Windscale Advanced Gas-cooled Reactor Research Articles

Characterisation of corrosion products from degraded uranium dioxide fuel

A number of cans of spent nuclear fuel from the UK's Windscale advanced gas-cooled reactor have been recovered from over four decades of pond storage to be repackaged. All the cans were found to be flooded and those with cut sections of pins contained significant quantities of sludge; cans containing only intact and/or punctured pins were free from sludge. These observations necessitated a study of the sludge to understand its composition. The results will help inform the likely long-term behaviour of irradiated UO 2 in contact with water during interim or extended storage, or disposal. Inductively coupled plasma – mass spectrometry found the sludge to be heterogeneous between cans but to be largely uranium based. Iron dominated the composition of one sample. Scanning electron microscopy – energy dispersive X-ray, power X-ray diffraction and thermogravimetric analysis indicated that the uranium phases were almost entirely crystalline UO 2. No other uranium phases were detected and the UO 2 was assigned as shards of spent fuel. In contrast, the iron phases consisted of a mixture of different oxides and oxy-hydroxides that appear to have precipitated from solution. Caesium was commonly associated with the iron phases. The sludge was associated with a substantial amount of moisture when investigated by TGA.

The status quo on HTGR decommissioning

Five High-Temperature Gas-Cooled Reactors, which have been operated in the past, are now in different stages of decommissioning. The present status, technical approaches and experiences are described for AVR, DRAGON, FSV, Peach Bottom Unit 1 and THTR.After defueling the core, the Reactor Pressure Vessel (RPV) of AVR has been grouted with low-density cellular concrete and removed from the containment building. The RPV is now stored in a new interim storage building, on the site of the Juelich research center. The DRAGON reactor was operated in UK from 1965 until 1976 when it was defueled and put into safe storage. It was partially decommissioned in 2005 and is now facing further removal steps with a target decommissioning completion date of 2022. The 330 MWel Fort Saint Vrain (FSV) HTGR in US was shut down in 1989 and undergone immediate dismantling with a fixed-price contractor. The plant site was released for unrestricted use in August 1997. The Peach Bottom Unit 1 was shut down in 1974. The spent fuel was transferred to INL. Decommissioning is not expected until 2034, when the other LWR units at this site are scheduled to shut down. The German 300 MWel Thorium High-Temperature Reactor (THTR) was closed-down in 1989 and resides under Safe-Enclosure conditions since 1997, after having evaluated the radiological consequences of direct dismantling vs. 30 years safe enclosure.The 32 MWel Windscale Advanced Gas-Cooled Reactor (WAGR) prototype shows some relevant design similarities to modular HTGR. Lessons learned from the WAGR and other relevant decommissioning projects are therefore addressed, too.As a general conclusion, it can be stated that the decommissioning procedures need to be thoroughly reflected already during the design of any new HTGR project.The paper has been elaborated within the European GEMINI+ project and is based on the deliverables of the Task ‘Decommissioning and Waste Management of the GEMINI+ system’.

R.E. Hester and R.M. Harrison (Eds.): Nuclear Power and the Environment

As volume 32 in the RSC series entitled ‘‘Issues in Environmental Science and Technology’’ it follows the general series pattern of editors selecting a range of experienced authors to collate relevant facts that will thoroughly cover the subject matter. Divided into nine chapters, all principal environmental issues in the nuclear industry are dealt with, but there is a distinct UK-centric bias. Each chapter is well referenced, but surprisingly poor cross-indexing makes it difficult to move easily from subject to subject. Globally 436 nuclear power plants have been built, 60 more are under construction and 131 proposed; but now comes the contradiction—the eye-watering estimates for decommissioning, with a prediction of $1 trillion for the USA alone. It is widely assumed that the cost of decommissioning a nuclear generator is the same as its construction cost—but then there is an actual cost, declared in the press but not included in this book. The Windscale advanced gas-cooled reactor (AGR) has just been totally stripped, leaving the building available for reoccupation, at a cost of £110 M—quite a contrast with prediction. It is also claimed the next decommissioning will cost even less. In contrast high level waste (HLW) disposal is in a mess, with most nations continually pushing the problem into the future, exceptions being Sweden and Finland. Sellafield alone has 100 tones of separated plutonium to either use or dispose of; so who will take the decision to store it safely; or better, utilise it as a fuel? The one route clearly not in consideration is to research and build reactors known as accelerator driven systems (ADS) where thorium replaces uranium as the primary fuel. These reactors also consume HL wastes as fuel. Creating what will be in effect a new industrial sector is already well advanced in China and India—but not anywhere in Europe. The lack of a mention of this possible opportunity for European countries is very surprising in a book with this title. With over a century of accumulated knowledge, more is probably known about the effects of radiation on humans than there is about the effects of any other hazard. The average lifetime background dose from natural exposure is 2.2 mSv. In a study of 174,500 workers just 6% had experienced a lifetime dose exceeding 100 mSv, and it was shown statistically that this group had no increase in cancer over that indicated from national rates. There was, however, a slightly elevated risk of leukaemia. In contrast, many European health services restrict mammograms to bi-annual assessment, which have an individual exposure level of 0.4 mSv. This compares to a 0.1 mSv contribution from a 10-h flight, and 16 mSv from a CT heart scan. Bibliography Nuclear Power and the Environment R.E. Hester and R.M. Harrison (Eds.) RSC Publishing, Cambridge, UK, 2011, pp 246 ISBN 978-1-84973-194-2 GBP 65.00

