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Massive Hydrogen Production with Nuclear Heating, Safety approach for coupling a VHTR with a Massive Hydrogen Production with Nuclear Heating, Safety approach for coupling a VHTR with a Iodine/Sulfur Process Cycle Frédéric BERTRAND, Anne BASSI Dominique BARBIER, Patrick AUJOLLET et Pascal ANZIEU CEA (Commissariat à l’energie atomique), DEN (Nuclear Energy Division) frederic. [email protected] fr ICHS, Pisa september 8 -10 th 2005 Session production and storage (ref-210048) 1/18

OUTLINE ü Economical and technical background ü Presentation of the whole plant (coupled facilities) OUTLINE ü Economical and technical background ü Presentation of the whole plant (coupled facilities) ü Safety approach proposed ü Implementation of defence in depth (Di. D) to the whole plant ü Conclusion ICHS, Pisa september 8 -10 th 2005 Session production and storage (ref-210048) 2/18

Economical and technical background ü Investigation on energy production without fossil energy ü No Economical and technical background ü Investigation on energy production without fossil energy ü No release of green house effect gases ü Thermochemical Iodine/Sulfur (IS) cycle requiring a high temperature supply possible with a VHTR Other H 2 production processes are also under investigation at CEA (HTE, Westinghouse cycle) in order to explore different solutions ü Safety approach taking into account nuclear safety constraints and conventional industry safety constraints as well Main safety principles : progressiveness, homogeneity, diversity and safety architecture built to face all kind of risks in the whole plant ü ICHS, Pisa Final objective : safety strategy for the whole plant and design of the coupling system taking into account safety constraints september 8 -10 th 2005 Session production and storage (ref-210048) 3/18

Brief presentation of the whole plant (VHTR/HYPP) ü Main reference assumptions H 2 Unit Brief presentation of the whole plant (VHTR/HYPP) ü Main reference assumptions H 2 Unit 1 1000°C IHX 1 Core IHX 2 H 2 Unit 3 400°C H 2 Unit 2 H 2 Unit 4 H 2 Unit 5 VHTR containment He circulation Overall coupling Partial coupling of each H 2 unit Nuclear power (600 MWth) fully devoted to H 2 production Around 10 H 2 units (exact number still to determine) ICHS, Pisa september 8 -10 th 2005 Session production and storage (ref-210048) 4/18

Presentation of VHTR and of IS process ü Main VHTR features Fuel : ceramic Presentation of VHTR and of IS process ü Main VHTR features Fuel : ceramic coated particles Moderator : Graphite Coolant : helium (400/1000°C) Large thermal inertia : intrinsic feature improving safety ü H 2 production process with IS Cycle ICHS, Pisa september 8 -10 th 2005 Session production and storage (ref-210048) 5/18

Presentation of the safety approach (nuclear and conventional) ü Nuclear safety approach ü Specificities Presentation of the safety approach (nuclear and conventional) ü Nuclear safety approach ü Specificities ü Fission product accumulation and decay heat to remove ü Short time constant for controlling the reactivity ü Solutions retained ü Presence of successive physical barriers ü Main safety function to protect the barriers (scram to fast control of reactivity) ü Defence in depth (Di. D) concept (implemented in 5 levels) Prevention of incidents and accidents and limitation of their consequences ü Conventional industry approach ü Main features ü Diversity of hazardous substances ü Diversity of accidental effects : toxics dispersion, pressure wave, heat flux, missiles, … ü Solutions retained ü Presence of at least one barrier associated to safety distances ü Assessment of safety distances resulting from scenario calculations of major representative accidents ; scenarios selected according their likelihood and their severity Prevention of incidents and accidents and limitation of their consequences (Di. D implicitly applied, and eventually SEVESO II Directive) ICHS, Pisa september 8 -10 th 2005 Session production and storage (ref-210048) 6/18

Presentation of the safety approach (VHTR/HYPP) üMain safety functions of the coupled facility To Presentation of the safety approach (VHTR/HYPP) üMain safety functions of the coupled facility To prevent excessive loading of barriers To protect the barriers ücontrol of the nuclear reactivity and of the chemical reactivity üextraction of the nuclear power, of thermal power (heat release by chemical reactions, phase changes) and of the mechanical power (compressors, pumps, pressure wave associated to phase changes or very rapid gas expansion due to heat release) üconfinement of hazardous substances : fission product and chemical substances üConcept of Defence in Depth (Di. D) Hierarchical deployment of different levels of equipment and procedures in order to maintain the effectiveness of physical barriers if the provisions of a level fails to control the evolution of a sequence, the subsequent level will come into play the levels are intended to be independent as far as possible the general objective is aimed to prevent that a single failure at a level or even combinations of failures at different levels propagate and jeopardize Di. D at subsequent levels ICHS, Pisa september 8 -10 th 2005 Session production and storage (ref-210048) 7/18

