CANDU Reactor & Reactivity Devices

Transcription

1 CANDU Reactor & Reactivity Devices B. Rouben UOIT Nuclear Plant Systems & Operation NUCL-5100G 2016 Jan-Apr 2016 January 1

2 Contents We study the CANDU reactor and its reactivity devices in CANDU reactors, and their uses. Some specifics are for the CANDU 6, but they apply in a similar way to other CANDUs January 2

3 A Nuclear Generating Station 2016 January 3

4 Components of a Nuclear Plant What are the basic components of a nuclear generating station? They consist of the nuclear reactor and the Balance of Plant. The reactor must contain: Nuclear fuel Coolant (Heat-Transport System) Moderator (in thermal reactors only) Control and Shutdown Mechanisms cont d 2016 January 4

5 Components of a Nuclear Plant The Balance of Plant must contain: One or more Steam Generators (Boilers) to turn water into steam (unless the primary coolant is turned into steam in the reactor itself, and unless a gas coolant is used) A Turbine-Generator to turn mechanical energy into electricity Connections to the outside electrical grid January 5

6 CANDU-6 Plant Reactor Containment Building Turbine Building 2016 January 6

7 Main CANDU Reactor Systems Reactor Assembly Fuel and Fuel Channel Heat Transport System Shutdown Cooling System Pressure and Inventory Control System Moderator System Special Safety Systems 2016 January 7

8 CANDU Reactor face 2. Reactor coolant pump 3. Steam generator 4. Fuelling machine carriage 5. Moderator heat exchanger 6. Dousing water system 7. Dousing water tank January 8

9 CANDU-6 Reactor Vault 2016 January 9

10 CANDU-9 Reactor Vault 2016 January 10

11 Reactor Face End Fittings and Feeders 2016 January 11

12 CANDU 6 Heat Transport System 2016 January 12

13 Reactor Assembly The reactor assembly contains the reactor core and the reactivity control devices. Major components of the reactor assembly are: Calandria Vessel End-Shields Shield Tank Fuel Channels Reactivity Control Devices 2016 January 13

14 Fuel-Channel Arrangement Heavy-Water Coolant Heavy 2016 January 14

15 CANDU-9 Fuel-Channel Heavy 2016 January 15

16 Fuel-Channel Arrangement Heavy 2016 January 16

17 Fuel-Channel Spacers 2016 January 17

18 CANDU 37 Element Fuel Bundle 2016 January 18

19 CANDU-6 Reactor Assembly 2016 January 19

20 Reactivity Management Every reactor needs ways of controlling and manipulating reactivity in both the positive and the negative directions. Here we will look specifically at CANDU reactivity management January 20

21 Long-Term Reactivity Control For long-term maintenance of reactivity, refuelling is required because reactivity eventually decreases as fuel is irradiated: fission products accumulate and total fissile content decreases. In the CANDU 6 reactor, the average refuelling rate is ~ 2 channels per Full-Power Day (FPD), using the 8- bundle-shift refuelling scheme (8 new bundles pushed in channel, 8 irradiated bundles pushed out). 4-bundle-shift and 10-bundle-shift refuelling schemes have also been used in other CANDUs. Selection of channels is the job of the station physicist January 21

22 Fuelling machines at both ends of the reactor remove spent fuel, insert new fuel January 22

23 Reactor Regulating System The reactivity devices used for control purposes by the Reactor Regulating System (RRS) in the standard CANDU-6 design are the following: 14 liquid-zone-control compartments (with H 2 O) 21 adjuster rods (24 in CANDU-9) 4 mechanical control absorbers moderator poison January 23

24 Special Safety Systems There are in addition two spatially, logically, and functionally separate special shutdown systems (SDS): SDS-1, consisting of 28 cadmium shutoff rods which fall into the core from above (32 in CANDU-9) SDS-2, consisting of high-pressure poison injection into the moderator through 6 horizontally oriented nozzles. Each shutdown system can insert > 50 mk of negative reactivity in approximately 1 s. Next Figure summarizes the reactivity worths and reactivity-insertion rates of the various CANDU reactivity devices January 24

25 REACTIVITY WORTH OF CANDU-6 REACTIVITY DEVICES (Table Credit for CANDU-6: CANTEACH Document , Fig. 2.1) Function Device Total Reactivity Worth (mk) Maximum Reactivity Rate (mk/s) Control Control Control 14 Zone Controllers 21 Adjusters (24 in CANDU-9) 4 Mechanical Control Absorbers (17 in CANDU-9) (driving) (dropping) Control Moderator Poison (extracting) Safety Safety 28 Shutoff Units (32 in CANDU-9) 6 Poison-Injection Nozzles ~ January 25

26 CANDU Reactivity Devices All reactivity devices are located interstitially between rows of calandria tubes (see next Figure), i.e., in guide tubes permanently positioned in the low-pressure moderator environment. There exists no mechanism for rapidly ejecting any of these rods, nor can they drop out of the core. This is a distinctive safety feature of the pressure-tube reactor design. Maximum positive reactivity insertion rate achievable by driving all control devices together is about 0.35 mk/s, well within the design capability of the shutdown systems. See Plan, Side, and End views of device locations in following Figures January 26

27 Interstitial Positioning of Reactivity Devices Here, a vertical device is shown in its interstitial location half-way between a horizontal fuel channel and its neighbour January 27

