atw 2015-01

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atw Vol. 60 (2015) | Issue 1 ı January ENERGY POLICY, ECONOMY AND LAW 28 | | Fig. 1. Strategy of GSR Development for SFR. Art. Title Art. Title Art. Title 1 Definition 21 Use of qualified equipment 41 I&C system 2 Radiation Protection 22 Human factors 42 Electric power system 3 Defense-in-Depth 23 Prevention of harmful effects between systems 4 Interfaces of safety with security and safeguards 5 Physical Protection / Safeguards 24 Protection against sodium reactions 6 Proven technologies 26 Inherent protection of reactor | | Tab. 1. General Safety Requirements (Articles of Technical Standards) for SFR. 43 Control room, etc. 44 Alarm devices, etc. 25 Reactor design 45 Optimization of radiation protection 7 Assessment of Design Safety 27 Suppression of reactor power oscillation 8 Construction and operating experiences 46 Radioactive waste processing & storage systems 47 Radiation protection provision 28 Reactor core, etc. 48 Fuel handling & storage facilities 9 Decommissioning 29 Fuel rod and assembly 49 Auxiliary systems 10 Postulated initiating events 30 Protection against flow blockage 50 Power conversion system 11 Design bases accidents 31 Reactivity control system 51 Emergency response facilities and equipment 12 Design extension conditions 32 Reactor protection system 52 Intermediate cooling system 13 Safety classes and standards 33 Use of computerized system 53 Liquid sodium handling system 14 External events design bases 34 Diverse protection system 54 Sodium heating system 15 Fire protection 35 Reactor coolant boundary 55 Protection against sodium freezing 16 Design bases for environmental effects 36 Reactor cooling system 56 Purification control of cover gas and supply 17 Reliability 37 Overpressure protection 57 Operating experiences and safety research 18 Sharing of facilities 38 Residual heat removal system 19 calibration / test / inspection/ maintenance 20 Startup, shutdown, and low power operation 58 Limiting conditions for operation 39 Ultimate heat sink 59 Initial tests 40 Reactor containment, etc. new requirements to be added, we have referenced the international documents like IAEA SSR-2/1, Safety Design Criteria of GIF and draft version of SFR GDC under development by ANS. Fukushima action items and applicability of Risk Informed Regulation(RIR) are also considered. Utilizing this strategy and process, we have developed a draft version of SFR GSR containing 59 articles. The title of the articles are listed in Table 1. 3. Development of OPT for SFR reactivity control safety function The OPT is a top-down method with a tree structure for each DID level describing objectives and barriers, safety function, challenges to maintain safety functions, mechanisms of safety function degradation, and provisions for each degradation or failure mechanisms to maintain safety functions. Reference [2] describes conceptually how to apply this methodology to development of safety requirements for innovative reactors, specifically for the modular high temperature gas cooled reactors. In general, we have three safety functions to fulfill the safety objectives, i.e., control of reactivity, core heat removal and containment integrity. Among these three safety functions, we have developed the OPT for the safety function of “reactivity control”. Because the design of PGSFR is not mature yet, we have developed the OPT modelling the KA- LIMER-600 [4] reactor which is conceptually designed by KAERI and is an SFR of 600 MWe size. OPT is a qualitative methodology whose development relies mainly on experiences of experts using the design documents like probabilistic safety assessment report. Because the SFR GSR we are developing is a general one which should not be reactor or design specific, we have developed the OPT for KALIMER even if the target reactor to apply the GSR in reviewing is the PGSFR. The detailed description of the system is not included in this paper since it is not necessary to understand the developed OPT. Example of the Level 3 OPT we have developed for the safety function of “reactivity control” is shown in Figure 2. In Figure 2, safety function means the essential function necessary to ensure the safety objectives by maintaining DID and barrier integrity. Challenge is the phenomenon which threatens the successful achievement of the safety function and the possible challenges to the safety function Energy Policy, Economy and Law Assessment of General Safety Requirements for SFR ı Namduk Suh, Moohoon Bae, Yongwon Choi, Bongsuk Kang and Huichang Yang
