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Comparison of different strategies for decommissioning a tritium laboratory
Fusion Engineering and Design 88 (2013) 2655–2658
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Comparison of different strategies for decommissioning a tritium laboratory Kris Dylst ∗ , Frederik Slachmuylders, Bart Gilissen SCK•CEN, Dismantling, Decontamination and Waste, Boeretang 200, 2400 Mol, Belgium
h i g h l i g h t s Decontamination to below the free release limits is very labour intensive. Disposing of contaminated steel to a nuclear melting facility is cost effective. It can be advantageous to invest in decontamination of non-steel materials.
a r t i c l e
i n f o
Article history: Available online 11 January 2013
a b s t r a c t Between 2003 and 2009 two rooms that served as tritium laboratory at SCK•CEN and its ventilation system were decommissioned. Initially, the decommissioning strategy was to free release as much materials as possible. However, due to the imposed free release limit this was very labour intensive. Timing restrictions forced us to use a different strategy for the ventilation system. Most of the steel was disposed of to a nuclear melting facility. As a result there was a significant decrease in the required man labour. For the second laboratory room a similar strategy as for the ventilation was used: contaminated steel was disposed of to a nuclear melting facility and other materials that could not be easily decontaminated were disposed of as nuclear waste. At the expense of extra waste generation compared to the first laboratory the decommissioning was done using merely one third of the man hours. Comparison of the used strategies indicated opportunities for cost optimization. Even in absence of time constraints it is best to foresee a safe disposal of metals to a nuclear melting facility, whilst it is worth to invest in the labour intensive decontamination of the other materials to free release them. © 2013 Elsevier B.V. All rights reserved.
1. Introduction The tritium laboratories at SCK•CEN were commissioned in 1975 for a maximum tritium inventory of 37 TBq (1000 Ci) tritium. Major tritium research for both fission and fusion has been executed. The tritium laboratory consists of 2 adjacent rooms with a total floor space of nearly 130 m2 . The lab refurbishment was necessary to improve the infrastructure, the capabilities and the safety. The refurbishment will not increase the surface of the laboratories. A new ventilation system should allow for a higher tritium inventory of 370 TBq (10,000 Ci). One room has been refurbished whilst the other was in operation. Despite this increased complexity of the refurbishment project, it was important for SCK•CEN to keep some tritium research capabilities available. Between 2003 and 2009 two rooms that served as tritium laboratory at SCK•CEN and its ventilation system were
∗ Corresponding author. Tel.: +32 (0) 14 33 26 30. E-mail address: [email protected] (K. Dylst). 0920-3796/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.fusengdes.2012.12.009
decommissioned. Initially, the decommissioning strategy was to free release as much materials as possible. Timing restrictions forced us to use a different strategy for the ventilation system and the second laboratory: steel and other materials that could not be easily decontaminated below the free release limit were disposed to a nuclear melting facility and other materials that could not be easily decontaminated were disposed as nuclear waste. The different strategies used on two comparable laboratories give us a unique learning opportunity for optimizing tritium facilities decommissioning.
2. Manners for disposing of nuclear materials from the tritium laboratory The three main ways to dispose of decommissioned materials from the tritium laboratories were: 1. free releasing the materials, 2. disposing the materials as nuclear waste, or 3. recycling steel in a nuclear melting facility.
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Table 1 Measured H3 activity on a hot spot on the painted metal floor of a process cell in laboratory 1. Layer (from exposed to deep)
Paint (exposed layer) Metal just below paint Bulk metal
Measured H3 contamination (Bq/dm2 )
(Bq/g)
5000 800 n.a.
3000 n.a. 4
Free released materials can be disposed of or recycled as nonnuclear materials. They contain less radionuclides than what is considered to be a risk for public health. In Belgium there is a law that lists the maximum concentration for a large number of radionuclides contained in materials. These concentrations are free release limits. The waste disposal cost for free released materials is negligible compared costs associated with nuclear waste. Materials that cannot be free released should be disposed of as nuclear waste. However, most of the metals which cannot free released can be recycled in a nuclear melting facility. In Belgium there are several different types of nuclear waste defined mostly depending on the composition, physical form and radio nuclear contamination of the waste [1]. 3 types of waste were generated whilst dismantling the tritium laboratories: (i) compressible waste, (ii) burnable waste and (iii) filters. The costs connected to disposing nuclear waste are high. Contaminated steel can be recycled in a nuclear melting facility at only a fraction of the cost in comparison to disposing it as compressible nuclear waste.
