The risk of core damage at a nuclear power plant can be reduced if inspectors systematically concentrate on the parts of the facility where the probability and consequences of damage forecast the worst risk scenario.The weakest points in the plant can then be focused on. Costs can be reduced, as can the radiation exposure to plant personnel.
This project is being financed by SKI, the Swedish Nuclear Power Inspectorate, together with the nuclear power plant operators in Sweden. Several nuclear plants both in and outside Sweden are showing an interest in it. 'The results of the pilot study are so good that we can see great potential in putting this technology into practice in the future,' says Roger Axelsson, head of the mechanical engineering unit at Oskarshamn.
'We know more now than we did ten years ago. Developments in IT have prepared the new way of thinking. This can give us financial advantages as well as increased safety.' The pipe systems that provide the power reactors with cooling water are inspected regularly at all nuclear power plants. No pipe ruptures or large leakages can be allowed, since they may end up causing core damage. With the help of risk-based inspection, the experts judged the main risk of core damage to reactor 1 at Oskarshamn to be from a leakage or a pipe rupture in the main circulation and cooling-water pipe systems.
Non Destructive Examination (NDE) is used to inspect the pipe systems. In reactor 1 alone at least 1,200 locations where there is a potential for damage are potential sites for inspection.
Concentrate on highest risk
The most common cause of damage is stress corrosion cracking, which causes cracks to develop in the vicinity of welds, primarily in the stainless steel piping. The cracks may turn into leaks and possible pipe ruptures if not discovered in time. Risk-based Inspection allows the plant personnel to concentrate on the most risk significant locations in the plant instead of examining everything continuously. The experts at DNV Technical Consulting have therefore used methods to assess where the safety risks are highest.
'Now, thanks to our study last year, we've developed a quantitative risk model for each component,' says Björn Brickstad. 'Focusing inspections at the high-risk locations is good for overall safety, but it's also an important working environment issue as regards the radiation exposure that the inspectors are subject to.'
The results of the pilot study show that, while the current selection system is good at picking out all the high-risk components for inspection, it also chooses a relatively high number of areas that have a low risk. These latter areas only marginally decrease the risk of core damage and mean that the inspection personnel are subject to unnecessary radiation exposure.
The power plant staff conduct their safety work in the same way as has been done for many years. But now, according to DNV Technical Consulting, they can modify their risk assessments thanks to the experience gained over the years, and to new research.
The risk of core damage is assessed in a nuclear plant by using a probabilistic and consequence analysis. This is an approach that has been developed over many years.
Says Brickstad, 'We supplement our probabilistic fracture mechanics analyses with the plant's own analyses of the consequences to the rest of the system if anything should happen in the pipe system - so-called probabilistic safety analyses (PSA). The risk can then be expressed as probability times consequence.'
Thanks in part to the requirements of SKI, Sweden has had a unique tradition of risk-based thinking in nuclear technology since the end of the 1980s. DNV Technical Consulting has for many years carried out research and consultancy services for the Swedish nuclear power industry. In some cases, DNV Nuclear Technology carries out third-party reviews of these services.
The operations that are gathered under DNV's responsibility are organised into separate companies so that the third-party role is uncompromised.
Says Brickstad, 'DNV has been prominent in the field of risk analysis for a long time, particularly in the offshore and petroleum industry. Now we can exchange our experiences and learn from each other, which is good for our customers.'
Date: 2000-08-15