Level 1 : Prevention of abnormal operation and failures ü Appropriate design rules ü Level 1 : Prevention of abnormal operation and failures ü Appropriate design rules ü Adapted to operating conditions and to chemical substances ü ü Thermodynamical nominal conditions and possible transients Corrosive substances (H 2 SO 4, HI) Hydrogen embrittlement Tritium and Hydrogen diffusion (purity of H 2) ü Solutions retained ü Materials foreseen to resist to corrosion (tantale, glass coated steels, ceramics, steel alloys, …) ü Barriers and/or purification system to prevent tritium from entering HYPP ü Rule of the art regarding engineering sizing for nuclear and process industries ü Provisions regarding parameter variations transmitted via the coupling system from HYPP to VHTR and vice versa ü Conditions to fulfill ü Keeping the two facility in their normal operating domain energy exchanges with controlled P, T, Q ü Controlled hot Helium T to HYPP Controlled cold Helium T to VHTR ü Possible solutions matching coupled system behaviour Phase changing temperature control (steam generator of JAERI) Cold source of variable power for normal starting and shutdown transients ICHS, Pisa september 8 -10 th 2005 Session production and storage (ref-210048) 8/18

Level 2 : Control of abnormal operation ü Objective ü To avoid that an Level 2 : Control of abnormal operation ü Objective ü To avoid that an excursion out of normal operating domain propagate to other facility or degenerate from incident to accident ü Abnormal operations could occur in nominal or transient regime ü Protection systems of level 3 must not be triggered at level 2 ü Solutions envisaged ü Simulation of coupled facilities to assess dynamic behaviour ü Definition of the limits of the normal operating domain ü Appropriate design of control system of the whole facility Scram of VHTR must be avoided ICHS, Pisa september 8 -10 th 2005 Session production and storage (ref-210048) 9/18

Level 2 : Control of abnormal operation ü Control of abnormal operation occurring in Level 2 : Control of abnormal operation ü Control of abnormal operation occurring in HYPP ICHS, Pisa september 8 -10 th 2005 Session production and storage (ref-210048) 10/18

Level 2 : Control of abnormal operation ü Control of abnormal operation occurring in Level 2 : Control of abnormal operation ü Control of abnormal operation occurring in VHTR ü HYPP should be able to match fluctuations coming from VHTR ü Due to high thermal inertia of VHTR core such an abnormal fluctuation should be less probable than fluctuations induced by HYPP ü Abnormal energy supply from Helium must be controlled to avoid : ü Emergency shutdown of HYPP ü Spontaneous stopping of H 2 SO 4 decomposition ü Solution envisaged ü Prevention and control of fluctuations based on VHTR control system design ü Three-way valves associated to ternary or secondary He recirculation loop ICHS, Pisa september 8 -10 th 2005 Session production and storage (ref-210048) 11/18

Level 3 : Control of accidents progression and limitation of their consequences üObjectives of Level 3 : Control of accidents progression and limitation of their consequences üObjectives of level 3, assuming that despite provisions of previous level, accidents can occur Remark : the accidents assumed here should be controlled within the design basis conditions and should not induce large leakages through the ultimate barrier nor induce significant domino effects control of accidents ü reach of a safe withdrawal state (safety functions fulfilled durably) ü uncoupled state of the facilities üFulfillment of safety functions ü Nuclear and chemical reactivity ü Emergency shutdown of VHTR (Control rod insertion) Emergency shutdown of HYPP (cutoff of reactors feedings + inerting) ü Power extraction Radiative and conductive extraction (cooled screens) of DH for VHTR Pressure venting and equipment cooling in case of reaction runaway ICHS, Pisa september 8 -10 th 2005 Session production and storage (ref-210048) 12/18

