6.6.1 Boilers

The primary function of the boilers is to transform the heat from the reactor into a form which is suit­able for electrical power generation. In this respect the AGR represents a major advance from the magnox reactors in that the boilers are designed specifically to provide steam to 660 MW turbines of exactly the same design as those used on the modern fossil-fired stations. The boiler temperatures and pressures are therefore higher than on the magnox stations and in addition there is an integral gas-heated reheater. The reactor itself has a heat output of 1550 MW and delivers carbon dioxide gas at 6l5°C to the boilers, which raise 500 kg of steam per second at 54I°C and 166 bar for expansion in the turbine HP cylinder and subsequently reheat it to 539°C at 40.7 bar for expansion in the IP cylinder. This duty requires 300 kilometres of boiler tubing in each reactor and the whole boiler system accounts for some 10% of the capital cost of the station.

Apart from its function in transferring power, the main boiler system plays a major role in the safety of the reactor in that it forms part of the primary cir­cuit pressure boundary and is the principal means of removing the heat produced within the reactor vessel post trip. An additional and much smaller decay heat boiler is installed at the base of the main boiler. This provides the essential diverse post-trip cooling system and is served by completely independent feed and steam systems.

The boiler systems in the various AGRs are all similar in principle, but two basically different types of boiler tube bank configuration have been devel­oped and are in service: the serpentine boiler shown in Fig 2.88 and the helical pod boiler shown in Fig 2.89. The description which follows applies in general to both types of boiler but in detail primarily to the serpentine type of boiler as installed at Heysham 2, Hinkley Point В and, broadly, Dungeness B. The helical pod type of boiler is installed in the Hartlepool and Heysham l reactors.

The boilers are located within the reactor pressure vessel in an annulus formed between the gas baffle and the reactor pressure vessel wall. This is a de­velopment of the design principles originally estab­lished for Oldbury and Wylfa magnox stations. The boilers are of the once-through subcritical type, chosen in order to minimise the number of reactor pressure vessel penetrations, and a single stage reheater bank is located above each high pressure unit. The boiler annulus is partitioned into four quadrants, each con­taining three rectangular boiler units and two gas circulators. Each quadrant is operated separately and any one may be taken out of service while the reactor is on load. Each of the twelve individual boiler units (eight for helical pod boilers), weighing 120 tonnes, is in a 16 m tall vertical rectangular case, containing banks of closely packed boiler tubes. The tubing is arranged in horizontal rows with vertical serpentine return bends; the horizontal tube axes are arranged in the vessel circumferential direction to maximise the length of straight tubing.

Hot gas from the reactor core is drawn down over the banks of boiler tubing by the gas circulators lo­cated underneath the boilers at the bottom of the annulus. The gas is constrained to flow down over the banks of boiler tubing by means of the casings which surround each boiler unit and by the gas seals which form a floor across the annulus around the base of the boiler casings just above the circulators. The feedwater connections to the boilers pass through penetrations in the walls of the concrete pressure vessel and enter the unit casing from below, while the superheated steam outlet connections pass through penetrations near the top of the casing.

The serpentine tubes, which make up the decay heat

boiler and the main economiser/evaporator/superheater

sections, are assembled in pairs by means of welded spacers to form vertical platens. Forty-four such pla­tens hang side-by-side in the module casing and are supported at regular intervals across and down the module by links attached to beams which span the casing. That part of the module which comprises these platens and their stainless steel casing is supported at its base on mild steel beams. The beams are slung from flexible links anchored to the gas baffle skirt on the one side and to the reactor pressure vessel wall on the other. The load is transferred from the module casing to the beam through a pin joint. This permits the anchor points of the beam to move rela­tive to one another without themselves causing the module to tilt, whilst at the same time accommodat­ing the slight radial tilt which is produced by thermal expansion of the superheater tailpipes.

The reheater section of the module is located above the superheater section and is hung from the top slab of the reactor pressure vessel. The tubing is again arranged in the form of platens, but in this case there are 36 platens each of which consists of four parallel steam paths in the same vertical plane. The casing can move relative to that of the lower part of the module and a gas-tight seal is provided by means of a flexible foil.

The tube materials are chosen to avoid excessive gas or waterside corrosion. They are therefore graded from lCr^Mo in the economiser, through 9Сг 1 Mo, to 316H austenitic steel for the superheater and reheater where the gas temperatures are high. The роіпГ of transition from the 9CrIMo ferritic steel to the austenitic material is particularly sensitive. It Js necessary to ensure that the gas temperature does not exceed 550?C in order to avoid excessive gas-side oxidation of the ferritic steel, and that the steam is superheated to a temperature sufficiently above sat­uration temperature to avoid any risk of stress cor­rosion in the austenitic steel. The junction between the two materials is achieved with a short transition section of Inconel 600, which has a coefficient of expansion roughly halfway between those of the fer­ritic and austenitic tubing materials.