3. The tritium free release limit Free release limits are the key in decommissioning nuclear facilities. It is easier to decontaminate to below higher free release limits than to decontaminate below lower free release limits. Another aspect is the applicability of the free release limits: i.e. is it possible or even meaningful to check against the existing free release limits? The latter considerations are very valid in the case of our tritium laboratory. The tritium free release limit for solid radioactive waste products in Belgium is 100 Bq/g [2]. This limit is not practical in our case for several reasons: • A limit expressed per weight material averages the tritium contamination at the surface with the tritium in the bulk. In our case the materials that needed to be checked were solid materials which were contaminated at room temperature by tritiated liquid spills or by tritiated gasses. Table 1 shows that despite tritium can migrate into substance, in our case [3] contamination is concentrated in or at the surface layers. Incorporated tritium contamination is more an issue for materials in which tritium would be generated due to activation or materials in which tritiated species can penetrate easily. • Due to the low energy level (-radiation, Emax = 18.7 keV) the radiation emitted by tritium incorporated in the bulk of a material has no effect on the environment. In this perspective it becomes clear that, especially in the case of surface contaminated materials, the tritium surface contamination (Bq/dm2 ) is a better safety criterion than the average tritium content of a material (Bq/g) [4]. • Another, practical, reason to prefer surface contamination based tritium free release limits for solid materials is that nondestructive measurement techniques can be used. The low energy level of tritium radiation prevents ‘incorporated’ tritium measurement with non-destructive techniques.
The above arguments for a tritium free release limits based on surface contamination are valid for the equipment and furniture that were present in our lab. An application was made to the Belgian Federal Nuclear Control Agency to introduce an exceptional free release limit of 250 Bq/dm2 removable tritium for surface contaminated material. Although all parties agreed that the removable tritium contamination was a valid criterion for free release there was no will to treat tritium as an exceptional low radiotoxic element. As a result we were forced to use an interpretation of the international transport regulations [5] for beta–gamma emitters as a free release limit. The materials from the tritium lab could be free released when the measured removable surface contamination was less than 4 Bq/dm2 . 4. Decommissioning the first laboratory The two tritium laboratory rooms and their common ventilation system were decommissioned one after the other between 2003 and 2009. Before the rigorous surface contamination free release limit was approved it was decided that, to decommission the first laboratory the goal was to free release as much materials as possible. Items like closets, tables and furniture that were not suspected to have been exposed to medium or highly tritiated species were cleaned and checked for residual tritium contamination. The items that did not meet the free release criterion after a couple of decontamination sessions were disposed of as nuclear waste. This was justified because of the limited volume of these items. The items in the laboratory in which tritium was handled in medium to high concentrations were a large process cell, a glovebox, and a hood. These items were mostly composed of painted carbon steel and had plexiglas windows. These items together with their ventilation shafts represented large volumes of waste. These materials were free released as much as possible. Due to the low free release limit a lot of effort was necessary to decommission the first laboratory. It took almost 4000 man hours over a period of 2.75 years to decommission the laboratory. The decontamination of the metal from the process cells alone (without dismantling or decommissioning) took nearly 1100 man hours. 