SE0100215 March 2001 ISSN 1650-1942 SWEDISH DEFENCE RESEARCH AGENCY Methodology report

FOI-R-0054

Hans Tovedal

Simulation of Fission Products from Nuclear Weapons Explosions

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Division of NBC Defence SE-901 82 UMEA

2/ SWEDISH DEFENCE RESEARCH AGENCY FOI-R--0054-SE Division of NBC Defence March 2001 SE-901 82 UMEA ISSN 1650-1942

Hans Tovedal

Simulation of Fission Products from Nuclear Weapons Explosions Issuing organization Document ref. No., ISRN Swedish Defence Research Agency FOI-R—0054-SE Division of NBC Defence Date of issue Project No. SE-901 82 UMEA March 2001 A413 Project name (abbrev. If necessary) SWEDEN Author(s) Initiator or sponsoring organization Hans Tovedal Project manager Goran Agren Scientifically and technically responsible Kenneth Lidstrom Document title in translation Simulation of Fission Products from Nuclear Weapons Explosions

Abstract The simulation method MARS, Mathematical Radiac Simulation, is primarily intended for preparedness exercises in nuclear fallout areas and simulates the ionizing radiation doserates from fission products deposited on the ground, i e fallout from a nuclear weapons explosion or from a release of radioactive materia from a nuclear reactor. MARS gives at any time after the fictitious explosion or reactor release the doserates at any position in the fallout area.

A special method, in this report called FOSIM, Fallout Simulation, based on MARS values, has also been developed. This is a technique to produce a standardized solution containing fission products of about the same composition as that caused by a nuclear weapons explosion.

The MARS method and the FOSIM technique were both applied in a preparedness group exercise in a nuclear weapons scenario, LOTTA, in a northern region of Sweden in March 1998. This report gives a short description of the FOSIM method and a fairly detailed evaluation of the application results. From these, it is obvious that the technique produces a set of fission products that, from a couple of hours after the explosion and further, is quite similar to that expected in the fallout from a nuclear weapons explosion. The samples, taken in the fallout area, and prepared by addition of certain amounts of the FOSIM solution, offers unique opportunities for the preparedness laboratory to check and further develop its analysing methods. Some steps of the FOSIM technique need to be further developed, but already in this exercise it gave very realistic sample conditions for the benefit of the laboratory analyses. Key words MARS, mathematical, radiac, simulation, nuclear, fallout, doserate, fission, explosion, FOSIM

Further bibliographic information Language

ISSN 1650-1942 ISBN

Pages 28 Price Ace. to pricelist Dokumentets utgivare Dokumentbeteckning, ISRN Totalförsvarets forskningsinstitut FOI-R—0054—SE Avdelningen för NBC-skydd Dokumentets datum Uppdragsnummer 901 82 UMEÅ Mars 2001 A413 Projektnamn (ev förkortat)

Upphovsman(män) Uppdragsgivare Hans Tovedal Projektansvarig Göran Ågren Fackansvarig Kenneth Lidström Dokumentets titel Simulering av fissionsprodukter från kärnvapenexplosioner

Sammanfattning Den tidigare utvecklade simuleringsmetoden MARS, Mathematical Radiac Simulation, är huvudsakligen avsedd för beredskapsövningar i områden med radioaktivt nedfall. Den simulerar den joniserande strålningens dosrat från fallout, dvs markdeponerade fissionsprodukter från en kärnvapenexplosion eller efter ett utsläpp av radioaktiv materia från en kärnreaktor. Mars ger kontinuerligt dosraten i varje enskild del av nedfallsområdet.

En speciell metod, i denna rapport kallad FOSIM, Fallout Simulation, baserad på MARS-data, har nyligen utvecklats och tillämpats. Den består av en teknik att producera en standardiserad lösning av fissionsprodukter med samma sammansättning som de som skapas vid kärnvapenexplosioner. MARS- och FOSEVÍ-metoderna har båda tillämpats i samband med en beredskapsövning, LOTTA, Mars 1998, i norra delen av Sverige, med ett kärnvapenscenario. Föreliggande rapport ger en kortfattad beskrivning av FOSIM och en mer detaljerad uvärdering av resultaten och metoden som sådan. Det framgår att FOSIM-tekniken ger en uppsättning fissionsprodukter som, från några timmar efter den tänkta kärnvapenexplosionen och i fortsättningen därefter, överensstämmer mycket väl med den som förväntas i nedfallet efter en verklig kärnvapenexplosion. Mark- och vegetationsprover, tagna inom det simulerade nedfallsområdet och preparerade med FOSEVI-lösningen i enlighet med MARS-värdena, ger unika möjligheter för beredskapslaboratorierna att testa och eventuellt vidareutveckla sina analysmetoder.

Vissa delar av FOSIM-metoden behöver förbättras något, men redan vid den genomförda övningen erhölls mycket realistiska prover, till nytta för övningen av analyslaboratorierna.

