The ABB Atom Criticality Safety Handbook

Transcription

1 The ABB Atom Criticality Safety Handbook Gordana Dodig-Crnkovic ABB Atom AB Nuclear Fuel Division, Dept. BCD SE Västerås, Sweden

2 CSH-A Table of Contents 1 Introduction 2 Organization of Work for CSE 3 CSE Step by Step: Order Form-Check Lists-Evaluation Report-Implementation 4 Modeling Methods and Calculation Tools used for CSE (4.1 CSE Methods; 4.2 Precautionary Principle; 4.3 Benchmarks and Validation) 5 CSE for Fuel Fabrication Plant 6 CSE for Transports 7 CSE for Fuel Storage 8 Standards, Handbooks and Standard Literature 9 Common Material Compositions 10 Most Important Physical Factors affecting Criticality Safety 11 Reflection and Interaction 12 Criticality Safe Parameters for Isolated Systems (12.1 Isolated Homogeneous Systems; 12.2 Pipes; 12.3 Cylinders and Slabs with Different Reflector and Absorber Materials; 12.4 Heterogeneous Systems) 13 A short Review of Criticality Accidents 14 Glossary of Terms in Nuclear Criticality Safety 15 Handbook administration

3 Criticality Safety Handbook Mission strengthen the company criticality safety culture set standards for making criticality safety analyses document our organization, routines, and methods of work in the field of criticality safety. standardize and document analysis procedures, methods, and acceptance criteria. provide general guidance in matters of NCS principles and practices at ABB Atom. serve as a reference source in the broadening and strengthening of the safety culture in the organization.

4 The Regulatory Basis In assessing the criticality safety at ABB Atom we apply the methodology outlined in SKIFS 1998:1 as well as ANSI/ANS Standards, and ISO Safety in SKIFS 1998:1 is described in terms of defense in depth, which means that physical barriers against e.g. inadvertent criticality must exist. Barriers are also necessary for the mitigation of the consequences of a possible criticality event in systems in which criticality is judged as a scenario not too far-fetched.

5 THE CONCEPT OF DEFENSE IN DEPTH 2 Control of Abnormal Operation, Detection of Failures by permanent surveillance, periodic testing, protection systems, understanding of abnormal behavior in safety repports 1 Prevention of Failures and Abnormal Operation by robust design and high quality in construction, operation and maintenance 5 4 Severe Accident 3 Design Basis Accident 2 Abnormal Operation 1 3 Control of Accidents within the Design Basis by engineered safety features and accident procedures, bounding incident and accident cases within categories with maximal radiological consequences Normal Condition 1 Abnormal Operation 2 Design Basis Accident 3 Severe Accident Mitigation of consequences of significant off-site radiological releases by elaboration of an emergency response plan 4 Control of severe plant conditions by prevention of accident progression and mitigation of accident consequences

6 The CSE-Piece of Nuclear Safety Puzzle SKI s independent reviewer Safety Advisory Board (BERNS) SKI (Swedish Nuclear Power Inspectorate) Nuclear Fuel Factory Workshops (Production) SÄK Safety Committee Criticality Safety Group, KSG SAFETY & SAFEGUARDS Department CSE-responsible group Transports Fuel Storage (Nuclear Power Stations)

7 ABB Atom Fuel Division s Safety Committee: SÄK Criticality Safety Radiation Protection Fire-fighting Conventional Safety Environmental protection Criticality Safety Group: KSG enables direct communication between CSE, workshops- and operative safety personnel improve long-term planning of CSE

8 Order Form and Checklist A request for a criticality safety analysis requires filling in of an order form with the necessary characterization of the system/process in question. To assure that all relevant scenarios and types of possible problems are taken into account, we have a Checklist for Systems Containing Uranium. The Checklist consists of a number of questions, which must be answered with yes/no or a description, and a responsible person must sign each of them. Complete Request for CSE includes both a filled-in order form and a signed Checklist.

9 Modeling Methods and Calculation Tools used for CSE The Precautionary Principle The precautionary principle described in CSH-A as applied to CSE means that the risk of an inadvertent criticality must not be underestimated under any circumstances! (The Precautionary Principle in Maastricht treaty states that the absence of certainty given our current scientific knowledge, should not delay the use of measures preventing a risk of large and irreversible damages to the environment, at an acceptable cost.)

10 Criticality safe: Risk landscape k eff defines only one single point! Analyzing safety against criticality is like exploring a risk landscape, which can have a very detailed structure. Our aim is to identify relevant regions of significant risk (Risk = Probability x Consequence). Judgement of what can be considered as likely/unlikely or possible/impossible is made in accordance with existing experience and our best knowledge. Corresponding PRA is used to support the choice of proper scenarios.

11 What does it mean criticality safe? k eff Säkerhetsmarginal Säkerhetsmarginal 0.86 parameter k1 k2 säker gräns kritisk k eff < 0.95 is an common definition. k eff < 0.98 can be accepted for accidental scenarios, under the condition that PRA (probabilistic risk analysis) can confirm extremely low expected frequency for that type of events. k eff as a rule is a function of several parameters. It is of great importance to understand its behavior with different parameter variations. It is the slope of k eff -curve that indicates how fast one is approaching safety-relevant limits.