The boilers are designed to withstand earthquakes up to a specified severity. Vertical seismic forces are accommodated by the main support beams; radial forces by the superheater penetration sheath tube connections to the upper casing and to neighbouring units via the annular ring at the base of the casing; and circumferential forces by special upper and lower seismic restraints which link the unit casing to dummy penetrations in the vessel wall.

The flow/pressure drop characteristics of once- through boilers can cause large flow and tempera­ture differences to occur between tubes and between boiler units if the design is not undertaken carefully. The temperature differentials would be undesirable

REHEATER-MAIN BOILER GAS SEAL

SUPERHEATER TAIL PIPES

SUPERHEATER SUBHEADERS

AUSTENITIC STAINLESS-STEEL SECONDARY SUPERHEATER tubes

SPECIMEN ACCESS 1 TV ACCESS TUBE

Fic. 2.88 Serpentine main boiler and reheater unit

because they would lead to thermal stress or even to the carryover of water into the turbine. The explana­tion of this potential flow instability is as follows. If the water flow in a boiler tube is increased the pressure drop will increase in proportion to the square of the mass flowrate (Fig 2.90 (a)). However, the length of tube occupied by water will also increase relative to that occupied by steam. This will increase
the gravitational pressure drop but, since the pressure drop per unit length is greater in the superheater, there will also be a reduction in pressure drop. The fiow/pressure drop characteristic may therefore be cubic (Fig 2.90 (b)) which means that for certain pressure drops three different flowrates may exist. In addition to this ‘static’ instability, it is possible for a periodic oscillation of the flow to occur in any one

Fic. 2.89 Helical pod boiler unit

tube and this is usually coupled out of phase with other tubes.

In the AGR boilers, large numbers of tubes are connected in parallel between common feed and steam headers and would therefore operate at substantially different water flowrates and temperatures despite the fact that they all have the same pressure drop. These problems are controlled by the installation of an ori­fice at the inlet to each boiler tube such that the total pressure drop/flowrate curve has a continuously positive gradient (line C in Fig 2.90 (b)). However, the small size of orifice which is required to provide control at low loads would lead to prohibitively high pumping costs at full load. A compromise is there­fore accepted in which control is provided at full power but some instabilities are tolerated for brief periods during start-up and post trip. A complex suite of computer hydrodynamic models is used to predict the steady state, static/dynamic stability and transient responses of the boilers.

Feedwater passes through the boilers at the rate of half a tonne per second throughout most of their 30 year working life. However, the tube materials are made of ordinary steels which are actually corroded in this environment. The steel is in fact protected only because the initial corrosion products form a thin surface film which effectively inhibits further attack. The film is magnetite (FejC>4) and is virtually insoluble in pure water. But if contaminant solutes or oxygen are allowed to enter the boiler, very high local concentrations of acids, alkalis or salts will be generated at the boiling dryout zones. Under these conditions the magnetite film becomes much more soluble and breaks down. Corrosion then proceeds rapidly and unchecked because the magnetite produced by this on-load corrosion is porous and unprotective. The austenitic steels in the superheater are susceptible to ‘stress’ corrosion in these same aggressive solution environments, particularly in the presence of oxygen. Whilst short-term wetting of the austenitic is accept­able if the water quality is good, a superheat margin is normally maintained at the 9Cr-316 transition joints to ensure that the austenitic sections run under dry steam conditions.

In regions of high water velocity, a tube metal wast­age phenomenon known as ‘erosion-corrosion’ may occur and can lead to extremely high rates of metal loss. This process is a function of velocity, pH, temperature, oxygen concentration and the chromium content of the tube metal. Severe damage due to this corrosion mechanism has been experienced on both magnox and AGR stations in the past. The boiler tube inlet orifices and the bends in the economiser are the most susceptible areas. However, the phenomenon is now better understood and is controlled by raising the pH and/or oxygen dosing. The inlet orifices and erosion sites downstream are made of stainless steel and, in the latest stations, 1 Cry Mo is used in place of mild steel for the economiser and decay heat boiler tubing in early designs.