Nyckelord MARS, radioaktiv, nedfall, joniserande strålning, dosrat, fissionsprodukt, kärnvapenexplosion, FOSIM Övriga bibliografiska uppgifter Språk

ISSN 1650-1942 ISBN

Omfång 28 sidor Pris Enl. prislista Distributör (om annan än ovan) FOI-R--0054--SE

Contents

Introduction 6

General aspects 6

The fission product simulation method 7

Theoretical basis for the exercise 12

Preparatory test 16

Data and results from the exercise 19

The remaining analyses 21

The remaining nuclides 27

Conclusion 27

References 28 FOI-R--0054--SE

Introduction

A group for radiac preparedness at the Swedish Defence Research Agency, FOI, has been trained and tested in a simulated nuclear weapons explosion scenario. One important task for the group is to maintain a well-trained and competent preparedness regarding the radiation effects of fallout from nuclear weapons explosions, nuclear reactor accidents and any other incident that may lead to the release of huge amounts of radioactive materia. The measurement of radiation doserates in the environment and laboratory analyses of samples collected in the fallout area, are high priority operations to be performed by the group at an early stage. That would make a basis for decisions of authorities regarding eventual measures in order to reduce the doserate to the population.

The exercise was intended to cover all types of preparedness competence in the group, which demanded a high degree of realism in the scenario. Two simulation procedures were applied in order to meet this demand, namely

1. The doserates in the fallout area was simulated by the use of the MARS method

2. The radioactive materia in environmental samples was simulated by fission products from neutron irradiated uranium.

This report is a technical description and evaluation of the second procedure, which may be called the FOSIM, Fallout Simulation, method. This is one in a series of reports1>2;3'4'5'6 which are describing the LOTTA exercise from other point of views.

General aspects

A nuclear weapons explosion in the air creates a big cloud containing a huge amount of radioactive materia, fission and neutron activation products. When the explosion takes place near the ground, a lot of ground material will be sucked up in the cloud, consisting of particles with a size distribution from rather coarse gravel to very fine particles. The radioactive materia will deposit on the particles, which are transported away by the winds and eventually returned to the ground. Big particles will soon leave the cloud while the smaller ones fall down much later and far away from where they were removed. This process creates a fallout area, where the activity per square metre and the ionising radiation doserate may be very high.

The content of different radionuclides in the radioactive materia and its distribution on different particle sizes will depend on the size and type of the nuclear load, the altitude of the explosion point and several other factors. While the temperature is still very high, big particles, which will soon fall to the ground, will be contaminated with elements that condense at the prevailing temperature. Smaller particles on the other hand will also carry elements that condense at much lower temperatures. Consequently, the nuclide composition will change in character along the direction of the cloud movement, partly because of these different condense processes, but mainly due to the physical decay of radionuclides with different decay constants. FOI-R--0054--SE

The expected mitial radionuclide composition after the splitting of fissile element may be taken from tables7. This report contains tables for individual and cumulative fission yields together with half-lives for occuring, significant isobars.

Prediction of the most probable radionuclide composition in the case of a real nuclear weapons explosion is of course not quite feasible. Therefore, for exercise purposes one have to choose a composition among those considered possible to anticipate. The difference between the anticipated set of radionuclides from the splitting of the two common fissile elements, U-235 and Pu-239, is so small that, from the exercise point of view, it has no significance on the choice. The competence requirements for a preparedness group, that has to survey the fallout area for extension and intensity of the radioactive materia, will not be dependent on the fissile element type in the bomb. It is only necessary to choose a scenario that contains the most conceivable and dominating radionuclides.

Fission products created by neutron irradiation of natural uranium in a nuclear reactor show in the initial stage approximately the same nuclide distribution as that produced by a nuclear weapons explosion, if the fissile elements are identical. The fission processes in the reactor will however go on for a certain time during which the radionuclide distribution successively changes, mainly because of the decay of nuclides with short half-lives.

Below, a method is described that makes use of fission products from a nuclear reactor irradiation of uranium to simulate fission products from a nuclear weapons explosion. The scenario is based on an imaginary nuclear weapons explosion near ground but at a long distance from the exercise area. The preparedness analytical laboratory was in that way anticipated to get the opportunity to analyse samples containing realistic quantities and qualities of fission products.

In the scenario, it took more than ten hours for the radioactive cloud to reach the exercise area, which meant that several short lived fission products had decayed to insignificant activity and doserate levels when the deposition started in that area. This was advantageous for the simulation procedure and improved the resemblance between the two types of fission product distributions.

