(521.5114) WISE Amsterdam - Incomplete fuel rod insertion is an important problem. It is important that control rods can be inserted into the fuel rods very rapidly to stop the chain reaction. In Doel the control rods are normally lowered in 1.3 seconds. If fuel rod deformaties take place this can mean that control rods can't be inserted totally, and this can have severe safety consequences.
The Belgians were fully aware of problems with Incomplete Rod Insertion (IRI) before they experienced those problems them- selves in 1996. Problems with IRI had occurred at many nuclear plants by 1995: at Almaraz, Spain; Ringhals, Sweden; South Texas and Wolf Creek in the US, and elsewhere. Since 1996 Electrabel suffered the most severe consequences of all utilities. One of its largest PWRs, Doel-4, was forced to close for nine months until safety authorities agreed on measures to cope with the problem.
In June 1996 following a reactor scram at Doel-4, it was discovered that five Rod Control Cluster Assemblies (RCCA) had failed to insert completely into the `dashpot' and one was stuck above the dashpot. Tihange-3 had to be stopped in 1996 too after 14 months of continued operation, three months earlier than planned, following another reactor scram in which three RCCA were stuck in the dashpot. A rod drop test at the end of the fuel cycle showed eight control rods blocked, one above the dashpot. At both reactors the same measure was taken: loading only fresh or one-cycle fuel assemblies under the RCCA positions. In some cases fuel assembly deformation reached 2,5 centimeter, consuming all the gaps between adjacent assemblies.
In June 1997 a new rod drop test at Doel-4 showed nine RCCA blocked, one out of the dashpot. Doel-4 had to be shut down immediately, five months earlier than planned. The blockages occurred in both one-cycle fuel and fresh fuel asemblies. This is an indication that the fuel deformaties were not burn-up related as was widely postulated earlier.
Tihange-3 too had renewed rod drop problems between August 1997 and April 1998 and had to be shut a month earlier. A rod drop test in May 1998 at Doel-4, imposed by nuclear safety authorities, showed a risk of IRI within two months and Electrabel decided to shut the reactor two months earlier. Rod drop tests at Doel-4 and Tihange-3 this year have shown only slightly improved rod drop times.
The incomplete fuel rod problem was a nightmare for Electrabel and it's not yet over. Due to fuel deformation, fuel assemblies are harder to handle during reactor outages, there are `significantly more damaged and leaking assemblies' since 1995 in Doel-4 and Tihange-3 reactors, than in the Electrabel's other five PWRs. Even now, more than a year after the last incomplete rod insertion at Doel-4, Electrabel claims it is still paying for the problem, through shortened fuel assembly life, spent fuel pools congested with damaged fuel assemblies and old fuel skeletons of recaged assemblies.
The strict position of the Belgian safety authority on rod drop testing means that the reactors are at constant risk of an earlier outage and fresh fuel may not be available when needed.
The consequences of those problems on nuclear fuel management were huge, Goethals of Tractebel Energy Engineering said. Fuel loading had to be optimized technically rather than economically. Electrabel doesn't dare to refuel by replacing only a part of all assemblies at the two affected reactors, as is being done normally, but instead unloads the complete cores at each outage. Worse, Doel-4 had to change to annual fuel cycles, abandoning the more economic longer cycles, because of the lower-enrichment fuel loaded to deal with the IRI problem. Numerous fuel assemblies had to be prematurely discharged: 48 fuel assemblies at Doel-4 will stay only for three cycles in the reactor core, rather than the planned four cycles. Spent fuel pools are filling up with partially spent fuel assemblies and old skeletons, also creating additional nuclear waste.
Electrabel also decided to load massively stiff and straight fuel assemblies at Doel-4. It recaged 25 assemblies during the winter of 1997/98. Doel-4 is the only reactor where massive recaging of assemblies has been done to fight IRI. The cost of incomplete fuel rod insertion for Electrabel up to now is estimated at between US$30-US$50 million, excluding the cost of the unplanned earlier closures. Electrabel hopes the situation will be back to normal from the safety point of view in 2000, but the economic repercussions are still being felt. The Belgians say that no fully proven generic root cause has yet been identified and they have become warry of modified fuel designs. Electrabel and its engineering firm Tractebel said that, although they were forced to accept new designed fuel rods to remedy the rod-drop problems, they are more and more unwilling to adopt new designs that have not been proven to be reliable.
It is not unexpected that Electrabel is mainly concerned about the financial consequences of the incomplete fuel rod insertion problem. But what is known about the environmental consequences of the damaged and leaking spent fuel rods? How much more radioactivity is released in the environment? What about the increased contamination of the spent fuel pools?
Sources:
- Nuclear Fuel, 1 November 1999
- Doelbewust, April 1998
Vooruitgangstraat 317, 1030 Brussel, Belgium.
Tel: +32-2-201 1944
Fax: +32-2-201 1950
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