5. Decommissioning the ventilation system When the first laboratory was refurbished, both laboratories were connected to a new ventilation system making the old ventilation system obsolete. For a number of reasons the free releasing strategy from the first laboratory could not be reapplied for the old ventilation system. The first consideration was safety: the ventilation and pulsation shafts crossed non-nuclear corridors in the BR1 building. For the decommissioning the hallway was divided into a passage zone and a decommissioning zone by installing temporary walls. To limit the risk of exposure the shafts needed to be packed and removed as soon as they were dismantled; leaving no time for thorough decontamination. Second consideration was a timing restriction: The old ventilation fan and filtering system was located in a technical story that had to be removed to start with the construction of the reactor building for the new experimental Guinevere reactor. Another timing aspect was that this time the decommissioning team was preoccupied with other projects. The decommissioning needed to be done by the tritium lab R&D team. To not compromise too much on our R&D timings the work needed to be done in as little time as possible. And finally economic aspects played a role. After the decommissioning the first laboratory analyses was made [6] indicating
K. Dylst et al. / Fusion Engineering and Design 88 (2013) 2655–2658
that more than 20% of the direct costs could be saved when the metals would be disposed of to a nuclear melting facility instead of free releasing them. As the ventilation system consisted mainly out of carbon steel it could almost entirely be dispatched to the nuclear melting facility. This exercise was done within half a year and required only 860 man hours. 6. Decommissioning the second laboratory Despite that the second laboratory area was only 2/3 of the size of the first (51 m2 compared to 78 m2 ); both laboratories were comparable in terms of decontamination: • The process cells were about the same size and weight in both laboratories. • The first laboratory contained a glovebox, but in the second laboratory were two chemical hoods installed vs. one in the first. • When the first laboratory was decommissioned a lot of furniture moved into the second. This furniture was decommissioned together with the second laboratory. As a result both laboratories were comparable both in terms of metallic and non-metallic items. The decommissioning of the second laboratory was almost exclusively done by members of the tritium lab R&D team instead of the decommissioning team. As we were more eager to research than to decontaminate we chose work differently: • In the former lab the decommissioning team put tremendous efforts in several decontamination sessions of non-metallic items to get them below the free release limit. We limit the maximum number of decontamination sessions to two per item. If after that the item would not be below the free release limit the item would be disposed as nuclear waste. • The metals were decontaminated only once to remove the most mobile tritium; then they were disposed of to the nuclear melting facility unless the one decontamination resulted in a tritium level below the free release limit. At the expense of generating some extra waste the second laboratory was decommissioned with merely 1/3rd of the man hours in less than 40% of the calendar days. A more detailed comparison will be made in the next paragraph. 7. Comparison of the different strategies For the laboratories the comparison is pretty straight forward: both rooms contained comparable installations and furniture which needed to be either free released or dispatched as nuclear waste. In laboratory one the exclamation was on free releasing as much as materials as possible; whereas in laboratory two the goal was to decommission the room as fast as possible with the least effort. From Table 2 can be concluded that each team was successful in complying with their decommissioning targets. The team that worked on the first laboratory was able to free release more than 75% of the metallic items and the furniture. The team that worked on the second laboratory and the ventilation system did not weigh the items when it was not required. However the count of the items from which the removable tritium contamination was measured indicates a lot smaller portions of the metals (27%) and other materials (65%) were free released. The R&D team that decommissioned the second laboratory room and the ventilation system obtained their goals as well: the second laboratory was decommissioned in less than 40% (1 year vs.
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Table 2 Comparison of labour and waste volume by decommissioning of different tritium infrastructures. Laboratory 1 Man hours (kh) Timespan (years) Nuclear waste Compressible (m3 ) Burnable (m3 ) Nuclear melt (tonne) Free release Metals (%) Other materials (furniture, etc.) (%)
Laboratory 2
Ventilation system
4.0 2.75
1.3 1.0
0.9 0.5
4.0 3.0 0.3
5.7 3.5 5.2
1.1 1.5 1.7
79 (weight) 75 (weight)
27a (parts) 65a (parts)
a Indicative numbers: report of the number items for which the removable tritium contamination was measured.
