CANDU Reactor Safety Features

• Chapter 2 is an overview of reactor and moderator systems within CANDU nuclear power plants.
• It covers the use of natural uranium fuel bundles.
• It highlights the horizontal calandria and pressure tube design.
• It details the employment of heavy water for moderation and cooling, with circuits kept separate.

Reactor Assembly: Functional Requirements

• The calandria is a large, horizontal cylinder containing the low-pressure moderator
• Pressure tubes run through the calandria, housing the fuel, the high-pressure heavy water coolant, and enabling online refuelling.
• Reactivity control mechanisms, and vertical in-core flux measuring devices enter the calandria from above
• Ion chambers and liquid poison injection nozzles are horizontally mounted, and they enter the calandria from the side
• End shields and concrete vault walls offer structural support and radiation shielding.
• The main job of the calandria is to hold the fuel channels and contain the moderator, in order to facilitate a controlled nuclear fission chain reaction for heat production.
• The calandria shell is enclosed and supported at each end by end shields and the walls of the reactor vault support these components.
• Pressurized heavy water coolant removes the heat generated, and it flows around and through the fuel bundles.
• Each fuel channel holds 12 fuel bundles. Coolant flow is bi-directional. The CANDU 6 reactor contains 380 fuel channels; the CANDU 9 reactor has 480.
• Zirconium pressure tubes connect to stainless steel end fittings at the ends of the fuel channels to allow for mechanical connections to the fuelling machines.
• On-line refuelling occurs by using two identical fuelling machines, which attach remotely to both ends of the fuel channel while the reactor operates.
• The calandria vessel is stainless steel and fabricated remotely then designed to withstand temperatures, pressures, fields of radiation, loads, and earthquakes.
• Installation of equipment like pressure tubes, reactivity mechanisms, and flux detectors take place at the power plant site.
• Vertical and horizontal reactivity control devices for regulation/shutdown, and neutron flux detectors are positioned in the calandria and attached at the bottom.
• The Calandria must support fuel channels and facilitate a chain reaction, remove heat, allow for online fuel replacement, and withstand environmental stressors.
• It houses reactivity control, measurement, and shutdown devices, gives radiation and thermal shielding, and allows component replacements/refurbishments.
• End shields offer axial core radiation along with thermal shielding, containing inner and outer tubesheets, lattice tubes, and a space filled with steel balls and ordinary water for cooling.
• Light water provides radial shielding, with vault water in CANDU 6, or a separate shield tank in CANDU 9.

CANDU 9 Reactor Assembly

• The CANDU 9 reactor assembly has a shield tank, with air contained in the vault. The CANDU 6 reactor assembly had a vault filled with water as a radiation shield.
• The reactor vault is about 20 m high, 20 m wide, and 12.5 m deep.
• All the flux measuring and controlling devices that penetrate the calandria are held by a reactivity mechanism deck
• Horizontal flux measuring devices and reactivity control units penetrate the calandria from its side.
• The shield tank is 13.3 m in diameter and 8.1 m long, the light water and steel ball filled end shields and shield tank around the calandria allow maintainers to work in the vault in shutdown.
• The Over-pressure rupture disc and piping assembly are components of the shield tank.
• The calandria has a diameter of 8.5 m and a length of 6 m, and it holds the moderator ( heavy water) and forms the shield tank's inner shell.
• The volume, that contains the fuel, defined by the pressure tubes inside the calandria, 7-meters in diameter and 6 m long, it is considered the reactor core.
• With the core 1.5 m less in diameter than the calandria, the heavy water between the core and calandria wall reflects thermal neutrons.

Calandria & Fuel Channel Assemblies

• The calandria shell and the two end shields make up the calandria vessel, decreasing the diameter of the calandria shell to enable thermal flexing and optimize the neutron reflection of heavy water.
• Each end shield has inner and outer tubesheets, joined by lattice tubes, to create a closed vessel containing carbon steel balls and shield cooling water.
• End fittings serve many purposes, such as connecting pressure tubes to the heat transport system via feeder pipes and enabling on-power refuelling by removing the Channel Closure Plug.
• Liner tubes extend through end fittings to help movement for fuel bundles and coolant flow, while shield plugs within the liner tubes offer radiation shielding.
• Each end fitting has a positioning assembly to hold the assembly, accommodating pressure tube elongation during operation.
• Pressure tubes in the calandria hold the fuel in the reactor core for pressurized heat transport coolant.
• The pressure tube is surrounded by the calandria tube, and an annulus exists between them maintained by spacers and filled with gas.

