I.CHEM.E. SYMPOSIUM SERIES NO. 110

PREVENTION OF MAJOR PROCESS ACCIDENTS WITHIN THE UK AND OVERSEAS

J .G. C o l l i e r *

The nuclear and chemical industries share a common imperative to prevent major accidents which could harm the public, produce major financial losses and result in an adverse public image. Given this commitment to preventing accidents, where do we best put our money to ensure this objective? This address examines that question in respect of the design, construction and operation of plant. The use of accident modelling to identify the major contributors to risk and therefore, the most effective and economic means of preventing accidents is highlighted.

INTRODUCTION

Given that this conferenoe is concerned with accidents in the chemical industry you may be wandering why someone from the nuclear industry has been awarded the considerable prestige of giving this opening address. Of course, I cannot be sure what was in the organising committee's mind when they made the invitation, but I believe there were a number of considerations.

Firstly, although I have spent all my career in the nuclear industry I am qualified as a chemical engineer and much of my early career was concerned with safety related research; secondly, I was for a period of time in the early 80's Director of the Safety and Reliability Directorate of the UK Atomic Energy Authority which is not only concerned with nuclear safety but also advises the Health and Safety Commission on major chemical plant hazards. But perhaps the major reason why I am addressing you this morning is the similarity between the chemical and nuclear industries in respect of safety and environmental matters and in respect of public concern. Both the nuclear industry and the chemical industry operate plants which have the potential for large accidents involving significant pollution and contamination leading to major economic consequences, loss of life and grave public concern. Moreover this contamination and pollution may in some instances like Chernobyl and Sandoz be of such magnitude as to cross national boundaries. The close relationship between the nuclear and chemical industries in the United Kingdom is fostered not only by the requirement for chemical plants to prepare and reprocess nuclear fuel, but also by a two-way flow of information and know-how on safety matters between

* United Kingdom Atomic Energy Authority, 11 Charles II Street, London.

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the two industries; the nuclear industry has adopted the HAZOP (Hazard and Operability Studies) methodology for the design review of its process plants, whilst the techniques of probabilistic risk assessment, developed in the nuclear industry and initially applied in assessing the risk from the complex of chemical, oil and gas plants at Canvey Island, has now been more widely adopted throughout the chemical industry.

That having been said it must be stressed that the chemical industry and the nuclear industry in the UK share an excellent safety record; no member of the general public outside the factory gate has ever been killed directly as a result of an accident at a UK nuclear or chemical facility - and that includes both the Flixborough and Windscale fires. Despite that record they also share increasing public apprehension, regulatory control and planning inquiries.

The Nature of the Problem

Whilst the chemical industry has been growing steadily from the start of this century, since the second World War a number of factors have combined to exacerbate and highlight the problem of major chemical hazards. Firstly the improved understanding of scale up of chemical processes together with better materials and fabrication technology meant that during the 60's and 70's unit sizes of chemical plants increased significantly - in some cases up to 20 times - with the consequential economies of scale but, of course, with inventories of products and intermediates increased correspondingly together with the consequence of any potential major accident.

Secondly, new processes were developed and the ranges of chemicals, especially those derived from petroleum products, widened significantly. The technology to liquify and store petroleum and other gases such as methane, oxygen, nitrogen and ethylene in large quantities was established. A third factor has been the increasing concern over the period both nationally and internationally about the environment and public safety leading to increasingly stringent requirements on both the nuclear and chemical industries. This increased public concern has drawn attention to the evolving regulatory situation and caused an upsurge in the interest paid to public inquiries and planning consents.

Thus, whilst the likelihood of an accident at a particular plant has been reducing with more plants and greater inventories of toxic or hazardous materials the consequences of any individual major accident have been steadily increasing. Hence the importance of this conference.

It is not necessary to dwell for long on the need for "safe" chemical plants. Apart from the potential for harm to the public an accident can have major economic implications for the operator with the loss of the plant and the need for compensation as well as having an adverse effect on the public image of the company. All companies therefore have a major incentive to prevent accidents; it is worth spending considerable sums of money to achieve

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this aim. The major question for this conference to answer is "GIVEN OUR COMMITMENT TO PREVENTING ACCIDENTS WHERE DO WE BEST PUT OUR MONEY TO ENSURE THIS OBJECTIVE".

