Modern Power Station Practice

[Kena Sugrue]

Welcometo the world of power generation, where innovation and engineering excellence converge to provide us with the energy that powers our modern lives. In this comprehensive volume, Modern Power Station Practice: Boilers and Ancillary Plant Volume B, brought to you by the experts at British Electricity International, we delve into the heart of power stations, focusing on the intricate and vital components that make up the backbone of electricity generation. This volume provides an invaluable resource for engineers, researchers, and enthusiasts alike who seek to understand the intricacies of power station boilers and the critical ancillary systems that support their operations. These pages uncover the secrets of heat transfer, combustion, and thermodynamics.

There are many reasons why the safe handling of irradiated AGR fuel needs to be linked to decay heating. The implications of stringer or element overheating arising from various postulated fault conditions has been rigorously studied and, together with a general requirement to restrict fuel tempermures at other stages during handling, has led to the s.;ict application of maximum heating limits to the individual operations. Of all the likely fault conditions which are thought could arise in the fuel route, the most severe is that of a dropped irradiated fuel stringer in which gross damage to some of the tuel elements could be compounded by fuel can melting and the consequent release of radioactive fission products. Although such an occurrence, either in the reactor or at any of the fuel route facilities, is considered to be extremely unlikely, it is nevertheless necessary from a safety viewpoint to be able to guarantee that, even if it were to happen, an unacceptable release of radioactivity to the environment would not take place.

In this context the withdrawal of an irradiated stringer is forbidden even from a shut down reactor unless the total stringer decay heating is less than predetermined limits. The absolute values depend upon the available cooling capacity provided by the charge machine during the discharge process. The most common off-load refuelling regime adopted involves compliance with a maximum stringer heating of around 40 kW, which in practice results in post reactor shutdown delays of up to 12 or 15 hours before refuelling can begin. A similarly derived limit is designed to protect against the consequences of a dropped stringer within a buffer storage decay tube. Other potentially serious implications of overheated fuel have been considered. Within the irradiated fuel dismantling (IFD) cell, for example, dismantling is not permitted unless the total stringer decay heating is less than 40 kW; this ensures that fuel temperatures do not reach levels which would significantly enhance the oxidation rate of any exposed UO2, at possible pin failure sites, to U3O8. UjOg would be produced as a fine powder, therefore constituting a ready source of cell contamination.

Routine estimation of AGR fuel decay heating is a highly complex matter. Since the very high fuel ratings and irradiations produce considerable levels of post-discharge heating, it is important that changes in stringer conditions during service are properly accounted for in the calculational process, so that heating levels are neither under nor over estimated. Therefore sophisticated techniques have been developed which allow the retrospective examination of ratings seen over the entire operating histories of individual stringers, prior to their removal from the reactor. By this means, recommended minimum cooling periods can be provided in order to ensure that the various heating limits are not exceeded in practice.

A limited disposal of sludges to sea in sealed packages has taken place. The area selected for sea disposal is subject to international agreement and is approximately 600 miles from the south-eastern point of England. The packages consist of steel drums containing the sludge which has been immobilised in cement. Similar packages have been designed for ion-exchange resins using a polymer matrix in place of cement.

Metallic wastes, for example valves, pipes and reactor control machinery, are stored in concrete cells at magnox and AGR power stations. These cells have thick walls which act as radiation shields in addition to providing containment. Like the resin and sludge cells, the walls are typically 1 m thick. The design of the facilities is such as to ensure that the waste is kept dry with provision for the checking of adventitious water ingress and removal.

