Structure, System and Component Designs of JSFR in relation to Sodium Chemical Reaction Issues. Yasushi Okano Japan Atomic Energy Agency (JAEA)

Transcription

1 Structure, System and Component Designs of JSFR in relation to Sodium Chemical Reaction Issues Yasushi Okano Japan Atomic Energy Agency (JAEA) 1

2 Contents Introduction SDC Criteria & Paragraphs General Approach to Sodium related hazard Sodium-Water Reaction Phenomena, Potential effects, Design measures Sodium Fire Phenomena, Potential effects, Design measures Remarks 2

3 Sodium Chemistry Related Exp. Operation Experiences & Experiments Troubles, including sodium leaks and sodium-water reactions, had been experiencedand solved from the early stage of SFR development ca The troubles, incl. sodium-water reaction and sodium fire phenomena, had been thoroughly investigated, and the causes of such troubles were clarified. In JAEA: Several sodium test facilities have/had been operated. 50MW-scale SG test loop, CCTL & PLANDTL sodium loops, AtheNa facility, SWAT facilities (Sodium Fire)9 Technical results are considered in the SFR designs: Monju, JSFR 3

4 SDC related to Sodium Chemical Reaction Criterion 17: Internal and external hazards Internal hazards: para The design shall take due account of internal hazards such as fire, explosion, flooding,9., and sodium chemical reaction with air, water and other materials, including associated pressure waves and product releases, e.g. hydrogen. Appropriate features for prevention and mitigation shall be provided to ensure that safety is not compromised. Criterion 42bis: Plant system performance of a sodium-cooled fast reactor (e) Sodium is chemically active and opaque, and it is solid below 98 ºC. (g) Due to chemical risk of sodium which burns in air and reacts with water, impact of such chemical reactions to items important to safety must be prevented. 4

5 SDC related to Sodium Chemical Reaction Criterion 47: Design of reactor coolant systems Para. 6.14bis. Inert gas shall be used as a cover gas in sodium-filled components to prevent chemical reaction at the free surface of sodium, Para. 6.14ter. Provisions shall be made to detect sodium leaks and to mitigate the consequence of sodium chemical reaction in case of postulated sodium leaks from the reactor coolant systems. 6.16ter. Chemical reactions between sodium and water/steam or other working fluids shall be considered in the design of the secondary coolant system. Provisions to prevent and/or mitigate such chemical reactions shall be incorporated in the design: (a) Provisions shall be made to detect leaks of working fluids,.. (b) A pressure relief system shall be employed in the secondary coolant system. (c) The fundamental safety functions shall be maintained under postulated design extension conditions for severe chemical reactions between the sodium and the working fluid. 5

6 SDC related to Sodium Chemical Reaction Criterion 74: Fire protection systems 6.54ter. Compartments with sodium components shall be protected from water ingress to prevent sodium-water chemical reactions, especially from water used in case of fire fighting in an adjacent compartment. 6

7 Safety Approach for Sodium-Chem. Fundamental Safety Approach Sodium reaction after leakage Internal Hazard Anther points: coolant inventory for cooling, containment function Prevent the progress of such effect to safety functions by minimizing the consequences of sodium water/air reaction Key for Gen-IV SFR system To ensure reliability of sodium contained SSC, based on the operation experiences; Prefer more preventive approach on sodium related hazard Incorporate the progresses of knowledge & R&D results in the design & assessment on sodium-water reaction and sodium fire; especially Design Extension Conditions and hazard initiators/propagations 7

8 Image of Safety Approach for Sodium-Chemical Reactivity Prevention Internal Hazards Internal Events Initiator: e.g. Leakage from piping/tube Mitigation Pressure release, Extinguishment, Catch pan (steel floor), etc Prevention e.g. Reactor coolant boundary function Mitigation 8

9 Sodium-Water Reaction Sodium reaction with water, producing H2, heat, and corrosive products. Steam generator (SG) tube functions: Boundary between water and sodium (of secondary coolant system) Major reactions 1 Na + H2O NaOH + H2 2Na + H2O Na2O + 2 Three types of phenomena and propagation - Categorized based on existing experiments and past accidents - H 2 Target wastage Multi-wastage via propagation 部 Pressure wave [pulse] Damaged 破損伝熱管 SG tube Water/steam 水 蒸気 H2 Adjacent tube 隣接管 Water/steam 水 蒸気 Wastage by high temperature reaction products Na ウェステージ Wastage Water/steam 水 / 蒸気破損伝熱管管外 : ナトリウム SG tube creep 伝熱管全体が expansion/rupture 高温にさらされる High 高温領域 temperature Pressure wave Reacting 反応部 part High 高 pressure 圧 力波圧力波圧Pressure wave SG tank 蒸気発生器胴 9

