Release of Radioactive Materials from a Degrading Core (RADECO)47

The main objective of iodine behavior studies was to assess if the paints applied in Finnish

nuclear power plants will contribute in formation of organic iodides under the chemical

containment conditions prevailing in Finnish nuclear power plants during a severe accident.

Organic iodides are highly volatile compounds which are difficult to remove from atmosphere with the existing filter technology.

The progress of severe accident phenomena during a severe accident have not been investigated in the same extent as the severe accident phenomena starting during operation.

The oxidation of metals in oxygen-rich atmosphere and release of fission products may be different from those during normal operation. A scoping study of the integral effects of severe accident starting at shutdown conditions should be first performed. The air ingression to the pressure vessel is a key point e.g. to the release of ruthenium in volatile form.

Specific goals in 2008

The formation of organic iodides in gas phase was performed by impregnating the paint surfaces with elemental iodine and then exposing the test blocks to gas atmosphere with or without radiation field to see if any iodine is revolatilized from the painted surfaces. In addition, VTT participate OECD/BIP project, which starts 1.7.2007. The information from the project is useful to compare our own experimental iodine test results, especially the modelling part could be helpful to understand the phenomena.

Figure 1. The painted blocks adsorbed gaseous iodine on the surface from the air.

In all performed experiments, part of the iodine was released from the surface, but it is immediately adsorbed on the other surfaces, reaction vessel, and gas lines. The reaction on the surface was so rapid that it was impossible to determine the chemical form of iodine. The amount of organic or elemental iodine trapped in the filters was negligible.

According to these experiments the formation of the organic iodine is very limited. The

painted containment surfaces exposed to atmosphere can be effective traps for iodine in

severe accident conditions. However, some migration of iodine on the surfaces may be

possible by sequential revolatilization and re-trapping. Furthermore, the effects of flooding

the painted surfaces which carry iodine with moderate to high pH water still needs to be

studied.

The target of investigations of severe accidents during shutdown conditions was to examine overall performance of MELCOR code for accident scenarios occurring during maintenance outage. Olkiluoto 1 and 2 were selected for the reference plant. Special goals of the work were to assess air ingression into opened pressure vessel, the effects on cladding oxidation and potential hydrogen combustion and ruthenium release. Concerning the ruthenium issue, the strategy was to adjust the existing ruthenium release models in MELCOR to account for the recent measured data on ruthenium behavior under oxidizing conditions. The modifications were limited to the available sensitivity coefficients and material data specified in the MELCOR input. The calculations were purposed to be scoping studies for envisioning the possible effects of ruthenium behavior to plant scale. Increasing the accuracy of the results would need modifications to the MELCOR source code.

The results of the calculation with MELCOR code suggest that even a small hole in the RPV Bottom Head may allow some air ingress to the core region. If the Lower Head is intact, no air ingress to the RPV would take place. Station blackout scenario resulted in high zirconium oxidation fraction, about 60 %, and hydrogen generation. The oxidation fraction in the Bottom Head LOCA case remained lower, less than 14 %. The reason for lower oxidation is the drainage of coolant through the RPV leak and injection of cold coolant to the RPV by shutdown cooling system, which suppressed steam production. Air ingression was not sufficient to support strong oxidation.

In the calculated cases Ru release from the core equaled to 8 – 23 % of the initial core inventory. The highest Ru release was obtained in the station blackout scenario. If a large portion of the released ruthenium is volatile RuO

, the vapor is easily transported to the reactor pool and reactor hall residing outside the containment barrier. In the performed calculation a total of 41 kg of Ru was released from the containment and reactor building. The activity release with Ru is of the order of 2700 TBq. Due to several conservatisms and modeling simplifications this exercise can be considered only as crude approximation and encouragement to study further the behavior of Ru during severe accidents during plant shutdown conditions and its effects on source term.

Deliverables in 2008

• Zilliacus, R., Kekki, T., Formation of organic iodide on painted surface in gas phase, VTT-R-00503-09, VTT, Espoo, 2009, 7 p.

• Suopajärvi, A., MELCOR calculations for accidents occurring during maintenance outage in Olkiluoto units 1 & 2, VTT-R-00933-09, VTT, Espoo, 2009, 60 p.

• Travel report of 2

Meeting of the BIP (Behaviour of Iodine) Project Programme Review Group has been sent to the reference group.

• Travel report of 3

Meeting of the BIP (Behaviour of Iodine) Project Programme Review Group has been sent to the reference group.

• Information related to the progress of the OECD/BIP Programme was distributed.

2.5.2 Primary circuit chemistry of fission products (CHEMPC)

The objective of the project is to study the behaviour of iodine in a severe accident conditions.