The fission product simulation method

The main part of the method is neutron irradiation of a small amount of uranyl nitrate in a nuclear reactor in order to produce fission products. The set of fission products in the irradiated uranyl nitrate sample, henceforth called the source, at the end of the irradiation was intended to simulate what would have been created in a nuclear weapons explosion with U-235 as the fissile load. In the case of an explosion, the fission products are created momentarily. What radionuclides that are created and what their respective activity will be is principally governed by their individual and cumulative fission yields and the decay constants for their isobars 7. The individual yield may be used for estimation of the radionuclide activity immediately after the explosion while the cumulative yield has a growing influence on the activity during the following lapse. FOI-R--0054-SE

Geographical and meteorological conditions at the time for the exercise were calculated by using a model for atmospheric transport4. The conditions were chosen in a way to make the transport time for the radioactive cloud, from the explosion location to the exercise area, to be about ten hours. Fission products with very short half-lives would then be expected to have decayed down to insignificant levels before the radioactive materia reached the exercise area. Consequently, radionuclides with short half-lives and also those with very low fission yields were excluded from the theoretical set of radionuclides that were regarded in the planning of the exercise. They were considered not to be possible to analyse by available analysis methods. Also excluded were the radioactive noble gases because they would never deposit in the exercise area. The tabled data7 indicated that a suitable lower level for acceptable half-lives would be of the order of 2 hours and for the cumulative fission yield 1 %. These levels led to a limitation of the realism, showing itself as a deviation from the theoretical decay function for the activity and the doserate, t"1' , that is valid for quite a long time after the explosion. Nevertheless, these limitations appeared not to be of any practical importance and could easily be neglected.

The U-235 fission rate, when a certain amount is irradiated with thermal neutrons in a reactor, is governed by the neutron flux, the fission cross section and the number of U-235-nuclei. The qualitative and the relative quantitative content of fission products in the source at the end of the irradiation would be in accordance with that created momentarily in a nuclear weapons explosion, if no radionuclide decay occurs during the irradiation. The activity, At, of a given radionuclide, Xi, would then be

Al = q>-e1.X|.tb Bq (1) where (p = the number of fissions per s e'j = individual fission yield for the nuclide X; A,; =the decay constant for the nuclide Xi, s"1

tb = the irradiation time, s

All created radionuclides are however radioactive and will decay during the irradiation and that makes the final set of radionuclide activities different from what it would have been without decay. The activity build-up for a radionuclide that decays during the production process is given by (2).

Al=q>.e1-[l-exp(-A,,-tb)] Bq (2)

These two expressions are illustrated in Figure 1. FOI-R--0054-SE

-•—With decay —§8— No decay 0.02 -

0.015 - S 0.01 -

0.005 -

oepi_ 1 1 1 1 1 0.2 0.4 0.6 0.8 1 1.2 tb, h

Figure 1. Activity build-up during neutron irradiation

The following values were used:

tb= 3600s=lh which means that a total of 1E4 fission product nuclei of each kind, light and heavy resp, were produced during the irradiation. The activity will decrease when the irradiation has finished, in this case after one hour, and the combined build-up-decay-process is described by (3) and (4):

A,=

A1=9.e!.[l-exp(-Vtb)].exp[-V(t-tb)], (4) and their graphical illustrations are shown in Figure 2. FOI-R--0054--SE

0.016

m

10 12 Time t, h

Figure 2. Activity during irradiation 1 h followed by decay to 10 h.

From the exercise point of view, it is the radionuclide data at the time for the preparedness group efforts that is of importance. At that time, most of the short-lived nuclides have decayed to a more long-lived isobar and therefore, the individual fission yield for many of the radionuclides is of less importance and should be replaced by the cumulative fission yield in the calculation of radionuclide activities. This introduces a slight overestimation of the activity for a few radionuclides, mainly Pm-149 and Ce-141, but they have very low gamma-ray abundances and are therefore not probable to be detected in the samples. 1-133 will also be slightly overestimated, but not by more than a few percents a couple of hours after the irradiation.

All further calculations will therefore be based on the cumulative fission yields, e^, for all radionuclides. That means that the individual fission yield e] in the mathematical expressions above should be substituted by e*.

Half-life and fission yields are parameters that determine the radionuclide activity and its variation as a function of time. The fission yield in the example above was e* = 0.01 or 1 %, but most of the radionuclides of any significance have yields of the order of 5 %.

The activity distribution among the fission products as a function of their half-lives will change as time goes. The fast decay of the short-lived nuclides displaces the dominating activity towards nuclides with longer half-lives. Figure 3 is an illustration of the activity distribution for nuclides with half-lives up to 10 h. The values represent the situation ten hours after the reference point of time, the time for the termination of the irradiation and for the imaginary nuclear weapons explosion. All nuclides are supposed to have the cumulative fission yield 0.01 and the activity is given in Bq/fission, which shall be interpreted as the nuclide activity after a single nuclear fission at the reference time.