Ensuring a Safety Culture

Major operators accept that they are uniquely responsible or accountable for the safe operation of their plants. In the nuclear industry that responsibility or accountability is absolute - ie. the operator is liable irrespective of whether he has been negligent or not. To instil the appropriate "culture" in a company requires a commitment to safety from the highest levels of the company and a management process which allows and encourages the setting of safety objectives and targets, the issuing of guidelines for design, construction and operation and the establishment of appropriate "independent" (independent of the operating arm) monitoring and auditing activities.

A "safe" plant requires a "womb to tomb" approach; safety starts with the choice of the process and ends when the plant is finally decommissioned. We can recognise three separate phases.

The design activity should evaluate alternative processes and flowsheets with the objective of reducing toxic or hazardous inventories; the use of modular plants or a "make it when you want" approach. Plants should be designed to be as simple as possible relying to the maximum extent on passive "safety" systems (perhaps involving simple physical processes like natural convection or fluidic devices) rather than active "engineered" safety systems (relying on pumps, valves, switches, operator action etc). The most effective use of probabilistic risk assessment (PRA) comes at the design stage when the findings can be used to reiterate the plant design to eliminate any potential weaknesses. Operator error is still a major cause of accidents. Here the need is for more automated process plant control with better displays and better ergonomic design. These displays need to concentrate on system parameters like pressure, flow, temperature, as well as the status of the plant - valves, pump, inventories etc.

The steady growth of a wide range of design standards and oodes, together with modem quality asssurance/quality control methods, has ensured that plant can be constructed and commissioned to the required high standards.

Thus, careful attention to design and construction results in a plant which conforms to the latest standards and requirements. Nevertheless it is equally important to ensure that the plant is correctly operated and maintained. This is particularly so for the case of plant built some years ago to less demanding standards. It is therefore vital that operators are trained to a high standard including the use of plant simulators and receive and obey clearly defined operating instructions and emergency procedures. It is also important that safety systems are properly and regularly tested and

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that emergency exercises are carried out at specified intervals. Industry initiated safety audits are becoming more common and appreciated in both the nuclear and chemical industries.

A major aspect I have not touched on thus far is the matter of siting. Perhaps because of the novelty and therefore greater uncertainty about the technology nuclear plants have always been located on carefully selected sites where there has been some control on the growth of development close to the site as part of the licensing process. Certainly inappropriate siting and inadequate control of housing development close to the site of the plant were major factors in the magnitude of the casualties at both Bhopal and Mexico City. With the question of siting comes the important matter of emergency planning. It is vital to be absolutely clear where accountabilities lie. Basically it is for the operator to ensure that adequate plans have been drawn up for the on-site situation including the evacuation of all non-essential personnel, the alerting of the emergency services, the setting up of an emergency control/communication centre sufficiently far away from the plant not to be affected. The off-site situation, certainly in the EEC countries, is the responsibility of the local authority emergency planning officer. He will draw up the necessary plans for any evacuation of the general public.

The Use of Accident Modelling

All of the factors I have just outlined can be effectively captured by the use of severe accident modelling techniques. These techniques require, amongst other things, a detailed understanding of the physical, chemical and engineering characteristics of accident situations; common cause failures need to be understood and identified and one cannot underestimate also the value of data banks for evaluating reliability in order to assign realistic probabilities to the detailed events being considered. Basically starting from a series of postulated initiating events related to the reliability of the plant, it is possible to establish a series of scenarios involving either major releases of toxic materials, fires or explosives. Par a given site, population distribution and weather pattern, the consequences of each scenario can be established. Knowing the probability of each scenario it is possible to establish a probability/consequence relationship and identify an overall "risk" from a particular plant or complex. Risk is usually expressed in terms of the risk of death or serious injury to individual members of the public and also to society in general. This was first done in the UK in relation to a proposed further development of the Canvey Island complex. This initial study came out with a risk to people living close to the complex of 1.3 x 10-3/yr which must be considered an unacceptable public risk. A series of measures were identified and implemented so that by the time a reassessment was carried out in 1981 the risk has been reduced significantly to 6.5 x 10-5/yr as a "worst case" and 3.5 x lO-5yr as a mean. These figures can be compared with the annual risk of an accident at a nuclear

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power station (cf Sizewell) or that due to overtopping of the Thames barrier, as shown in Table 1 below.

