Quantifying the Unimaginable - the Case for Human Performance Limiting Values

摘要

Most Human Error Probabilities are within the range 10~(-4) to 1.0, and there are certainly data to support these levels of human reliability. However, sometimes in quantified safety cases or probabilistic safety assessments, scenarios arise which require quantification of a human error sequence which seems incredible, or extremely unlikely. The question is how to deal with such scenarios. Furthermore, in conventional safety assessments use is often made of common cause limiting values, which account for unpredicted interactions between sub-system elements. The logical question is therefore whether the equivalent should exist for human reliability. Human Performance Limiting Values have been in use in some UK nuclear licensees and companies for more than a decade, but their usage and application is not necessarily uniform or even accepted across the nuclear industry. A workshop was therefore convened with representatives from several nuclear utilities and the regulator, to discuss the issues surrounding HPLVs. This paper documents the main elements of the guidance arising from this work, based on best current practices in the UK nuclear energy industry.