Human errors related to maintenance and modifications

The focus in human reliability analysis (HRA) relating to nuclear power plants has traditionally been on human performance in disturbance conditions. On the other hand, some studies and incidents have shown that also maintenance errors, which have taken place earlier in plant history, may have an impact on the severity of a disturbance, e.g. if they disable safety related equipment. Especially common cause and other dependent failures of safety systems may significantly contribute to the core damage risk. The first aim of the study was to identify and give examples of multiple human errors which have penetrated the various error detection and inspection processes of plant safety barriers. Another objective was to generate numerical safety indicators to describe and forecast the effectiveness of maintenance. A more general objective was to identify needs for further development of maintenance quality and planning In the first phase of this operational experience feedback analysis, human errors recognisable in connection with maintenance were looked for by reviewing about 4400 failure and repair reports and some special reports which cover two nuclear power plant units on the same site during 1992–94. A special effort was made to study dependent human errors since they are generally the most serious ones. An in-depth root cause analysis was made for 14 dependent errors by interviewing plant maintenance foremen and by thoroughly analysing the errors. A more simple treatment was given to maintenancerelated single errors. The results were shown as a distribution of errors among operating states i.a. as regards the following matters: in what operational state the errors were committed and detected; in what operational and working condition the errors were detected, and what component and error type they were related to. These results were presented separately for single and dependent maintenancerelated errors. As regards dependent errors, observations were also made about weaknesses in audits made by the operating organisation and in tests relating to plant operation. The number of plant-specific maintenance records used as input material was high and the findings were discussed thoroughly with the plant maintenance personnel. The results indicated that instrumentation is more prone to human error than the rest of maintenance. Most errors stem from refuelling outage periods and about a half of them were identified during the same outage they were committed. Plant modifications are a significant source of common cause failures. The number of dependent errors could be reduced by improved co-ordination and auditing, post-installation checking, training and start-up testing programmes.Inhimillisen luotettavuuden analysointi ydinvoimalaitoksissa on perinteisesti keskittynyt ihmisen suorituskyvyn tutkimiseen häiriötilanteissa. Eräät tutkimukset ja tapahtumat ovat toisaalta osoittaneet, että häiriöitä edeltävät kunnossapitovirheet voivat vaikuttaa häiriön vakavuuteen esimerkiksi estämällä turvallisuuden kannalta tärkeän laitteiston toiminnan tarvetilanteessa. Erityisesti moninkertaiset turvallisuusjärjestelmien viat (dependent failures) voivat merkittävästi vaikuttaa reaktorisydämen vaurioitumisen todennäköisyyteen. Tutkimuksen ensimmäinen tavoite oli tunnistaa ja tuottaa esimerkkejä moninkertaisista inhimillisistä virheistä, jotka ovat läpäisseet laitoksen moninaiset turvallisuuden puolustusrakenteisiin kuuluvat virheiden havaitsemis- ja tarkastusprosessit. Toinen tavoite oli tuottaa turvallisuuteen liittyviä tunnuslukuja kuvaamaan ja ennakoimaan kunnossapidon tehokkuutta. Yleisempi tavoite oli tunnistaa tarpeita kunnossapidon ja sen suunnittelun laadun kehittämistä varten. Käyttökokemusanalyysin alkuvaiheessa etsittiin kunnossapidon yhteydessä tunnistettavia inhimillisiä virheitä käymällä läpi noin 4400 vika- ja korjausraporttia sekä erikoisraportteja kahdelta saman laitospaikan ydinvoimalaitosyksiköltä vuosina 1992–94. Erityisesti pyrittiin etsimään moninkertaisia inhimillisiä virheitä, koska ne seuraamuksiltaan ovat yleensä kaikkein vakavimpia. Syvällinen perussyyanalyysi tehtiin 14 inhimilliselle moninkertaiselle virheelle, jotka käytiin läpi haastatteluina laitoksella kunnossapitotyönjohtajien kanssa ja analysoitiin seikkaperäisesti. Kunnossapidon yhteydessä havaitut yksittäisvirheet tutkittiin yksinkertaisemman menettelyn avulla. Tulokset esitettiin virheiden jakaumina muunmuassa sen suhteen, missä käyttötilassa virheet oli tehty ja missä havaittu; missä käyttö- ja työtilanteissa virheet oli havaittu sekä, missä laite- ja virhetyypeissä niitä oli esiintynyt. Tilastot esitettiin erikseen kunnossapidon yhteydessä esiintyneille yksittäis- ja satunnaisvirheille. Satunnaisvirheiden osalta tehtiin myös oman käyttöorganisaation suorittamien tarkastusten ja käyttötoiminnan kokeiden heikkouksia koskevia havaintoja. Lähtöaineistona käytettyjen laitoskohtaisten kunnossapitoraporttien määrä oli suuri ja havainnoista keskusteltiin yksityiskohtaisesti laitoksen kunnossapitohenkilöstön kanssa. Tulosten mukaan instrumentointi on muuta kunnossapitotoimintaa alttiimpaa inhimillisille virheille, joista pääosa on peräisin vuosihuoltoseisokkijaksoilta, ja joista noin puolet havaittiin saman seisokin aikana kuin ne tehtiin. Laitosmuutokset aiheuttavat huomattavan paljon yhteisvikoja. Satunnaisvirheiden määrää voidaan jatkossa vähentää parantamalla muutostyöprojektien koordinointia ja katselmointia, sekä asennustarkastuksia, koulutusta ja käyttöönotto-ohjelmia

