HTGR Safety Design Fundamental Safety Functions Safety Analysis Decay heat removal Criticality

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1 HTGR Safety Design Fundamental Safety Functions Safety Analysis Decay heat removal Criticality Frederik Reitsma IAEA Course on High temperature Gas Cooled Reactor Technology Oct 22-26, 2012

2 Content / Overview The GEN-IV safety goals Fundamental safety functions Inherent safety philosophy Stability principle of reactor unit design Safety assessment Safety analysis Treatment of uncertainties Selected results SSE Detection of inadvertent CR insertion Water ingress Concluding remarks Oct 22-26, 2012 IAEA Course on High temperature Gas Cooled Reactor Technology 2

3 Generation IV Goals Sustainability 1.Generate energy sustainably, and promote long-term availability of nuclear fuel 2.Minimize nuclear waste and reduce the long term stewardship burden Safety & Reliability 3.Excel in safety and reliability 4.Have a very low likelihood and degree of reactor core damage 5.Eliminate the need for offsite emergency response Economics 6.Have a life cycle cost advantage over other energy sources 7.Have a level of financial risk comparable to other energy projects Proliferation Resistance & Physical Protection 8.Be a very unattractive route for diversion or theft of weapons-usable materials, and provide increased physical protection against acts of terrorism Oct 22-26, 2012 IAEA Course on High temperature Gas Cooled Reactor Technology 3

4 Any design must: Safety Functions ensure the performance of safety functions prevent, limit, control, or mitigate the consequences of anticipated operational occurrences and postulated accidents. The Molular pebble bed reactor designs includes a combination of inherent and passive safety features rely more on inherent characteristics and passive design features that prevent significant fission product transport from the fuel, core, and reactor pressure boundary Oct 22-26, 2012 IAEA Course on High temperature Gas Cooled Reactor Technology 4

5 Design for SAFETY must demonstrate: Fundamental Safety Functions Reactivity Control Heat Removal Confine Radioactivity Oct 22-26, 2012 IAEA Course on High temperature Gas Cooled Reactor Technology 5

6 Engineered SAFETY or the preferred approach, Inherent Safety. Oct 22-26, 2012 IAEA Course on High temperature Gas Cooled Reactor Technology 6

7 Inherent safety philosophy Safety does not rely on engineered systems that may fail but on the inherent design and the laws of physics. Increased safety: smaller plant that allows for reactor cooling by passive heat transfer mechanisms following an accident prevents the fuel temperatures from increasing to levels where significant radioactive fission products can be released from the fuel and thus eventually into the atmosphere, the type of accident that is most feared by the public. Oct 22-26, 2012 IAEA Course on High temperature Gas Cooled Reactor Technology 7

8 Inherent safety design characteristics Large negative temperature feedback effects Automatic shutdown with loss of coolant Strong negative temperature coefficient limits reactivity excursions Reactivity Control Low excess reactivity On-line reloading Excess reactivity only to overcome power changes / load follow Reactivity Design functionalities Reactor shutdown during operation and maintaining sub-criticality for cold conditions Reactivity control during operation and daily load-follow Damped xenon oscillations, also for all operator actions Inherently safe features during operation and licensing events Fuel storage sub-criticality for fresh, used and spent fuel Oct 22-26, 2012 IAEA Course on High temperature Gas Cooled Reactor Technology 8

9 Power (%) Temperature (C) Automatic reactor shutdown Safety demonstration: Stop coolant flow and no control rod movements Reactivity Control Total Power (%) Fission Power (%) Average Fuel Temperature Average Moderator Temperature Time (seconds) Oct 22-26, 2012 IAEA Course on High temperature Gas Cooled Reactor Technology 9

10 Automatic power reduction Reactivity Control Safety demonstration at HTR-10 reactor during HTR conference in Beijing, September 2004 Events: 1. Single control rod withdrawn 2. Coolant circulation stopped 3. Power initially increases due to reactivity insertion but then decreases 4. Automatic shutdown due to reactivity feedback effects (increased temperatures) Oct 22-26, 2012 IAEA Course on High temperature Gas Cooled Reactor Technology 10