Level 3 : Control of accidents progression and limitation of their consequences üFulfillment of Level 3 : Control of accidents progression and limitation of their consequences üFulfillment of safety functions ü Confinement function protection against external aggressions dynamic confinement and double walls isolating procedure for leaking part of circuit ü Role of the coupling system regarding safety functions plays a role of barrier between the plant and the atmosphere and between VHTR and HYPP (IHXs wall and coupling/decoupling gates) permits to control reactivity and extract power via VHTR/HYPP interfacial control and regulation of common parameters) Coupling system contributes to fulfill safety functions and is involved at least in level 1 to 3 of Di. D. Therefore it must include redundancies and high reliability (classified ? ) equipments ICHS, Pisa september 8 -10 th 2005 Session production and storage (ref-210048) 13/18

Level 3 : Control of accidents progression and limitation of their consequences üAccidents relating Level 3 : Control of accidents progression and limitation of their consequences üAccidents relating to level 3 of Di. D, prevention and protection measures ü Main accidents considered loss of supporting systems (electric, pneumatic, products evacuation) failure or rupture of coupling system as an initiating event DBA in VHTR limited leakage without ignition in HYPP Prevention and protection : Stand-by support systems to foresee (loss prevention) Leak detection and equipment designed to prevent ignition of mixtures emergency shutdown of VHTR and HYPP and uncoupling of VHTR and HYPP ü Particular case of cumulated rupture of IHX 1 and IHX 2 Depressurizing wave resulting from a breach on He circuit could induce simultaneous breaches in IHX 1 and IHX 2 due to high temperature and pressure difference ICHS, Pisa september 8 -10 th 2005 Session production and storage (ref-210048) 14/18

Level 3 : Control of accidents progression and limitation of their consequences ü Accidents Level 3 : Control of accidents progression and limitation of their consequences ü Accidents relating to level 3 of Di. D, prevention and protection measures ü Particular case of cumulated rupture of IHX 1 and IHX 2 Breach A or A’ : risk of corrosive and flammable substances ingress in VHTR containment Breach B or B’ : risk of radioactive materials ingress in HYPP Provisions aimed to control such accidents to avoid that they degenerate in severe accidents Emergency insulation gates of the coupling system (independent from others) Simulation of those accidents as DBA to determine reliability allocation for safety systems and IHXs Inerting provisions in the containment ICHS, Pisa september 8 -10 th 2005 Session production and storage (ref-210048) 15/18

Level 4 : Control of severe plant conditions and mitigation of severe accidents consequences Level 4 : Control of severe plant conditions and mitigation of severe accidents consequences ü Objectives and accidents relating to level 4 of Di. D ü Despite upstream levels of Di. D, severe accidents are considered here low probability sequences including multiple failures Complementary provisions are elaborated in order to limit the consequences of severe accidents, especially regarding the integrity or the by-pass of the last barrier : containment of VHTR, last wall and safety distance for HYPP (regarding VHTR and regarding the surrounding) Provisions to limit consequences of Domino effects due to the proximity of VHTR and HYPP ICHS, Pisa september 8 -10 th 2005 Session production and storage (ref-210048) 16/18

Level 4 : Control of severe plant conditions and mitigation of severe accidents consequences Level 4 : Control of severe plant conditions and mitigation of severe accidents consequences ü Investigation required to settle level 4 provisions ü ü Support studies to perform in order to assess the consequences of severe accidents and to verify if the probabilities/consequences permit to reach safety objectives Sizing of VHTR containment to a external pressure wave (less pessimistic approach than TNT equivalent method possibly to foresee) ü Possible provisions ü Reduction of energetic ignition sources ü Absence of confinement and obstacles (pipe agglomerate) to avoid flame acceleration ü Inerting or igniting systems in containment ü Venting systems, physical barrier between VHTR and HYPP (deflectors, distance, etc) ü Grounding of coupling system and/or VHTR ü Training of rescue teams and internal emergency plans to define ICHS, Pisa september 8 -10 th 2005 Level 5 off-site response still Session production and storage (ref-210048) 17/18

CONCLUSIONS ü A safety approach based on the Di. D has been proposed for CONCLUSIONS ü A safety approach based on the Di. D has been proposed for the coupling of a VHTR with a hydrogen production plant by IS thermochemical cycle ü Extension of main safety functions adopted in nuclear reactors to the VHTR/HYPP coupled facilities ü The coupling system has been identified as an essential part of the safety architecture ü It takes a part of successive levels of Di. D ü It contributes to fulfill the main safety functions ü Investigations (simulation end tests) are needed to understand the behaviour and the accidents of the coupled facilities and to design safety systems (coupling) and barriers taking into account accidents relating to each level of Di. D ICHS, Pisa september 8 -10 th 2005 Session production and storage (ref-210048) 18/18