Frédéric BERTRAND , Anne BASSI Dominique BARBIER, Patrick AUJOLLET et Pascal ANZIEU
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Transcript of Frédéric BERTRAND , Anne BASSI Dominique BARBIER, Patrick AUJOLLET et Pascal ANZIEU
ICHS, Pisa september 8-10 th 2005 Session production and storage (ref-210048) 1/18
Frédéric BERTRAND, Anne BASSIDominique BARBIER, Patrick AUJOLLET et Pascal ANZIEU
CEA (Commissariat à l’energie atomique), DEN (Nuclear Energy Division)
Massive Hydrogen Production with Nuclear Heating,Safety approach for coupling a VHTR
with a Iodine/Sulfur Process Cycle
ICHS, Pisa september 8-10 th 2005 Session production and storage (ref-210048) 2/18
OUTLINE
Economical and technical background
Presentation of the whole plant (coupled facilities)
Safety approach proposed
Implementation of defence in depth (DiD) to the whole plant
Conclusion
ICHS, Pisa september 8-10 th 2005 Session production and storage (ref-210048) 3/18
Economical and technical background
Investigation on energy production without fossil energy No release of green house effect gases Thermochemical Iodine/Sulfur (IS) cycle requiring a high temperature
supply possible with a VHTR
Other H2 production processes are also under investigation at CEA (HTE, Westinghouse cycle) in order to explore different solutions
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
Final objective : safety strategy for the whole plant and design of the coupling system taking into account safety constraints
ICHS, Pisa september 8-10 th 2005 Session production and storage (ref-210048) 4/18
Brief presentation of the whole plant (VHTR/HYPP)
Main reference assumptions
Nuclear power (600 MWth) fully devoted to H2 production
Around 10 H2 units (exact number still to determine)VHTR containment
Core IHX1 IHX2
H2 Unit 1
H2 Unit 2
H2 Unit 5
H2 Unit 3
H2 Unit 4
Overall couplingPartial coupling of each H2 unit
1000°C
400°C
He circulation
ICHS, Pisa september 8-10 th 2005 Session production and storage (ref-210048) 5/18
Presentation of VHTR and of IS process
Main VHTR features
Fuel : ceramic coated particlesModerator : GraphiteCoolant : helium (400/1000°C)Large thermal inertia : intrinsic feature improving safety
H2production process with IS Cycle
H2O H2 + ½ O2
Obtained by the sum of :
H2SO4 H2O + SO2 + ½ O2 (T > 850°C)
I2 +SO2 + 2H2O 2HI + H2SO4 (T ~ 100°C)
2HI H2 + I2 (T ~ 400°C)
ICHS, Pisa september 8-10 th 2005 Session production and storage (ref-210048) 6/18
Presentation of the safety approach (nuclear and conventional)
Nuclear safety approach Specificities
Fission product accumulation and decay heat to remove Short time constant for controlling the reactivity
Solutions retained Presence of successive physical barriers Main safety function to protect the barriers (scram to fast control of reactivity) Defence in depth (DiD) concept (implemented in 5 levels)
Prevention of incidents and accidents and limitation of their consequences Conventional industry approach
Main features Diversity of hazardous substances Diversity of accidental effects : toxics dispersion, pressure wave, heat flux, missiles,…
Solutions retained Presence of at least one barrier associated to safety distances 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 (DiD implicitly applied, and eventually SEVESO II Directive)
ICHS, Pisa september 8-10 th 2005 Session production and storage (ref-210048) 7/18
Presentation of the safety approach (VHTR/HYPP)
Main safety functions of the coupled facility
control of the nuclear reactivity and of the chemical reactivity extraction of the nuclear power, of the 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) confinement of hazardous substances : fission product and chemical substances
Concept of Defence in Depth (DiD)
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 DiD at subsequent levels
To prevent excessive loading of barriers
To protect the barriers
ICHS, Pisa september 8-10 th 2005 Session production and storage (ref-210048) 8/18
Level 1 : Prevention of abnormal operation and failures
Appropriate design rules Adapted to operating conditions and to chemical substances
Thermodynamical nominal conditions and possible transientsCorrosive substances (H2SO4, HI)Hydrogen embrittlementTritium and Hydrogen diffusion (purity of H2)
Solutions retainedMaterials foreseen to resist to corrosion (tantale, glass coated steels, ceramics, steel alloys,…) Barriers and/or purification system to prevent tritium from entering HYPPRule of the art regarding engineering sizing for nuclear and process industries
Provisions regarding parameter variations transmitted via the coupling system from HYPP to VHTR and vice versa
Conditions to fulfillKeeping the two facility in their normal operating domain
energy exchanges with controlled P, T, QControlled hot Helium T to HYPP Controlled cold Helium T to VHTR
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) 9/18
Level 2 : Control of abnormal operation
Objective
To avoid that an excursion out of normal operating domain propagate to other facility or degenerate from incident to accident
Abnormal operations could occur in nominal or transient regime Protection systems of level 3 must not be triggered at level 2
Solutions envisaged
