Radioactive Inventory at the Fukushima NPP

Transcription

1 Radioactive Inventory at the Fukushima NPP G. Pretzsch, V. Hannstein, M. Wehrfritz (GRS) Gesellschaft für Anlagen- und Reaktorsicherheit (GRS) mbh Schwertnergasse 1, Köln, Germany Abstract: The paper presents calculated inventories of radio nuclides of the Fukushima Daiichi NPP. In a first step the calculation method and the computer code system were adjusted and evaluated by performing a Benchmark with experimental results. Further, the fuel rods in the UO2- and MOX fuel elements were characterized and burn up and decay calculations were performed for the spent fuel at the different units of the Fukushima Daiichi NPP, i.e. at the reactor cores as well as at the spent fuel pools. The total activities were averaged according to their ratio and decay time with respect to the date of the accident 11 March A selction process resulted in 25 nuclides out of the total nuclide inventory of 572 nuclides with essential contribution to the potential radiation exposure of the population after release. 1 INTRODUCTION After the accident at the Fukushima Daiichi NPP in Japan calculations were performed at GRS to estimate the radio nuclide inventory at the reactor units 1-4 and at the corresponding spent fuel pools. Subsequently those nuclides had to be selected from the calculated nuclide inventory essentially contributing to the radiation exposure of the population in case of release considering predominantly the pathways groundshine, inhalation and ingestion. In a first step benchmark calculations were carried out to evaluate the calculation code. In a second step the inventory of a typical Fukushima NPP fuel element was calculated. The third step considered the decay calculations of the real fuel elements contained in the spent fuel pools at the date 11 March 2011 of the accident. 2 CALCULATION PROCEDURE The calculation of the nuclide inventory was carried out with the computer code system KENOREST-2008 /HES 99, HES 08/. This system comprises the three dimensional Monte Carlo Code KENO-Va solving the Boltzmann transport equation and the one dimensional burn up code OREST consisting of the spectrum code Hammer to calculate cross sections and the code ORIGEN to calculate the nuclide inventory for different decay times of radio nuclides. 2.1 Benchmark Calculations Burn up calculations for boiling water reactors are much more difficult to perform compared to those for pressurized water reactors and more uncertainties are involved due to the neutron spectrum unknown in detail. Thus in a first step a Benchmark inventory of known experimental data of the NEA Spent Fuel Isotopic Composition Database (SFCOMPO) /NEA 11/ was calculated to quantify the uncertainties of the code system.

2 Data of the Boiling Water Reactor (BWR) of Fukushima Daini NPP comparable to those of Fukushima Daiichi NPP were available. The fuel element parameters of both reactor types are almost identically. The fuel element (FE) consists of six fuel rod types different in their initial U 235 enrichment (see Tab. 1 and Fig. 1). Another fuel rod type G additionally consists of 4,5 wt. % Gadolinium oxid (Gd 2 O 3 ). Tab. 1: U 235 enrichment of the six different UO 2 fuel rod types and the Gadolinium fuel rod used in the model /JAE 02/ Fuel Ros Number G Enrichment U 235 [%] 3,90 3,45 3,40 2,90 2,00 3,40 The nuclide inventory of fuel rod SF98 was calculated for samples SF98-3 to SF98-7 where the axial neutron flux density can be assumed to be homogeneously (see Fig. 2). Fig. 1: Schematic of the fuel element model (FE-Box outer dimensions: 138,1 x 138,1 mm 2 ), fuel rod types 1-5, /NEA 11/ 2

3 Fig. 2: Measurement positions of the samples of fuel rod SF98 /NEA 11/ The position of fuel rod SF98 in the fuel element is shown in Fig. 1. The burn up data were taken from the power history given in /JAE 02/ and scaled with respect to the sample position. To verify the results the calcultated and experimental data were compared. 2.2 Fuel Data of Fukushima Daiichi NPP The inventories were calcultated for reactors 1-3 of Fukushima Daiichi NPP, the reactor of unit 4 was unloaded at the moment of the accident, and for the spent fuel pools of units 1-4. The corresponding numbers of fuel element are shown in Tab. 2. Tab. 2: Inventories to be considered for units 1-4 at Fukushima Daiichi NPP (No of fuel elements and tons of Heavy Metal (thm) /JAI 11, NEI 10/) Unit FE/Core thm/core FE/FE Pool thm/fe Pool , , ,3 The reactos can be operated with UO 2 - as well as with MOX-fuel elements. The data available to calculate the inventory of MOX-fuel elements were insufficient. Thus additional data available at GRS for a generic MOX-fuel element were used with a conservative approach /GRS 11/. The Plutonium compositions of the six fuel rod types consisting in the fuel element are shown in Tab. 3. 3

