RASTEP (RApid Source TErm Prediction) is a computerized tool for use in the fast diagnosis of accidents in nuclear power plants and analysis of the subsequent radiological source term. The tool is based on a Bayesian Belief Network that is used to determine the most likely plant state which in turn is associated with a pre-calculated source term from level 2 PSA. In its current design the source term predicting abilities of RASTEP are not flexible enough. Therefore, the purpose of this thesis is to identify and evaluate different approaches of enhancing the source term module of RASTEP and provide the foundation for future implementations. Literature studies along with interviews and analysis have been carried out in order to identify possible methods and also to rank them according to feasibility. 4 main methods have been identified of which 2 are considered the most feasible in the short term. The other 2 might prove useful when their maturity level is strengthened. It is concluded from the study that the identified methods can be used in order to enhance RASTEP.

Highly stressful situations have a prominent influence on the human psyche and it is crucial to take this into account when designing and evaluating software used predominantly in nerve wrecking situations. Lack of principles for design and evaluation of design of emergency handling software programs allows for graphic interface solutions that leave room for mistakes and slips in an environment where making an error can have catastrophic consequences. The study proposes a table of principles for evaluation and (re)design of user interface of programs used in highly stressful situations. The principles are grouped based on the human capability hindered by stress. Said principles were evaluated with the help of a case study on a tool for decision support (RASTEP) when facing a nuclear emergency. Statements on usability issues with RASTEP were collected through open group interviews and participant observation and pattern matched with proposed principles. Result of the pattern matching showed the disposition of usability issues per hindered capability following the same pattern as the one of the principles. Furthermore, all but one collected statement were identified as belonging to a group of hindered capabilities and were identified as violating at least one of the proposed principles. This points to the table of principles being able to serve as a tool in evaluating usability of graphic user interface of an existing emergency handling software and identifying its usability issues. / Mycket stressiga situationer kan ha en betydande påverkan på det mänskliga psyket, och det är avgörande att ta hänsyn till när man skapar och utvärderar mjukvara som främst används i pressade situationer. Brist på principer för utvärdering av design av nödhanteringsprogram möjliggör gränssnittslösningar som kan ge upphov till misstag i en miljö där felsteg kan ha katastrofala konsekvenser. Studien föreslår en tabell med principer för utvärdering och (re)design av användargränssnitt för program som används i mycket stressande situationer. Principerna är grupperade efter mänskliga förmågor som hindras av stress. Nämnda principer utvärderades genom en fallstudie av ett verktyg för beslutsstöd (RASTEP) vid kärnkraftsolycka. Vittnesmål om användbarhetsproblem med RASTEP insamlades genom öppna gruppintervjuer och deltagande observation (participant observation), och patternmatchades med de föreslagna principerna. Resultatet av patternmatchningen visade att fördelningen av användbarhetsproblem per hindrad förmåga följde samma mönster som fördelningen av principerna. Dessutom identifierades alla insamlade vittnesmål utom ett tillhöra en grupp av hindrade förmågor och identifierades bryta mot åtminstone en av de föreslagna principerna. Detta tyder på att tabellen med principerna kan användas som ett verktyg för att utvärdera användbarheten av gränssnitt hos ett befintligt nödhanteringsprogram och identifiera dess användbarhetsproblem.

This master's thesis is an investigation and evaluation of MELCOR (a software tool for severe accident analyses regarding nuclear power plants), or more correctly of the (ASEA-Atom BWR 75) reactor model developed for version 1.8.6 of MELCOR. The main objective was to determine if MELCOR, with the reactor model in question, is able to produce satisfactory results in severe accident analyses compared to results made by MAAP, which is currently the only official software tool for this application in Sweden. The thesis work is related to the RASTEP project. This project has been carried out in several stages on behalf of SSM since 2009, with a number of specific issues explored within an NKS funded R&D project carried out 2011-2013. This investigation is related to the NKS part of the project. The purpose with the RASTEP project is to develop a method for rapid source term prediction that could aid the authorities in decision making during a severe accident in a nuclear power plant. A software tool, which also gave the project its name, i.e. RASTEP (RApid Source TErm Prediction), is therefore currently under development at Lloyd's Register Consulting. A software tool for severe accident analyses is needed to calculate the source terms which are the end result from the predictions made by RASTEP. A set of issues have been outlined in an earlier comparison between MAAP and MELCOR. The first objective was therefore to resolve these pre-discovered issues, but also to address new issues, should they occur. The existing MELCOR reactor model also had to be further developed through the inclusion of various safety systems, since these systems are required for certain types of scenarios. Subsequently, a set of scenarios was simulated to draw conclusions from the additions made to the reactor model. Most of the issues (pre-discovered as well as new ones) could be resolved. However the work also rendered a set of issues which are in need of further attention and investigation. The overall conclusion is that MELCOR is indeed a promising alternative for severe accident analyses in the Swedish work with nuclear safety. Several potential benefits from making use of MELCOR besides MAAP have been identified. In conclusion, they would be valuable assets to each other, e.g. since deviations in the results (between the two codes) would highlight possible weaknesses of the simulations. Finally it is recommended that the work on improving the MELCOR reactor model should continue. / RASTEP