Collection and Characterization of Emissions from Metal Cutting in CAORSO Nuclear Power Plant Decommissioning

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18th International Conference on Structural Mechanics in Reactor Technology (SMiRT 18) Beijing, China, August 7-12, 2005 SMiRT18-W01-3

COLLECTION AND CHARACTERIZATION OF EMISSIONS FROM

METAL CUTTING IN CAORSO NUCLEAR POWER PLANT

DECOMMISSIONING

Franco Giuseppe Cesari

University of Bologna

Phone: +39 0516441709, Fax: +39

0516441747

E-mail:

Luigi Andrea Terzi*

University of Bologna

Phone: +39 0516441713, Fax: +39

0516441747

E-mail:

Angelo Giostri

Sogin Caorso

Cristina Bernini

INFM – Genova

Enzo Sirito

SAFIM – SIGE

Massimo Sirito

SAFIM - SIGE

ABSTRACT

The Caorso’s Nuclear Power Plant (BWR, 870 MWe) has just started the decommissioning process, with the intent to reach, by almost ten years, the “green field” conditions for the site. The plant has fully worked for a very short period of time, by 1981 to 1986, being shut down after 1987 Italy’s poll that abrogated nuclear power use. The dismantling of the components and of the structural materials has already begun in the Turbine Building.

The University of Bologna, on indication of the NPP’s management, has started an experimental campaign to test the cutting processes and its filtering plant.

The starting phase is the qualification of the cutting methods chosen by Caorso’s management, oxyfuel and plasma cuttings. This campaign is set over no contaminated material, or, better, material with a contamination under the level of free release, and is now running in the University Labs. Next phase are filtering tests.

This part of the qualifying campaign is set to highlight the kind of trouble that can emerge in the cutting processes, not yet taking into account radioactivity.

Caorso’s BWR is a plant designed and built in the 70s. Possible decommissioning problems weren’t considered during the design phase so the cutting processes will be quite difficult, even on the conventional side.

The final phase is settled in the plant. Cutting tests, following indications made by previous campaigns, will be conducted in the Turbine Building, where tests can be conducted on a low level of radioactivity (only some little part of it has a contamination level over the free release limit of 1 Bq/cm2). The intent is to develop an extensive cutting procedure, with the obvious option of remote control, able to face difficulties connected with cutting processes in a nuclear plant like Caorso. This means handling with radioactivity and with not airy narrow rooms, fulfilled with pipes

Keywords: Plasma cutting, Oxycutting, emissions, decommissioning, Aerosol, experimental campaign.

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1. DECOMMISSIONING ACTIVITIES

The Caorso Nuclear Power Plant was the greatest Italian nuclear facility. It is equipped with a Boling Water Reactor of almost 850 MWe, with an efficiency of the 33%. The plant was stopped in 1986, due to planned maintenance, and has never been made start back, following the poll that in 1987 sanctioned Italian exit by nuclear power use. Caorso’s NPP worked properly for almost seven years and , by now, keeps most side of Italian nuclear radwaste.

Decommissioning activities of Caorso BWR nuclear facility started in 1999 and had a progressive acceleration in the last years. The schedule foresees reaching the “green field” conditions for the site in 2017. The plant, whose construction was completed at the end of the 70s, was not conceived and built thinking about the final dismantling process. Thus, all the operations regarding dismantling and cutting activities can be quite troublesome. The difficulty is due not only to the nuclear contaminated materials, but even to the poor manoeuvrability, to narrow and poorly lighted places and also, sometimes, to the difficulty in obtaining and keeping an adequate air circulation in site where the dismantlement features are operating.

The dismantling process has started removing components and materials in the Turbine Building, which is a section of the controlled zone. This is because the plant is a BWR and so the coolant, obviously water, goes to the turbines directly after the in core channels exposition. Radioisotopes carried by the cooling water to the plant and even toward turbines leave some contamination by deposition on the inner surface of piping and plant components. Some part of the plant, especially inside the Reactor Building, is still a lot contaminated; the specific contamination can even reach values higher then thousands of Bq/g, due to neutron activation of materials.