10 FOI-R--0054--SE

1.00E-07-

8.00E-08 - c g 6.00E-08 - Im 4.00E-08 - —•— Explosion.theory / / —s— Irradiation, theory 2.00E-08 - /

O.OOE+00 - * 1 1 1 -i 1 0 2 4 6 8 10 12 Half-life VA, h

Figure 3. Relative activities 10 h after the reference point of time, as a function of the half-life

These curves show the biggest differences in activity between the imaginary explosion case and the simulated case. In the next figure, the corresponding curves for the time range up to Ty2 = 100 h is shown. It is obvious that the difference decreases with increasing half-lives.

i.^uc-u/ •

• Explosion, theory 1.00E-07 - —H—Irradiation, theory 8.00E-08 • TV io n 1 ^L » 6.00E-08

CO 4.00E-08 -

2.00E-08 -

O.OOE+00 - 20 40 60 80 100 120 Half-life VA, h

Figure 4. Relative activities 10 h after the reference point of time, as a function of the half-life

11 FOI-R--0054-SE

The exercise went on for about two days and an appreciable change in the activity distribution, as a function of the half-lives, could be observed during that time. Figure 5 below shows these distributions at different times.

—•—10h -a- 20 h 1.00E-07 -• 30 h 40 h 8.00E-08 - -*— 50 h c o

mf 4.00E-08 -

2.00E-08 - • * \T j^ 0.00E+00 - £&&L. ; 1 1 1 _l 20 40 60 80 100 120 1 Half-life T /2, h

Figure 5. Relative activity at different times after the reference point of time

Theoretical basis for the exercise

The table below shows data for the fission products that were chosen to be regarded as significant in the exercise and which, according to tabled data 7, could be expected to be found in the radioactive materia. As mentioned above, only fission products with cumulative yields greater than 1 % and with half-lives greater than 2 h will be considered. The noble gases, i e Xe-133 and Xe-135, have been excluded because they will not deposit on the ground.

12 FOI-R--0054--SE

Table 1 The data for the fission products that were chosen to be regarded as significant in the exercise.

Nuclide Half-life Decay Cumulative Bq/fission constant fission yield JVi, h

La142 1.54E+00 1.25E-04 5.85E-02 7.31 E-06 Sr-92 2.71 E+00 7.10E-05 5.94E-02 4.22 E-06 La141 3.90E+00 4.94E-05 5.85E-02 2.89E-06 Pr-145 5.98E+00 3.22E-05 3.93E-02 1.27E-06 Sr-91 9.50E+00 2.03E-05 5.83E-02 1.18E-06 Y-93 1.02E+01 1.89E-05 6.35E-02 1.20E-06 Zr-97 1.68E+01 1.15E-05 5.98E-02 6.85E-07 1-133 2.08E+01 9.26E-06 6.70E-02 6.20E-07 Ce-143 3.31 E+01 5.82E-06 5.96E-02 3.47E-07 Pm-149 5.30E+01 3.63E-06 1.08E-02 3.92E-08 Mo-99 6.60E+01 2.92E-06 6.11E-02 1.78E-07 Te-132 7.80E+01 2.47E-06 4.30E-02 1.06E-07 1-131 1.93E+02 9.98E-07 2.89E-02 2.88E-08 Nd-147 2.64E+02 7.29E-07 2.25E-02 1.64E-08 Ba-140 3.07E+02 6.27E-07 6.21 E-02 3.89E-08 Ru-103 9.43E+02 2.04E-07 3.03E-02 6.19E-09 Sr-89 1.21E+03 1.59E-07 4.73E-02 7.51 E-09 Zr-95 1.54E+03 1.25E-07 6.50E-02 8.15E-09 Ce-144 6.84E+03 2.81 E-08 5.50E-02 1.55 E-09 Sr-90 2.55E+05 7.55E-10 5.78E-02 4.37E-11 Cs-137 2.65E+05 7.28E-10 6.19E-02 4.51 E-11

Many of the nuclides decay to radioactive daughter nuclides that have very small individual fission yields and are for that reason not included in the set of primary fission products in Table 1 above. This applies above all to the daughter products from the decay of:

Sr-90, Sr-91, Sr-92, Zr-95, Zr-97, Mo-99, Te-132, Ba-140, La-141 and Ce-144

These daughter nuclides will successively be built up and complete the primary set of nuclides that eventually may deposit on the ground. They are included in Table 2, where the final, relative activity distribution, as Bq/fission, has been estimated for a number of times, in relevance in the exercise.

13 FOI-R--0054-SE

Table 2 The primary set of nuclides that eventually may deposit on the ground.

Nuclide Half-life Cumulative Bq/fission Bq/fission Bq/fission Bq/fission Bq/fissic fission yield Vk, h % t=0 t=1Oh t = 20h t=30h t = 40h

