Essential and emergency power systems Operational safety a t nuclear power stations is the most important consideration should a complete loss of electrical supplies occur during accident or transient conditions. Independent supplies are vital and must be guaranteed for reactor cooling purposes. This report describes a number of expert views on the subject.
by Eur. Ing. F. John L. Bindon
The design of a power stations electrical system to support generation requires very careful consideration t o be paid to its back- up essential and emergency supply systems. On grounds of safety and for providing sufficient independence of all external electrical connections for start-up (and black- start) purposes, these systems form a vital role in all power station installations.
The complexity of the design and the installation depends to a large degree on the type of generation. Nuclear power plants are sophisticated technology and because safety is of paramount importance, such plants have to have high levels of redundancy.
nuclear power stations, a half-day Colloquium was held recently at Nuclear Electrics Ba rnwood Headquarters. The Colloquium set out to address the current developments in essential and emergency supply systems and their compliance with licensing and regulatory requirements. The event was organised on behalf of the Working Party on Nuclear Power, supported by I E E Professional Groups P9 and P I 0.
presentation which dealt with a description of a generic electrical distribution system for nuclear power station. It was given by John Orton (Nuclear Electrics Training Centre, Oldbury) and made clear a number of principal considerations, amplified during the meeting .
It was felt important to have a review of the regulatory regime in the UK, in order to identify the licence conditions which are most relevant to Nlls safety assessment of essential power systems. A paper given by Eric Dixon (HM Superintending Inspector, NII) set out in broad detail the Regulators main areas of interest when examining such systems and how the licensees must clearly demonstrate their proposals for the operation and maintenance of such systems.
they apply to essential power systems in nuclear power stations:
To examine the current position in British
The Colloquium opened with a
There are three key licence conditions as
to be made for the production and assessment of safety cases to justify the plans operation in all its phases.
0 Secondly, adequate procedures must be in hand for the periodic and systematic review and reassessment of safety cases.
0 Finally, the systems must be subjected to regular and systematic examination, inspection, maintenance and testing of all plant which may affect safety.
These three conditions mean that whenever essential supply systems are needed to ensure safety of the plant they must meet the requirement of the safety case, subject to period i c reviews, exa mi nation , ma i n-te n a n ce and testing .
The safety case for a nuclear plants justification encompasses considerat.ion of all credible faults within the design basis. It implies that there must be always a clear definition of the requirements of an essential power system, supported by an unambiguous statement of what constitutes the essential power system of the plant. The system must be shown to have sufficient integrity and be of a standard to be able to deliver i ts essential safety function under all operational conditions, including faults. Particular attention must be paid to the vulnerability to a single fault in addition to common cause effects or external hazards.
3.3 kV gas circulator pony motor in the foreground; 41 5 V A C switchboard, 11 0 V and 440 V DC chargers, switchboards and batteries in the background [photo: NNC Ltd.]
0 The first requires adequate arrangements
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Four decades A paper by Richard Davidson (BNFL)
examined the changes in four decades of operation a t Calder Hall and Chapelcross. Calder Hall is now approaching i t s 40th year of operation and i t s essential system is of particular interest.
At Calder Hall, the loss of main electrical supplies which results in the shutdown of the reactor and the tripping of the main gas coolant circulator motors, demands the coolant to be continually circulated by standby pony motors. These motors are powered by the essential supply system, in addition to a number of other vital pieces of plant, e.g. gas seal and lubricating pumps, feed water pumps, control rods, cooling fans for the biological shielding system, steam valves, gas isolation valves and emergency lighting. The total essential load is relatively heavy but can be supplied by two diesel driven 500 kW generators.
have occurred over some 40 years, the first major modification coming in 1967. It had been seen that the Grids reliability was of an exceptionally high level and so it was decreed that only one diesel generator was needed. However, it was soon realised that back-up provision was necessary for the diesel generator, and modifications were undertaken to reconfigure the essential system in order to guarantee certain loads. Ten years on from 1967, it was decided that one diesel did not conform with modern standards of safety. The system reverted to two diesels and the whole installation was reinforced by a high-capacity interconnector designed to limit the fault capacity of the system to 55 kA.
In the late 19808, the reliability of the system was further examined as part of the Nlls Long Term Safety Reviews. Various improvements were put into place, such as the auto-closing equipment on existing diesel generators with a capability to install mobile standby alternators. In safety case terms, all deterministic and probabilistic standards of system availability and reliability have been applied to Calder Hall which now conforms to current standards.
over its life but the essential supply system does conform to the highest standards as embodied in the HSEs Safety assessment principles for nuclear plants and Tolerability of risk from nuclear power stations.
Wylfa was the last of the Magnox stations to be built, coming into operation in 1971. The station electrical supplies are provided by five gas turbines and a battery-backed DC system brought into operation through a selective tripping scheme. The licensing requirements called for a minimum level of gas circulation in order to achieve the necessary reactor core cooling, post-reactor trip. The essential electrical system was considered to be vulnerable to both fire and corn mon-mode fai Iu res.
John McMurtrie (NNC) described changes to the Wylfa essential system, whereby an
Mr. Davidson looked a t the changes which
Chapelcross has not seen as many changes
independent diesel generator would be overlaid on to the existing system. A new design now provides a selected back-up scheme for two motors capable of providing gas circulator drives and all the necessary oil seals etc. for maintaining a pressurised reactor. Three diesel generators are installed and sized to supply two 400 kW gas circulator pony motors in the event of a reactor trip.
Forced gas circulation is only required for two hours after a reactor trip, and thus operator intervention is always available. The main objective of the overlay system is to reduce the possibility of failure to operate essential plant and equipment. The number of interlocks is kept to a minimum, as is the use of automatic features. Mr. McMurtie discussed the main arguments for the new system, stating that it would be able to resist the effects of fire, station electrical faults, gas releases and seismic events.
Space prevents a full description of the Overlay project, including as it did design, installation, commissioning and testing and the incorporation into the stations full electrical system.
given by Russell Berridge (Rolls Royce & Associates) who showed how the requirements for supplying essential supplies to Royal Navy submarines compare with those of commercial land-based reactors. Today, all the UKs submarine fleet is nuclear powered using PWRs as the energy source for which the installation of essential system is of paramount importance.
Mr. Berridge described typical naval submarine electrical systems and how the standard of integrity was as high as, if not higher than, that required for nuclear power stations. With such vessels away from base for many months, coupled with a need to spend considerable amounts of t ime submerged, it was obvious that the technology must be not only of the highest quality but also supplies had to meet guaranteed standards of operation. A certain amount of confidentiality surrounded this particular presentation, although the comparisons between land-based PWR systems and the requirements of nuclear submarines were made apparent and were appreciated.
performance and functional safety characteristics of electrical supply installations on nuclear power plants:
The final paper at the Colloquium was
The following standards are specific to the
0 BS 7674 : 1993 IEC 1225 : 1993
0 IEE: 1996
John Bindon can be contacted a t PO Box 22, Bangor, Gwynedd 1159 5QU, UK. He is an I E E Member.
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