSIS CHALALLENGES FACE PASSIVE SYSTEMS CONCLUSION REFERENCES

Introduction Passive system : the system whose functioning does not depend on external input or human action such as actuation , mechanical movement or supply of power. 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.

Passive systems types and classifications 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 Active/Passive system may include: input signal , external power source, moving mechanical parts and moving working fluids.

Moving Mechanical Part Examples Moving Fluids Moving Mechanical Part External Power Source Input Signal System Class Physical barriers against releases of fission product (clad). × Class A Reactor shutdown systems based on injection of borated water produced by the disturbance of a hydrostatic equilibrium between the pressure boundaries and external water pool. √ Class B

Moving Mechanical Part Examples Moving Fluids Moving Mechanical Part External Power Source Input Signal System Class Accumulators equipped with check valves √ × Class C Core makeup tanks (CMT) Passive cooled steam generators. isolation condensers. ( from batteries or elevated fluids) Class D

REVIEW ON PASSIVE SYSTEMS IN SOME NUCLEAR POWER PLANTS Advanced Pressurized Water Reactors (APWR+) Developed by Japanese PWR utilities and Mitsubishi Heavy Industries. The APWR+ is a four loop type PWR with 1750 MW(e) output, 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 and natural circulation during accident Advanced Accumulators Advanced Boric Acid Injection Tank The core and the loop piping are submerged in case of LOCA

AP -1000 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). A four stage automatic depressurization system (ADS). Two accumulator tanks (ACC). A lower containment sump (CS). Passive containment cooling system (PCS).

VVER -1000 Reactor VVER employ the following passive features as shown in Fig. 4[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

Economic Simplified Boiling Water Reactor (ESBWR ) 4500 MwtH ESBWR are developed by General Electric and employ the following safety features in his design [2]: Gravity driven cooling system (GDCS) Automatic depressurization system (ADS), which consists of the depressurization valves (DPV) and safety relief valves (SRV), Isolation condenser system (ICS), Standby liquid control system (SLCS), Passive containment cooling system (PCCS), and Suppression pool (SP).

ROLE OF PASSIVE SYSTEMS TO STRENGTH SAFETY AND MITIGATE ACCIDENTS Table 2: Core Damage Frequency (CDF) and Large Early Release Accident Frequency for SMR (LERA) [1,2]. Reactor type SMART IRIS CDF/reactor .year 8.56x10-7 2x10-8 LERA/reactor .year < 10-8 6x10-10

Core Damage Frequency /reactor .year Large Early Release Accidents/ 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) WWER-1000 (1000 Mwe) ESBWR (1500 Mwe) Core Damage Frequency /reactor .year 10-5 1.6x10-7 < 10-7 5x10-7 < 10-6 ~ 10-8 Large Early Release Accidents/ reactor .year 10-6 < 10-8 5x10-8 < 10-8

DETERMINISTIC AND PROBABILISTIC ANALYSIS Deterministic analysis: Deterministic analysis model system design and phenomena which occurs during operation, focus on accident types , consequences and releases without considering the probabilities of different event sequences. Thermal Hydraulic computer codes are usually used for deterministic analysis of passive systems such as RELAP and ATHLET computer codes.

Probabilistic analysis (PSA) PSA 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 ). - event tree and fault tree for certain accident scenarios

CHALALLENGES FACE PASSIVE SYSTEMS The following challenges and problems still face passive systems [4,5,6 ]: 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. Aging of passive systems must be considered for long plant life.  

CONCLUSION Passive Systems decrease the Core Damage Frequency and Large Early Release Accidents. Passive systems strength safety of the reactor and it should be used in combination with active systems to prevent accidents .

REFERENCES Passive Safety Systems and Natural Circulation in Water Cooled Nuclear Power Plants. TECDOC -1624, IAEA, November, 2009. Status of advanced light water reactor designs 2004. IAEA TECDOC 1391, Vienna (2004). A.K. Nayak and R.K. Sinha , Role of Passive Systems in Advanced Reactors., Progress in Nuclear Energy 49(2007) 486-498. Natural Circulation Phenomena and Modeling for Advanced Water Cooled Reactors. IAEA, TECDOC. 1677, Vienna, 2012. Passive safety Systems in Advanced water cooled Reactors, Case Studies, IAEA TECDOC - 1705, Vienna (2013). Progress in Methodologies for the Assessment of Passive Safety System Reliability in Advanced Reactors. IAEA TECDOC 1752, Vienna (2014). 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 21-24 November 1994. A.K. Nayak .et.al., Passive system reliability analysis using APSARA methodol.ogy. Nuc. Eng. and Design , 238 (2008) 1430-1440

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