The radiation level inside the Turbine Building is instead quite low. Most of the material to be removed inside there is, in fact, already under the free release level, 1 Bq/cm2 referring to superficial contamination, even without applying any decontamination process.

2. EXPERIMENTAL ACTIVITY

University of Bologna, through the DIENCA – LINSEI laboratory, has started a co-operation with SOGIN SpA, which is charged by government authorities to prepare and manage all the dismantling programmes. A major disadvantage of delayed dismantlement, as applied for Caorso nuclear plant, is that the workers will be unfamiliar with the facility and will have to rely on the adequacy of archival records for structural and engineering details. Regardless of whether dismantling is delayed or immediate, it will involve segmentations and removal of metal components by means of adequate devices to be used in situ. A range of health hazards can be postulated for workers involved in these metal cutting. For material superficially contaminated due to internally deposited substances the inhalation is expected to be the predominant mode of exposure.

The research aim is to evaluate thermal cutting processes based on two options, plasma torch and oxyacetylene device, with intent to investigate the possible risks for the workers and to optimise the operative procedures. Firs of all a testing campaign to analyse emissions of the thermal cutting processes chosen has been settled.

The choice to analyse oxycutting and plasma cutting has been made by SOGIN SpA. Tubes and plates to be cut during the tests have been selected by the LINSEI lab. personnel, on advice of the Caorso’s panel. The experimental campaign was settled in a lab. outside the NCPP, because all the material tested, derived by the Turbine Building piping, had a level of contamination under the free release conditions. This first part of the working process refers only to the conventional side of the cutting phase.

2.1 Testing Campaign

This results obtained by the testing campaign until now will be shortly presented. A complete set of the experimental data collected during the campaign is not yet available, because chemical and physical analyses are on course. Any consideration here exposed is subject to further co-integration and review, when the full spectre of test response will be examined and compared in manner to be adequately diffused.

2.2 Cutting Processes Description

The total volume of the hall, where the cutting tests have been set, is of 1,750 m3.

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• ventilation capacity corresponding to one substitution per hour of cutting room total volume;

• use the same torches as during the cutting in nuclear plant dismantlement;

• ferritic steel (Carbon Steel) as structural material;

• simulacres (nickname for metallic pieces from Caorso nuclear facility used in this campaign) of different shape and thickness (in principle tubes, plates and flanges) from piping of main steam line or secondary cooling system or vacuum condenser servicing, all located in Turbine Building;

• structural steel contamination by internally deposited materials, if any;

• coats of protective paints of different type on the simulacre outside surfaces, if any;

• high quality chemical/physical analysis of the dust captured to air/fumes suspension during the cutting, performed by authorised labs possibly located in the same site of the cutting room;

• identification of aerosols and their aggregates before/after the HEPA.

Fig. 1 – Thermal cutting phase.

The cutting hall has no particular specifications to respect, in the sense that it doesn’t have to be a laboratory but it has to largely correspond to plant rooms. For that a mechanical workshop, equipped as requested by the preceding conditions, has been selected. Workers have been qualified following the cutting technical specifications elaborated by DIENCA-LINSEI before beginning the campaign.

Tubes to be cut were leaned on stands whose bases were placed on a rectangular plate with edges fold up in manner to collect any slag produced by cutting process, as shown in Fig. 1.

The photograph represented in

Fig. 2

shows, in particular, the cutting process of a 6” tube, that was placed with the axis in vertical position. The cut here was made with a plasma torch, as the picture shows.

Suction, in this case, was on the upper side of the tube, to use the chimney effect. Aspiration tube can be fairly seen in picture 1.

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Fig. 2 – Thermal cutting phase, vertical axis duct

Fig 3 shows the sampling system, mounted on the filtering and aspiration pipe, in which the nominal flow rate is of 1.500 m3 /h, reduced by loss of head at almost an half. The fan is placed in filtering box equipped by paper filters and HEPA unit.