2.75 years) of the calendar days necessary for decommissioning the first laboratory using merely one third of the man hours (1.3 kH vs. 4.0 kH). On the other hand, the faster strategy generated more nuclear waste. Comparing the decommissioning of the ventilation system with the decommissioning of the first laboratory is not straight forward. But we can assume that the decommissioning of the steel process cell from laboratory 1 and the decommissioning of the ventilation system were comparable: both structures were almost exclusively made from steel and each of them weighed about 2 tonnes. The first team decontaminated the highly tritiated walls and floor of process cell to below the free release limit. However, the decontamination and disassembling of the cell was spread over 5 quarters. It was estimated that the decontamination of the inside of the cell alone took nearly 1100 man hours in 2005 [6]. When this is compared to the 2 quarters and 860 man hours that were necessary to safely decommission and dispose of the metals from the ventilation system to a nuclear melting facility it can be stated that the second strategy required less than 80% of the man hours and 40% of the calendar days. In the paragraph above it was already indicated that the free release strategy required more man hours but resulted in less nuclear waste than the much faster nuclear melting option. Table 3 summarizes the financial consequences of these differences. For this comparison only the direct costs are taken into account; indirect costs like for example the cost of income due to the unavailability of a laboratory were not incorporated. The total cost for decommissioning the first laboratory serves as basis for further comparisons and is set to the indicative value of 100. The labour costs for decommissioning laboratory 1 were almost a threefold than those for laboratory 2 and the ventilation system. But the profit made on the reduced labour cost was almost completely consumed by the costs allocated to the extra amount of nuclear waste. Disposing of nuclear materials is very costly. When the disposal is paid for by the technical passive fund in Belgium surpluses are charged. The technical passive fund is a fund for decommissioning nuclear installations that is provisioned when a nuclear installation is commissioned. Compared to the costs for nuclear disposal, the Table 3 Direct cost for decommissioning the tritium infrastructures at SCK•CEN (reported costs are costs relative to the total cost of decommissioning laboratory 1). Post
Laboratory 1
Laboratory 2
Ventilation system
Labor Purchases Nuclear metal recycling Nuclear waste Total
46.2 3.1 0.4 50.5 100.0
16.8 0.8 6.0 66.5 89.9
12.1 0.6 1.9 18.1 32.7
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cost for recycling (melting) of contaminated steel metals and the costs for acquiring equipment for decontamination are negligible. Summing everything up it can be concluded that the fast strategy was 10% cheaper, almost 3 times faster and required 3 times less human resources than the free release strategy. The +20% direct cost reduction that was expected in [6] was not achieved but this can be explained by the surplus charged for using the technical passive fund, that at the time of the writing of [6] was not taken into account. 8. Could we have done better? Recommendations It is known that decontaminating the metal process cell took 1100 man hours. It can be assumed that the decontamination to below the free release limit of the other metallic items took 500 man hours extra versus safe disassembly and disposal to a nuclear melting facility. When the metals from the first laboratory would have been recycled, and the rest of the materials would still have been free released there would have been a saving of 1600 man hours or an indicative value of approximately 20. On the other hand the cost for disposing the metals would increase by an indicative value of approximately 6. As a result 14% would have been saved on the total cost of decommissioning laboratory 1. Also the number of necessary calendar days necessary would have decreased, but not down to the level of the number of days that were necessary for decommissioning laboratory 2. The total cost for decommissioning laboratory 1 would then have been around the indicative value of 86; which is a 5% saving versus the decommissioning of laboratory 2. The recommendation for future decommissioning depends therefore mainly on the timing restrictions: Even when there is no time constraint it is best to foresee the safe disposal of the metals to a nuclear melting facility. Without taking into account indirect costs it is worth to invest man hours in decontamination to below free release limit of non-steel materials.
In the case there is a tough time constraint, the fast strategy used for laboratory 2 and the ventilation system should be re-applied. Economically, it is not recommended to strive for free releasing all materials. 9. Conclusions The two rooms that were used as tritium laboratories at SCK•CEN were decommissioned as well as the old ventilation system. The decommissioning strategy for one room was to free release as much materials as possible. For the second laboratory a faster strategy was used: contaminated steel was disposed of to a nuclear melting facility, other materials that could not be easily decontaminated were disposed of as nuclear waste. In this paper a comparison has been made between the two strategies. The second option turned out to be 10% cheaper and could be achieved in less than half of the time. In the future further cost savings can be achieved by using the latter strategy for the metals and the first strategy for the other materials. References [1] SCK•CEN, Officiële richtlijnen voor het beheer van radio actief afval, Ref RC/ss/02-23, 2002-05-28. [2] Belgian Ministery of the interior, Koninklijk Besluit van 20 juli 2001 houdende algemeen reglement op de bescherming van de bevolking, van de werknemers en het leefmilieu tegen het gevaar van de ioniserende stralingen, 2001. [3] K. Dylst, Vrijgave methodologie voor de vrijgave van procescellen in de tritium laboratoria, Internal SCK•CEN document REF.IDPBW.0713, Mol, Belgium, 7 March 2005. [4] U.S. Department of Energy, DoE Handbook on tritium handling and safe storage, DoE-HDBK-1129-99, 1999. [5] IAEA, Regulations for the safe transport of radioactive material, par 508 from IAEA safety standard series No ST-1. [6] K. Dylst, J. Braet, A. Bruggeman, S. Vanderbiesen, Refurbishment of the tritium laboratories at SCK•CEN, in: European Working Hot Laboratories and Remote Handling Proceedings, Jülich, Germany, 19–21 September 2006/Forschungzentrum Jülich, Jülich, Germany, Forschungzentrum Jülich, 2007, p. 5.