Simulation of coupled facilities to assess dynamic behaviourDefinition of the limits of the normal operating domainAppropriate 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) 10/18
Level 2 : Control of abnormal operation
Control of abnormal operation occurring in HYPP
Initiating events
Increasing severity
Level 2 of DID
Level 3 of DID
Initial state
-Abnormal operation in H2 unit n1 And/or abnormal operation in H2 unit n2
…………
-Abnormal operation in HYPP and/or VHTR - failed H2 unit uncoupled
- Abnormal operation in VHTR and HYPP - uncoupled VHTR-HYPP
Uncoupling of H2 unit(s) out of normal operation domain
control of abnormal operation by means of regulating system
- Overall uncoupling of HYPP - normal shut down of HYPP
- Normal shut down of VHTR - normal shut down of HYPP if possible
Final state
Normal operation of coupled VHTR-HYPP
- Normal VHTR operation with small power redistribution - Reduced load HYPP operation - partially uncoupled VHTR-HYPP until repairs of H2 unit
Sub-level 1
Sub-level 2
Sub-level 3
Provisions Provisions
success
failure
failure
success
- Normal operation of VHTR with large power redistribution - HYPP stopped until repairs - uncoupled VHTR-HYPP
success
failure
success
failure
- VHTR stopped - HYPP stopped - uncoupled VHTR-HYPP - protection system have not been triggered
ICHS, Pisa september 8-10 th 2005 Session production and storage (ref-210048) 11/18
Level 2 : Control of abnormal operation
Control of abnormal operation occurring in VHTR
HYPP should be able to match fluctuations coming from VHTR
Due to high thermal inertia of VHTR core such an abnormal fluctuationshould be less probable than fluctuations induced by HYPP
Abnormal energy supply from Helium must be controlled to avoid :
Emergency shutdown of HYPP
Spontaneous stopping of H2SO4 decomposition
Solution envisaged
Prevention and control of fluctuations based on VHTR control system design
Three-way valves associated to ternary or secondary He recirculation loop
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
Objectives 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 reach of a safe withdrawal state (safety functions fulfilled durably) uncoupled state of the facilities
Fulfillment of safety functions Nuclear and chemical reactivity
Emergency shutdown of VHTR (Control rod insertion)
Emergency shutdown of HYPP (cutoff of reactors feedings + inerting)
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) 13/18
Level 3 : Control of accidents progression and limitation of their consequences
Fulfillment of safety functions Confinement function
protection against external aggressions
dynamic confinement and double walls
isolating procedure for leaking part of circuit
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 DiD. Therefore it must
include redundancies and high reliability (classified ?) equipments
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
Accidents relating to level 3 of DiD, prevention and protection measures
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
Particular case of cumulated rupture of IHX1 and IHX2
Depressurizing wave resulting from a breach on He circuit could induce simultaneous breaches in IHX1 and IHX2 due to high temperature and pressure difference
ICHS, Pisa september 8-10 th 2005 Session production and storage (ref-210048) 15/18
Level 3 : Control of accidents progression and limitation of their consequences
Accidents relating to level 3 of DiD, prevention and protection measures
Particular case of cumulated rupture of IHX1 and IHX2
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
VHTR containment
Core IHX1 IHX2
H2 Unit 1
H2 Unit 2
H2 Unit 5
H2 Unit 3
H2 Unit 4
1000°C
400°C
A
A’
B B’
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
Objectives and accidents relating to level 4 of DiD Despite upstream levels of DiD, 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
VHTR containment
Core IHX1 IHX2
H2 Unit 1
H2 Unit 2
H2 Unit 5
H2 Unit 3
H2 Unit 4
1000°C
400°C
C
B B’ B
A
Hazardous releases, Impact on containment
O2 leak
ICHS, Pisa september 8-10 th 2005 Session production and storage (ref-210048) 17/18
Level 4 : Control of severe plant conditions and mitigation of severe accidents consequences
Investigation required to settle level 4 provisions 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)
Possible provisionsReduction of energetic ignition sourcesAbsence of confinement and obstacles (pipe agglomerate) to avoid flame accelerationInerting or igniting systems in containmentVenting systems, physical barrier between VHTR and HYPP (deflectors, distance, etc)Grounding of coupling system and/or VHTRTraining of rescue teams and internal emergency plans
Level 5 off-site response still to define
ICHS, Pisa september 8-10 th 2005 Session production and storage (ref-210048) 18/18
CONCLUSIONS
A safety approach based on the DiD has been proposed for the coupling of a VHTR with a hydrogen production plant by IS thermochemical cycle
Extension of main safety functions adopted in nuclear reactors to the VHTR/HYPP coupled facilities
The coupling system has been identified as an essential part of the safety architecture
It takes a part of successive levels of DiDIt contributes to fulfill the main safety functions
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 DiD