Project WAGR: the UK Demonstration Project for Power Reactor Decommissioning

The United Kingdom Atomic Energy Authority (UKAEA) has built and operated a wide range of nuclear facilities since the late 1940’s. 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.The Windscale Advanced Gas-cooled Reactor (WAGR) was the UK’s prototype Advanced gas cooled reactor and became the forerunner of a family of 14 reactors built to generate cheaper and more efficient electricity in the UK. WAGR was constructed between 1957 and 1961 and was a carbon dioxide cooled, graphite moderated reactor using uranium oxide fuel in stainless steel cans. The reactor consisted of a graphite moderator housed in a cylindrical reactor vessel with hemispherical ends. The reactor and associated heat exchangers were enclosed in the iconic spherical containment building regularly used by the media in the UK as an illustration of the nuclear industry.The reactor first produced power in August 1962 and achieved full design output in 1963. It operated at an electrical output of 33 MW (E) for 18 years (average load factor of 75%). In 1981 the reactor was shut down after satisfactory completion of all the research and development objectives.In anticipation of the UK’s likely nuclear decommissioning needs the UKAEA decided to decommission WAGR to the International Atomic Energy Agency’s (IAEA) stage 3 (restoration of the area occupied by the facility to a condition of unrestricted re-usability) as the national demonstration exercise for power reactor decommissioning.Since 1998 the UKAEA and its contractors have been undertaking the dismantling of the reactor core components and pressure vessel in a series of 10 campaigns. These contain neutron activated components expected to produce dose rates well in excess of 1Sv/hr. To carry out the work UKAEA installed an £8M remote dismantling machine (RDM), a waste recovery and transport system and a shielded waste route through which to move, sort, assay condition and package the waste. The RDM and associated hoist have been used to deploy the numerous tools and end effectors necessary to effect the painstaking dismantling process.Now that the remote operations are reaching a conclusion it is possible to review the success of both these tools and the RDM itself.

Operational waste removal from Windscale advanced gas-cooled reactor (WAGR)

Operational waste removal from Windscale advanced gas-cooled reactor (WAGR)

WAGR Decommissioning: The Transport of WAGR Heat Exchangers to Drigg Low Level Waste Repository

The Windscale Advanced Gas-Cooled Reactor (WAGR) operated between 1963 and 1981 and was used as a prototype for experimental research and development related to the Advanced Gas-Cooled Reactor programme. WAGR is now the UK's demonstration project for power reactor decommissioning. A major operation in the decommissioning of the WAGR was the removal of the four heat exchangers (REs) from their concrete bioshields inside the secondary containment sphere and transporting them to Drigg low level waste repository. The issues involved with the transport of the four heat exchangers are described, covering details of the heat exchangers themselves as a transport package. Preparation of the route along public roads is described along with details of the transport vehicles and transport operations.

The Design of a Method and Equipment for Decommissioning the WAGR Pressure Vessel and Insulation

The paper is a comprehensive design study and describes a method for decommissioning the Windscale advanced gas-cooled reactor (WAGR) pressure vessel and insulation. The design of equipment to carry out the required operations is also given. The structure of the pressure vessel and insulation and its residual activity are described and assessment criteria are proposed to judge the suitability of possible decommissioning methods. Various ideas are then considered and compared using the assessment criteria. The most promising solution is developed and general arrangement drawings of equipment are given. A reserve method, using different technology, is identified in case the first method fails due to unforeseen circumstances. The method has been adopted for development in the WAGR decommissioning programme.