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Comparison of different strategies for decommissioning a tritium laboratory Kris Dylst ∗ , Frederik Slachmuylders, Bart Gilissen SCK•CEN, Dismantling, Decontamination and Waste, Boeretang 200, 2400 Mol, Belgium
h i g h l i g h t s Decontamination to below the free release limits is very labour intensive. Disposing of contaminated steel to a nuclear melting facility is cost effective. It can be advantageous to invest in decontamination of non-steel materials.
a r t i c l e
i n f o
Article history: Available online 11 January 2013
a b s t r a c t Between 2003 and 2009 two rooms that served as tritium laboratory at SCK•CEN and its ventilation system were decommissioned. Initially, the decommissioning strategy was to free release as much materials as possible. However, due to the imposed free release limit this was very labour intensive. Timing restrictions forced us to use a different strategy for the ventilation system. Most of the steel was disposed of to a nuclear melting facility. As a result there was a significant decrease in the required man labour. For the second laboratory room a similar strategy as for the ventilation was used: contaminated steel was disposed of to a nuclear melting facility and other materials that could not be easily decontaminated were disposed of as nuclear waste. At the expense of extra waste generation compared to the first laboratory the decommissioning was done using merely one third of the man hours. Comparison of the used strategies indicated opportunities for cost optimization. Even in absence of time constraints it is best to foresee a safe disposal of metals to a nuclear melting facility, whilst it is worth to invest in the labour intensive decontamination of the other materials to free release them. © 2013 Elsevier B.V. All rights reserved.
1. Introduction The tritium laboratories at SCK•CEN were commissioned in 1975 for a maximum tritium inventory of 37 TBq (1000 Ci) tritium. Major tritium research for both fission and fusion has been executed. The tritium laboratory consists of 2 adjacent rooms with a total floor space of nearly 130 m2 . The lab refurbishment was necessary to improve the infrastructure, the capabilities and the safety. The refurbishment will not increase the surface of the laboratories. A new ventilation system should allow for a higher tritium inventory of 370 TBq (10,000 Ci). One room has been refurbished whilst the other was in operation. Despite this increased complexity of the refurbishment project, it was important for SCK•CEN to keep some tritium research capabilities available. Between 2003 and 2009 two rooms that served as tritium laboratory at SCK•CEN and its ventilation system were
∗ Corresponding author. Tel.: +32 (0) 14 33 26 30. E-mail address: [email protected] (K. Dylst). 0920-3796/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.fusengdes.2012.12.009
decommissioned. Initially, the decommissioning strategy was to free release as much materials as possible. Timing restrictions forced us to use a different strategy for the ventilation system and the second laboratory: steel and other materials that could not be easily decontaminated below the free release limit were disposed to a nuclear melting facility and other materials that could not be easily decontaminated were disposed as nuclear waste. The different strategies used on two comparable laboratories give us a unique learning opportunity for optimizing tritium facilities decommissioning.
2. Manners for disposing of nuclear materials from the tritium laboratory The three main ways to dispose of decommissioned materials from the tritium laboratories were: 1. free releasing the materials, 2. disposing the materials as nuclear waste, or 3. recycling steel in a nuclear melting facility.
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Table 1 Measured H3 activity on a hot spot on the painted metal floor of a process cell in laboratory 1. Layer (from exposed to deep)
Paint (exposed layer) Metal just below paint Bulk metal
Measured H3 contamination (Bq/dm2 )
(Bq/g)
5000 800 n.a.
3000 n.a. 4
Free released materials can be disposed of or recycled as nonnuclear materials. They contain less radionuclides than what is considered to be a risk for public health. In Belgium there is a law that lists the maximum concentration for a large number of radionuclides contained in materials. These concentrations are free release limits. The waste disposal cost for free released materials is negligible compared costs associated with nuclear waste. Materials that cannot be free released should be disposed of as nuclear waste. However, most of the metals which cannot free released can be recycled in a nuclear melting facility. In Belgium there are several different types of nuclear waste defined mostly depending on the composition, physical form and radio nuclear contamination of the waste [1]. 3 types of waste were generated whilst dismantling the tritium laboratories: (i) compressible waste, (ii) burnable waste and (iii) filters. The costs connected to disposing nuclear waste are high. Contaminated steel can be recycled in a nuclear melting facility at only a fraction of the cost in comparison to disposing it as compressible nuclear waste.