TABLE 1 - Estimated Maximum Individual Risk to Members of the Public from accidents

However, these relatively early assessments included hazards, such as fire, cannon cause failures, human error both by operators of plant and the management but omitted, firstly, "external" initiating events like aircraft crashes, earthquakes, flooding, storms etc. and also human factors such as maloperation. A great deal of research effort has therefore gone into improving individual models used; the field of dense gas dispersion modelling is an obvious example. Considerable effort is also being expended on trying to understand the interaction between human beings and complex plants. A modem reassessment of the situation would now include these improvements. Thus we see that ensuring chemical plant safety is an evolutionary rather than revolutionary process. It involves both a) the evaluation of reliability information via data banks and b) the understanding of physical, chemical and engineering characteristics of accident situations. In the past the reporting of accidents and incidents at chemical plants was very patchy and incomplete and yet we need to learn from past mistakes. Reporting of incidents certainly in the EEC is now a legal requirement. The setting up of data bases on chemical accidents such as that operated by HSE/SRD (MHIDAS -Major Hazard Incident Data Service) will enable the lessons to be learned from past events. It is a fact of life that if one examines details of past incidents, many have occurred as a result of circumstances that would not have been considered if they had been suggested prior to the incident.

Some of these circumstances have been very strange indeed. A particularly bizarre accident occurred in Pennsylvania United States in 1960, when a driver took an LPG tanker under a railway bridge which he had done many times before. This time, however, due to the depth of snow and ice, the relief valve on top of the tanker was knocked off. He reversed free of the bridge and went to raise the alarm. In the meantime, sparks from a passing train ignited the gas cloud which had been released.

Another example of this type of incident occurred at Oppau in Germany in 1921, when 4,500 tons of a mixture of ammonium nitrate and ammonium sulphate exploded whilst it was being broken up by explosive charges. A crater was

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formed 400 feet in diameter and 95 feet deep. More than 500 people were killed including sane in Mannheim 4 miles away. Nearly three quarters of the houses in the town of Oppau were destroyed. The intriguing feature of this accident was that the decision to use explosives was taken only after exhaustive tests and thousands of actual blasting events, using the same procedures and explosive mixture, had been carried out before this particular tragedy.

A third example occurred at Perth Amboy in the United States in 1949, when a fire broke out in an asphalt tank, engulfing some adjacent tanks. Cne of these tanks, thought to have contained naphtha, failed at the bottom seam and the tank became reminiscent of a Saturn V in the spectacular manner shown in this slide.

One of the virtues of data bases such as this is that they give us a much better idea of just how bizarre some incidents really are. We can also expect greater international co-operation/harmonisation of safety practices between countries both on the governmental/regulatory side and between operators.

I hope by now that you can see how the use of accident modelling can identify the major contributors to risk and thereby answer the question that I posed earlier concerning where we best put our money to ensure our objective of preventing accidents.

Current Issues and Final Garments

I started by referring to the similarities between the chemical and nuclear industries on environmental and safety matters and I would like to finish by listing some common current issues;

Ageing Plant Emergency Planning Waste Disposal Transport of Dangerous Materials Regulatory Aspects.

Firstly, the question of ageing plant. Many of the chemical plants now in operation were built during the 60's and 70's. Like much of our nuclear plant in the UK they are showing signs of age. There is a clear need to requalify such plant against modem day safety standards. This means a re-examination of the safety case, identification of any "weak" items, -pressure vessels which can not be fully inspected and where a leak before break case is not possible; structural supports which might fail in a fire or an earthquake etc.

Secondly, the question of transfer of technology to the emerging nations and the need to build up a technological base of skilled, trained personnel to handle modern chemical plants. The build, operate and transfer (EOT) approach offers one way to achieve this build up of indigenous experience.

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Thirdly, the whole question of transport and waste disposal - a matter of public concern as regards both our industries.

My time is rapidly running out. I have tried in this overview to touch on some of the major topics which will be addressed by speakers at this conference. Whether we like it or not in both the nuclear and chemical industries safety costs money and bears on the economics of the industries. The trick is therefore to employ our professional skills to ensure maximum safety at minimum costs. I trust that our two industries will continue to learn from each other in a mutually beneficial way.

Thank you ladies and gentlemen.

Reference : VC Marshal l "Major Chemical Hazards" E l l i s Horwood/Jchn Wiley (1987)

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