Typical arisings of magnox and AGR sludges and resins are given by Bennett D (1983) and are shown in Table 4.10. PWR resins and sludges are given by Passant [9] and shown in Table 4.11. Fuel cladding wastes arise as the result of the removal of extraneous metal cladding and supports from the fuel before it is dispatched to a reprocessing facility. Removal of this material assists in the packing of the fuel in the transport containers.

special safety precautions. Two types of storage cell have been adopted. At some power stations the waste is stored under chemically-dosed water and at other sites is stored dry. The cells are of concrete to provide containment and radiation shielding. In the wet cells, forced ventilation is provided to prevent hydrogen build-up from the air/water reactive magnox, the air being filtered before discharge. In the dry cells, temperature probes are installed to warn of possible magnox overheating, the probes being coupled to alarms. Forced ventilation is again provided to remove any hydrogen.

AGR power station fuel yields steel and graphite debris, which is accumulated in dry concrete cells, providing containment and radiation shielding. This waste is not reactive and unlike magnox waste special. precautions against fire are not necessary.

Dry solid wastes have isotopic compositions related largely to neutron activation products with the radionuclides Co-60, Mn-54 and Fe-59 being prominent. The isotopic composition before disposal is time dependent in view of the long storage times. Ion exchange resins contain predominantly Cs-137 and sludges are often a mixture of activation products, fission products and actinides.

National policies on disposal of radioactive wastes are under review. Environmental concern over the dumping of solid radioactive waste at sea led to industrial action by the transport unions in 1983. This followed a resolution calling for a voluntary suspension ot sea dumping at the 1983 meeting of the London Dumping Convention (LDC), pending the completion ot an LDC scientific review. As a result the government suspended sea dumping and agreed to establish an independent review of the subject in conjunction with the Trades Union Council (TUC). This review by Holliday [10] was published in late 1984 and had the principal recommendation that the

A Radioactive Waste Management Advisory Committee (RWMAC), established by the government, is also examining issues relating to an overall policy for the management of radioactive wastes and its fifth report to government was published in 1984. RWMAC monitors the activities of the Nuclear Industry Radioactive Waste Executive (NIREX), an organisation set up in 1982 by the principal organisations that produce radioactive waste to manage the disposal of most solid low level and intermediate level radioactive waste. The activities of NIREX include the identification of land sites potentially suitable for the disposal of low and intermediate level wastes and managing the subsequent work.

Reactivity faults occur if there is an uncontrolled increase in the reactivity of the reactor either over the entire core, or locally. Such an increase inevitably gives power and hence temperature transients which must be limited by the protective equipment to less than the melting point of magnox. Indeed, because the strength of magnox decreases substantially at a few degrees centigrade less than 640C and because full coolant mass flow conditions usually exist, the acceptable maximum temperature is reduced to avoid distortion of the fins with a consequential reduction of heat transfer coefficient.

The reactivity increases because the control rods, either as groups of rods or locally as a few rods, run out in an uncontrolled fashion. The reactivity release rate depends upon the worth of the rods running out, and their speed of withdrawal. Both of these aspects are limited by design, by restricting the size of rod groups and the speed of withdrawal and by using interlocks to prevent the main rod groups being moved together. There are two types of reactivity faults to be considered, those where there is a uniform increase of reactivity over the core, symmetric faults, and those where only a few rods run out, asymmetric faults. These need to be considered separately using slightly different techniques because the protection for each 4s organised in a different way. In each case, however, it is necessary to demonstrate that the probability of exceeding the limiting clad temperature is acceptably low. As in the case of the depressurisation faults, this is done by considering the magnitude of the uncertainties in the manufacture of the fuel, the heat transfer properties and the model used in the study. For those faults where the gas circuit remains intact, the acceptable probabilities that any fuel temperature will reach the limit in the event of the fault occurring is 1 in 1000.

The new fuel, which is only slightly radioactive, is transported by road in strong industrial containers (magnox fuel) or in Type A containers (AGR fuel), being treated with care because uranium is a chemically toxic material and because even slightly damaged fuel is unsuitable for reactor use and costly to replace.

In the case of new AGR fuel, the stringent requirements of criticality control necessitate that the amount of uranium in each package, and the number of packages per vehicle are restricted. The criticality aspects of the packaging are subject to competent authority approval.

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