3. The tritium free release limit Free release limits are the key in decommissioning nuclear facilities. It is easier to decontaminate to below higher free release limits than to decontaminate below lower free release limits. Another aspect is the applicability of the free release limits: i.e. is it possible or even meaningful to check against the existing free release limits? The latter considerations are very valid in the case of our tritium laboratory. The tritium free release limit for solid radioactive waste products in Belgium is 100 Bq/g [2]. This limit is not practical in our case for several reasons: • A limit expressed per weight material averages the tritium contamination at the surface with the tritium in the bulk. In our case the materials that needed to be checked were solid materials which were contaminated at room temperature by tritiated liquid spills or by tritiated gasses. Table 1 shows that despite tritium can migrate into substance, in our case [3] contamination is concentrated in or at the surface layers. Incorporated tritium contamination is more an issue for materials in which tritium would be generated due to activation or materials in which tritiated species can penetrate easily. • Due to the low energy level (-radiation, Emax = 18.7 keV) the radiation emitted by tritium incorporated in the bulk of a material has no effect on the environment. In this perspective it becomes clear that, especially in the case of surface contaminated materials, the tritium surface contamination (Bq/dm2 ) is a better safety criterion than the average tritium content of a material (Bq/g) [4]. • Another, practical, reason to prefer surface contamination based tritium free release limits for solid materials is that nondestructive measurement techniques can be used. The low energy level of tritium radiation prevents ‘incorporated’ tritium measurement with non-destructive techniques.
The above arguments for a tritium free release limits based on surface contamination are valid for the equipment and furniture that were present in our lab. An application was made to the Belgian Federal Nuclear Control Agency to introduce an exceptional free release limit of 250 Bq/dm2 removable tritium for surface contaminated material. Although all parties agreed that the removable tritium contamination was a valid criterion for free release there was no will to treat tritium as an exceptional low radiotoxic element. As a result we were forced to use an interpretation of the international transport regulations [5] for beta–gamma emitters as a free release limit. The materials from the tritium lab could be free released when the measured removable surface contamination was less than 4 Bq/dm2 . 4. Decommissioning the first laboratory The two tritium laboratory rooms and their common ventilation system were decommissioned one after the other between 2003 and 2009. Before the rigorous surface contamination free release limit was approved it was decided that, to decommission the first laboratory the goal was to free release as much materials as possible. Items like closets, tables and furniture that were not suspected to have been exposed to medium or highly tritiated species were cleaned and checked for residual tritium contamination. The items that did not meet the free release criterion after a couple of decontamination sessions were disposed of as nuclear waste. This was justified because of the limited volume of these items. The items in the laboratory in which tritium was handled in medium to high concentrations were a large process cell, a glovebox, and a hood. These items were mostly composed of painted carbon steel and had plexiglas windows. These items together with their ventilation shafts represented large volumes of waste. These materials were free released as much as possible. Due to the low free release limit a lot of effort was necessary to decommission the first laboratory. It took almost 4000 man hours over a period of 2.75 years to decommission the laboratory. The decontamination of the metal from the process cells alone (without dismantling or decommissioning) took nearly 1100 man hours. 5. Decommissioning the ventilation system When the first laboratory was refurbished, both laboratories were connected to a new ventilation system making the old ventilation system obsolete. For a number of reasons the free releasing strategy from the first laboratory could not be reapplied for the old ventilation system. The first consideration was safety: the ventilation and pulsation shafts crossed non-nuclear corridors in the BR1 building. For the decommissioning the hallway was divided into a passage zone and a decommissioning zone by installing temporary walls. To limit the risk of exposure the shafts needed to be packed and removed as soon as they were dismantled; leaving no time for thorough decontamination. Second consideration was a timing restriction: The old ventilation fan and filtering system was located in a technical story that had to be removed to start with the construction of the reactor building for the new experimental Guinevere reactor. Another timing aspect was that this time the decommissioning team was preoccupied with other projects. The decommissioning needed to be done by the tritium lab R&D team. To not compromise too much on our R&D timings the work needed to be done in as little time as possible. And finally economic aspects played a role. After the decommissioning the first laboratory analyses was made [6] indicating