11 Automatic power reduction Safety demonstration at HTR-10 reactor during HTR conference in Beijing, September 2004 Reactivity Control Oct 22-26, 2012 IAEA Course on High temperature Gas Cooled Reactor Technology 11

12 Inherent safety design characteristics Heat Removal Passive heat removal post-shutdown decay heat removal is achievable through conduction, natural convection and radiation heat transfer, due to the core geometry, low power density of the core and high thermal capacity of the core structures Needs low power density! Centre Reflector Pebble Bed Side Reflector Core Barrel RPV RCCS Citadel Conduction Radiation Conduction Conduction Radiation Convection Convection Conduction Radiation Conduction Radiation Convection Convection Convection Conduction Radiation Oct 22-26, 2012 IAEA Course on High temperature Gas Cooled Reactor Technology 12

13 Decay heat after fission stops Passive heat removal Heat Removal * Heat removal under all reactor operation conditions and events * Two active heat removal systems: - Self-sustained PCU-Brayton thermodynamic cycle / Circulator and SG - Core Conditioning System (CCS) used during maintenance or Helium Purification system * Passive heat transfer from the core to the outer heat sink during loss of forced cooling * CCS designed as defense-in-depth to keep the core at normal operating temperatures during upset conditions Illustration of Fuel Temperatures behaviour for a DLOFC Event over 60 days Oct 22-26, 2012 IAEA Course on High temperature Gas Cooled Reactor Technology 13

14 Fuel containing radio-active fission products Confine Radioactivity Adequate Confinement of Radioactivity is ensured by: - High-quality ceramic coated-particle fuel of proven design - Sufficient Heat Removal Oct 22-26, 2012 IAEA Course on High temperature Gas Cooled Reactor Technology 14

15 Fuel containing radio-active fission products Confine Radioactivity Fuel elements with multi-coated fuel particles are used for optimum retention of fission products The silicon carbide layer has the ability to contain fission products Can withstand very high temperatures. Control radio-nuclides primarily at its source (within the coated particles) Oct 22-26, 2012 IAEA Course on High temperature Gas Cooled Reactor Technology 15

16 Additional barriers to fission products and radioactive releases Confine Radioactivity * Transport of radioactivity through two main mechanisms: - Neutrons and gammas originating at the reactor and activation - Radio-nuclides in the coolant having escaped from the fuel spheres Barriers: The pressure boundary, building, suppression pool, filters, etc IAEA Course on High temperature Gas Oct 22-26, Cooled Reactor Technology

17 Safety Functions - Recall The most important inherent characteristics of the PBMR which contribute to the fulfilment of the fundamental safety functions are: A fuel and core design with a low excess reactivity and an overall negative temperature coefficient of reactivity sufficient to accommodate any foreseeable reactivity insertions during start-up and power operations without damage to the fuel. A core design that ensures that post-shutdown decay heat removal is achievable through conduction, natural convection and radiation heat transfer, due to the core dimensions, low power density of the core and high thermal capacitance of the core structures. Peak temperatures remain below the structural design limits, and the fuel temperature is kept below the limit where serious degradation of the coated particles would lead to a significant activity release. High-quality ceramic coated-particle fuel of proven design, which adequately retains its ability to confine radioactive fission products over the full range of operating and accident conditions. Oct 22-26, 2012 IAEA Course on High temperature Gas Cooled Reactor Technology 17

18 Four Principles of Stability Incorporated Into Modular HTGR Design Core may never melt or be overheated to unallowable temperature Nuclear transients may never lead to unallowable power output excursions or cause unallowable fuel element overheating Fuel elements may never be allowed to corrode excessively Thermal stability Nuclear stability Chemical stability Reactor cannot melt, practically no release of fission products, catastrophe-free nuclear energy Core may never be allowed to deform or change composition Mechanical stability Oct 22-26, 2012 IAEA Course on High temperature Gas Cooled Reactor Technology 18

19 Large heat capacity leads to slow reactor thermal response The large thermal capacity of the core internals allows relatively fast load changes of the system without requiring fast response from the core the energy stored in the core can be withdrawn or stored with minimum core temperature changes Reactor geometry facilitates heat removal to heat sinks Heat sink can work in passive mode for long time The large negative temperature coefficient results in the reactivity and consequently the neutronic power changing to counteract temperature changes The reactor is nearly self-regulating minimum of control interaction is required to maintain the reactor outlet temperature at a given value No operator intervention is required for a few hours Oct 22-26, 2012 IAEA Course on High temperature Gas Cooled Reactor Technology 19