Pr-144 2.88E-01 O.OOE+00 O.OOE+00 1.55E-09 1.55E-09 1.55E-09 1.55E-09 Nb-97 1.23E+00 O.OOE+00 O.OOE+00 4.86E-07 3.23E-07 2.13E-07 1.41 E-07 La-142 1.54E+00 5.85E-02 7.31 E-06 8.12E-08 9.02E-10 1.00 E-11 1.11E-13 1-132 2.28 E+00 O.OOE+00 O.OOE+00 9.45E-08 9.11E-08 8.36E-08 7.65E-08 Sr-92 2.71 E+00 5.94E-02 4.22E-06 3.28E-07 2.54E-08 1.97 E-09 1.53E-10 Y-92 3.54E+00 O.OOE+00 O.OOE+00 8.76E-07 1.92 E-07 3.23E-08 4.97E-09 La-141 3.90E+00 5.85E-02 2.89E-06 4.88E-07 8.25E-08 1.39E-08 2.35E-09 Pr-145 5.98E+00 3.93E-02 1.27E-06 3.98E-07 1.25E-07 3.92E-08 1.23E-08 Tc-99m 6.00E+00 O.OOE+00 O.OOE+00 1.02E-07 1.24E-07 1.21 E-07 1.13E-07 1-135 6.57E+00 6.28E-02 1.84E-06 6.41 E-07 2.23E-07 7.77E-08 2.71 E-08 Sr-91 9.50E+00 5.83E-02 1.18E-06 5.68E-07 2.74E-07 1.32E-07 6.34E-08 Y-93 1.02E+01 6.35E-02 1.20E-06 6.08E-07 3.08E-07 1.56 E-07 7.89E-08 Zr-97 1.68E+01 5.98E-02 6.85E-07 4.53E-07 2.99E-07 1.98E-07 1.31 E-07 1-133 2.08E+01 6.70E-02 6.20E-07 4.44E-07 3.18 E-07 2.28 E-07 1.63E-07 Ce-143 3.31 E+01 5.96E-02 3.47E-07 2.81 E-07 2.28E-07 1.85E-07 1.50E-07 La-140 4.03E+01 O.OOE+00 O.OOE+00 6.08E-09 1.11E-08 1.51 E-08 1.84E-08 Prn-149 5.30E+01 1.08E-02 3.92E-08 3.44E-08 3.02E-08 2.65E-08 2.32E-08 Y-90 6.41 E+01 O.OOE+00 O.OOE+00 4.47E-12 8.49E-12 1.21 E-11 1.53E-11 Mo-99 6.60E+01 6.11E-02 1.78E-07 1.60E-07 1.44E-07 1.30 E-07 1.17E-07 Te-132 7.80E+01 4.30E-02 1.06E-07 9.70E-08 8.87E-08 8.12E-08 7.43E-08 1-131 1.93E+02 2.89E-02 2.88E-08 2.78E-08 2.68E-08 2.59E-08 2.49E-08 Nd-147 2.64E+02 2.25E-02 1.64E-08 1.60E-08 1.56E-08 1.52E-08 1.48E-08 Ba-140 3.07E+02 6.21 E-02 3.89E-08 3.80E-08 3.72E-08 3.64E-08 3.55E-08 Pr-143 3.26E+02 O.OOE+00 O.OOE+00 6.57E-09 1.18E-08 1.58E-08 1.90 E-08 Ce-141 7.80E+02 O.OOE+00 O.OOE+00 1.19E-08 1.38E-08 1.40E-08 1.40E-08 Nb-95 8.40E+02 O.OOE+00 O.OOE+00 6.66E-11 1.32E-10 1.97E-10 2.61 E-10 Ru-103 9.43E+02 3.03E-02 6.19E-09 6.14E-09 6.10E-09 6.06E-09 6.01 E-09 Sr-89 1.21 E+03 4.73E-02 7.51 E-09 7.51 E-09 7.51 E-09 7.51 E-09 7.51 E-09 Y-91 1.40E+03 O.OOE+00 O.OOE+00 4.12E-09 6.08E-09 7.01 E-09 7.43E-09 Zr-95 1.54E+03 6.50E-02 8.15E-09 8.11 E-09 8.08E-09 8.04 E-09 8.00E-09 Ce-144 6.84E+03 5.50E-02 1.55E-09 1.55E-09 1.55 E-09 1.55E-09 1.55E-09 Sr-90 2.55E+05 5.78E-02 4.37E-11 4.37E-11 4.37E-11 4.37E-11 4.37E-11 Cs-137 2.65E+05 6.19 E-02 4.51E-11 4.51 E-11 4.51 E-11 4.51 E-11 4.51 E-11

Total 6.28E-06 3.02E-06 1.87E-06 1.34E-06

The total activity of, and the total doserate from, the fission products that are created in a nuclear weapons explosion will with good accuracy decrease according to the function

,.-1.2 where

At = the activity or doserate, t hours after the explosion Ai= - " - , 1 hour -" - - " -

14 FOI-R--0054--SE

This function is valid from one hour up to several months after the explosion.

The summed up activity at different times, according to Table 2, is given in the last row in that table. Comparison with the t"1 ^-function can be made in two ways, of which one is shown in Figure 6.

