Roy Claxton says an emphatic no. As founder and managing partner of the eponymous Claxton International, he says his company had no nuclear experience whatsoever before its successful bid to build a £1million (nearly Euros 1.5m) large boiler lifting rig (BLR) now carrying out decommissioning work at the Trawsfynydd nuclear power station in Wales’ Snowdonia National Park. Claxton says it all came down to having the right idea at the right time. So well received has been Claxton’s novel approach to the lifting system, which is being used to remove 12 1,000t boilers in 100t sections that his company is now bidding for future nuclear work. It includes two bespoke rigs for spent fuel handling at a Lithuanian power plant.
Claxton’s big break into the nuclear business shows even the most conservative industry can be pervious to new entrants with the appropriate engineering solutions. Claxton’s design for the four-legged, tracked BLR was subsequently verified independently by both Dorman Long Technology in the UK and 2DM Associates in Houston, Texas, US.
Currently, only a handful of retired power stations and other nuclear installations are undergoing decommissioning in the UK. It is a similar picture elsewhere in the developed world where nuclear power is a mature industry. Yet globally current decommissioning work represents merely the tip of a very large iceberg. The US, Russia (along with the former Soviet Union countries), France, Japan, Germany and the Nordic countries have all either entered the decommissioning phase, or are about to embark upon it.
Still in its infancy, the dismantling business offers considerable opportunities to materials handling specialists in the removal of irradiated reactor parts and, more traditionally, benign boilers, other facilities such as turbine generators, associated infrastructure and buildings. There are also opportunities for more specialised robotics applications in areas where access is restricted.
It is estimated that in Britain alone, the cost of the nuclear clean-up is likely to rise significantly beyond the government’s official forecast of £70.2 billion. Last year, an all-party Trade and Industry select committee said the final cost of decommissioning and clean up was still unclear. But even the publicly stated figure could represent only half of what will be required, since it excludes decommissioning of seven nuclear power stations owned by British Energy, the UK’s biggest power producer, the first of which is due to close in 2011, and dealing with the Ministry of Defence’s nuclear sites and the long-term storage of the waste. Adding those all in would bring the total cost to a staggering £160 billion.
Reactors
Trawsfynydd nuclear station has two Magnox type reactors which were shut down in 1995 at the end of their working life. During normal operations two types of intermediate level waste (ILW) have accumulated on site; namely Miscellaneous Activated Components (MAC) and Fuel Element Debris (FED). MAC is predominantly components, which have been activated by the reactor core and then discharged. FED mainly consists of fuel cladding produced when fuel elements were prepared for dispatch to the reprocessing facility. As part of the decommissioning programme for the site, these waste streams are to be retrieved from storage vaults, monitored, packed and immobilised in a form suitable for on site storage in the medium term and for eventual disposal in the long term waste repository to be commissioned by the Nuclear Industry Radioactive Waste Executive (NIREX) at a later date. All of these activities require relatively sophisticated materials handling systems.
(The ownership of Nirex was transferred from the UK government departments Defra and DTI to the Nuclear Decommissioning Authority (NDA) in 2007).
The 12 boilers at the plant are being cut into manageable sections between 89t and 110t using conventional oxypropane hot cutting methods for lifting. The BLR consists of four legs travelling on a track that runs centrally over the boilers. Each track consists of a pair of I-section beams supported at the boiler box wall and incorporating end stops. Hydraulic motors within the travelling legs provide long travel motion.
The travelling legs carry a system of upper header beams, which in turn support a head sheave assembly and rope block incorporating lower sheave. The legs are connected at the lower level by link beams. Raising and lowering of the rope block is achieved using an electric double braked winch mounted on a linking beam at track level. Cross-travel of the load is provided via sliding parallel beams, driven by hydraulic side-shift cylinders. Hydraulic power pack units, located adjacent to the BLR and connected by hydraulic lines, provide supplies to the hydraulic motion systems.
Two proprietary control systems are used: an electrical processor-based control system located on the BLR energises the hoist system, while an electro-hydraulic system located within the hydraulic power packs is used to raise and lower the towers and energise both long- and cross-travel. The control systems include integral protective features, but in addition independent automatic protection has been included in the design as a further safeguard.
Boiler segments are attached to the BLR lifting block via a purpose designed three-point lifting frame. The frame is secured to a boiler segment using shackles that are connected at three lifting holes, cut at 120 degrees around the boiler casing.
Maximum hoisting speed is 0.5m/min with the hoist rope on the first layer of the winch drum, rising to 1.0m/min (eighth layer), while long travel proceeds at 600mm/min and cross-travel at 1.0m/min.