10 Sodium Leak Rate and Phenomena & (Water leak rate: e.g. 0.1 ~ 10 g/s) Adjacent tube damaged by erosion and corrosion in reaction flame Propagation Small-scale Water Leakage Mid-scale Water Leakage Large-scale Water Leakage (Water leak rate: e.g. 10 g/s ~ Several kg/s) Several tubes damaged by hightemperature during longer period (Water leak rate: more than several kg/s) Accumulated gas zone No wastage and overheating rupture Sodium Pass-through failure Sodium Water/Steam Reaction flame Reaction zone Failure tube Tube Inner pressure Water/ Steam Target wastage Overheating tube rupture Gas zone (e.g. hydrogen and steam) Water/Steam Water/Steam Sodium Water/Steam Pass-through failure Reaction flame Target wastage Water/Steam Sodium Water/Steam Sodium Water/Steam Pass-through 貫通欠陥 failure Water/Steam Large break: Reaction zone e.g. Double ended guillotine break Weakening Fast Reactor of Cycle Water/Steam Technology Development Project material strength 10

11 Effects on Secondary Coolant System (1) Damage progression (Thermal and chemical effects) Effect on adjacent tubes due to high temperature and corrosive water jet. Need to immediate control for termination, not to the failure propagate to adjacent and spread. (2) Mechanical effects on the secondary coolant system & components Initial spike-shaped pressure wave in a very short time, and quasi-steady pressure due to hydrogen and heat generation if sodium-water reaction continues. These pressures potentially become a risk to secondary coolant system and components, e.g. secondary main pipes, intermediate heat exchanger (IHX) tube. Pressure Initial spikeshaped pressure Time (milli-second) Rapid pressure generation and propagation Pressure wave 蒸気発生器力波圧力波圧Reacting part 反応部 High 高圧 pressure Pressure wave SG trunk 胴部Pressure Quasi-steady pressure ウェステージ 破損伝熱管 Outside 管外 : of ナトリウム tube: Sodium Time (Several tens of seconds) Multi-wastage and overheating Wastage Water/steam 水 / 蒸気 Damaged tube Whole SG tube is 伝熱管全体が exposed to high 高温にさらされる temperature High 高温領域 temperature 11

12 Potential Effects on Primary Coolant System (1) Progression to In-vessel event Potential effect on safety function via chemical, structural and neutronic effects, just in case that boundary between primary and secondary coolant system boundaries (e.g. IHX tube) fails due to long-/short-period pressure Gas ingress to the core region Could lead to reactivity insertion. Secondary pump To steam turbine Reaction products in primary coolant system Could lead to coolant flow blockage Could lead to material corrosion. If IHX is located near the core (e.g. pool type), pressure wave Could lead the configuration change of the reactor core Reactor vessel Built-in pump type IHX Failure of the primary/secondary boundaries From feed water heater Na-water reaction (SG tube rupture) Gas discharge Coolant flow blockage 12 SG

13 Potential Effects on Containment (2) Progression to In-/Out-Containment Via Thermal, chemical and mechanical effects Potential sodium fire, if the secondary coolant system & components damaged by e.g. pressure increase Potential contact of sodium and structural material (e.g. concrete) Note] An enclosure is applied so that sodium fire would not occur. Leaked liquid sodium is retained by the enclosure with inert atmosphere. Effects by Sodium-water reaction Hydrogen combustion Behaviour of reaction products Reactor vessel Built-in pump type IHX Secondary pump SG To steam turbine Sodium-water reaction (SG tube rupture) From feed water heater Failure of the Sodium fire * secondary boundary 13

14 Summary of L.L. from the Superheater(SG) accident in PFR PFR Pool type, 600MWt / 3 loops of Secondary circuit Superheatertube rupture propagation, 27 th February 1987 Initial failure (small trans-axial crack) in one tube, then subsequent tube failure by sodium-water reaction jet RelatedSSC Causes/Conditions Example of design resolutions Flow-induced vibration of tubes Fast steam dump system Hydrogen detection system Leak flow through gaps of central baffle No gap design (e.g. welding) Not installed in superheater Install in all Sodium-steam heat exchanger Out of order Operation rules(e.g. No operation if out-of-order) 14