20 Some examples Safe-shutdown earthquake initial conservative study Detection of inadvertent inserted control rod Water ingress Rankine cycle Largely excluded by design in the Brayton cycle Oct 22-26, 2012 IAEA Course on High temperature Gas Cooled Reactor Technology 20

21 Pebble Beds and earthquakes The impact of earthquakes on the PBMR design is investigated as part of the safety case Shaker-table experiments (SAMSON) located at the HRG (Hochtemperatur-Reaktorbau GmbH) site at Jülich, Germany used to postulate conservative compaction densities and times for use in the safety studies Focus of this study: compaction of the pebble-bed or fuel region only no radial disturbance in the core cavity dimensions - excluded by the core structure and graphite reflector design change in the bulk or average packing density during an earthquake Study core-neutronics and thermal-hydraulics behaviour of a postulated SSE Oct 22-26, 2012 IAEA Course on High temperature Gas Cooled Reactor Technology 21

22 SAMSON Facility SAMSON experiments at 0.4 g > (5 seconds) > (15 seconds) Oct 22-26, 2012 IAEA Course on High temperature Gas Cooled Reactor Technology 22

23 SSE postulated event Reactivity increase due to: Denser packing of fuel spheres Reduction of control rod effectiveness (decreased pebble bed height) The two major phenomena: neutronic response of the fuel due to the bed compaction (streaming, leakage, spectrum changes, temperature feedback) changes in the heat transfer (pebble bed packing fraction, reduced core height) This is an excellent case for multi-physics solvers Core compaction calculation Neutronics effect / feedback Thermal hydraulic effect / feedback Structural analysis: mechanical failures / stress analysis Oct 22-26, 2012 IAEA Course on High temperature Gas Cooled Reactor Technology 23

24 PBMR400 SSE postulated event Postulate: Only effect is pebble bed compaction Decrease in pebble bed or core effective height Very conservative assumptions for concept design Packing fraction increases: i) > 0.62 ii) > 0.64 No control rod movement Compaction duration: i) 5 seconds ii) 15 seconds typical range for the duration of strong shaking that results from large earthquakes Includes a PLOFC and DLOFC (beyond design base) Reactivity increase due to: Denser packing of fuel spheres Reduction of control rod effectiveness Oct 22-26, 2012 IAEA Course on High temperature Gas Cooled Reactor Technology 24

25 Fission power (SSE + PLOFC) (1 st very conservative results) Oct 22-26, 2012 IAEA Course on High temperature Gas Cooled Reactor Technology 25

26 Core average fuel sphere temperatures (SSE + PLOFC) Oct 22-26, 2012 IAEA Course on High temperature Gas Cooled Reactor Technology 26

27 Power (%) Actual SSE results: SSE Fission Power as a % of the Steady State Values (0 s to 30 s) with RPS Trip Initiated on the Reactor Power Control rod insertion begins at 1.73 s as a result of the power SCRAM set point Time (s) Oct 22-26, 2012 IAEA Course on High temperature Gas Cooled Reactor Technology 27

28 Detection of inadvertent RCS/RSS insertion Detection of a single control rod chain failure / insertion or SAS insertion during normal operation. Reactivity effects is significant but might be compensated by the rest of the bank. Question: Is detection by the 3 azimuthally dependent sets of detectors possible? Oct 22-26, 2012 IAEA Course on High temperature Gas Cooled Reactor Technology 28

29 Fast flux (>0.1MeV) [n.cm-2.s-1] on R=185.5 cm Detection of inadvertent RCS/RSS insertion Noticeable difference in the line-of-sight fast fluxes / neutron leakage from the core To be fully illustrated for RCSS positions in-between detectors Case: CR2-drop: One control rod dropped of CB2 (lower) to 990 cm x x Axial height [cm] from top of VSOP model Theta [radians] Oct 22-26, 2012 IAEA Course on High temperature Gas Cooled Reactor Technology 29