The following contaminants were searched

• Dust;

• Manganese;

• Nichel;

• Total Cromium;

• Crome exavalent;

• Zinc;

• Lead;

• Nitrogen oxides (NOx);

• Volatile Organic Substances (S.O.V.).

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Fig. 3 – Sampling system

2.3 Operation sequence

The schedule of the cutting programme has to be examined in some detail.

Every test requires three different samples; everyone of these corresponding to a cutting period of half an hour. So a single test lasts an hour and half.

The full schedule was completed within two days and half of work. First day was fully dedicated to the oxy-cut, the second one to the plasma torch process and the half day for test calibration. The complete series of simulacres and the number of samples are as follows

1. Tube spools, 4” Scd. 40S, placed in horizontal axis position a) oxyacetylene cutting (3 samples);

b) plasma cutting (3 samples);

2. Tube spool, 10” Scd. 40S, placed in horizontal axis position a) oxy-acetylene cutting (3 samples);

b) plasma cutting (3 samples);

3. Tube spool, 6”, placed in vertical axis position a) plasma cutting (3 samples);

4. Piping spools, 32’’, placed in horizontal axis position a) Two spools, oxy-acetylene cutting (3 samples); b) Two spools, plasma cutting (3 samples);

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5. Steel plate and two flanges

a) Plate, oxy-acetylene cutting (1 sample); b) Flanges, oxy-acetylene cutting (2 samples).

Tubes and plates subjected to the cutting test were, as much as possible, representative of the typology of fittings or simple components that are mounted individually or a part of a complex component on the plant. The structural material is always Carbon Steel with different superficial treatments (no stainless steel in piping and servicing for Turbine Building systems)

Fig. 4 – Cutting of the plate with oxy-acetylene torch (test numbered 5a)

4. RESULTS

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Tab. 1 – Specimen COD01

Parameter Specimen Concentration

[mg/Nm3]

1 6,1

2 12,4

Dust

3 5,4

1 0,009

2 0,019

Mn

3 0,009

1 0,009

2 0,012

Ni

3 0,007

1* 0,012

2* 0,012

Total Cr

3* 0,012

1* 0,004

2* 0,004

Cr VI

3* 0,004

1 0,031

2 0,049

Zn

3 0,023

1* 0,012

2* 0,012

Pb

3* 0,012

1 4,11

2 6,16

NOx

3 4,72

1* 0,34

2* 0,34

S.O.V.

3* 0,34

*t.n.d.= small traces lower to analytical method sensitivity

The content of Tab 2 refers to the test 2b), where the cutting operation is carried out by the plasma torch.

It is evident, in this case, the greater production of dust and generally of gaseous emissions. The other difference more evident consists on lead concentration. The outside surface of 10” tube has been covered by minium, which is red lead, causing for this test a substantial increasing in Pb concentration. Same consideration is for the Nitrogen oxides. The very high temperatures, reached under the plasma torch and falling in the range of 10-20,000 °C, could favour the production of this kind of products.

The most significant results are shown in the histogram of

Fig. 5

. The dust production with the plasma torch rises to a significant relevance.

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Tab. 2 – Specimen COD02

Parameter Specimen Concentration [mg/Nm3]

1 80,7

2 60,3

Dust

3 47,8

1 0,174

2 0,077

Mn

3 0,021

1 0,016

2 0,026

Ni

3 0,017

1* 0,012

2* 0,019

Total Cr

3* 0,017

1* 0,004

2* 0,004

Cr VI

3* 0,004

1 0,031

2 0,040

Zn

3 0,033

1* 1,047

2* 1,181

Pb

3* 1,721

1 26,69

2 57,48

NOx

3 36,95

1* 0,34

2* 0,34

S.O.V.