K. Dylst et al. / Fusion Engineering and Design 88 (2013) 2655–2658
that more than 20% of the direct costs could be saved when the metals would be disposed of to a nuclear melting facility instead of free releasing them. As the ventilation system consisted mainly out of carbon steel it could almost entirely be dispatched to the nuclear melting facility. This exercise was done within half a year and required only 860 man hours. 6. Decommissioning the second laboratory Despite that the second laboratory area was only 2/3 of the size of the first (51 m2 compared to 78 m2 ); both laboratories were comparable in terms of decontamination: • The process cells were about the same size and weight in both laboratories. • The first laboratory contained a glovebox, but in the second laboratory were two chemical hoods installed vs. one in the first. • When the first laboratory was decommissioned a lot of furniture moved into the second. This furniture was decommissioned together with the second laboratory. As a result both laboratories were comparable both in terms of metallic and non-metallic items. The decommissioning of the second laboratory was almost exclusively done by members of the tritium lab R&D team instead of the decommissioning team. As we were more eager to research than to decontaminate we chose work differently: • In the former lab the decommissioning team put tremendous efforts in several decontamination sessions of non-metallic items to get them below the free release limit. We limit the maximum number of decontamination sessions to two per item. If after that the item would not be below the free release limit the item would be disposed as nuclear waste. • The metals were decontaminated only once to remove the most mobile tritium; then they were disposed of to the nuclear melting facility unless the one decontamination resulted in a tritium level below the free release limit. At the expense of generating some extra waste the second laboratory was decommissioned with merely 1/3rd of the man hours in less than 40% of the calendar days. A more detailed comparison will be made in the next paragraph. 7. Comparison of the different strategies For the laboratories the comparison is pretty straight forward: both rooms contained comparable installations and furniture which needed to be either free released or dispatched as nuclear waste. In laboratory one the exclamation was on free releasing as much as materials as possible; whereas in laboratory two the goal was to decommission the room as fast as possible with the least effort. From Table 2 can be concluded that each team was successful in complying with their decommissioning targets. The team that worked on the first laboratory was able to free release more than 75% of the metallic items and the furniture. The team that worked on the second laboratory and the ventilation system did not weigh the items when it was not required. However the count of the items from which the removable tritium contamination was measured indicates a lot smaller portions of the metals (27%) and other materials (65%) were free released. The R&D team that decommissioned the second laboratory room and the ventilation system obtained their goals as well: the second laboratory was decommissioned in less than 40% (1 year vs.
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Table 2 Comparison of labour and waste volume by decommissioning of different tritium infrastructures. Laboratory 1 Man hours (kh) Timespan (years) Nuclear waste Compressible (m3 ) Burnable (m3 ) Nuclear melt (tonne) Free release Metals (%) Other materials (furniture, etc.) (%)
Laboratory 2
Ventilation system
4.0 2.75
1.3 1.0
0.9 0.5
4.0 3.0 0.3
5.7 3.5 5.2
1.1 1.5 1.7
79 (weight) 75 (weight)
27a (parts) 65a (parts)
a Indicative numbers: report of the number items for which the removable tritium contamination was measured.
2.75 years) of the calendar days necessary for decommissioning the first laboratory using merely one third of the man hours (1.3 kH vs. 4.0 kH). On the other hand, the faster strategy generated more nuclear waste. Comparing the decommissioning of the ventilation system with the decommissioning of the first laboratory is not straight forward. But we can assume that the decommissioning of the steel process cell from laboratory 1 and the decommissioning of the ventilation system were comparable: both structures were almost exclusively made from steel and each of them weighed about 2 tonnes. The first team decontaminated the highly tritiated walls and floor of process cell to below the free release limit. However, the decontamination and disassembling of the cell was spread over 5 quarters. It was estimated that the decontamination of the inside of the cell alone took nearly 1100 man hours in 2005 [6]. When this is compared to the 2 quarters and 860 man hours that were necessary to safely decommission and dispose of the metals from the ventilation system to a nuclear melting facility it can be stated that the second strategy required less than 80% of the man hours and 40% of the calendar days. In the paragraph above it was already indicated that the free release strategy required more man hours but resulted in less nuclear waste than the much faster nuclear