The Swiss Federal Nuclear Safety Inspectorate (ENSI) has issued Leibstadt Nuclear Power Plant (KKL) approval to restart subject to conditions. The conditions under which dryouts have occurred at KKL are known. KKL has instigated counter measures. Consequently operation is safe. In an interview Ralph Schulz, Director of the Safety Analyses Division, shares some background insights.
Was there ever a risk for mankind or the environment due to the dryout phenomenon at Leibstadt?
No. Cladding tube damage poses no risk to mankind or the environment. Fuel rod damage is a well-known phenomenon. Dryout is just one of the causes that can lead to fuel rod damage. The cladding tubes of the fuel rods form the first of several barriers that protect the environment against radioactive fuel and its fission products. Dryouts lead to accelerated oxidation of the cladding tubes and consequently reduce their strength. Obviously the barriers should not be weakened. Therefore dryouts are not permitted under normal operation. The dryouts at KKL resulted in actual cladding tube damage just once in 2014. In all other cases the result was solely reductions in the wall thickness or discolouration.
Can a reactor go out of control because of dryout?
Contrary to the claims of some nuclear power critics, dryouts do not result in an uncontrolled increase in reactor power and hence a safety problem. If fuel rod damage does occur, it is easily detected and the plant can be safely shut down. Normally however a NPP is not shut down in the event of fuel rod damage because its effects are marginal. Fuel rod damage is rare but not unusual. In KKL for example there are about 62,000 fuel rods in the core.
Today you issued an approval to Leibstadt Nuclear Power Plant to restart operation. Does that mean that you know the cause for the oxidation of the fuel rods?
We know the conditions under which a dryout occurs in KKL. Based on this, KKL has been able to configure the reactor and its operation in such a way that these conditions are specifically excluded. KKL has performed a cause analysis based on a systematic examination of more than 200 fuel elements. It is generally known that the power, design type and position of fuel elements together with the core flow, i.e. the quantity of water that is pumped through the core per unit of time, play a decisive role in dryout phenomena. Now the comprehensive investigations have provided actual values for these parameters. Other possible causes for the dryout, for example manufacturing errors, were previously unambiguously excluded.
Nevertheless KKL must continue with the analysis. Does that mean there are still unknowns?
In many technical applications individual physical phenomena play a role but are not fully understood and therefore mathematically can only be described to a limited extent. This applies for example whenever there is turbulent flow or for every heat transfer scenario. Nevertheless the processes taking place can frequently be explained in general even without consideration of the detailed physical processes and as such can be technically safely exploited because they are sufficiently investigated. In this way the detailed physical mechanisms leading to dryout in KKL are also not yet known. The findings so far are largely empirical. Therefore we have asked KKL to perform further investigations to improve the understanding of the detailed physical processes which, under defined boundary conditions, result in local cooling deficits and in this way provide further assurance for the findings obtained until now.
In 2015 measures had already been taken that should have prevented a dryout. However the 2016 overhaul outage revealed that these were not successful.
The measure to increase the margin relative to the critical boiling transition power in operating year 2015/16, was taken based on the computer model used up until now. This model is used globally in many boiling water reactors. As has now been revealed however, its does not cover all the individual phenomena. However thanks to the comprehensive analyses of the past few months, the operator now knows the most important parameters that play a role in dryout and can now take them into consideration in the configuration and operation of the reactor.
You mentioned the core flow. Are we talking about a cooling problem here?
No. The cooling technology of a boiling water reactor is tried and tested. This is apparent from plants with a similar design. The cause for the dryout is the interaction between the core flow, power and design characteristics of the plant and the fuel element.
Was the fact that there was too little water in the reactor pressure vessel responsible for the fuel rods being able to dry out?
No. The level was correct at all times, which means that the level was always well above the top of the fuel rods. Locally however there were conditions under which the normally present water film was lost on a few fuel rods at least temporarily and the heat was directly transferred to the steam. This led to a significant increase in cladding tube temperature.
Until now there has been one case, in 2014, in which a fuel rod dryout went so far that radioactive material escaped into the coolant. What exactly happens, if the cladding is corroded through?
How large was the fraction of the reactor core that was affected by corrosion?
The reactor core of the Leibstadt nuclear power plant comprises 648 fuel elements with a total of around 62,000 fuel rods. In total, of the fuel elements used in the 2015/16 operating year, 30 fuel rods in 13 fuel elements were affected by dryout to such an extent that they required replacement. That is about 0.05 percent of the fuel rods in the core.
Are there still any oxidised fuel rods in the reactor?
Oxidation on fuel rod cladding tubes is an entirely normal process. This mechanism is allowed for by the design of the fuel rods. As part of visual inspections and measurement tests, the adherence to limit values is regularly checked. For those fuel elements that have fuel rods exhibiting heavier signs of oxidation but which otherwise can remain inserted, these fuel rods are replaced by zirconium rods without uranium.
Can you state that there will not be any dryouts before the next annual overhaul?
As we now know the boundary conditions that led to the dryouts and the measures taken, we are convinced that a recurrence can be excluded. However KKL must check extensively during the next overhaul whether local cooling deficits have again occurred.
Leibstadt Nuclear Power Plant had already had problems with fuel rod damage at the end of the 1990s. Is there any connection with the latest incident?
No. The mechanisms that led to the fuel rod damage are different. The corrosion found at that time was in the area where the fuel rods are covered by the spacers. The failure mechanism at the time was so-called shadow corrosion, which could be traced back to an unfavourable combination of cladding tube and spacer material. Thanks to the continuous development of the material composition, this failure mechanism has not returned since then at KKL.
Since starting operation in 1984 KKL has increased its output several times. Is there a connection between the dryouts and these increases in output?
There are a multitude of factors that influence the occurrence of dryout. Undoubtedly the reactor output is one of them. However tracing the dryout solely back to the increased output would be incorrect and an over-simplification. The last increase in output took place in 2002. The analyses have shown that the dryouts first occurred in cycle 28, i.e. not until 2012/13. Between these times operating experience in respect of the reactor core was positive.
Have you called upon the assistance of external experts, for example the Expert Group on Reactor Safety (ERS)?
With its Reactor Core Section, ENSI has its own proven expertise in the field of reactor and fuel technology. For specific questions we called upon experts from TÜV Süd, the Paul Scherrer Institut PSI and Sten Lundberg Consulting SLC. We have used the Expert Group on Reactor Safety primarily to consider long-term strategic safety questions. The findings in respect of the fuel elements relate more to day-to-day operations.
Elements of the media have accused ENSI of covering up the problems with fuel rods in the nuclear power plant up to last December.
ENSI continuously informs the public about the state of the Swiss NPPs. Thus for instance ENSI reported on the findings on the fuel elements in its 2014 and 2015 Oversight Reports. Moreover we referred to the problem in our web article linked to the 2014 Oversight Report. According to the legislative authority the licence holder is responsible for the safe operation of an NPP. KKL informed the public repeatedly and promptly about the findings on the fuel rods during the 2016 annual overhaul. For its part ENSI informed the public after handling of the incident was completed. Up until this point, the Supervisory Authority was not in a position to communicate about the decisions or handling approaches that would have supplemented the information provided by the operator.
Leibstadt Nuclear Power Plant had already made it known in Autumn 2016 that it wanted to restart operation in mid-February 2017. Furthermore the lakes used for hydroelectricity appear emptier than ever. Was this a factor in your decision?
No. The core of ENSI’s work is the protection of man and the environment against the dangers of peaceful use of nuclear energy. In Switzerland nuclear power plants can only be operated if they fulfil the requirements of the legislative authority. This is the sole focus for ENSI.