atw Vol. 60 (2015) | Issue 1 ı January | | Fig. 2. Developed Level 3 OPT for “reactivity control” safety function. are dealt with by the provisions (inherent characteristics, safety margins, systems, provisions). Mechanism is defined as “specific reason, process or situation whose consequence might create challenge to the performance of safety function.” [3] 4. Completeness assessment of the developed general safety requirements The draft GSR developed, referencing the LWR GDC, SDC of GIF RSWG and ANS GDC for SFR is assessed from the DID point of view utilizing the OPT. The process is mainly to identify and check whether the mechanisms in the OPT are included in the draft GSR. In Figure 2, we have 4 challenges to the safety function of “reactivity control” and 6 mechanisms that could induce the challenges. For example fuel cladding chemical interaction (FCCI) is a mechanism that could induce the change in core geometry. Because the GSR is a top level requirement enforced by law, the contents are rather qualitative description, so the prevention of this mechanism and the ensuing challenge needs to be translated into requirements of GSR. On the other hand, how to achieve this mechanism should be handled in specific design by provisions, so the contents of provisions in Figure 2 need to be described in a lower level regulatory documents like review guides. The assessment whether the requirement to prevent this mechanism is included in the GSR or not is done as following: • Mechanism; Fuel-cladding chemical interaction. • Assessment; The paragraph (3) of Article 25 “reactor design” in our draft GSR stipulates that “the property change by the irradiation of the main core structure materials like control rod driving mechanism, core support structure etc. should not impair the structural integrity”. Thus the paragraph of Article 25 is the requirement to prevent mechanism of “fuel-cladding chemical interaction”. • Result; Requirement to prevent the FCCI is well implemented in the draft GSR. Another example we present is the mechanism of “control rod withdrawal from subcritical state” that challenges ‘the inability to maintain subcriticality”: • Mechanism; Control rod withdrawal from subcritical state. • Assessment; The subparagraph 5 under the paragraph 2 of Article 32 “reactor protection system” stipulates that “the protection system shall be designed to assure that the specified acceptable fuel design limits are not exceeded for any single malfunction of the reactivity control systems such as an accidental withdrawal of control rods.” Thus, the subparagraph of Article 35 is the requirement to prevent the mechanism of “control rod withdrawal from subcritical state”. • Result; Requirement to prevent the control rod withdrawal from subcritical state is well implemented in the draft GSR. In this way, we have confirmed that the draft GSR has all the requirements to prevent the challenges and mechanisms for the “reactivity control” safety function. We found no requirements to be added or revised by assessing this “reactivity control” safety function, but we expect that we might be able to find some requirements to be revised or added by assessing this way and it might confirm the utility of our approach. We found that utilizing the OPT in this way is a systematic and integral way to complement the development of GSR. We will continue to work for other two safety functions in a future. 5. Conclusion The draft version of GSR for SFR was developed, first by evaluating the applicability of the current LWR GSR to SFR and then taking into account other international requirements for SFR. The current requirements including the LWR GDC are coming from the long-year accumulated experiences of licensing and operation, but there are not enough experiences for SFR, so even if it is possible to develop a draft version of SFR GSR referencing the currently available requirements, there is need of a systematic and integral methodology to complement the developed GSR for SFR. The application of OPT is a good way to achieve the requirements. So the OPT have been developed for a safety function of reactivity control and applied in complementing the draft GSR. Assessing the completeness of the GSR in view of DID concept utilizing the OPT, it was found that the draft GSR includes all the requirements necessary to prevent the mechanisms which could challenge the safety function. The developed GSR will be applied in licensing review of PGSFR under ENERGY POLICY, ECONOMY AND LAW 29 Energy Policy, Economy and Law Assessment of General Safety Requirements for SFR ı Namduk Suh, Moohoon Bae, Yongwon Choi, Bongsuk Kang and Huichang Yang
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