28 Plan View of CANDU-6 Reactivity Device Locations (Figure Credit: CANTEACH Document , Fig. 2.3) 2016 January 28

29 Plan View of CANDU-9 Reactivity Device Locations 2016 January 29

30 Side-Elevation View of CANDU-6 Reactivity-Device Locations (Figure Credit: CANTEACH Document , Fig. 2.4) 2016 January 30

31 End-Elevation View of CANDU-6 Reactivity-Device Locations (Figure Credit: CANTEACH Document , Fig. 2.5) 2016 January 31

32 End-Elevation View of CANDU-9 Reactivity-Device Locations 2016 January 32

33 Liquid Zone Controllers For fine control of reactivity: 14 zone-control compartments, containing variable amounts of light water (H 2 O used as absorber!) The water fills are manipulated: all in same direction, to keep reactor critical for steady operation, or to provide small positive or negative reactivity to increase or decrease power in a controlled manner differentially, to shape 3-d power distribution towards desired reference shape Note: in the ACR, zone controllers will be mechanical, not water compartments 2016 January 33

34 Liquid Zone-Control Units (Figure Credit: CANTEACH Document , p. 90) 2016 January 34

35 Liquid Zone-Control Units 2016 January 35

36 Liquid Zone Level Control System 2016 January 36

37 Liquid Zone-Control Compartments (Figure Credit: CANTEACH Document , p. 91) ************** ************** 2016 January 37

38 Mechanical Control Absorbers For fast power reduction: 4 mechanical absorbers (MCA), tubes of cadmium sandwiched in stainless steel physically same as shutoff rods. The MCAs are normally parked fully outside the core under steady-state reactor operation. They are moved into the core only for rapid reduction of reactor power, at a rate or over a range that cannot be accomplished by filling the liquid zone-control system at the maximum possible rate. Can be driven in pairs, or all four dropped in by gravity following release of an electromagnetic clutch January 38

39 X = Mechanical Control Absorbers (Figure Credit: CANTEACH Document , p. 93) 2016 January 39

40 Adjuster Rods When refuelling unavailable (fuelling machine down ) for long period, or for xenon override: 21 adjuster rods (24 in CANDU-9), made of stainless steel or cobalt (to produce 60 Co for medical applications). Adjusters are normally in-core, and are driven out (vertically) when extra positive reactivity is required. The reactivity worth of the complete system is mk. Maximum rate of change of reactivity for 1 bank of adjusters is < 0.1 mk per second. The adjusters also help to flatten the power distribution, so that more total power can be produced without exceeding channel and bundle power limits. Some reactor designs (Bruce A) have no adjusters January 40

41 Top View Showing Adjuster Positions (CANDU-6) (Figure Credit: CANTEACH Document , p. 94) 2016 January 41

42 Face View Showing Adjuster Positions (CANDU-6) (Figure Credit: CANTEACH Document , p. 95) 2016 January 42

43 Adjuster Rod Configuration (CANDU-9) 2016 January 43

44 Role of Adjuster Rods to Shape the Neutron Flux, Optimize Reactor Power and Fuel Burn-up (CANDU-9) 2016 January 44

45 Withdrawing Adjuster Rods to Provide Positive Reactivity and to Compensate for Xenon (CANDU-9) 2016 January 45

46 Moderator Poison Moderator poison is used to compensate for excess reactivity: in the initial core, when all fuel in the core is fresh, and during and following reactor shutdown, when the 135 Xe concentration has decayed below normal levels. Boron is used in the initial core, and gadolinium is used following reactor shutdown. Advantage of gadolinium is that burnout rate compensates for xenon growth January 46

47 CANDU Special Shutdown Systems Two independent, fully capable shutdown systems: SDS-1 (rods enter core from top) SDS-2 (injection of neutron poison from side) Figure credit: CANTEACH Document , Safety Functions Shutdown Systems, by V.G. Snell, January 47

48 SDS-1 SDS-1: 28 shutoff rods (32 in CANDU-9), tubes consisting of cadmium sheet sandwiched between two concentric steel cylinders. The SORs are inserted vertically into perforated circular guide tubes which are permanently fixed in the core. See locations in next Figure. The diameter of the SORs is about 113 mm. The outermost four SORs are ~4.4 m long, the rest ~5.4 m long. SORs normally parked fully outside core, held in position by an electromagnetic clutch. When a signal for shutdown is received, the clutch releases and the rods fall by gravity into the core, with an initial spring assist January 48

49 Top View of CANDU-6 Core, Showing Shutoff- Rod Positions (SA 1 28) (Figure Credit: CANTEACH Document , Fig. 2.7) 2016 January 49

50 Shutdown Rod Configuration (CANDU-9) 2016 January 50

51 CANDU-9 Reactivity Control Devices 2016 January 51

52 SDS-2 SDS-2: high-pressure injection of solution of gadolinium into the moderator in the calandria. Gadolinium solution normally held at high pressure in vessels outside of the calandria. Concentration is ~8000 g of gadolinium per Mg of heavy water. Injection accomplished by opening high-speed valves which are normally closed. When the valves open, the poison is injected into the moderator through 6 horizontally oriented nozzles that span the core (see next Figure). Nozzles inject poison in four different directions in the form of a large number of individual jets. Poison disperses rapidly throughout large fraction of core January 52

53 Positions of Liquid-Poison-Injection Nozzles (CANDU-6) (Figure Credit: CANTEACH Document , Fig. 2.8) 2016 January 53

54 END 2016 January 54