15 Design Measures Abnormality detection, mitigation measures (e.g., reactor trip, water blow), pressure release (control) are essential. Diagram of example of measures for sodium-water reaction in JSFR Water leak signal Secondary system pump trip Water/steam isolation and blow Low pump trip: Reactor trip Steam isolation valve at SG outlet Turbine Steam release valve at SG outlet Environment Igniter Rupture disk Secondary pump : Cover gas pressure gauge Hydrogen meter : Rupture disk failure detection system Dump tank Separator Feed/bleed valve at SG inlet Blow tank Feed isolation valve at SG inlet Feed pump 15

16 Approach to DBA & DEC Design Basis Accident (DBA) Prevent failure of boundary between sodium and water Margin to external force by external hazards (i.e. earthquake) Mitigate measures to avoid a propagation e.g. flow vibration, thermal transient, aging degradation, steam and water pipe whip 9 Postulated initiator Random failure of tube = single tube failure in SG Design Extension Condition (DEC) Multiple failure (DBA + a loss of mitigation function) Object: Confirmation of no cliff edge effect. 16

17 Assessment and Criteria Assessment Key point: Spike-shaped pressure wave, Quasisteady pressure state, Propagation of tube failure. DBA] Conservative + Single Failure DEC] Best Estimate + Multiple failure Criteria [Same for DBA & DEC Integrity of boundaries (e.g. SG tube) between the primary and secondary coolant systems. Prevent significant damage of components in secondary coolant system. Integrity of reactor core and primary coolant system 17

18 Sodium Fire Type & Potential effect on CV Challenge to CV Assessment parameter Mode of Sodium leakage and fire Overpressure failure ofcontainment boundary Pressure in CV Large spray fire Localfailure of containmentboundary Temperature of CV steel Small/medium/large fire and following hydrogendeposition and combustion Aging degradation of CV Temperature of CV Large pool fire Sodium leakage mode Assessment parameter Large spray fire Small fire Large pool fire CV steel CV concrete Rapid temperature increase of CV atmosphere Rapid pressure increase of CV atmosphere Pressure in CV Local and longtime heating of CV steel CV steel failure (Sodiumconcrete reaction) Temperature of CV steel Temperature increase of CV concrete Loss of integrity of containment building Temperature of CV concrete and duration time 18

19 Measures to Sodium Fire Prevention of Hazard Sodium Coolant Boundary Double wall Primary Circuits for Loop type, Secondary Circuits for Loop & Pool types Limiting the break area in hypothetical guillotine break Operational issues still considered small leaks protected by steel liner, leak detection Mitigation of Hazard Early detection is indispensable (for primary & secondary circuits) Reduce/Eliminate the combustion Inner gas environment or injection in sodium-piping rooms and/or containment Reduce the amount of sodium leak Early Sodium Dorian (Cooling capability still to be assessed) For DEC, multi-failure to be considered 19

20 JSFR Designs Containment Boundary(SCCV) RV, Guard vessel, Doublewall pipe 1 次ポンプ 1 次アルゴンガス系 格納バウンダリ (SCCV) 蒸気 Adoption of SCCV 2 次ポンプ 2 次冷却系 1 次冷却系 格納容器空調系 Containment air-conditioning Air atmosphere Prevent sodium leak accident, Prevent deflagration/detonation of accumulated hydrogen which may be produced by sodium-concrete reaction Double boundary concept (double-walled primary & secondary pipes, a guard vessel for a reactor vessel) is adopted. 給水 20

21 Assessment: SPHINCS Code Sodium combustion evaluation was carried out by the SPHINCS code developed by JAEA. SPHINCS code is based on mechanical / phenomenological models to deal with sodium spray and pool combustion simultaneously. Code Validation with experiment (3m 3 steel vessel, leak rate of 12 kg/h) 21

22 Assessment & Measures against Sodium Fire in JSFR Sodium Fire Assessments Sodium Leakage from Primary Coolant System in Containment Sodium Leakage from Secondary Coolant System in Containment/Reactor Building Candidate measures against Sodium Leakage from Secondary Coolant System Double tube or Sodium Drain + Nitrogen Gas Injection + Catch pan etc. 22

23 Remarks Approach to Sodium Chemical Reaction Sodium reaction after leakage Internal Hazard Prevent the progress of the effects on safety functions by mitigating the consequences by sodium water/air reaction Key for Gen-IV SFR system Ensure reliability of sodium contained SSC More preventive approach to hazard occurrence Incorporate progresses of knowledge & R&D results; Hazard initiator, Hazard propagation9 DEC (i.e. multiple-failure) 23