Magnox steels

Here to is the ageing time, when CEC achieve the saturation in the absence of irradiation. For Magnox steels this time in years is determined from Eq. (18) ... 

M R Wootton, R Moskovic, C J Bolton and P E J Flewitt, Experience of surveillance schemes adopted for Magnox steel reactor pressure vessels. Energy Mater, 2008, 3, (1), 45-56, available from www.maney.co.uk/joumals/ema and www.ingentaconnect.com/content/maney/ema. 

The large physical size of the later Magnox stations, such as Wylfa, led to the development of the more compact advanced gas-cooled reactor (AGR) design [31] that could utilize the standard turbine generator units available in the UK, Stainless-steel clad, enriched uranium oxide fuel can tolerate higher temperatures... 

Cladding. The Magnox reactors get their name from the magnesium-aluminium alloy used to clad the fuel elements, and stainless steels are used in other gas-cooled reactors. In water reactors zirconium alloys are the favoured cladding materials. 

W.A. Westall et al.. Determination of and Li in Magnox reactor steels. Environmental Radiochemical Analysis, RSC 10 Int. Symposium, Oxford, U.K.,13-15 September, 2006. 

Magnox Reactor waste streams include a wide range of materials such as ion exchange (IX) resins, sludge, Magnox fuel element debris (FED), reactor graphite and carbon and stainless steels. Some wastes will exhibit heterogeneity and for many construction materials such as steels there will be radionuclides present which are neutron activation products of trace impurities and were un-quantified at the time of manufacture. 

DETERMINATION OF TRITIUM RADIONUCLIDE AND LITHIUM PRECURSOR IN MAGNOX REACTOR STEELS... 

Deuterium is in very low concentration. Lithium has an atomic weight of 6.94 and the abundance of Li is around 7% in natural Li. The main reaction product of B is Li which does not generate but there are other, minor reactions that do. Except in boron steels, the activation of Li predominates. Another source of in fission reactors is the low yield, ternary fission of fuel (-130 x 10 atoms per fission product pair). In Magnox gas-cooled reactors, from ternary fission is mainly retained in the metallic uranium fuel and its cladding but some is released into the coolant circuits, where it may possibly diffuse into structures within the primary vessel. Tritium is a low energy /5 emitting radionuclide of low radio-toxicity and with a half life of 12.3 years. 

Determination of Tritium Radionuclide and Lithium Precursor in Magnox Reactor Steels 139... 

The intention of this work was to measure the concentration of Li in examples of unirradiated reactor steel from which the likely activity concentration could be calculated using suitable values for the neutron flux. Calculated values could then be compared with the activities measured in examples of steel irradiated in a Magnox reactor. From this it was hoped to ascertain whether, given the concentration of Li, the activity could be reliably calculated or whether the effects of formation from ternary fission and subsequent diffusion would prevent this. Whilst the activity in activated steel was measured in a straight forward manner, the measurement of Li in steel was difficult. The conventional analytical techniques of ICP-OES and ICP-MS respectively failed to achieve an adequate detection limit, so alternative techniques were tried. SIMS was sensitive enough to detect Li but is a microscopic technique and so was prone to large uncertainties when used to predict the bulk concentration. NAA followed by radiochemical determination allowed the determination of but initially suffered interference from another activated radionuclide ( S). 

In summary, four experimental/calculational techniques, ICP-OES, ICP-MS, SIMS-SEM-EDX and NAA with radiochemical analysis, were used to determine the concentration of Li in reactor steels either by direct measurement or by calculation from measured values. Li measurements from the first two techniques, ICP-OES, and ICP-MS, were discarded due to excessive values/excessive limit of detection. SIMS/SEM-EDX measurement of Li and two approaches to neutron activation of steels samples with radiochemical measurement of values used to calculate Li concentrations yielded 10 valid data points spread over one order of magnitude, see Table 4. In particular, the two neutron activation approaches using differing flux, time and temperature conditions yielded good consistency of calculated values of Li. Consequently, the average Li concentration in Magnox RPV steel is considered to be < 1 ng g" and estimated as 0.4 0.2 ngg- x(lB). 

The concentration of natural lithium (tritium-precursor) in un-irradiated Magnox RPV steel has been estimated directly using SIMS-MS and SEM-EDX and indirectly from measuring tritium induced by neutron activation of mainly Li. Tritium was measured in surveillance specimens irradiated during the 26 year operational life of a Magnox reactor followed by radioactive decay of approximately 15 years. For comparison purposes, inactive archive RPV steel was also irradiated in the CONSORT reactor followed immediately by radioanalysis for tritium. In this way, the possibility of diffusive transfer of tritium into or out of the steel during residence in the reactor could be evaluated. 

Based on various approaches, analytical techniques, calculations and available samples, the average lithium concentration in Magnox RPV steel is considered to be < 1 ng g and estimated as 0.4 0.2 ng g (IB)... 

Given the dependence on trace impurity levels and their distribution, further analytical data are needed to increase the confidence in the estimated value of the average concentration of natural lithium in Magnox RPV steel. 

Preliminary indications are that tritium activity may be predicted from the natural lithium concentration alone and no evidence was observed of significant tritium diffusion into or out of steel activated in a Magnox reactor. 

The Advanced Gas-cooled Reactors (AGR) are built into PCPV with internal boilers and gas circulators. The uranium-enriched oxide fuel is clad in stainless steel so that the risk of a charmel fire or fuel meltdown under fault conditions inherent in the Magnox design has been greatly reduced. 

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