Nuclear Power Plants (NPPs)

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1 (NPPs) Laboratory for Reactor Physics and Systems Behaviour Weeks 1 & 2: Introduction, nuclear physics basics, fission, nuclear reactors Critical size, nuclear fuel cycles, NPPs (CROCUS visit) Week 3: Neutronics (reactor physics design) + Reactor heat transfer (fuel rod) Week 4: Reactor heat transfer (cladding, coolant) + Time-dep. reactor behaviour Week 5: Long-term reactivity changes + Principal types of nuclear power plants Week 6: Environmental aspects, nuclear safety, advanced systems Long-term reactivity changes + Principal NPP types.. 1

2 Summary, Week 4 Axial temperature variations: max. fuel, cladding temperatures not far from core centre 2 nd technological constraint: cladding temp. for chemical reactions with coolant (corrosion) Convective heat transfer between clad and coolant 3 rd technological constraint: critical heat flux (DNB) All 3 constraints have to be satisfied fixes maximal power density at hot spot Neutronics provides local power values, considerable incentive to flatten neutron flux Different types of reactivity variations, need to keep k eff = 1 Reactor kinetics: importance of delayed neutrons, ρ needs to be << β (for +ive ρ) Short-term reactivity variations: fuel temp., moderator temp., void (liquid coolant) Reactivity coeffs. (feedbacks) need to be negative Medium-term reactivity variations: effects of Xe 135 Long-term reactivity changes + Principal NPP types.. 2

3 Long-term Reactivity Effects Laboratory for Reactor Physics and Systems Behaviour ρ-variation of largest magnitude in power reactor, also slowest (not really, kinetics ) Fuel composition changes with burnup (fuel evolution ) Determines, for given initial ρ-excess, max. burnup achievable (from neutronics viewpoint) Involved phenomena: Consumption of fissile material (U 235, ) Production of new fissile material from fertile (Pu 239 from U 238, etc.) Appearance of non-fissile nuclides such as U 236 ( also transuranics ) Accumulation of stable FP s All the above (except 2 nd ) cause ρ One needs to determine the fuel composition as function of irradiation time, via Fuel Evolution Equations (Bateman Equations) Neutronics analysis, corresponding to each different reactor state, gives new values for: k eff, ρ-coefficients, control rod worths, power distribution, etc. Long-term reactivity changes + Principal NPP types.. 3

4 Fuel Evolution Equations For thermal reactor burning enr. U : Even if power is constant, Φ(t) varies because of variation of Σ f Preferable to consider the variable fluence (time-integrated flux): Equation for U 235 becomes with solution: (units: cm -2 ) (analogy with radioactive decay θ : time, σ a5 : decay constant) Considering other reactions, one has for the other nuclides N.B.: For the fissiles, σ a = σ c + σ f etc., etc. Long-term reactivity changes + Principal NPP types.. 4

5 Comments For the FP s, fuel evolution equations more complex Radioactive decay chains also need to be considered Only few FP s need to be treated explicitly Fuel burnup: (contributions of all fissile isotopes to be considered) Average values: One can express all parameters ( k eff, ρ-coefficients, etc.) in function of W sp Long-term reactivity changes + Principal NPP types.. 5

6 Example LWR fuel ~ 3.4% U enr W sp ~ 30,000 MWd/t (today, > 4% enr, W sp ~ 50,000 MWd/t) Solution of Fuel Evolution Equations gives In example, ~ 30% of fissions in Pu (in-situ) For a U nat reactor (CANDU) can be ~ 50% Pu- quality at discharge (~ 70% fissile) poor for nuclear explosive ( civil Pu ) For production of military Pu, one needs to strongly reduce W sp (< 5000 MWd/t) Nuclear power plants too costly for this (one uses cold power reactors) Long-term reactivity changes + Principal NPP types.. 6

Consequences for Reactor Control Laboratory for Reactor Physics and Systems Behaviour For considered example (U enr, LWR): For a U nat reactor (CANDU, ): N.B.: Scales different Large reactivity variation in LWR case demands partial charging, discharging of core e.

7 Consequences for Reactor Control Laboratory for Reactor Physics and Systems Behaviour For considered example (U enr, LWR): For a U nat reactor (CANDU, ): N.B.: Scales different Large reactivity variation in LWR case demands partial charging, discharging of core e.g. with 3 segments (zones) in the equilibrium situation: 1 new fuel (at t = 0) 2 fuel with one cycle of residence in core 3 fuel with 2 cycles of residence Long-term reactivity changes + Principal NPP types.. 7

8 Consequences for Control (contd.) Laboratory for Reactor Physics and Systems Behaviour After time T/3, one discharges Segment 3 and displaces the fuel between the zones Reactivity variation: Long-term reactivity changes + Principal NPP types.. 8