1 H —•—Table 2 —S3— The function (t+3)A-1.2 \ a) 0.8 \ \ §> 0.6

" 0.4

0.2 ^~~-—-*—-#~^

0 1 1 1 1 1 1 1 1 0 5 10 15 20 25 30 35 40 45 Time after the explosion, h

Figure 6. Comparison between the function (t + 3)"1'2 and the theoretical Table 2-values

The curves have been adapted to each other by displacing the t"12-curve 3 h, which made the two curves agree very well. Therefore, the real time 5 h corresponds to the time 8 h in the new function, which consequently is written (t + 3)"1'2. In practice this means that the theoretical activity should be considered to be three hours older than the real time activity, if it should be used in any connection. This effect is probably a result of the choice of fission products, where certain limits was adopted for half-lives and fission yields and also that the noble gases were omitted.

A direct comparison of the t~L2-function in real time scale and the table 2-values shows that a fairly good agreement is obtained from about t =10 h, and that this improves with time, Figure 7.

15 FOI-R--0054--SE

1.2 - Table 2 - The function tA-1.2

0.8 _2

0.4

0.2-

10 15 25 30 35 40 45 Time after the explosion, h

Figure 7. Comparison between the function t"-1. •2 and the theoretical Table 2-values

It is obvious that the shortage of short-lived fission products is responsible for the difference between the two curves during the first ten hours after the explosion. In this exercise, the radioactive cloud did not arrive in the exercise region before that time. The conclusion is therefore that the irradiation time of one hour was appropriate and gave a reasonably good set of fission products for the simulation of the corresponding products from a real nuclear weapons explosion.

Preparatory test

A test irradiation of a small amount of Uranium, 4 mg uranyl nitrate, for one hour and in a neutron flux of 3E13 n cm"2 s"1 was carried out in the research reactor R2 at Studsvik. The main purpose was to compare experimentally produced fission products with tabled data7 and also to choose appropriate irradiation conditions for the exercise. The source was analysed by gamma- ray spectrometry the day after the irradiation and again after two weeks. These data are shown in Table 3. It was not possible to measure it earlier than one day, because of the very high source activity. Consequently and unfortunately, it was not possible to obtain any data on very short-lived fission products.

16 FOI-R--0054--SE

1.20E-07

1.00E-07 • Irradiation, experimental —H—Irradiation, theory — -A — Explosion, theory °- 8.00E-08

6.00E-08 -

4.00E-08

2.00E-08 -

O.OOE+00 0.00E+0 1.00E+0 2.00E+0 3.00E+0 4.00E+0 5.00E+0 6.00E+0 7.00E+0 8.00E+0 9.00E+0 0 111111111

Half-life, h

Figure 8. Theoretical and experimental activity as functions of half-lives

The agreement between the curves is quite acceptable and constitutes a very good support for the fission product simulation model.

Data and results from the exercise

Aliquots of uranium nitrate were irradiated for one hour in the reactor. They were aimed to be dissolved and diluted to form standardized radioactive liquid sources. Unfortunately, it was very difficult to dissolve the material and the absolute specific activity in Bq/ml was therefore not exactly known. However, the gamma-ray spectrometers and the measuring geometries used for the analyses are accurately calibrated what makes the obtained sample analyse results very good estimations of the qualitative and quantitative contents of the radioactive materia. A kind of evaluation of the results from carried out analyses was performed in order to assess the composition and strength of the standard solutions. It was based on a comparison of analyse results for some occuring radionuclides. It concerns the primary fission products Ce-143, Ba-140, Zr-95, Sr-91 and Ru- 103. They were chosen because of their fairly long half-life, they are not extremely volatile, not daughters to long-lived, primarily created fission products with high fission yields, they occur as reference nuclides in the nuclear weapons literature and, finally, they are quite easy to analyse by gamma-ray spectrometry.

The comparison data is presented in the following Table 5, together with Figure 9, which illustrate the character of the relations between the different samples and nuclides.

19 FOI-R--0054--SE

Table 5 A comparision between the different samples and some occuring radionuclides.

Sample Added vol Ce-143-act Ba-140-act Zr-95-act Sr-91-act Ru-103-act ml Bq/ml Bq/ml x10 Bq/ml x10 Bq/ml Bq/ml

L420J 0.97 1.25E+05 3.92E+04 4.69E+04 1.72E+05 5.12E+04 L523J 0.64 1.45E+05 5.42E+04 6.44E+04 1.92E+05 5.97E+04 L524J 0.29 1.51E+05 4.38E+04 6.24E+04 1.97E+05 6.34E+04 L421J 0.86 1.28E+05 4.59E+04 6.07E+04 1.70E+05 4.90E+04 L422J 1.23 1.15E+05 4.10E+04 5.82E+04 1.77E+05 4.67E+04 L06V 0.2 1.41 E+05 4.03E+04 5.45E+04 1.82E+05 6.40E+04 L117T 0.13 1.18E+05 4.45E+04 5.63E+04 1.63E+05 5.05E+04 L519T 0.06 1.32 E+05 4.65E+04 6.07E+04 1.78E+05 6.68E+04 L218T 0.14 1.17E+05 4.14E+04 5.76E+04 1.49E+05 3.78E+04 L114T 0.13 1.25E+05 4.62E+04 4.87E+04 1.68E+05 5.65E+04 L416T 0.15 1.19E+05 3.84E+04 6.35E+04 1.74E+05 4.79E+04 L215T 0.08 1.18E+05 3.95E+04 5.04E+04 1.45E+05 4.86E+04