The British nuclear legacy
The United Kingdom Atomic Energy Authority (UKAEA) has built and operated a wide range of nuclear facilities since the late 1940s. UKAEA’s mission is to restore the environment of its sites in a safe and secure manner. This restoration includes the decommissioning of a number of redundant research and power reactors. They include the Windscale Advance Gas-Cooled Reactor (WAGR), a forerunner to the commercial AGR power reactors that are in operation today at various coastal locations around Britain.
UKAEA is also progressing with the decommissioning of Winfrith in Dorset. Winfrith was the site of nine reactors including the experimental Dragon reactor and a large Steam Generating Heavy Water Reactor (SGHWR) feeding the UK National Grid. The agency has been restoring the environment since the early 1990s. Around half of the civil nuclear research site has already been released for commercial use, and decommissioning will be fully complete by 2020. Many of the procedures developed during the decommissioning of WAGR and at Winfrith are being put into practice elsewhere.
The decommissioning methodology – and the equipment used to carry out the work – are subjected to rigorous scrutiny by the Nuclear Installations Inspectorate (NII) of the Health and Safety Inspectorate (HSE). A Technical Assessment Guide (TAG) of nuclear lifting operations issued last July sets out the issues that NII inspectors are meant to consider when assessing the adequacy of licensees’ safety cases “in the exercise of their professional regulatory judgement.” It is also a useful indication of the issues the NII deems to be important safety criteria.
The guide notes the main regulations used to control lifting operations and equipment including the statutory requirements relating to the design, supply and use of lifting equipment. It underlines the role of the statutory requirements in helping to identify any potential nuclear hazard. These include: The Lifting Operations and Lifting Equipment Regulations 1998 – LOLER, (Statutory Instruments 1998 No. 2307); The Provision and Use of Work Equipment Regulations 1998 – PUWER, (Statutory Instrument 1998 No. 2306); The Management of Health and Safety at Work Regulations 1999; The Supply of Machinery (Safety) Regulations 1992, (Statutory Instrument 1992 No. 3073).
However, it stresses that nuclear lifting operations create unique hazards due to the nature of the material being lifted or the nuclear plant equipment or services that may be damaged in the event of a failure in the lifting operation. Most lifting equipment will simply be designed to meet normal operating loads in accordance with design codes such as BS2573. Such codes usually require limited consideration to be given to generic internal faulted conditions, and for the structure or mechanism to survive such conditions. Other codes and standards give advice on operating such equipment with the objective of avoiding failures – for example, BS7121.
But the NII SAPs require a far more detailed and structured analysis of operations and equipment on nuclear licensed sites. This requires a comprehensive consideration of fault sequences and events. This approach will be familiar territory to those involved in high-end lifting applications in the guise of deterministic safety assessment, (DSA).
The term “high integrity” and how it has been applied over the years to nuclear lifting, considering the safety functions attributed to lifting devices and how these are implemented to create lifting solutions that are safe, continues to define the boundaries in design. In addition, safe design engineering philosophy engenders a conservative approach with added redundancy backup. Detail such as the pros and cons of standard hoist reeving systems, cross-reeved systems or dual reeving systems, and which is the better design philosophy are the currency of contemporary debate.
Safety functional requirements and the ingenious work designers do to combat the many problems that can occur are an important part of crane design and development. The important argument of “hangman’s drop,” (prevention or withstand) and how no one solution fully solves the problem remain key issues.
The prospect of new build is still further advanced in the US, where there is a strong feeling of optimism regarding the potential for building a new generation of nuclear plants. Several utilities have already filed for combined construction and operating licenses (COLs) based on specific sites, some of which are locations of existing nuclear plants with space for additional units. Most of the prospective new plants are based on new generation nuclear steam supply system (NSSS) technology, such as the Westinghouse AP1000 advanced passive reactor, as PaR Nuclear crane product manager Jim Nelson explains: “Chances for US success of this reactor are bolstered by the recent selection of the AP1000 by the Chinese government for four new reactors there. In the US there is talk of the real possibility of one or more actual orders for new plants during 2007.”
Nelson says PaR, which specialises in fuel flask handling equipment, says there has been much activity of late in the regulatory climate and applicable crane standards which will affect future business in the sector for hoist manufacturers. “My observation is that an ongoing trend in the direction of heightened expectations in the areas of quality assurance, documentation, and safety- and reliability-related features in cranes in nuclear plants will continue,” he tells Hoist.
Last year the US Nuclear Regulatory Commission (NRC) issued its Regulatory Issue Summary covering standards and guidelines for design and operation of cranes and related lifting equipment in US nuclear power plants. While not introducing significant new guidelines, says Nelson, the RIS emphasises the importance of existing guidelines for heavy load handling, and draws on recent industry experience that calls attention to certain aspects of crane design and operation deserving of particular attention. The RIS also recommends that the regulatory agency adopt a crane design standard, NOG-1, published by the American Society of Mechanical Engineers (ASME), as an acceptable basis for conformance to the guidelines of NRC NUREG-0554, Single-Failure-Proof Cranes for Nuclear Power Plants. A number of other industry codes and standards apply to nuclear plant cranes. These are to be summarised in a new document to be published by the ASME later this year: A Guide to American Crane Standards.