9 Comments Reactivity variation reduced by factor of ~ 3 (in case considered) Initial enrichment needed ( as also control ), significantly reduced Fuelling, refuelling needed more frequently (but each time quantity ~ 1/3) In the limit, one can have continuous refuelling (not possible in LWR ) One can profit also from flux flattening Material properties deteriorate with irradiation One has technological constraints to maximum burnup in LWRs (U enr ) For systems using U nat, e,g, CANDU, neutronics provides principal constraint Long-term reactivity changes + Principal NPP types.. 9

10 Means of Control Control rods (of different types) Compensation rods, e.g. for Xe-buildup, cold-to-hot, etc. Higly absorbing materials (B 4 C, Ag-In-Cd, ) Pilot rods, for power regulation, automatic piloting, etc. (low ρ-worths, steel often used) Safety rods, (normally withdrawn, fall rapidly in accidental situation ( scram ), B 4 C, etc. Soluble poison (liquid moderator/coolant) For long-term effects, adjustable concentration (H 3 BO 3 in PWRs) Not used in BWRs (influence too large on α v ) Reduces need for compensation control rods (advantage also of better power distribution) Requires special chemical processes and control Burnable poison Solid, strong absorbers (Gd, B, ), mixed with a certain fraction of fuel rods (BWRs, ) Disappears (is burnt) during irradiation, with density reduction: Again, mainly for long-term effects (fuel evolution) Can be optimised to flatten ρ-curve (cf. Slide 28) Long-term reactivity changes + Principal NPP types.. 10

11 Reactor Instrumentation, Power Regulation Laboratory for Reactor Physics and Systems Behaviour Power of NPP known via thermal balance for coolant, bur power regulation needs rapid monitoring (possible via neutron flux measurements, with prior calibration ) Neutron detectors: fission chambers, BF 3 counters, etc. (with, without γ-compensation) Start-up chains (< 10-6 P 0 ) Minimal count-rate necessary at start (sensitive detectors, external n-source if necessary ) Logarithmic chains (~ 10-7 P 0 to P 0 ) Several decades covered, current mode; derivative provides period measurement Linear chains (~ 10-2 P 0 to 10 P 0 ), for piloting reactor Allow fine regulation; feedback loop (connected to pilot rods) and servo-mechanism Safety chains, for triggering insertion of safety rods (often in mode 2-out-of-3 ) Fixed criteria, e.g. P 1.15 P 0, T T min, N.B.: Detectors often in reflector; however NPPs also have in-core instrumentation, e.g. series of miniature chambers, which can provide a detailed flux map. Long-term reactivity changes + Principal NPP types.. 11

12 Principal NPP Types About 440 NPPs worldwide 370 GWe (14% electricity) Switzerland: 5 NPPs 3 GWe (40% electricity) LWRs dominate > 85% of all NPPs Manufacturer NPPS GWe Various manufacturers over the years Main players in current renaissance : - Areva (Framatome ), Westinghouse, General Electric Long-term reactivity changes + Principal NPP types.. 12

13 LWRs: Pressurized Water Reactor (PWR) Laboratory for Reactor Physics and Systems Behaviour Indirect cycle (boiler: steam generator) Long-term reactivity changes + Principal NPP types.. 13

14 LWRs: Boiling Water Reactor (BWR) Laboratory for Reactor Physics and Systems Behaviour Direct cycle (boiler: reactor itself) Long-term reactivity changes + Principal NPP types.. 14

15 LWRs: General Characteristics Laboratory for Reactor Physics and Systems Behaviour Characteristics of the steam produced, very similar in the two cases η th 33 to 35% Large numbers, e.g. typically 270 m 3 moderator/coolant, flow rate of 14 t/s, 8 MWe pumps, Very different arrangements of components within reactor pressure vessel (RPV) Long-term reactivity changes + Principal NPP types.. 15

16 RPV Internal Arrangements Laboratory for Reactor Physics and Systems Behaviour PWR BWR Separation of water entering and exiting core External pumps Water at core exit separated from steam and mixed with feed-water (supplementary circuit needed internally jet pumps) For the steam: separator, dryer Long-term reactivity changes + Principal NPP types.. 16

17 PWR Primary Circuit Steam Generator - Primary coolant in U-tubes - Steam separators, dryer on top Pressurizer - Liquid/vapour mixture - Heater/spray for pressure control Long-term reactivity changes + Principal NPP types.. 17

18 BWR Pressure Vessel: Inside View Laboratory for Reactor Physics and Systems Behaviour In spite of lower pressure, cost of BWR not any less Complexity of pressure vessel internals Supplementary circuits, etc. Control rods enter from bottom Steam separators, dryer, etc. on top Helps flatten axial flux shape Long-term reactivity changes + Principal NPP types.. 18

