Behaviour of Carbon-14 Released from Activated Steel in Repository - - PowerPoint PPT Presentation

Power / Tapani Eurajoki 1

Behaviour of Carbon-14 Released from Activated Steel in Repository Conditions – a Key Issue in the Long-term Safety of Decommissioning Waste

NKS Seminar on Decommissioning of nuclear facilities, Studsvik, Nyköping, Sweden, September 2010

Power / Tapani Eurajoki 2

Contents of the presentation

• origin and activity of C-14 in activated metal components
• final disposal of activated components in Loviisa NPP
• corrosion as the release mechanism
• chemical speciation of the released carbon
• some comments on the conducted experimental work
• approach chosen for the safety assessment of Loviisa NPP

decommissioning plan

• possible mechanisms to affect the speciation
• ideas for further experimental work - and related problems

Power / Tapani Eurajoki 3

Origin and activity of C-14 in activated metal components

• Neutron activation, three

reactions:

– N-14 (n,p) C-14 – C-13 (n,g) C-14 – O-17 (n,a) C-14

• In an LWR, in stainless

steel the reaction from N-14 dominates

Induced activity in the decommissioning waste

1E+2 1E+3 1E+4 1E+5 1E+6 1E+7 1E+8 1E+9 1 10 100 1000 10000 100000 1000000

time, yrs activity, GBq

TOTAL C-14 Ca-45 Mn-54 Fe-55 Co-60 Ni-59 Ni-63 Eu-152 H-3

Induced activity in the decommissioning waste of the Loviisa NPP

Power / Tapani Eurajoki 4

Final disposal of activated components; case: Loviisa NPP, reactor pressure vessel with internals

• One-piece removal of large

components

• Reactor internals are packed in

the RPV

• The nozzles are sealed.
• Surrounding concrete silo

structure

• Low corrosion rate

Power / Tapani Eurajoki 5

Release from steel and chemical speciation of carbon

• Carbon is assumed to be released from steel, as the steel corrodes.
• In anaerobic conditions in cementitious environment the corrosion rate is

low (~0.1 µm/a).

• Diffusion from steel is believed to be low compared to corrosion.
• Chemical speciation in the activated steel (carbide?; what happens if C-

14 is formed from N-14 as nitride? High neutron energy involved.)

• Chemical speciation after being released from the activated steel?

– carbonate? => low solubility and large amount stable carbonate in the cementitious repository conditions => OK – soluble organic form? => possibly very mobile – gaseous organic form? => solubility, mobility

• According to Pourbaix diagram the organic forms should not be stable in

the alkaline and reducing repository conditions; still there are indications

• f their existence.

Power / Tapani Eurajoki 6

Possible migration routes for C-14 from the repository to the biosphere

C-14 ORGANIC INORGANIC DISSOLVED CARBONATE

• NEGLIGIBLE

RETAINED IN CARBONATE FORM RETAINED IN REPOSITORY WELL GAS LIQUID WELL TRANSPORTATION IN GAS PHASE DISSOLVED GAS METABOLISM - PLANT UPTAKE ATMOSPHERIC RELEASE ATMOSPHERIC RELEASE ATMOSPHERIC RELEASE

Power / Tapani Eurajoki 7

Some comments on the available literature and the conducted experimental work

• Only few relevant articles on experimental results are available.
• Experimental works by Deng et al. (1997) and Kaneko et al. (2003) are often

cited

– open questions regarding coverage, representativeness and relevance for the repository issue

• phenomenology to some extent unclear
• redox conditions may not be representative
• Both gaseous and soluble organic compounds were detected
• No straightforward quantitative conclusions can be drawn.
• A master’s thesis (Kuitunen 2007) was financed by Fortum for the safety

assessment of Loviisa NPP

– Literature survey – …even though the formation of organic species has not been confirmed, their existence cannot be denied and this should be taken into account in the future safety assessments.

• Research is going on…

Power / Tapani Eurajoki 8

Approach chosen for the safety assessment of Loviisa NPP decommissioning plan

• Release from the metal according to the corrosion rate
• Soluble and mobile chemical form assumed
• Sensitivity analysis: gaseous chemical form
• => C-14 is the dominant nuclide

1.0E+03 1.0E+04 1.0E+05 1.0E+06 1.0E+07 1.0E+08 1.0E+09 1.0E+10 1.0E+11 1.0E+03 1.0E+04 1.0E+05 1.0E+06 Time (a) Release rate (Bq/a) C-14 Ni-59 Tc-99 Nb-94

Release rates as 1000 years’ averages compared to regulatory constraints (dashed lines)

Power / Tapani Eurajoki 9

Possible phenomena/mechanisms to tackle the problem

• chemical reactions between organic forms and carbonate; kinetics?
• some catalysts to enhance the reactions
• isotopic exchange
• radiolysis to decompose the organic molecules
• microorganisms
• …
• behavior of the organic species: sorption, diffusion?
• => Need for further experimental work

Power / Tapani Eurajoki 10

Questions and ideas for further experimental work - and related problems

• A long phenomenological chain from the steel to the biosphere; which

phenomena are to be investigated?

• Is C-14 chemically similar to C-12 in steel => research work with radioactive

substances.

• Accelerated corrosion is probably needed in the experiments, but how not to

disturb the speciation of carbon?

• The reported experiments should be repeated with additional measurements.
• All the chemical species should be measured.
• If assumed to be organic => possible reactions and their kinetics (possible to

experiment with inactive substances).

• Long timescales characterize the repository conditions => how to accelerate the

experiments.

• Research within NKS?

Power / Tapani Eurajoki 11

Conclusions

• C-14 in activated steel components may be one of the major dose

contributors in the decommissioning waste.

• Chemical speciation of C-14 in repository conditions is not completely

clear.

• Organic species may be significantly more mobile than inorganic ones.
• The problem is characterized by a long phenomenological chain from the

steel to the biosphere.

• Further research is necessary to reveal the main chemical reactions and

their kinetics.

• A more detailed – and possible less conservative – modelling can be

applied only if the phenomenology is sufficiently well known.

Power / Tapani Eurajoki 12

Thank you for your attention!