On-load refueling

Components for this design are relatively small. They thus require no large unique facilities, can be made in a number of small factories, and pose no special problems for transport and installation. As in the case of the British gas-cooled reactors, on-load refuelling enables a high burnup to be obtained from low-enriched fuel. The channel tube design provides... 

All UK nuclear power stations are designed for on-load refuelling, and safety during operation is important if the station is to continue to generate at full load over the days and weeks ahead. During operations close liaison between the Central Control Room (CCR), engineers and industrial staff is essential. 

Other training programmes on the simulator include reactor shutdown, on-load refuelling and turbine run-up, as well as abnormal or fault situations, including the loss of auxiliary plant with the reactor at power, such as ... 

The tour of the station included a visit to the Central Control Room, Cas Circulator House and Turbine Hall. The pile cap was viewed from the visitors observation gallery. At the time of the visit. Reactor 3 was at 30% power for on-load refuelling operations and Reactor 4 was at full power. 

The British and French natural uranium gas-cooled reactors have been designed for on-load refueling. This has resulted from the small excess reactivity available in natural uranium systems and, therefore, the frequent plant shutdown that would be required if new fuel additions could not be made while the plant was in operation. The refueling machines that have been developed for this purpose are large (49) and represent a significant fraction of the plant cost. 

The helium coolant flows downwards through the core. On-load refuelling is obligatory. The fuel pebbles are passing the core 15 times. Due to the limited core diameter no metallic structure is required to support the top graphite reflector, and the control/ absorber units can be installed within the radial reflector, acting by gravity. 

The plant is designed for on-load refuelling. The fuel is natural uranium. However, there are several options for alternative fuel cycles which improve the economy of the plant, e.g. slightly enriched uranium up to 1.2% enrichment, recycling of plutonium in the form of spiked-fuel elements or homogeneous mixed oxide fuel, tandem operation together with light water reactors etc.). 

The plant is designed for on-load refuelling. Increasing the aluminium concentration in the uranium fuel rod to reduce the rate of swelling has permitted longer irradiation. An average discharge burnup of 5300 MWd/t without any limit on residence time in the reactor core is now achieved. 

Each reactor has six heat chambers supplying steam at two pressures to six 52.4 MW turbo alternators. For on-load refuelling universal charge and discharge machines are used. Plates 1.3 and 1.4 show the pictoral view and outline sectional elevation of the station, respectively. 

The TNPG, a consortium, installed two gas-cooled graphite-moderated type, using natural uranium as fuel with on-load refuelling. The number of gas circuits has been reduced to four back pressure steam turbines providing gas circulator drive. The turbine house contains 4No. 142.5 MW turbo alternators. 

The main source of radiation under accident conditions for which precautionary design measures should be adopted consists of radioactive fission products. These are released either from the fuel elements or from the various systems and equipment in which they are normally retained. In Annex III examples of methods for assessing radiation sources for selected accidents are described. The scenarios are selected for illustrative purposes and cover all the major categories of designs for nuclear power plants with LWRs, CO2 cooled reactors with UOj metal clad fuel, HWRs and reactors with on-load refuelling. 

The irradiated fuel has a very high radionuclide content owing to the fission products and transuranics that accumulate in it. For on-load refuelling systems, delayed neutrons that are emitted from the fuel while it is in the... 

V. On-load refueling with a machine of simpler design than that used in the magnox stations maximum fuel burn-up of 18,000 MW d/tonne. 

Once an equilibrium cycle has been achieved, the reactivity is maintained at a suitable value by on-load refueling. The average rate of refueling required is nine fuel bundles per day, to compensate for a reactivity decline due to burn-up of about 0.04% per day. 

While the coolant void coefficient is normally positive, the degree of subdivision of the piping allows any power excursion arising from a loss of coolant accident to be easily terminated by the control rods. An added advantage is that, with on-load refueling, the excess reactivity of the core can be maintained at a much lower level than is required for a light water reactor. 

The Individual standpipes, each serving its own channel and fitted with seal plugs, simplify refuelling, whether for on-load or off-load, On-load refuelling machines are complex and expensive, and call for high standards In operation and maintenance. 

In Table 1, the capital cost change Item Is related to changes In core size stemming from changes in channel form factor. The extra capital cost of the on-load refuelling machine is not Included. In the case shown, this would need to be less than 600,000 to be economically justified. 

This figure Is near the lower end of the estimated cost range for the additional equipment cost and thus on-load refuelling Is unlikely to be attractive In, reactors of 500 MWe or less. The off-load schemes seem to show optimum behaviour In the vicinity of 9-cycles. 

For on-load refuelling, the bum-up loss with respect to the ideal continuous fuel cycle is negligible. In the offload cycles, on the contrary, burn-up decrease is signifle tnt about 300 MWd/t with respect to the ideal cycle. 

All these comparisons are based on the assumptions that only VJL of the on-load refuelling operations produces some trouble leading to scram and that in 50 of such events it is possible to re-start very quickly, i.e., within half an hour. The picture could change quite substantially, if these assumptions are not verified. 

Feature 4, In the above list, on-load refuelling, has economic advantages particularly for natural uranium reactors where frequent or continuous refuelling reduces fuel cycle costs. In addition. It should reduce outage times for planned refuelling operations and the forced removal of faulty assemblies. 

The fuel burn-up at 8OOO MWd/t is not unattractive for a natural uranium reactor. The quoted thermal efficiency of the station is the minimum guaranteed. I cannot accept that the on-load refuelling system is complex. 

Mr Johnson raised a question on superheat. Nuclear superheat has never been a feature of Swedish commercial designs, but it is Incorporated in the Marviken reactor as an experimental feature to permit the possibilities to be investigated. The main purpose of Marviken is to demonstrate the most important features of the HHWR, such as direct cycle, natural circulation and on-load refuelling, to the stage at which commercial offers can be based on these features. The nuclear superheat facility could be incorporated into the reactor at a relatively low additional cost, and our calculations have... 

Simplified on-load refuelling should ensure high availabifity by enabling maintenance shutdowns to be planned for the most advantageous time and by giving the flexibility to adjust the fuel cycle according to the electrical load requirements. ... 

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