19 LWR Fuel Assemblies PWR: A 15x15 B&W Fuel Assembly - 17X17 indicated in table Long-term reactivity changes + Principal NPP types.. 19

20 LWR Fuel Assemblies (contd.) Laboratory for Reactor Physics and Systems Behaviour BWR Smaller assemblies, each in own channel box Boiling within channels Single-phase (liquid) by-pass flow between boxes Cruciform-shaped control rods positioned in by-pass water gaps Complex fuel lattice (different enrichments, use of burnable poison (Gd), etc.) to flatten flux distribution Long-term reactivity changes + Principal NPP types.. 20

21 Modern BWR Assemblies Increased heterogeneity Water holes, part-length rods, etc. Higher enrichments, more poisoned pins for achieving higher burnups Water holes Part-length rods Inner water channels Water boxes Long-term reactivity changes + Principal NPP types.. 21

22 Control Rods PWR: Control-rod cluster - rods come in from above BWR: Cruciform control blades - come in from bottom - Other means of control soluble boron in coolant (PWRs), burnable poisons in fuel rods (BWRs) Long-term reactivity changes + Principal NPP types.. 22

23 Automatic control (limited possibilities) Laboratory for Reactor Physics and Systems Behaviour The negative α m in a PWR gives it the character of a load follower If demand, steam valve opens Pressure in steam generator, temperature also (T sat ) Heat flux (primary-to-secondary), T mod $ Reactivity, power system follows demand (Finally, T c, T m, power stabilizes at new level) If demand, one has the reverse effects Power quasi-automatically In the case of a BWR, the cycle is direct If demand, pressure but this time in reactor Boiling, power (α v negative) A possible remedy: coupling to the feed-water pumps If demand, pump speed is increased Void, hence power (stabilizes at new level) Long-term reactivity changes + Principal NPP types.. 23

24 Heavy Water Reactors Due to large migration area (M 2 ) of moderator, large size of reactor core Need to have system employing pressure tubes (for coolant at high pressure) Moderator itself can be at low (atmospheric) pressure As coolant, one has various possibilities Organic liquid, CO 2, H 2 O, D 2 O, Canadians built prototypes with each of these Most successful commercially proved to be D 2 O Allowed use of natural U System: CANDU CANadian Deuterium Uranium Long-term reactivity changes + Principal NPP types.. 24

25 CANDU Schematic Secondary circuit as in PWR Large number of fuel/coolant channels Possibility of on-line fueling Headers necessary to collect coolant flowing into and out of individual channels Long-term reactivity changes + Principal NPP types.. 25

26 Typical Characteristics (199t D 2 O coolant, 263t moderator Gentilly) Long-term reactivity changes + Principal NPP types.. 26

27 CANDU Fuel Bundle Power density (ϖ d ) 10 times lower than in PWR (ϖ l ) max nevertheless comparable ϖ l = V. (ϖ d /N) Cross-sectional area of unit cell No. of rods per fuel bundle Burnup 8,000 MWd/t Better than PWR with 40,000 (1kg of 3-4% U enr 6kg U nat ) Long-term reactivity changes + Principal NPP types.. 27

28 Gas-Cooled Reactors (HTR) HTR is latest type of gas-cooled reactor (moderator: graphite), preceded by: Magnox with U nat fuel and CO 2 as coolant First commercial nuclear power station: Calder Hall (UK), 1956 AGR (Advanced Gas-cooled Reactor), developed in 1960 s (steel cladding, U enr ) Much higher temperatures achievable with HTR (> 700 C) Made possible by use of unique fuel type ( coated particles no metallic cladding) (Apart from use of gas as coolant, helium in his case no boiling crisis, etc.) Energy conversion efficiency high (η th > 40% for electricity production) Opens new possibilities for nuclear energy utilization High temperature heat for chemical processes, e.g. H 2 production Long-term reactivity changes + Principal NPP types.. 28

29 HTR (contd.) Fuel Cycle At beginning, reference fuel cycle was HEU (high enriched uranium), i.e. U 235 as fissile, Th 232 as fertile (coated particles of different diameters, 200/500µ) Today, reference is LEU (U 235 <20%, due to non-proliferation considerations) Two HTR types developed Block type: prismatic (hexagonal) graphite blocks with separate holes for gas flow and fuel (graphite pellets containing coated particles) Pebble-bed type: graphite pebbles (6cm diameter) with central region containing coated particles; fresh pebbles from top, burnt pebbles discharged from bottom Today, main attraction of HTRs: inherent or passive safety characteristics Low power density, single-phase coolant, negative α m, large thermal inertia, ceramic fuel/cladding, etc. Long-term reactivity changes + Principal NPP types.. 29