——•——Ce-143 Bq/ml —M—Ba-140 Bq/ml 2.50E+05 T . - A--Zr-95 Bq/ml x 10 — Sr-91 Bq/ml 2.00E+05 -- —3K—-Ru-103 Bq/ml x 10

> 1.50E+05 - • "§ o E 1.00E+05

5.00E+04

0.00E+00 L420J L523J L524J L421J L422J L06V L117T L519T L218T L114T L416T L215T Sample Figure 9. Nuclide relations in various samples

The activity is given in Bq/ml of the added standard solution volume. There seems to be a variation common for at least Ce-143, Sr-91 and Ru-103. A somewhat different pattern would be identified for Zr-95 and Ba-140, which is remarkable because all nuclides were present in the same solution. The reason for that has not been looked for, but it may possibly indicate that there might be some variations due to the handling of the material in the adding procedure. The variations, however, are so small that it is possible that they are a result of purely random variations and in that sense not significant deviations.

20 FOI-R--0054--SE

A simple analysis of data in Table 5 and their relation to theoretical and experimental data is compiled in Table 6.

Table 6 An analysis of the data in Table 5 and their relation to theoretical and experimental data.

Ce-143-act Ba-140-act Zr-95-act Sr-91-act Ru-103-act Bq/ml Bq/ml Bq/ml Bq/ml Bq/ml

Average, Bq/ml 1.28E+05 4.34E+04 5.70E+03 1.72E+05 5.35E+03

Std dev 1.21E+04 4.47E+03 5.85E+02 1.52E+04 8.62E+02

Theory, rel 6.13E+04 1.00E+04 2.17E+03 7.37E+04 1.64E+03

Average / Theory, rel 2.09 4.34 2.63 2.33 3.26

Experiment, rel 6.17E+04 1.00E+04 2.17E+03 7.38E+04 1.64E+03

Average / Exp, norm 0.477 1 0.606 0.537 0.751

Theory, rel / Exp, rel 1 1 0.998 1 0.998

The first and the second row contain average value and standard deviation respectively for the really measured activities, as activity per ml added solution.Row number three shows the theoretical, relative activity values7 after a nuclear weapons explosion.Measured and theoretical relative values are shown in row four. Absolute values are of no interest, only their mutual relation. The Ce-143, Zr-95 and Sr-91 are roughly on the same level, while Ru-103 and, above all, Ba-140 deviate appreciably. Ba-140 is of special interest because it has the highest value and it will therefore be used as the reference nuclide. In row five are given the relative, experimental data, obtained at the test irradiation of 4 mg uranyl nitrate, measured directly on the irradiation ampoule, without dissolving. Measured average values are compared with those obtained at the test irradiation in the next row. It is obvious that the ratios are of the same character as those in row four. The last row contains the relations between the theoretical values and those obtained from the test irradiation. The agreement is almost complete, which is interpreted as a very strong support for the method to use neutron irradiation of uranium for production of fission products that can simulate those created by a nuclear weapons explosion.

The remaining analyses

More complete analysis results for soil and tape samples containing data for the activity of additional nuclides, primarily created as well as daughter products, are compiled in table 7 and 8. These tables contain relative values that have been estimated from a calculation of the amount of activity added to the respective samples, from data in Table 4, and compared with data from the analyses. It was necessary to choose a reference nuclide, since all radioactive material was not dissolved when the standard solutions were prepared. Ba-140 was chosen to constitute the reference nuclide, mainly because it had the highest activity. There were no certain indications

21 FOI-R--0054-SE on the amount of Ba-140 in the solid material that was dissolved and the amount of radioactive materia immediately after the irradiation was therefore not well known. The Ba-140 activity has been assigned the relative value 1, and for the present it may be assumed that the Ba-140 was completely dissolved.

Data in the tables are illustrated in diagrams in connection with the tables, showing the nuclide distributions in the analysed samples as well as their mutual relation.

Table 7. The relative values that have been estimated from a calculation of the amount of activity added to the respective soil samples, from data in Table 4, compared with data from the analyses.