Fuel Channel

• The key dimensions for the CANDU 6 fuel channel include a 12.4m overall length, 6.35m pressure tube length (103mm inside diameter, 4.2mm wall thickness), and 5.9m calandria tube length.
• The channel Closure Plug, made of stainless steel, is positioned at the outboard end, and it provides pressure and boundary closure.
• A bolted connection is used for the Feeder Pipe of the Heat Transport System being coupled to the fuel channel near the outboard end of each end fitting.
• Roll-expanded joints connect each end fitting to to both ends of the pressure tube, each end fitting is made of stainless steel at 2.4m long/160mm in diameter
• Each fuel channel of the reactor contains 12 fuel bundles.
• Zirconium with 2.5% Niobium is used to create the pressure Tubes, that hold fuel bundles, they require great strength for good ductility/creep and corrosion
• To minimize the heat transferred from the reactor coolant, The Calandria Tube surrounds the pressure tube. The annular space that is filled with CO2 gas acts as an insulator
• During refueling and after fuel bundle placement the fuelling machine removes and inserts the shield plug

Arrangement of Fuel Elements, Pressure and Calandria Tubes

• Fuel channels are arranged on square lattice, 286 mm pitch, chosen for neutron and mechanical considerations, and the gap allows placement of flux/reactivity devices.
• The wide fuel channel spacing promotes neutron thermalization and proper machine access.
• Thin fuel element bundles allows fast neutrons to escape from the fuel and the moderator between the channels slows them.
• 37 pencils are used for each fuel bundle to produce a large surface area which promotes heat transfer from elements to the coolant..

Fuel Design

• Functional requirements for fuel bundles in CANDU reactors is to allow on-power refuelling, maintain integrity/stability, have correct coolant flow, and deliver specified fission power.
• Functional attributes for fuel sheaths have to contain uranium dioxide, minimize neutron absorption/corrosion, mitigate strain, and maintain hydraulic head loss.
• Uranium dioxide pellet designs are aimed for maximizing fissile material, minimizing volume change through radiation, and reducing gas releases/economic

Main Features of the Fuel Bundle

• CANDU 6 and 9 reactors use 37 pencil fuel bundles comprising sheaths and end caps made of Zircalloy-4 to enclose UO2 fuel pellets.
• Stainless Zircaloy-4 builds all the structural components such as Fuel Sheath, the End Caps, the End Plates, Inter Element Spacers to keep neutron absorption low.
• The fuel is uranium dioxide with 0.71% U235 content pellets typically with 30 high density pellets in a fuel pencil/CanLub (graphite) applied that reduces interactions between the pellets with cladding
• Shield Plugs/Pressure Tubes in the horizontal fuel elements don't need additional components/fully loaded fuel bundle is 24 kilograms with 90% that come from uranium fuel.

Fuel Channel Spacers (Garter Springs)

• Fuel channel spacers (garter springs) are smallest components for fuel channel assembly that ensure pressure tubes are not in contact with calandria tube.
• Spacers are designed to better separate tubes/maintain a diameter of 10 cm/6 meters long held at two ends where sagging would cause contact without support. Since calandria tube is supported by the moderator.
• Garter springs block deuterium ingress by maintaining specific thermal insulation/preventing hydride cracking and loss of coolant event.
• The spacers are made with iconel wire into coiled helical springs with a spring coil of 4.8 mm in diameter and do not impede the flow of carbon dioxide.
• 4 spacers required for each fuel channel to transfer weight into calandria tube/ensure consistent value as increase in pressure tube diameter that leads to up 5% the size of the annular gap/rolling assists in the reduction of wear.
• Correct positioning of the spacers occurs with specialized equipment to maintain installation.

Reactivity Control Devices: Introduction

• Devices regulate/protect CANDU core. Devices are used for regulation and protection by being inserted from the top, and system devices are vertically operated rods with poison injection nozzles being horizontal.
• These systems affect flux shape which matters due to dimensions related to neutron travel/flux disturbances leading to increased maximum extraction of energy from the fuel

Liquid Zones

• The core divides into 14 zones for spatial flux using reactivity control devices (requiring measurements/reactivity control) and high neutron probability that centers in the core and causes fission during flux.
• Heavy Water moderated Reactors rely on over 99%/purity of D2O and Light water in the given location of the core to control neutron absorption.
• Systems of compartments are called liquid zone control system's distributed in the core with varying light water through flow differential.
• The diagram shows five of the enlarged 14 zones, which are enlarged, containing level variations and flow differential, with volumes accumulating in each zone.
• Both axial halves in 14 zones have 7 zones, 3 compartments, and 4 travel zones throughout all zones being in the location.
• Reactor regulation controls the water level to induce reactivity in each compartment being all bulk/indicating uniform flux distribution for all the zones.
• Reactor regulation changes water in the zones for different values/zone changes leading to side flux/overall differences in levels.

