Wigner energy

During a planned release of Wigner energy from graphite in Windscale No. 1 Pile, it became overheated. Oxidation of the graphite raised the temperature further, despite attempts to restrict access of air, and part of the reactor core reached an estimated temperature of 1300°C (Penney, 1957). About 6 to 8 tonne of uranium melted, but, in contrast to the previous operational experience, there was remarkably little dissemination of particulate uranium oxide (Chamberlain Dun-ster, 1958 Chamberlain, 1981). The high temperature and restricted air flow probably caused a skin of sintered oxide to form on the uranium. 

Are other factors such as Wigner energy or criticality relevant Is there the potential for reactivity excursions In practice it is unlikely that either criticality or Wigner energy will affect the control factor, since the RHP relates to steady state storage rather than operations. 

As a result of the fire, the residual Wigner energy in Pile I is considered to be less that in Pile 2 the several anneals which had previously taken place in each reactor also mean that the distribution of the residual energy within the cores is not easy to predict. 

Figures 2 and 3 illustrate the typical channel structure, showing the vertical block junctions which embrace the fuel channels, the horizontal block junctions, and the slot feature in the base of the channel in which the graphite fuel boats were located. Figure 3 shows an example of extruded material, possibly lead, which has reached the fuel channels at a number of locations from adjacent isotope channels either during normal operation or during Wigner-energy anneals.

A total of 134 samples approximately 15 mm diameter by 20 mm long were recovered from a total of 13 channels previously identified as completely fi ee of obstructions. The associated dust was also recovered. 11 of these channels were located along lines joining the centre of the core to the base (vertical section) and to the right-hand side (horizontal section). One criterion by which the channels were selected was to attempt to include regions in which Wigner energy was likely to be maximised on the basis of irradiation dose and temperature. 

The correlation of these results with the perceived dose and temperature distributions within the core during operation is not very successful this is attributed to the several Wigner-energy anneals which took place towards the end of the reactor s operating period. 

In no case has any result been obtained which alters the previous perception of the status of Pile 2. This is extremely encouraging since it is considered that it will be relatively simple to prepare a safety case to modan requirements which will justify the continued safe storage of this reactor core without the imposition of special conditions. The experience gained from the dismantling of Pile 1 (where the potential problems associated with Wigner energy are less than in Pile 2) will ultimately be employed in the disposal of this reactor... 

In addition each fuel element is separately contained and supported within a graphite sleeve - a refinement which eliminates bowing and the sticking of the elements and gives protection against the build-up of Wigner energy. 

The second-order Brillouin-Wigner energy has the form... 

The Brillouin-Wigner energy expansion has the form of a simple geometric series in Tf 1. Explicitly, the coeliBcients in the BriUouin-Wigner energy expansion are therefore... 

Atomic Energy Office (1958). Final Report of the Committee appointed by the Prime Minister to examine the Technical Evaluation of Information Relating to the Design and Operation of the Windscale Piles, and to Review the Factors Involved in the Controlled Release of Wigner Energy, [White Paper] London HMSO (Cmnd 471). 

The new piles were certainly not trouble free, but worked efficiently for several years. Until Calder Hall became available in 1956, they were the only source of plutonium for the UK. In a much quoted phrase, Hinton described them as monuments to our initial ignorance .Many problems that arose had to be solved on an ad hoc basis and none more so than the problem of Wigner energy. The technique that was evolved to solve the problem had one major flaw, which would not emerge until 1957. 

The first time a spontaneous release of Wigner energy occurred was after the pile had been running for some time, and was completely unexpected. It must have come as a considerable shock to the operators at the time. This is how it was described ... 

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