DISCUSSION MEETING, 11th FEBRUARY, 1987.
“CHERNOBYL AND ITS AFTERMATH”
Introduced by John Fowler (Member)
at Durrington Community Centre.
John Fowler first gave a precis of his very extensive personal involvement with nuclear power plant construction including many years concerned with the development of nuclear safety, quality assurance standards and programs for many types of reactor. He represented the U.K. for eleven years at the International Atomic Energy Agency in Vienna, which developed the Q.A. and safety standards that are now commonly used world wide – including the Soviet Union. John assisted in developing the Q.A. for the Standard Nuclear Power Plant (S.N.V.P.P.S.) design originating in the U.S.A. upon which the Sizewell ‘B’ design is based and worked for the C.E.G.B. in the Sizewell public enquiry.
To set the scene John explained the classification of reactors – Magnox, Advanced Gas Cooled (A.G.R.), High Temperature (H.T.R.), Boiling Water (B.W.R.), Canadian (Candu), Steam Generating Heavy Water (S.G.H.W.R.), Fast Breeder (F.B.R.), Pressurised Water (P.W.R.) etc. and gave some principal features. He also touched on the future fusion reactors as distinct from the current fission types.
The Magnox, the first U.K. reactor for power generation used natural uranium fuel, had a graphite moderator and carbon dioxide gas took the heat generated to heat exchangers/boilers. The A.G.R. is the more advanced current U.K. build of this type (at Heysham etc.) now with a concrete pressure vessel with integral pods for steam generation. The F.B.R. at Dounreay is not yet fully developed. The P.W.R. developed in U.S.A. is the reactor in greatest use worldwide and will probably be adopted for Sizewell B. It uses enriched uranium fuel in stainless steel containers with light water as moderator and coolant in a steel pressure vessel, with four heat exchangers external to the vessel; the whole is enclosed in a secondary concrete containment. The design is based on S.N.V.P.P.S.
The H.T.R. being developed in the U.K., Germany and the U.S.A. uses enriched uranium fuel pellets which embody also the graphite moderator, helium gas in the coolant and the pressure vessel is of concrete. The B.W.R, developed in 1950’s in Japan, U.S.A. and U.S.S.R. is of the same family as the P.W.R. but the light water moderator around the core boils to produce steam for the turbines direct; in effect it acts as a recirculating boiler with steam separators and driers situated in the reactor. The Candu is proven to be efficient, it avoids the large pressure vessel by having fuel in zirconium tubes which are horizontally mounted in a large tank called a callendria filled with deuterium oxide (heavy water) as moderator and coolant.
Now, the Chernobyl reactor – the R.B.M.K. designed in 1954 is technically a B.W.R. rated 1000 MW with pressure tubes like the Candu but with a graphite moderator and operating at the higher core temperature of 700 deg C. Very signifcantly – it has no secondary containment. We would have found the design unacceptable for a number of reasons, including (a) in having high temperature graphite in close proximity to boiling water and fuel elements with consequent possibilities of combustion and generation of dangerous elements such as hydrogen and (b) the lack of secondary containment. Even so, the disaster can be largely attributed to operator failure, inadequate design review and lack of proper safety operating standards and systems. A totally unnecessary experiment was embarked on – unnecessary because many had done it before, but under safe conditions. The Chernobyl operator disconnected the turbine from the reactor, leaving the generator to supply the auxiliary system using its kinetic energy.
But it all went wrong. The decreasing flow of cooling water produced a dry boiler situation, called a positive void coefficient. With steam generated at rising temperatures and decreasing water supply the graphite caught fire and, at several thousand °C, the top of the reactor blew off releasing radio-active material.
John described in some detail the timetable of events – power reduced over a 12 hour period to 50% – disconnection of the turbine – 2 hours later emergency cooling disconnected. The test was delayed and the reactor left thus for 8 hours (VIOLATION OF RULES) as power dropped to 30 MW because controls were incorrectly set. In this condition xenon, a neutron absorber, builds up and moderates the reactor so inhibiting the restoration of power. Next day the operates stabilised power at 200MW continuous operation (VIOLATION OF RULES) and after 30 minutes started standby pumps to avoid loss of coolant water but this reduced steam so, to stimulate the reactor, he withdrew the control rods and simultaneously disconnected the safety trip (VIOLATION OF RULES). Even though he was worried that the rods were too far out he continued the experiment, despite the positive void problem, and as the turbine ran down water flow reduced – dry boil – disaster. Power rose from a fraction of full power to one hundred full power in 4 seconds. The operator tried manual shut-down but there were two explosions – first from steam and second from chemical reactions
The Russians subsequently made design changes in all their R.B.M.K. reactors – adding more absorbent control rods, increased the speed of their insertion, improved protection systems, operating rules and training.
In contrast the plant proposed for Sizewell B has had the most highly developed design review of any reactor in the world and conforms to the requirements of our regulatory bodies and international Nuclear Safety Standards.
In the open discussion Lionel Hill developed the theme that the event was really an accident from which lessons are learnt and one does not progress by always adopting a completely safe approach, the speaker agreed and deplored the media attempt to compare the event with an atomic bomb – actually it was millions of times less powerful and slower than a bomb – and stressed the safety procedures now adopted. Messrs. Hammond and Carey referred to the size of the pressure vessel, sea water cooling etc. and Bill Bailey asked about total energy release. Stanley Purkis spoke strongly against referring to the event as a valuable experience in view of the enormous damage to health and the ecology. Don Fife continuing this line referred to an estimate, made by an academic, of a possible major accident every seven years. Mark Markwell elaborated on the need to eliminate human error, which Jock Milway supported and included a plea for better publicity in order to reassure the general public, particularly about waste disposal. Tom Smith raised the question of limited supplies of uranium and use of breeder reactors. Mark Markwell referred to fusion as the safe way into the future and Lionel Hill finally asked about direct generation of electricity by magneto hydrodynamics (M.H.D.) in the long term.
The Speaker in answering and commenting on all the contributions said ‘Chernobyl’ was due to a number of features including loss of cooling water, and although the damage has yet to be fully assessed it was not really greater in overall terms than, for example, road traffic accident or smoking. The Russians attended the regular international safety meetings but although they had not been so forthcoming as the Western Bloc in some aspects it is to their credit that they have been completely open to the I.A.E.A. about the facts relating to Chernobyl. Most countries follow the I.A..E.A. principles but these are not mandatory. Whilst U.S.A., Sweden and some other countries have a moratorium against nuclear generation it is probable they will not be able to sustain this stand indefinitely as electric power is an essential service. There are really no significant problems for the U.K. and it is necessary to remember there are problems with other fuels also and that radioactive waste comes also from hospitals and other sources. As regards uranium shortage, breeder reactors will help and then fusion will solve fuel and waste disposal to a significant degree.
The Chairman expressed to John the meeting’s appreciation for the intimate, comprehensive and detailed account of this accident or disaster, which never should have happened, and for his handling of the discussion.
