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)

[email protected]

Massive Hydrogen Production with Nuclear Heating,Safety approach for coupling a VHTR

with a Iodine/Sulfur Process Cycle


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


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


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


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)


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)


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


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


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



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



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


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



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



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



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’


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


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


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

Frédéric BERTRAND , Anne BASSI Dominique BARBIER, Patrick AUJOLLET et Pascal ANZIEU

Documents

Transcript of Frédéric BERTRAND , Anne BASSI Dominique BARBIER, Patrick AUJOLLET et Pascal ANZIEU