3* 0,34

* t.n.d.= small traces lower to analytical method sensitivity

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Polveri Pb Nox

Ossitaglio Plasmataglio

Fig. 5 – Plasma and oxy-acetylene cutting comparison

Tab. 3 – Environmental analyses (Hall)

Parameter Specimen Concentration

[mg/m3]

Limit value [mg/m3]

Pers P1 5,6

Dust

Amb A1 2,8 10

Pers P1 0,023

Mn

Amb A1 0,010 0,2

Pers P1 t.n.d. < 0,004 Ni

Amb A1 0,003 0,1

Pers P1 t.n.d. < 0,010 Total Cr

Amb A1 t.n.d. < 0,002 0,5 Pers P1 t.n.d. < 0,002

Cr VI

Amb A1 t.n.d. < 0,0004 0,01

Pers P1 0,015

Zn

Amb A1 0,362 10

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Pers P1 0,260

Pb

Amb A1 0,059 0,05

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Pers P1 t.n.d. < 0,607

NOx _{Amb A1} _{t.n.d. < 0,607}

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Pers P1 t.n.d. < 0,08 S.O.V.

Amb A1 t.n.d. < 0,04

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* t.n.d.= small traces lower to analytical method sensitivity

(2)_{Limiting data referring to Zinc oxide in powder}

(3)_{For the allowable limit of Pb see Italian Decree of 26/02/ 2004}

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(4)_{For the volatile organic matter the threshold data are peculiar and different for each substance. No limiting data is listed due to very small}

concentration of the organic compounds in the collected dust.

Figure 6 and the following ones are images taken from the SEM analyses, made with a SEM LEICA CAMBRIDGE 360, equipped with an EDS probe for microanalyses OXFORD PENTAFET. The SEM analyses were performed on a Teflon filtering support, that was mounted on the sampling system. From the morphologic examination by SEM the electron photomicrographs reveal a carpet of ultra fine particles with some well separated spherical bodies. From their quality analysis the constitutive elements are mainly Fe (and Pb for tubes covered by minium coats) with traces of Mn and Cu (the last one could be probably caused by migration from condenser tube alloy elements). About the definition of particle dimensions we remember that size range can be roughly classified in three groups. The first group, which is called coarse particles, extends above 10 µm and are not generally a concern for inhalation. The second group, which is named fine particles with high respirable capability, extends from 0.1 to 10 µm aerodynamic diameter. Particles less than 0,1 µm, called ultra fine particles, have behaviour dominated by diffusion.

Fig. 6 – SEM analyses. It is evident here the ultra fine particles carpet, with spots

5. CONCLUSIONS

Aerosol produced by high temperature cutting techniques, which are markedly different from aerosols produced by mechanical cutting devices, consist of spheres and branched chain ultra fine aggregates. The aerosol produced in oxy-acetylene torch cutting contains a quite large fraction of spheres in the micrometer size range. In the plasma torch cutting techniques prevails an ultra fine branched-chain aggregate aerosol with primary particle size ranging from 0.05 to 0.15 µm diameter with occasional larger spherical particles.

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Copyright © 2005 by SMiRT18 higher in plasma torch cut than for the other hot technique. In any case a probable production rate could be assumed around 1,000 mg/min at the maximum for both techniques in cutting 2”-4” tubes.

This estimate could be used to determine adequate ventilation rates in the cutting room to limit the air concentration of respirable particles below a prescribed level. This is an important point to be considered attentively. The dust production with the plasma torch rises to a significant relevance. The atmosphere in the cutting hall was dramatically invaded by fumes to unbearable levels during the plasma torch cutting. To reduce the fog in the ambient air to a tolerable mixture for the sight and mainly for the inhalation -even if the personnel has applied protective mask on his mouth- the opening of large door in the workshop had to be requested, strongly increasing the ventilation rate.

Fig. 7 – SEM analyses, TEFLON filter

Considering for the same tubes the kerf width of 0.7 cm with oxy-acetylene torch and 0.5 cm for plasma one, the wall thickness and material density, it is possible to calculate the total mass of material ejected in each cut. Comparing it to the mass of respirable aerosol created per cut, the fraction of the kerf released as respirable aerosol could be determine ranging about 5-7 % for the two hot cutting techniques (the upper one is valid for plasma cutting).

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Collection and Characterization of Emissions from Metal Cutting in CAORSO Nuclear Power Plant Decommissioning