Nuclear Energy for Power

by J. FOWLER, member.
At the Durrington Community Centre – 8th February, 1989

The Greek phyolospher Democrates postulated the concept of the atom about 400 BC – an individual particle of matter that was not “Solid”, but having spaces within it. He seems to have been alone, other prominent thinkers – Socrates, Aristotle and Plato stuck to the earth, air, fire and water idea!

Boyle, in England, and others adopted Democrates’ concept in early statements of atomic theory – which still largely apply.

Enrico Firmi constructed an atomic pile in Chicago to produce continuous fission in 1941.

The Russians built the first nuclear reactor in 1954 to produce plutonium for military purposes, while in 1955 Britain built the Calder Hall plant for plutonium production and to generate electricity. Now there are over 900 reactors world wide, with a remarkably high level of operational safety, not with-standing Chernobyl etc.

Mr. Fowler presented a simplified description of the uranium atom which can exhibit nuclear fission and thereby release a lot of heat. The stable form of uranium atomic nucleus has 92 protons and 146 neutrons. The latter are electrically neutral and so unaffected by the 32 encircling electrons. Isotopic uranium has a different number of protons which become disruptive among the atoms around causing fission. Some 1% of natural uranium is of this nature. To increase this nuclear activity for heat production the natural uranium is enriched in its preparation for power station use.
In order to slow down the otherwise very rapid activity the uranium is surrounded by a moderator of water or graphite, which absorbs the heat in slowing down the neutron activity. The heat, is removed by a circulating fluid (C02 or water for example) which then passes through a heat exchanger to produce steam. The rate of heat release is controlled by rods of Boron or Cadmium inserted in the moderator. The immense concentration of energy is illustrated by one pin head of uranium having a heat equivalent of several hundred tons of coal or oil.
A number of types of reactor for power generation were described:-

1) . Magnox using natural uranium (U235) pellets in metal tubes forming rods which are inserted in 30,000 channels in the graphite core, and circulating C02 to extract the heat. The heat exchangers are outside the concrete reactor containment vessel (biological shield). The total weight of reactor, container and heat exchangers is about 1,000 tonnes. Calder Hall is of this design and each reactor is rated at 50 MW electrical output generated, steam at 17 bar and about 300°C, on load re fuelling by a 1000 Tonne re fuelling machine. More modern reactors of about the same size have an output of 500 MW electrical.

2) . The Advanced Gascooled Reactor (A.G.R.) up to 600 MW (Electrical). This uses 2% or 3% enrichment of the U23S content of the uranium rods.
There are larger tubes and rods which increases the gas flow and larger fuel elements. The heat exchangers are situated within the concrete container, which has 6 pods into which 6 exchangers are slid into place.

3) . The High Temperature Reactor. A development of the A.G.R. producing steam at 700°C and 49 bar. The fuel is 85% enriched with U235 in the form of coated pills – consisting of fuel plus modulator and in Germany thorium is added. The circulating coolant is Helium.

4) . The Pressurised Water Reactor (P.W.R.) which enjoys World-wide popularity and is now being installed at Sizewell. It is an American design. At Sizewell each reactor will steam two 600 M.W. generators. The same concept is used to drive nuclear submarines. Its great advantage is that it is factory built in sections for on site assembly, so that better quality control can be maintained. Its design is similar to the Magnox, with 2% or 3% fuel enrichment, and steam conditions of 325 °C and 15 bar. The concrete containing vessel may range from 400 tonnes to as much as 1 ,000 tonnes, each housing the reactor and 4 heat exchangers. These containers are designed to withstand a large aircraft crash as a direct hit.

5) . The Boiling Water Reactor – The “Kettle”! In this the reactor is immersed in ordinary water. This design is not used in this country.
The steam is raised within the container, and on load re-fuelling is not possible.

6) . The Canadian Deuterium (or heavy water) Reactor known as CANDU. The water contains deuterium oxide, having “heavy” hydrogen. Natural uranium is placed in pressure tubes, as in a locomotive fire tube boiler. Tubes can be individually maintained and arranged horizontally. The reactor has collandric support, suspended from the top, and the heat exchangers are within the containment vessel. This is a very safe reactor. However, it is constructed on site, with attendant quality control problems.

7) .The Steam Generating Heavy Water Reactor (SGHWR). This is similar to CANDU, but the tubes are arranged vertically.

8) . Breeder Reactor. This produces some very dangerous elements in its by-products – Amorisium and plutonium. It is described as a fast flux reactor, being likened to the welding arc plasma. Cooling and heat transfer is by liquid sodium. It has a core of depleted uranium (U238) which becomes U239 and then decays to plutonium and neptunium, it is a very complex process. The heat exchangers are outside/inside (take your choice!) the containment vessel.

9) .The Leningrade Reactor (RBMK) is the design used at Chernobyl. It is rated at 1,000 MW (electrical output) with steam at 270 deg C and 70 bar. It is a pressure tube reactor (like CANDU) with graphite moderator. Problems occur with close contact between the graphite and boiling water.
A design strongly criticised even before the disaster.

Thermofusion – the vision of the future. This concept is to fuse light atoms to make heavy ones. A temperature of 3 x 10 10°C is required, in
which hydrogen is converted into helium. This process is virtually the reverse of fission, is the one taking place in the sun. A long way off it seems.