Liquid Zone Level Control System

• Light zone volume/reactivity is varied by cylindrical water zone amount/neutron and level of water needing to be monitored before flux/ensure that system behaves as flux intends.
  • A flux detector at the centre of each zone measures local signal processed by Digital Control Computer/signals are compared from Regulating System in same computer to see if there are any errors.
• Reactor Trip: Computes control single for .5%FP/Sec rate/reactor setback occurs at .15%/Transducer Air Pressure.
• The control signal, as air, varies the amount of water to the zone compartment as an on/off controller (A/C symbol).
• Outflow constantly achieved by Helium for the amount in water/level are measured at equal values. Measured through helium surface system and constant gas.
• Measurement of the zone ensures fully/empty state.

Spatial Control of Neutron Flux

• Spatial control means the amount power output vs demanding rate/various situations during different power levels and control local disturbance from other devices.
• Reactors are divided into Section 2's 14 zones/flux control via adjustments at spatial level. Differential adjustments are achieved at a single spatial level.
• Zone controllers of compartmentalized vertical tubes traverse the core for connections to all zones connected near the top which means only mechanical connections apply at the shell.
• Detector assemblies come with detector, and they are center located throughout zones.
• 7.2 mk is reactivity worth for system of zone control is zero to equilibrium.
• • The total level around the liquid control is .5mk between 15/80%/reactivity coefficient is mk .077 in correspondence.

Adjuster Rods

• Liquid zones maintain fine bulk reactivity/spatial flux control/other methods that are coarse are implemented to ensure safety beyond liquid's use. Solids are implemented in adjusters /MACS to influence position and design.
• 3 adjusters are shown in the diagram in arrangement where 3 rows have symmetry due to the shape of the core and CAND U values contain amount listed.
• Neutron is always consistent/maximum center power production bundles requires constant adjustment. This does leads to fuel loss during fission/zones for safety/delays for control.
• If reducing with a given rate xenon will accumulate withdrawal and be compensated with adjustments from .35 min due tripping/known as "poison override" time.
• . Adjuster rods provide shape for optimal burnup/power, control as negative ranges/compensates for low ranges

Mechanical Control Absorbers

• Neutron solid/liquid system increases beyond the liquid zone, these bars are adjusted (or in adjuster rods) while describing how absorption bars can be added to the core. In order to provide additional negative relativity
• Absorber rods are implemented as bars/cadmium tubes to provide safety. Always staying out core till used for high temperatures.
• Driven into core used in coordination with zones/40FP as drop off /manual. Absorb in two in varying ranges/releases with 3 seconds

Shutdown Rods

• Reactors contain two for independency from rods, being absorbing for the 1st and 2nd shut off in the core via safety from 1/2 are not dropping completely/shutting of process.
• Speed is increased from springs in travel at seconds/ regulated to be safe due to manual switch and trip of the rod that doesn't not pull fast. Is the design safe?

Summary of Reactivity Control Devices

All reactivity components are installed above the deck at location with equipment of water/helium made for the mechanism. From that is, the mechanism/safety need spacing/deck amount. In 6 and 9 systems with solid systems and absorption material, there are implemented effects that take place from device.

• Maintaining the same level of negative and positive reactivity allows power, xenon, large amounts of the injection liquid during drops. Rod location is affected here at high rate is to follow.

Moderator Systems

The moderator in the calandria thermalizes the fast neutrons produced by fission then circulate, while the heat exchangers removes the heat in heavy water.

• Systems separated /can provide alternate cooling. In the primary, auxiliary, and emergency core cooling systems
• Two primary pieces of equipment are the circulation piping, two pumps, plus two heavy/four light heat exchangers located below the reactor.
• 75% come from moderators while production has product in the form of decay to account for 20& transfer/conventional transfer for 5% of the pump amount.

Main Circuit Layout

• The CANDU 6 layout includes with heavy water outflow the bottom of the calandria and has circuits implemented are 2 and four amount on CANDU 9.
• Moderator components implement layout, interconnect into a symmetrical that are symmetrical. At high output to low, they have valves which operate at 60C normally.
• The head tank is under supply and normal operations. Level is maintained in all operations.
• Loss of pressure will need a safety valve in order to reduce pressure.

Moderator Cover Gas System

The reactor contains several atmospheric compounds that need radiation protection and protection from water with help from the Moderator in order to prevent corrosion.

• The surfaces implemented/pipes through systems. Prevented of hazardous amounts during catalyst/high connect for safety by implementing 2 compressors equipped for instrumentation.

Moderator Liquid Poison System

Implementation to help reduce regulation from Module of safety by injection. However, when reducing, no transfer is created/injection used safely by the Reactor of 6.

• Negative reactivity compensators increase and decrease xenon for no critical output. Boric acid (B) and Gadolinium (G) are used safely for short operation and tank operations.

Moderator Purification System

• These filters help remove any impurities/help operate safely by maintaining safe operations. This system keeps radiation/corrosion and levels throughout the different types via trips

This lesson covers key safety features of CANDU reactors. It discusses the importance of pressure tube and calandria tube separation, shutdown systems, and design considerations for deuterium ingress. It also explores the function of spacers to accommodate pressure tube expansion and the implications of shutdown rod failure.