It would be difficult to overstate the importance of using the safest crane design for handling heavy loads in nuclear facilities. Dropping or otherwise losing control of a load in an area where the result could be damage to safety-critical equipment or spent fuel is unacceptable. With this in mind, the ASME Committee on Cranes for Nuclear Facilities (CNF) has written standards for overhead cranes – the equipment that handles the great majority of heavy loads.
No other industry or government standards, including those of the Crane Manufacturers Association of America (CMAA), have addressed issues important for nuclear facility cranes such as quality assurance, dynamic seismic analysis, and SFP crane features. NOG-1 and NUM-1 cover all these and more.
Many cranes and hoists supplied into this specific market place often utilise sophisticated control systems and remote operation via pre-programmed PLC controllers, necessitated by the extremely hazardous working environments in certain areas.
PaR has supplied its own interpretation of the NRC’s “Single-Failure-Proof” concept crane for handling flasks of spent nuclear reactor fuel assemblies in the fuel storage building at a US plant. It operates like a folding robot, its legs extending and retracting by means of screw jacks, and the girder extensions have mechanised 180-degree swing from folded back to extended positions. This enables the hoists to travel beyond the wall of the spent fuel storage pool for flask loading. When not in use, the crane folds into its smaller envelope, allowing the other cranes in the fuel building to operate over their normal travel ranges.
Both hoists include the PaR Nuclear/Ederer Nuclear X-SAM (Extra Safety And Monitoring) system, which enables them to withstand all known causes of crane failures that can result in dropped loads when ordinary crane features are supplied. Features include precise positioning and laser-assisted verification that the fuel flask is positioned accurately inside a tight clearance envelope within the spent fuel pool. Operational interlocks prevent the operator from handling a load in violation of specified procedures.
The two hoists have concentric load centres, enabling the crane to insert a canister of spent fuel into a huge transfer flask while continuing to support the flask itself. The crane was designed and built under a strict Quality Assurance Program and is seismically qualified to withstand the strongest design earthquake for its location. In accordance with normal procedures it was fully assembled and load tested at the factory before delivery.
Nevertheless, despite a greater tendency for manufacturers to build in safeguards, an NRC study makes several observations regarding strengths and weaknesses exhibited by crane operating experience and programmatic control of heavy load movements at nuclear power plants. The human error rate for crane operating events has significantly increased in recent years. The proportion of crane issue reports caused by poor human performance has increased over time, by between 70 and 80%. Unfortunately, little seems to have improved since similar human error results were reported in a 1996 Department of Energy (DoE) report: Independent Oversight Special Study of Hoisting and Rigging Incidents Within the Department of Energy [DOE]. Human error, whether directly associated with supervisors or equipment operators represented some 94% of DoE hoisting and rigging incidents.
The human error rate for crane operating events has significantly increased in recent years. The proportion of crane issue reports caused by poor human performance has increased over time, by between 70 and 80% nukes 11 Currently, only a handful of retired power stations and other nuclear installations are undergoing decommissioning in the UK nukes 6 The US, Russia (along with the former Soviet Union countries), France, Japan, Germany and the Nordic countries have all either entered the decommissioning phase, or are about to embark upon it nukes 7 Last year, an all-party Trade and Industry select committee said the final cost of decommissioning and clean up was still unclear nukes 8 It is estimated that in Britain alone, the cost of the nuclear clean-up is likely to rise significantly beyond the government’s official forecast of £70.2 billion nukes 9 Still in its infancy, the dismantling business offers considerable opportunities to materials handling specialists in the removal of irradiated reactor parts and, more traditionally, benign boilers, other facilities such as turbine generators, associated nukes 10 The UKAEA is progressing with the decommissioning of Winfrith in Dorset nukes 1 Winfrith was the site of nine reactors including the experimental Dragon reactor and a large Steam Generating Heavy Water Reactor (SGHWR) feeding the UK National Grid nukes 2 Many of the procedures developed during the decommissioning of WAGR and at Winfrith are being put into practice elsewhere nukes 3 The decommissioning methodology – and the equipment used to carry out the work – are subjected to rigorous scrutiny by the Nuclear Installations Inspectorate (NII) of the Health and Safety Inspectorate (HSE) nukes 4 Around half of the civil nuclear research site has already been released for commercial use, and decommissioning will be fully complete by 2020 nukes 5