melting option. Table 3 summarizes the financial consequences of these differences. For this comparison only the direct costs are taken into account; indirect costs like for example the cost of income due to the unavailability of a laboratory were not incorporated. The total cost for decommissioning the first laboratory serves as basis for further comparisons and is set to the indicative value of 100. The labour costs for decommissioning laboratory 1 were almost a threefold than those for laboratory 2 and the ventilation system. But the profit made on the reduced labour cost was almost completely consumed by the costs allocated to the extra amount of nuclear waste. Disposing of nuclear materials is very costly. When the disposal is paid for by the technical passive fund in Belgium surpluses are charged. The technical passive fund is a fund for decommissioning nuclear installations that is provisioned when a nuclear installation is commissioned. Compared to the costs for nuclear disposal, the Table 3 Direct cost for decommissioning the tritium infrastructures at SCK•CEN (reported costs are costs relative to the total cost of decommissioning laboratory 1). Post
Laboratory 1
Laboratory 2
Ventilation system
Labor Purchases Nuclear metal recycling Nuclear waste Total
46.2 3.1 0.4 50.5 100.0
16.8 0.8 6.0 66.5 89.9
12.1 0.6 1.9 18.1 32.7
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cost for recycling (melting) of contaminated steel metals and the costs for acquiring equipment for decontamination are negligible. Summing everything up it can be concluded that the fast strategy was 10% cheaper, almost 3 times faster and required 3 times less human resources than the free release strategy. The +20% direct cost reduction that was expected in [6] was not achieved but this can be explained by the surplus charged for using the technical passive fund, that at the time of the writing of [6] was not taken into account. 8. Could we have done better? Recommendations It is known that decontaminating the metal process cell took 1100 man hours. It can be assumed that the decontamination to below the free release limit of the other metallic items took 500 man hours extra versus safe disassembly and disposal to a nuclear melting facility. When the metals from the first laboratory would have been recycled, and the rest of the materials would still have been free released there would have been a saving of 1600 man hours or an indicative value of approximately 20. On the other hand the cost for disposing the metals would increase by an indicative value of approximately 6. As a result 14% would have been saved on the total cost of decommissioning laboratory 1. Also the number of necessary calendar days necessary would have decreased, but not down to the level of the number of days that were necessary for decommissioning laboratory 2. The total cost for decommissioning laboratory 1 would then have been around the indicative value of 86; which is a 5% saving versus the decommissioning of laboratory 2. The recommendation for future decommissioning depends therefore mainly on the timing restrictions: Even when there is no time constraint it is best to foresee the safe disposal of the metals to a nuclear melting facility. Without taking into account indirect costs it is worth to invest man hours in decontamination to below free release limit of non-steel materials.
In the case there is a tough time constraint, the fast strategy used for laboratory 2 and the ventilation system should be re-applied. Economically, it is not recommended to strive for free releasing all materials. 9. Conclusions The two rooms that were used as tritium laboratories at SCK•CEN were decommissioned as well as the old ventilation system. The decommissioning strategy for one room was to free release as much materials as possible. For the second laboratory a faster strategy was used: contaminated steel was disposed of to a nuclear melting facility, other materials that could not be easily decontaminated were disposed of as nuclear waste. In this paper a comparison has been made between the two strategies. The second option turned out to be 10% cheaper and could be achieved in less than half of the time. In the future further cost savings can be achieved by using the latter strategy for the metals and the first strategy for the other materials. References [1] SCK•CEN, Officiële richtlijnen voor het beheer van radio actief afval, Ref RC/ss/02-23, 2002-05-28. [2] Belgian Ministery of the interior, Koninklijk Besluit van 20 juli 2001 houdende algemeen reglement op de bescherming van de bevolking, van de werknemers en het leefmilieu tegen het gevaar van de ioniserende stralingen, 2001. [3] K. Dylst, Vrijgave methodologie voor de vrijgave van procescellen in de tritium laboratoria, Internal SCK•CEN document REF.IDPBW.0713, Mol, Belgium, 7 March 2005. [4] U.S. Department of Energy, DoE Handbook on tritium handling and safe storage, DoE-HDBK-1129-99, 1999. [5] IAEA, Regulations for the safe transport of radioactive material, par 508 from IAEA safety standard series No ST-1. [6] K. Dylst, J. Braet, A. Bruggeman, S. Vanderbiesen, Refurbishment of the tritium laboratories at SCK•CEN, in: European Working Hot Laboratories and Remote Handling Proceedings, Jülich, Germany, 19–21 September 2006/Forschungzentrum Jülich, Jülich, Germany, Forschungzentrum Jülich, 2007, p. 5.