A general layout of the coolant control plant in a AGR is shown in Fig 1.41. The flow rate through the plant is achieved by utilising the pressure drop across the primary coolant circulators and is typically 50-100 t/h CO2, leaving the primary circuit at approximately 280°C. Minor variations occur from reactor to reactor dependent on the details of the plant duty required but normally this plant consists of:

• An inlet filter to remove activated metal oxide debris and prevent it from reaching the bypass plant in order to maintain a low activity level. The filter can be of the sintered stainless steel type with a typical efficiency of 100% for a 4 fim panicle.

Fic. 1.41 CAGR coolant control plant

• The flow is split and typically 10-20 t/h CO: is fed to the recombination unit at 280°C, Oxygen from either the electrolysis plant or the bottle sup­ply is also fed to the unit to reform carbon di­oxide. Typical oxygen flow rates are 5-15 kg/h aivine a 20-50^0 conversion of carbon monoxide across the bed. This both maintains the required carbon monoxide concentration in the main circuit and ensures that no oxygen, which can react rapidly with the araphite fuel sleeves and also more slowly with the graphite core, is fed back to the main coolant circuit.

The recombination unit catalyst is a 0.3-0.5% platinum on alumina pellet of 3.2 mm right cy­linders with a total bed weight of 0.8 t. The catalyst will remove up to l. O^o of its weight of sulphur which exists in the reactor coolant as carbonyl sul­phide at a typical concentration of 100 vpb, higher levels affecting steel oxidation and oxide spalling. The sources of sulphur are ingress of circulator oil, the graphite core and, where installed, the carbon shield. The adsorbed sulphur will slightly reduce the catalyst efficiency and this has to be allowed for in the recombination unit bed design.

• The recombination unit flow is recombined with the main bypass flow and then passes through one side of a regenerative heat exchanger which cools the gas to 120°C, the other side being the returned reactor gas. The bypass flow is then further cooled to 35°C by passage through a water-cooled heat exchanger.

• The gas is passed to the drier system which con­sists of two drier towers, one on duty and the other either being regenerated or on standby. The drier material is silica gel spheres of 3 mm diameter hav­ing a total bed weight of 5.5 t. The capacity of silica gel decreases with number of regenerations and is dependent on the temperature of adsorption, moisture concentration and regeneration tempera­ture. Typical design parameters are adsorption tem­perature 35°C, regeneration temperature 200°C when its capacity at 300 vpm H2O is 2-4%. The beds are operated on a 8-24 h cycle and can be changed either on a time basis or a moisture break­through basis. The beds are regenerated by taking a How of 10 t/h at 200°C from the regenerative heat exchanger with reverse flow through the bed, then through a water cooler and separator at 40°C to condense and remove the water from the circuit; linallv the flow recombines vvith the main flow and is passed through the drier bed in use.

• The required quantity of methane, typically 1-5 kg h is ted to the drier outlet flow from either the methanation plant or bottle supply.

• The gas is passed through the bypass plant outlet liber which may be of the porous stainless steel t>pe and mas comprise either a single stage or a dual coarse/fine filter. This filter will prevent any recombination unit or drier dusts being fed to the reactor and for a dual stage filter operates at effi­ciencies of 100% for 3 and 1 diameter particles respectively.

Mechanism of drier adsorption The breakthrough of a constant feed of adsorbate from a fixed bed is characteristic of the equilibrium relationship between adsorbate and sorbent and the mechanisms controlling mass transfer from the gas to the solid phase. The variation of the exit adsorbate concentration with time in a fixed bed at constant inlet concentration is affected by several factors Involving gas and solid properties and bed dimensions. These are all implicitly included in three main headings:

• The mass transfer step (or steps) which controls the rate of flow from the gas phase to the internal surfaces of the adsorbent. This may be gas film diffusion, intraparticle diffusion, surface adsorption or any combination of these.

• The equilibrium behaviour between gas and solid (the isotherm).

• The mass balance across the bed.

The adsorption of water by silica gel in the presence of high pressure carbon dioxide gives a linear isotherm and for such a system.

c 1 1 .

—- = — + — eaUx) sin a](x) x dx/x

Co 2 гг J о

where

Co is the bed inlet water concentration C is the bed outlet water concentration о і and аг are functions of the integration parameter x is the integration parameter

The input data required to solve this equation are:

• Gas properties — linear flow rate, density,

viscosity, Schmidt number (д/pD)

• Solid properties — particle density, average

radius, bulk density, bed depth.

• Gas/solid properties — particle Reynolds number,

adsorption isotherm gra­dient, intraparticle diffu­sion coefficient.

10.2 Graphite

Graphite is a pure crystalline form of carbon vvith a crystal density of 2.26 g/cm3. The properties of the
bulk graphite can be significantly varied, being de­pendent on the choice of raw materials and the manu­facturing parameters. The basic process is similar for all artificial graphites and starts with the conversion of a high molecular weight hydrocarbon, either a natural pitch or a residue of crude oil refining, to a coke by heating in the absence of air at 800°C — I000°C. The coke is then calcined, crushed and sieved to give the required particle sizes, typically 1-2 mm and below. The coke is mixed with hot pitch and either extruded or moulded to the size required. The brick at this stage is not dimensionally stable and is heated to 900°C-1000°C to coke the pitch and give dimensional stability. Volatiles are released during this process, leading to an extensive network of pores throughout the brick and to a relatively low bulk density. The brick may then be further impregnated with pitch and carbonised, several times, to increase the bulk density to the required value and each time affecting the distribution and size of the open and closed porosity. The material is graphitised by passing a current through a bed of the carbonised bricks surrounded by coke which heats the bricks to 2800°C — 3000°C. This process also leads to the volatilisation of most impurities. Purification can be assisted by the addition of either solid or gaseous halides to the graphitising furnace leading to the volatilisation of the more volatile halides, most importantly boron.

The properties of the three types of graphite used in the main cores of the magnox and AGR reactors are given in Table 1.16.