In particular, the aim is to increase understanding of revaporisation and transport of iodine in primary circuit and containment of a nuclear power plant. The primary circuit study is a joint project with IRSN Caradache research centre (2006-2010) for the determination of iodine chemistry in the primary circuit. The objective of the study at VTT is to determine iodine compounds released due to the reactions on the surface of primary circuit piping. At the same time IRSN focus in the gas phase chemistry of iodine in similar experimental conditions. In this study, novel analysis techniques for quantification of chemical reaction kinetics are developed.

Such measurements provide information on high temperature chemistry and enable validation of for example iodine chemistry codes. Radiolytic oxidation of elemental iodine in containment conditions is studied together with Chalmers University of Technology. The objective is to verify the possible iodine oxide aerosol particle formation. The facility build at VTT is applied in this study.

Another objective of the project is to continue to follow up Phebus FP and International Source Term Programmes (ISTP). A study of applying analysis techniques developed at VTT in ISTP will be initiated. VTT participates also in follow up meetings of ISTC EVAN project.

Specific goals in 2008

The main goal in 2008 was to study the oxidation of iodine in conditions similar to the containment of a nuclear power plant during a severe accident. The amount of experiments was increased compared to original plan. Therefore it was possible to study several experimental conditions more and achieve a wide database for modelling. The total number of experiments was 26. Because of that the experimental work in primary circuit studies was postponed to 2009. However, the facility and the sampling system are already tested. As a result of a co-operation with IRSN, VTT participated in testing of CHIP facility at Cadarache.

The objective in containment experiments was to determine the influence of oxygen, ozone and iodine concentration as well as that of radiation intensity on the possible formation of iodine oxide aerosols from elemental iodine gas. The atmosphere was either air or a mixture of nitrogen and oxygen. The volume fraction of oxygen in a gas flow was 50%, 21% or 2%.

Experiments were conducted primarily in moist gas flow. Four revaporisation experiments were conducted in dry gas flow. In these experiments, the aim was to study the effect of ozone and radiation on the possible evaporation and subsequent nucleation of iodine. In the experiments ozone concentration was varied, while iodine concentration depended on the production procedure. The total gas flow through the flow furnace ranged from 6 l/min to 24 l/min (NTP). The total flow rate was varied in order to study the effect of residence time on iodine reaction products.

The second goal was to participate in international experimental program meetings. It

included interpretation circle meetings of Phebus FP and ISTP programmes and participation

in working group reviewing the progress of ISTC EVAN project. Several sampling

instruments and online detection techniques were studied at VTT to apply in EPICUR and

CHIP facilities.

0.00 3.00 6.00 9.00 12.00 15.00

0 50 100 150 200 250 300

O3 [ppm]

I2 [ppm]

0.00 0.10 0.20 0.30 0.40 0.50

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08

O3 [ppm]

I2 [ppm]

Figure 1. The effect of ozone concentration on iodine concentration transported through the facility. The results are from two experiments (green and purple). The source of ozone in

“green” experiment is an ozone generator. In “purple” experiment ozone is a product from a reaction between UV radiation and oxygen. It seems that a significant fraction of iodine reacts with ozone on the surfaces.

Figure 2. The EDX analysis of iodine and oxygen in collected particles on copper/carbon grid. The particles on carbon side of the grid contain both iodine and oxygen. These particles appear to be either liquid or at least partially melted. This is not a surprise as iodine oxide particles are known to vaporise easily under the electron beam. According to elemental map particles on copper side of the grid do not contain essentially any oxygen. The likely reason for the difference in particle morphology and composition is that iodine oxide particles deposited on copper have reacted to copper iodate. The possible formation of metal iodates from deposited iodine oxide particles is however interesting as such reactions would likely to take place also in containment building.

Deliverables in 2008

• A scientific paper “Progress on ruthenium release and transport under air ingress

conditions” was published in Nuclear Engineering and Design Vol. 238 (2008).

• An abstract “Progress in understanding key aerosol issues” was published in Proceedings of The Third European Review Meeting on Severe Accident Research (ERMSAR 2008), 23 – 25.9.2008.

• An abstract “Ruthenium behaviour under air ingress conditions: main achievements in the SARNET project” was published in Proceedings of The Third European Review Meeting on Severe Accident Research (ERMSAR 2008), 23 – 25.9.2008.

• An abstract “A computer controlled particle sampling system with an adjustable and constant dilution ratio and low sample losses” was published in Proceedings of International Aerosol Conference 2008, 24-29.8.2008.

• A final report “Experimental study on iodine chemistry (EXSI) – Containment experiments with elemental iodine” was published 23.2.2009.

• A progress report “Experimental study on iodine chemistry (EXSI) – Facility for primary circuit experiments” was published 15.4.2009.

• A summary report “Primary circuit chemistry of fission products (CHEMPC)” was published 13.3.2009.

• A report “Phébus FPT2 Final Report” was published 19.9.2008.