30 Water ingress DPP450 Oct 22-26, 2012 IAEA Course on High temperature Gas Cooled Reactor Technology 30

31 Mass flow rate [kg/s] Water mass in RCS [kg] Mass flow rate [kg/s] Water mass in RCS [kg] 5.4 HTR-MODUL Single SG Tube Rupture in Lower SG Plenum With Mitigation Fig.1.1. Water Ingress Rate From Both Nozzles of Ruptured SG Tube HTR-MODUL Single SG Tube Rupture in Upper SG Plenum With Mitigation Fig.2.1. Water Ingress Rate From Both Nozzles of Ruptured SG Tube Feedwater header side Steam header side Mass of water in RCS Feedwater header side Steam header side Mass of water in RCS Time [min] Time [min] 0 The amount of water that enters RCS when a SG tube is ruptured near the feedwater header is 92 [kg], (Fig.1.1). This is significantly less than the ingress when the SG tube is severed near the steam outlet header, 220 [kg], (Fig.2.1). Reverse flow, from SG shell side into the SG the ruptured tube, is caused by cooling the fluid in the SG tubes. Oct 22-26, 2012 IAEA Course on High temperature Gas Cooled Reactor Technology 31

32 Power [%] Reactivity [$] Power [%] Reactivity [$] 110 HTR-MODUL Single SG Tube Rupture in Lower SG Plenum With Mitigation Fig.1.2. Reactor Power and Core Reactivity HTR-MODUL Single SG Tube Rupture in Upper SG Plenum With Mitigation Fig.2.2. Reactor Power and Core Reactivity Reactor power Reactivity of core Reactor power Reactivity of core Time [sec] Time [sec] Reactor scram signal Moisture > 800 [vpm] is generated in Case #1 at time T O [s] and in Case #2 at time T O [s]. Control rods drop delayed until T O +11 [sec]. A rupture of SG tube near the feedwater header results in adding a larger amount of positive reactivity (Fig.1.2), compared to the case when a SG tube is cut near the steam header (Fig.2.2) when hot vapor with temperature of 530 [ o C] is mixed with helium of 250 [ o C]. Mild power rise, no fuel overheating, no challenge to fuel integrity The reactor power does not increase at a rate that will pose any threat of rapid fuel overheating and its consequent failure to retain fission products. 32

33 Other safety aspects Decay heat DIN standard with uncertainties given Applicable to pebble-bed reactors Derived for OTTO cycle but later shown to be adequate also for MEDUL cycles by FZJ If the decay heat is explicitly calculated pay attention to spectrum effects The SCALE procedure will not work. Ex-core criticality Same approach can be followed as for LWRs margin to k-eff uncertainties criticality handbook benchmarks limited Burnup credit (bit more tricky) Remember that the spectrum to perform burnup is not the spectrum if the fuel Cannot use the standard SCALE procedure Oct 22-26, 2012 IAEA Course on High temperature Gas Cooled Reactor Technology 33

34 Conclusions HTGRs has advanced safety characteristics passive safety achieved by the inherent design characteristics. The design displays the safety characteristics of future nuclear plants Follow the HTR-Modul safety philosophy. Oct 22-26, 2012 IAEA Course on High temperature Gas Cooled Reactor Technology 34

35 Thank you! IAEA Course on High temperature Gas Cooled Reactor Technology Oct 22-26,

36 Source material used: HTGR Technology Course for the Nuclear Regulatory Commission, May 24 27, 2010 HTR/ECS 2002 High temperature Reactor School, 2002 MUA 784: Reactor Physics, F Reitsma, Mechanical Engineering Post-Graduate: Nuclear Theme, University of Pretoria, 2012 Advanced Reactor Concepts Workshop, PHYSOR 2012 Workshop at PHYSOR 2010 Advances in Reactor Physics to Power the Nuclear Renaissance: The Pebble Bed Modular Reactor: From V.S.O.P. (Very Superior Old Product) to Generation IV candidate. Creation of the equilibrium core PBMR ORIGEN-S cross section library, C.C. Stoker, F. REITSMA and Z. Karriem, HTR2004. Oct 22-26, 2012 IAEA Course on High temperature Gas Cooled Reactor Technology 36