4 Tab. 3: Plutonium composition oft the different fuel rods in the MOX-FE [wt.% in Natural Uranium (0,7 wt.% U 235)] /GRS 11/ Rod Number Pu-fiss Pu 238 Pu 239 Pu 240 Pu 241 Pu ,77 0,02 0,64 0,25 0,13 0, ,19 0,03 0,99 0,38 0,20 0, ,69 0,05 1,40 0,55 0,29 0, ,04 0,09 2,53 0,98 0,51 0, ,97 0,11 3,30 1,29 0,67 0, ,69 0,16 4,73 1,84 0,96 0,32 MOX-fuel elements were exclusively used in unit 3 and only for the period of 5 months until the accident at 11 March 2011 /ANS 11/. The burn up was calculated assuming a uniform maximum burn up of 35 GWd/tHM for UO 2 - fuel elements and of 5 GWd/tHM for MOX-fuel elements. For the spent fuel pools the radioactive decay of the inventories was considered. It was assumed that the fuel elements have been irradiated three cycles with the a period of one and a half years. Thus in the different fuel pools are loaded 1/3 of the reactor load, correspondingly, with different decay times. 3 RESULTS 3.1 Benchmark Calculations The comparison of the ratio Calculated/Experimental nuclide masses (C/E-value) for the different sample positions 3-7 of fuel rod SF98 are depicted in Fig. 3 for fission products and in Fig. 4 for actinides. Fig. 3: Ratio of Calculated/Experimental nuclide masses (C/E-value) for fission products, Experimental: measured values of NEA-Benchmarks /NEA 11/ 4

5 For the Nd-isotopes a good agreement can be stated meaning that the burn up calculations are of good quality. U 235 and Pu 239 are slightly overestimate which indicates that in reality the neutron spectrum is slightly weaker than in the calculations. Bigger deviations in case of e.g. Ru 106, Sb 125, Cs 134 and some Sm-isotopes may be due to uncertainties in the cross sections. However, the average deviations of about 10 % are acceptable for the chosen approach. Fig. 4: Ratio of Calculated/Experimental nuclide masses (C/E-value) for actinides, Experimental: measured values of NEA-Benchmarks /NEA 11/ 3.2 Inventory Calculations for Reactor Units 1-4 of Fukushima Daiichi NPP The inventory calculations for UO 2 - and MOX-fuel elements resultet in a total of 572 nuclides for different decay times. For UO 2 -fuel elements the inventory was the result of averaging the calculated result of each fuel rod according to its weighting factor show in Tab. 4. One fuel element consists of about 170 kg heavy metal. Tab. 4: Fuel rod weighting factors for inventory averaging of a UO 2 -fuel element Fuel rod number G Weighting factors For MOX-fuel elements the averaging procedure was analogeously. The MOX-fuel element has an 9x9 array with a total of 80 fuel rods and one water channel with the same outer dimensions as for the UO 2 -fuel element. Two types of Gadolinium fuel rods are used. The first type (eight rods - Gd1) consists of 3,00 wt.% U 235 and 1,5 wt.% Gd 2 O 3 and the second type (four rods - Gd2) consists of 3,95 wt.% U 235 and 1,5 wt.% Gd 2 O 3. For MOX-fuel elements the inventory was the result of averaging the calculated result of each fuel rod according to its weighting factor show in Tab. 5. 5

6 Tab. 5: Fuel rod weighting factors for inventory averaging of a MOX-fuel element Fuel rod number Gd1 Gd2 Weighting factors At the moment of the accident at 11 March 2011 only unit 3 of Fukushima Daiichi NPP was loaded with MOX- fuel with 6 % of the total load, i.e. with 32 fuel elements /ANS 11/. The irradiation time of 5 months resulted in a mean burn up of 5 GWd/tHM. 3.3 Inventory of the Spent Fuel Ponds of Units 1-4 of Fukushima Daiichi NPP To calculate the inventories of the spent fuel pools the following circumstances were taken into account. A spent fuel pool has a capacity of about 2,4 reactor loads (see Tab. 2). Assuming three one and a half years cycles of a fuel element in the reactor there is alwas 1/3 of the reactor load of one specific cycle period in the spent fuel pool. The last unloading was carried out about 6 months before the accident (see Tab. 6). Thus the first decay time was chosen to be 6 months. The unloading of unit 4 was performed in November 2010, so that for conservativity reasons no decay time was supposed. The next 1/3 has a decay time of 6 months, two years etc. For unit 1 spent fuel pool of Fukushima Daiichi NPP the stored fuel element are e.g. distributed as shown in Tab. 7. For a reactor load of 400 fuel elements and a total of 292 fuel elements in the spent fuel pool subsequently 1/3 of the reactor load are distributed according to the different decay times beginning with the smallest one. Unit 4 shows a special situation because of the reactor was completely discharged before the accident 11 March Thus for the total reactor inventroy a decay time of 0 years was supposed. Tab. 6: Dates of last discharge of Units 1-4 of Fukushima Daiichi /NIS 11/ Unit Date of last discharge Tab. 7: Schemes of loading of spent fuel pools of Units 1-4 of Fukushima Daiichi NPP Unit Storage time [a] 0 0,5 1,5 2,5 3, Number/FE

