Codes for nuclear safety
23 Feb 2005
Simulation programs to prevent critical failures in nuclear reactors are being updated by researchers from Purdue University in Indiana, Sandia National Laboratories and US engineering companies.
Originally developed in the wake of, and using data from, the Three Mile Island accident in 1979, the three programs, known as reactor safety codes, are also being adapted for use in large-scale hydrogen generation reactors.
'These codes were all developed for modelling the TMI accident and seeing under what conditions a reactor can be operated safely,' says Karen Vierow, the leader of the research at Purdue. The codes can be used both to determine how an accident occurred, and in the design of new reactors to ensure that they will operate safely under a variety of failure conditions.
'What we are trying to predict with these codes is what does it take to damage the core of a reactor, then we design the reactors so that those accident scenarios can never happen,' Vierow says. 'Since we don't have many accidents like the types we are trying to analyse, we need these computer codes, but it's hard to validate them or to make sure the computer codes are really correct.'
The codes are tested regularly, and in the most recent research, each program was used to simulate a severe nuclear accident where the electricity supply in a nuclear power plant fails completely. This is in reality very unlikely, as all reactors have back-up diesel-powered generators to ensure that the coolant system keeps operating.
The team discovered that all the codes lacked information about the physics needed to specify the conditions. For example, one code did not model accurately a situation where a coolant pipe heated up excessively; in particular, it couldn't predict the changes to the structural integrity of the welds as the steam in the pipe became hotter.
'We made a lot of conservative assumptions, including a situation in which a certain valve sticks open and there is no backup valve, but in reality you would have several valves in the system in case one failed,' Vierow says. Making these assumptions, however unlikely in the real world, allowed the team to test 'every single possibility' of the physics involved, she explains.
The codes will find a new application in another form of energy — fuel cells. The large-scale use of these to generate electricity will require equally large reactors to produce hydrogen, and Vierow is modifying one of the programs so that it can be used to test the design of these reactors.
