VISIT TO DUNGENESS POWER STATION ‘A’
27th April, 1982
Thirty-one attended the visit; 28 travelling by coach, of these 23 were members of our Association and 5 were wives; one member travelled separately. Two guests also attended. Messrs. C.T. Melling and A.T. Ford, members of the Society of Retired Chartered Engineers, South East Kent,
Mr. Melling being Chairman.
As was arranged, the party arrived at the gates at 2 p.m. We were shown a film, to start with, and this covered generally to-day’s conception of the atom and the means for harnessing the power generated by fission. Animated diagrams showed the intense activity of the protons, neutrons and electrons, tracing out orbits as in a solar system like our own, the open spaces between them being relatively as large, though microscopic in size. Collisions that took place between invading neutrons and nuclei, was also illustrated, with the break-up of the atom as a consequence, to the accompaniment of the release of much heat and the formation of two totally different elements together with the detachment of two or three neutrons, the outcome of the new element formations. Returning to the simile of planets in a universe, the nucleus representing the sun, neutron/ nuclei collisions can only be few and far between; to boost this the fuel elements in a nuclear pile are surrounded by a ‘moderator’, generally graphite, which returns unspent neutrons to the core by bouncing them back, at a reduced velocity. If the chain-reaction was allowed to run wild, disaster would follow, so both a temperature and a reaction control is exercised. The first is achieved by cooling the core and, in the case of U.K. designed reactors, this is by continuous CO2 gas recycling. The gas, in turn, cooled by passage through heat-exchangers to give up sufficient of its heat to generate high-pressure superheated steam at 98.5 atms. the object of the exercise. Fission control is effected by the use of boron rods which can be
moved up or down in the neutron field. It has the reverse effect of the
moderator since the boron absorbs and holds neutrons by forming isotopes.
With increased neutron capture, pile activity will reduce, finally reaching shut-down, the restart available by withdrawal.
Uranium presents itself as an ideal fuel for a variety of reasons.
It is mildly radio-active and comparatively storage-safe, but, inherent in its composition, the ore carries an isotope which is strongly fissile and acts as ‘tinder’ to set off a chain of reactions, at temperature.
Though only 0.7 % of the whole, it provides most of the fuel- supply in the life of the element and this is extended, if enriched to 2.3%
A feature of such ores is the high ‘atomic mass’,usually indicated by a number alongside the chemical symbol which, in this case, will appear with ‘U’ as U238, denoting the sum of the protons and neutrons, two thirds of which being neutrons, the ‘ammunition’ of atom fission. Such atoms are, also, termed ‘heavy atoms’.
The reactors in station ‘A’ are of the original ‘Magnox’ design, named after the alloy cladding around the uranium fuel rods. Station ‘B’ will have the new ‘Advanced Gas Reactor’ design but both designs are essentially the same. Capacity of ‘A’ is 550MW, ‘B’ to be 1200.
An engineer/operator was made available to us for questioning. Briefly some of the quest ions put are as follow, and the answers received,
Q. How is the carbon dioxide gas coolant circulated ?
A. This is by steam turbine driven blower mounted directly below a
heat-exchanger (boiler) drawing ‘cooled’ gas (350 C) from the heat exchanger and returning it directly to the base of the furnace, the whole of this being situated outside the radio-active area. There are four of these.
Q. What is the overall thermal efficiency of the Station ?
A. This is 29% for Station ‘A’, with an expectancy of 41% for ‘B’.
The improvement is based on the higher temperature differential across the heat-exchange intended for ‘B’; ‘A’ being very low but even then comparing favourably with a coal-fired installation, in actual output.
Q. It would appear that the fuel elements now in use are only
marginally self-sufficent even after enrichment by ‘breeding’. Have any attempts been made towards isolating the isotope (U235) for enriching the ore, by addition.
A. No way has been found of doing this yet, but, since the life of
the elements now in use can be gauged in months, there seems little urgency to improve upon this.
Q. What were the main causes of shut down?
A. Welding failures ; but, so far, those detected
had been accessible for repair, with safety. Welding testing technique had been well improved since the first reactors went in and there was a great deal of confidence that such failures would be greatly reduced.
These failures were likely most where there were ‘live’ stresses, such as bellows pieces in ducting, but such parts are usually accessible and repairs can be effected without shut down.
Q. What is the maximum life expectancy of a gas-cooled station ?
A. This was in the order of 40 years and was based on the time
estimate, for the core to roach full saturation.
We were then escorted round the station, in groups of 8. So good had been our briefing we could, doubt less, have managed on our own, but for the way around. We were, of course, excluded from areas which could have required special clothing or precautions but we were able to spend some time looking down on the loading bay from a specially constructed visitors platform behind glass panels and see the operators in their special protective clothing though, apparently, that was not all that essential normally. From there we saw the refuelling machine and the moving gantry which carried the machine over the 18 inch cube concrete blocks which covered the tops of the fuel elements. In another two rooms were the four boilers with the gas circulating blowers mounted below them.We passed through the control rooms with its walls and consoles crammed with instrumentation. From here the depth of insertion of the boron rods is regulated. We also visited the turbine hall and various intermediary rooms and on emerging from all these we were checked against radio-activity.
Finally, we were treated to tea and biscuits and given further opportunity to put questions to the same gentleman as before, who came round the tables. In all a truly memorable visit and one that helped us to realise the extent to which we are now part of the nuclear age, from which there can be little chance of turning back. The visit ended with an expression of thanks, by the President, for the thought and hospitality so generously extended to us.
VISIT TO DUNGENESS POWER STATION ‘A’