HTR Fuel Block Pebble (200-500 µm) column of pellets

30 HTR Fuel Block Pebble ( µm) column of pellets Long-term reactivity changes + Principal NPP types.. 30

31 Block-type HTR Long-term reactivity changes + Principal NPP types.. 31

32 Pebble-Bed HTR Hundreds of thousands of pebbles in core of large HTR Currently, concept of modular systems, e.g. 4 x 250 MWe, instead of a single 1000 MWe NPP, more popular Commercial interest in certain countries, e.g. South Africa Long-term reactivity changes + Principal NPP types.. 32

33 Fast Breeder Reactors Main aim: utilisation of fertile materials (much more abundant) U 238 Pu 239, Th 232 U 233 (produce more new fissile than is destroyed) Breeding (η >2) easiest to achieve with fast neutrons (η of Pu ) Th-U 233 breeder possible with thermal neutrons (η of U ) Fast systems: Liquid Metal Fast Breeder Reactor (LMFBR), Gas-Cooled Fast Reactor (GCFR) Thermal systems: Molten Salt Breeder Reactor (MSBR), Light Water Breeder Reactor (LWBR) Industrial realisation has been mainly of LMFBRs, with various prototypes BN-350, 1972 (Russia); Phénix, 1973 (France); PFR, 1974 (UK), Monju, 1995 (Japan) Second LMFBR phase in 80s BN-600, Russia, 600 MWe, 1981 still operating successfully Superphénix. France, 1200 MWe, 1983 shutdown in 1998 (mainly economic reasons) Long-term reactivity changes + Principal NPP types.. 33

34 Sodium-Cooled Fast Breeders Laboratory for Reactor Physics and Systems Behaviour Several advantages of sodium as coolant Excellent thermal-hydraulics characteristics (power density can be >600 kw/litre!) Boiling temp. 880 C at atmospheric pressure (low risk of pipe ruptures at low p) Primary coolant temperatures of 600 C possible (energy conversion 40%) Disadvantages Chemical reactions: Na / H 2 O, Neutron activation: Na / air (sodium fires) Melting temperature 98 C (sodium circuit needs to be heated up at beginning) Because of radioactive Na in primary system, 3 circuits necessary Primary (Na), intermediate (Na), tertiary (H 2 O) Risk of Na/H 2 O reaction in core (or with radioactive Na in steam generator) eliminated Long-term reactivity changes + Principal NPP types.. 34

35 LMFBR Types Loop type Easier access to components (pumps, heat exchangers, etc.) for maintenance work Similar circuitry as in PWR, but at low pressure Certain disadvantages Pool type Large thermal inertia of the Na provides enhanced safety Radioactive Na in primary circuit, well confined Long-term reactivity changes + Principal NPP types.. 35

36 Superphénix (Pool-type LMFBR) Long-term reactivity changes + Principal NPP types.. 36

37 Phénix, Superphénix Characteristics Laboratory for Reactor Physics and Systems Behaviour Long-term reactivity changes + Principal NPP types.. 37

38 Plutonium Burners Today, too much plutonium (produced in LWRs) For appropriate control on Pu-inventories, one needs flexibility, even for breeders Not difficult to reduce conversion factor to <1, e.g. replace fertile blankets by reflectors of structural materials For Pu-burning, however, much higher Pu-enrichment needed for fuel in core ( 45%) Important to ensure that reactivity coefficients remain adequate, e.g. α c ( U 238!) Superphénix, before its shutdown (1998), was a large research instrument for research on advanced fuel cycles ( actinide management ) In principle, LWRs (responsible for current Pu-excess ) can also serve as Pu-burners MOX (mixed-oxide, i.e. PuO 2 /UO 2 ) fuel assemblies replace part of UO 2 fuel Use of inert-matrix Pu-fuel UO 2 in MOX replaced by e.g. ZrO 2 + poison, no new Pu produced Long-term reactivity changes + Principal NPP types.. 38

39 Summary, Week 5 Long-term reactivity variations Changes in fuel composition with burnup largest k eff variation, but slow (not safety related) Means of control, instrumentation Different types of NPPs (electricity production) LWRs dominate current scene PWR (primary-side pressure 150 bars) indirect cycle, steam produced on secondary side BWR (pressure 75 bars) direct cycle, steam produced directly in core Gas-cooled reactors HTR, most recent type (unique fuel concept: coated particles) Fast breeders Neutron spectrum kept fast (no moderator), η Pu sufficiently >2 Sodium-cooled systems (LMFBR) relatively well developed Long-term reactivity changes + Principal NPP types.. 39

Workshop on PR&PP Evaluation Methodology for Gen IV Nuclear Energy Systems. Tokyo, Japan 22 February, Presented at

PR&PP Collaborative Study with GIF System Steering Committees A Compilation of Design Information and Crosscutting Issues Related to PR&PP Characterization Presented at Workshop on PR&PP Evaluation Methodology