7 4 NUCLIDES OF RADIOLOGICAL RELEVANCE The selction of nuclides of radiological relevance out of the total nuclide inventory after a possible release essentially contributing to a potential radiation exposure of the population especially through the pathways of inhalation, ingestion and groundshine was carried out by means of procedure developed at GRS /PRE 03/. From the total of about 570 nuclides contained in the spent fuel element for further calculations of atmospheric dispersion and radiological consequences a selection was performend in such a way that for the source term only those nuclides essentially contributing to the potential radiation exposure were considered. For all nuclides of the inventory resulting from the burn up and decay calculations a normalized dose for inhalation and for groundshine, respectively, was calculated. This normalized dose is related for inhalation to the short term dispersion factor χ = 1 s/m³ and for groundshine to the fallout-/washout factor F+W = 1 m -2. The normalized dose was calculated for different organs. For groundshine exposure times of 7 days and 50 years were considered. The normalized dose subsequently was muliplied by the activity of the nuclide under consideration. Afterwards a ranking of this value was made for all nuclides. For further considerations only those nuclides were selected contributing at least 1/1000 of the maximum value. The ranking found for the inhalation pathway was analogously taken for the ingestion pathway. The selction of nuclides determined in this way with their total activity per 11 March 2011 taking into account all reactor loads and spent fuel load is provided in Tab. 8. Tab. 8: Total inventory of radio nuclides of the Fukushima Daiichi NPP with radiological relevance averaged over the different reactor and spent fuel pool loads with respect to 11 March 2011 Fission products Actinides Nuclide Activity [Bq] Nuclide Activity [Bq] KR 85 2,584E+17 PU238 7,917E+16 SR 89 9,796E+18 PU239 8,367E+15 SR 90 2,102E+18 PU240 1,464E+16 Y 90 2,148E+18 PU241 3,429E+18 ZR 95 1,908E+19 AM241 8,677E+15 NB 95 1,965E+19 AM243 5,533E+14 RU106 9,603E+18 CM242 8,949E+17 SB125 1,826E+17 CM243 5,698E+14 TE132 1,666E+19 CM244 6,320E+16 I 131 1,186E+19 XE133 2,378E+19 CS134 3,801E+18 CS137 2,988E+18 CE144 1,720E+19 PM147 3,509E+18 EU154 1,419E+17 Total Activity 2,586E+21 Bq 7

8 5 SUMMARY AND CONCLUSIONS In the present paper the inventories of radio nuclides of the spent fuel at the different units of the Fukushima Daiichi NPP, i.e. of the reactor cores as well as of the spent fuel pools, were calculated. In a first step the calculation method and computer code system was adjusted and evaluated by performing a Benchmark with experimental results. Next the fuel rods in the UO 2 - and MOX fuel elements were characterized and burn up and decay calculations were performed. The total activities were averaged according to their ratio and decay time with respect to the date of the accident 11 March A selction process resulted in 25 nuclides out of the total nuclide inventory of 572 nuclides with essential contribution to the potential radiation exposure of the population. 6 REFERENCES /ANS 11/ ANS Special Committee on Nuclear Non-Proliferation /GRS 11/ GRS Internal fuel data base, 2011 /HES 99/ /HES 08/ /JAE 02/ /JAI 11/ /KIL 05/ /NEA 11/ /NEI 10/ /NIS 11/ /PRE 03/ U. Hesse, S. Langenbuch KENOREST, eine direkte Kopplung von KENO und OREST, GRS-A-2783, 1999 U. Hesse, W. Denk, H. Deitenbeck, E. Moser OREST Eine direkte Kopplung von HAMMER und ORIGEN zur Abbrandsimulation von LWR-Brennstoffen, GRS-63, November 1986, Version 2006 Yoshinori Nakahara et.al Translation of Technical Development on Burn-up Credit for Spent LWR-Fuels, JEAERI-Tech (ORNL/TR-2001/01) Webpage des Japan Atomic Industrial Forum, Inc. (JAIF) April 2011 R. Kilger, B.Gmal Burn up calculations with KENOREST 03T01 and associated criticality studies for spent fuel samples from Takahama-3 reactor, ANS NCSD Meeting, Knoxville TN, USA, Webpage des OECD-NEA-Benchmarks Post Irradiation Examination for the Spent Fuel Samples Fukushima-Daini-2, April 2011 Nuclear Engineering International World Nuclear Industry Handbook 2010 The 2011 off the Pacific coast of Tohoku Pacific Earthquake and the seismic damage to the NPPs, Nuclear and Industrial Safety Agency (NISA) und Japan Nuclear Energy Safety Organization (JNES), G. Pretzsch, R. Maier German approach to estimate potential radiological consequences following a sabotage attack against nuclear interim storages, EUROSAFE, Paris,