Initiating Event Analysis of a Lithium Fluoride Thorium Reactor

The primary purpose of this study is to perform an Initiating Event Analysis for a Lithium Fluoride Thorium Reactor (LFTR) as the first step of a Probabilistic Safety Assessment (PSA). The major objective of the research is to compile a list of key initiating events capable of resulting in failure of safety systems and release of radioactive material from the LFTR. Due to the complex interactions between engineering design, component reliability and human reliability, probabilistic safety assessments are most useful when the scope is limited to a single reactor plant. Thus, this thesis will study the LFTR design proposed by Flibe Energy. An October 2015 Electric Power Research Institute report on the Flibe Energy LFTR asked “what-if?” questions of subject matter experts and compiled a list of key hazards with the most significant consequences to the safety or integrity of the LFTR. The potential exists for unforeseen hazards to pose additional risk for the LFTR, but the scope of this thesis is limited to evaluation of those key hazards already identified by Flibe Energy. These key hazards are the starting point for the Initiating Event Analysis performed in this thesis. Engineering evaluation and technical study of the plant using a literature review and comparison to reference technology revealed four hazards with high potential to cause reactor core damage. To determine the initiating events resulting in realization of these four hazards, reference was made to previous PSAs and existing NRC and EPRI initiating event lists. Finally, fault tree and event tree analyses were conducted, completing the logical classification of initiating events. Results are qualitative as opposed to quantitative due to the early stages of system design descriptions and lack of operating experience or data for the LFTR. In summary, this thesis analyzes initiating events using previous research and inductive and deductive reasoning through traditional risk management techniques to arrive at a list of key initiating events that can be used to address vulnerabilities during the design phases of LFTR development

Comparative Analysis of Nuclear Event Investigation Methods, Tools and Techniques

Feedback from operating experience is one of the key means of enhancing nuclear safety and operational risk management. The effectiveness of learning from experience at NPPs could be maximised, if the best event investigation practices available from a series of methodologies, methods and tools in the form of a ‘toolbox’ approach were promoted. Based on available sources of technical, scientific, normative and regulatory information, an inventory, review and brief comparative analysis of information concerning event investigation methods, tools and techniques, either indicated or already used in the nuclear industry (with some examples from other high risk industry areas), was performed in this study. Its results, including the advantages and drawbacks identified from the different instruments, preliminary recommendations and conclusions, are covered in this report. The results of comparative analysis of nuclear event investigation methods, tools and techniques, presented in this interim report, are of a preliminary character. It is assumed that, for the generation of more concrete recommendations concerning the selection of the most effective and appropriate methods and tools for event investigation, new data, from experienced practitioners in the nuclear industry and/or regulatory institutions are needed. It is planned to collect such data, using the questionnaire prepared and performing the survey currently underway. This is the second step in carrying out an inventory of, reviewing, comparing and evaluating the most recent data on developments and systematic approaches in event investigation, used by organisations (mainly utilities) in the EU Member States. Once the data from this survey are collected and analysed, the final recommendations and conclusions will be developed and presented in the final report on this topic. This should help current and prospective investigators to choose the most suitable and efficient event investigation methods and tools for their particular needs.JRC.DDG.F.5-Safety of present nuclear reactor