The Windscale Advanced Gas-Cooled Reactor Stage 3 Decommissioning Project

The planning and the progress to date of the United Kingdom’s first full-scale decommissioning project, which is dismantling the Windscale Advanced Gas-Cooled Reactor to stage 3 (the “greenfield” site concept), are described. The plant and the facilities that are being constructed to dismantle it are described. Methods used to separate and pack for disposal nonradioactive, low-level, and intermediate-level radioactive waste are summarized.

UKAEA and CEGB Development Programmes

The best method of identifying and solving the problems that might arise while decommissioning a nuclear reactor is to pursue a real project. The Windscale advanced gas-cooled reactor (WAGR) project has already highlighted several topics and thus indicated the direction in which the development programme should go. It is recognized that the concrete pressure vessels, which are not part of the WAGR structure, present special problems and so most of the CEGB work to date has been aimed in that direction. Both programmes are described.

Decommissioning Windscale Advanced Gas-Cooled Reactor

This paper describes the UKAEA project to decommission the Windscale advanced gas-cooled reactor. The plan is to take down the reactor itself, the heat exchangers and the sphere in which they are all contained. A special remote-handling machine is being developed which will cut up the radioactive items, and a facility is being built to package the waste for storage and disposal.

Measurement of Sulfur-35 in the Coolant Gas of the Windscale Advanced Gas-Cooled Reactor

Sulfur is an important element in some food chains and the release of radioactive sulfur to the environment must be closely controlled if the chemical form is such that it is available or potentially available for entering food chains. The presence of /sup 35/S in the coolant gas of the Windscale Advanced Gas-Cooled Reactor (WAGR) warranted a study to assess the quantity and chemical form of the radioactive sulfur, in order to estimate the magnitude of the potential environmental hazard that might arise from the release of coolant gas from civil advanced gas-cooled reactors. This paper describes the identification of the sulfur species and both its chemical and radiochemical determination. 6 refs.

The irradiation performance of magnesia-doped UO 2 fuel

Large-grained UO 2 doped with 5 mol% magnesia has been irradiated in the Windscale Advanced Gas Cooled Reactor to 28 500 MWd/ te( U). Fission gas release from the doped fuel was reduced by a factor greater than 2.5 × in comparison to that from standard UO 2 irradiated under identical conditions. Micro-gamma scanning showed that the retention of 137CS was also considerably greater in the doped fuel. Although computer calculations suggest that the improved fission product retention at this burn-up can be accounted for by differences in grain size, microstructural examination of the irradiated fuel showed considerable differences in the morphology of fission gas precipitation between the doped and undoped fuels. In particular, large fission gas bubbles ~0.2 μm in diameter were observed within the grains of doped fuel. It is suggested that these are stabilised by interaction with the MgO precipitates and that this may reduce diffusion of gas to the grain boundaries at higher burn-ups.

The measured emission of fission-product gases from operating uo 2 fuel

The fission-product gases released from a sample of uranium dioxide during irradiation in a capsule to 4.1 × 10 20 fissions/cm 3 in the Windscale advanced gas-cooled reactor (WAGR) have been measured continuously. The steady-state release rate was principally due to knock-out from the fuel surface and showed a strong dependence on isotopic half-life, in good agreement with theory. Occasional bursts of release of long-lived gases were detected, the quantities of which lend support to current theories of the mechanism of stable fission gas release. The release rates of gaseous fission products were also measured in an atmosphere of reactor coolant gas.

Paper 2: W.A.G.R. Fuel-Handling Equipment: Design, Operation, and Contribution to Future Designs

The ability to charge and discharge fuel and other reactor channel components with the reactor on load is a key feature of all the British Electricity Boards' nuclear power stations. The system adopted on the Wind-scale Advanced Gas-cooled Reactor (W.A.G.R.), where each core channel is provided with its own standpipe (single-channel access), simplifies the operation of locating a particular channel; there is no need to insert any moving parts into the reactor circuit, and the critical operation of coupling the fuel to the machine is carried out at ambient temperature with no radiation problems. For five years the W.A.G.R. fuelling machine has demonstrated the ease of fuelling with a single-channel access system, and the integrity and reliability of the equipment is demonstrated by its record, which exceeds 7000 operations. By taking advantage of this operating experience on the W.A.G.R. it is clear that changes can be made in the design of fuelling machines and equipment for future commercial stations to improve efficiency and reduce costs.

The windscale advanced gas-cooled reactor

The main purpose of the reactor development programme of the United Kingdom Atomic Energy Authority is to develop reactor systems that are economically more attractive than the present series of nuclear stations. The Windscale A.G.R. forms one of the main lines of development undertaken by the Authority over the past five years.

The Windscale Advanced Gas-cooled Reactor

The Windscale Advanced Gas-cooled Reactor