The carbon dioxide gas surrounding the boiler can be equally aggressive and, if gas temperatures are too high, may cause gross metal loss of the ferritic boiler steels through an accelerated breakaway type of oxidation. Mild steel had been used for the whole boiler in the magnox stations, but this has a tem­perature limitation of 350°C due to oxidation in the

Fig. 2.90 Flow/pressure drop characteristics of once-through boilers

carbon dioxide. This material (or ІСгтМо) is there­fore used only for the economiser and the decay heat boiler in the AGRs. Austenitic stainless steel is suit­able for all surfaces exposed above this temperature and is therefore used for the reheater and superheater which are exposed to the hottest gas at up to 615°C. However, because of the risk of stress corrosion in the stainless steel if any wetness is present in the steam, high chrome ferritic steel 9CrlMo is chosen for the larger, intermediate region of the boiler where evapora­tion and the primary stage of superheating takes place. Gas side oxidation of this material is acceptable up to about 550°C, beyond which the oxide weight gain induces breakaway and the oxide layer is no longer protective. The corrosion rate then increases by one or two orders of magnitude and would lead to pre­mature failure of tube supports. Since there are major power output benefits in running at higher gas tem­perature, the boilers are run as close as possible to this constraint. The judgement is based on very exten­sive oxidation research programmes and upon detailed computation of boiler temperatures and gas mixing.

The gas flow through the boilers has a high energy and density and is extremely noisy. The boiler tubes,

which are elastically suspended across this fluid flow, are liable to be excited into vibration via a number of ~ different fluid elastic mechanisms. This may lead to fretting damage at clamped tube supports or to fatigue damage at welded tube supports. This is a common problem in shell and tube heat exchangers, but is a cause for particularly careful scrutiny in the design of the AGR boilers because the extremely limited access allows virtually no repair or modification during the 30 year station life. The main excitation phenomena are vortex shedding, fluid elastic whirling, turbulent buffeting and acoustic resonance. However, these are complex phenomena and are not yet fully understood despite many years of theoretical and experimental investigation.

In vortex shedding, inwardly spiralling vortices are shed into the wake alternately from one side of the tube and then the other. An alternating counter­circulation is induced around the tube and this pro­duces a fluctuating side thrust (aerodynamic lift) and also a fluctuating drag force. The transverse force is several times greater than the stream-wise force. The frequency with which vortices are shed is characterised by the Strouhal number:

<5 = is the logarithmic decrement (logn yn/yn + i where у is the amplitude), q = is the fluid density К = is a constant between 3.3 and 9.9 (where equality at 9.9 indicates the onset of whirling and equality at 3.3 is the ‘rock bottom* safe design value).

The Strouhal number has a value of 0.2 for an iso­lated tube but is different for a closely packed tube bank (as is the true nature of the wake) and is there­fore normally determined from tests on rigid tube banks. If the vortex shedding frequency coincides with the natural response frequency of the boiler tubes, then coupling occurs and the tube motion feedback enhances the driving force such that unacceptably large vibration amplitudes are induced. Once coupling has occurred, the vortex shedding frequency ‘locks-in’ to the tube frequency and coupling persists over a broad velocity range (about ±30%). Experience has shown that, in order to avoid coupling, the designer should ensure that the natural response frequency of the boiler tubes is about three times higher than the vortex shedding frequency.

Fluid elastic whirling is a vibrational instability phe­nomenon which can arise in a steady fluid flew and which is in fact more common than vortex shedding resonance. When a tube within an array is displaced, the aerodynamic pressure forces around the tube are altered in such a way as to induce movement of the tube. If the tube damping is relatively low, such that it is unable to dissipate the fluid energy being sup­plied, the tube motion will increase and so will the fluid forces. This divergent instability will occur at the natural response frequency of the tube and is characterised by an elliptical whirling tube motion which is coupled out-of-phase with the similar motion of adjacent tubes in the array. A guide to a satisfac­tory relationship between fluid elastic excitation and tube damping has been obtained by Connors:

V/fD < K(m6/eD2)T

where V = is the maximum velocity between tubes

F = is the tube lowest natural frequency

D = is the tube diameter

M = is the tube mass plus added mass of entrained fluid per unit length

Turbulent buffeting may be regarded as a decayed and incoherent form of vortex excitation, generally in­volving much smaller amplitudes over a much broader frequency spectrum. For these reasons it rarely presents a problem, provided that the structural natural response frequency and the prospective vortex shedding frequency are widely separated.

Apart from these examples of potential fluid force excitation of body resonance, if the boiler casing dimen­sions are such that the frequency of the natural acoustic standing wave coincides with the vortex shedding fre­quency, then it is possible for the sound to become amplified to the extent that it can cause damage to the steam generator structures. It is for this reason that the AGR boiler units are each divided in half by a vertical acoustic baffle division plate.