Nucl L09J L010J L011J L012J L013J L127J L420J L421J L422J L523J L524J Ave

Nb-97 0.47 0.49 0.48 0.53 0.52 0.57 0.57 0.50 0.57 0.46 0.56 0.52 1-132 0.45 0.45 0.43 0.49 0.46 0.51 0.52 0.47 0.47 0.40 0.52 0.47 1-135 0.25 0.25 0.25 0.29 0.23 0.22 0.25 0.24 0.30 0.22 0.26 0.25 Sr-91 0.55 0.54 0.49 0.61 0.63 0.63 0.61 0.52 0.60 0.50 0.63 0.57 Zr-97 0.48 0.47 0.48 0.52 0.52 0.56 0.57 0.50 0.56 0.46 0.56 0.52 1-133 0.24 0.24 0.24 0.26 0.25 0.25 0.28 0.22 0.28 0.20 0.27 0.25 Ce-143 0.50 0.49 0.48 0.53 0.52 0.53 0.51 0.45 0.45 0.43 0.56 0.50 La-140 0.72 0.74 0.76 0.71 0.75 0.81 0.83 0.78 0.79 0.67 0.82 0.76 1-131 0.40 0.45 0.32 0.48 0.33 0.37 0.34 0.39 0.35 0.24 0.34 0.37 Ba-140 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 Ru-103 0.81 0.83 0.72 0.92 0.86 0.76 0.89 0.72 0.77 0.74 0.98 0.82 Zr-95 0.57 0.77 0.62 0.63 0.76 0.72 0.58 0.64 0.69 0.58 0.69 0.66

1.20

1.00 -

a, 0.80 - _3 (0 >

«J 0.60 -•

& 0.40 -

0.20-• 0.00

Sam pie

Figure 10. Relative yield per nuclide in soil samples

22 FOI-R--0054--SE

1.20

0.00 L09J L010J L011J L012J L013J L127J L420J L421J L422J L523J L524J Sample

Figure 11. Relative yield per soil sample

Table 8. The relative values that have been estimated from a calculation of the amount of activity added to the respective tape samples (Table 4) compared with data from the analyses.

Nuclide L08T L114T L117T L215T L218T L416T L426T L519T L525T Average

Nb-97 0.45 0.46 0.48 0.52 0.47 0.53 0.55 0.46 0.49 0.49 1-132 0.46 0.39 0.40 0.40 0.36 0.40 0.45 0.36 0.34 0.39 1-135 0.18 0.17 0.18 0.25 0.19 0.21 0.21 0.16 0.20 0.19 Sr-91 0.52 0.51 0.51 0.56 0.51 0.64 0.64 0.54 0.57 0.55 Zr-97 0.46 0.47 0.50 0.52 0.47 0.54 0.57 0.48 0.49 0.50 1-133 0.21 0.17 0.18 0.18 0.17 0.19 0.20 0.16 0.17 0.18 Ce-143 0.48 0.44 0.43 0.49 0.45 0.50 0.59 0.46 0.48 0.48 La-140 0.78 0.72 0.78 0.74 0.69 0.80 0.83 0.75 0.72 0.76 1-131 0.30 0.19 0.27 0.21 0.29 0.30 0.33 0.30 0.26 0.27 Ba-140 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 Ru-103 0.67 0.83 0.77 0.83 0.62 0.84 0.96 0.97 0.94 0.82 Zr-95 0.60 0.51 0.61 0.62 0.67 0.80 0.65 0.63 0.72 0.65

23 FOI-R--0054--SE

1.20

0.00 Nb-97 1-132 1-135 Sr-91 Zr-97 1-133 Ce- La- 1-131 Ba- Ru- Zr-95 143 140 140 103 Nuclide

Figure 12. Relative yield per nuclide in tape samples

1.20 - Nb-97

1.00 -1-132 -1-135 -Sr-91

•-X- - Zr-97 -1-133 -Ce-143 -O-- - La-140 -1-131 Ba-140 -Ru-103 -Zr-95 0.00 L08T L114T L117T L215T L218T L416T L426T L519T L525T

Sample

Figure 13. Relative yield per tape sample

24 FOI-R--0054--SE

It is obvious that the iodine isotopes exhibit the lowest yields, which could be expected since iodine is often released in various chemical and physical processes. 1-132 is the daughter to Te-132, which probably is the explanation to why it shows a higher value than the other iodine isotopes. Te-132 has a comparably long half-life and goes on producing 1-132 during the whole exercise. 1-131 shows a slightly higher value than 1-133 or 1-135 and that is probably an effect of a minor production from Te-13 lm. All the remaining nuclides have yields between 0.5 and 1 relative to Ba-140.

The consensus in the results may be interpreted as a support for the assumption that the obtained result represents the real content of the standard solution, at least for the nuclides in the comparison. Furthermore, it is very probable that what is true for one isotope is also true for the other isotopes of the same element, with exception for isotopes that are daughters to more long-lived nuclides. The probable composition of the standard solution, for nuclides with theoretical and experimental support, is shown in the last column of Table 9. The activity is referred to the time t = 20 h, to allow for the daughter products to be built up to near equilibrium.

Table 9 Comparision between the theoretical and measured radioactivity content for the chosen radionuclides.

Nuclide Half-life Yield, Bq/fission Bq/fission Measured Total yield Bq/ml at T%, h atoms per Att = O at t = 20 h Bq/ml relative to t = 20h fission t = 20h Ba140 Bq/ml

Pr-144 2.88E-01 1.55E-09 8.86E+02 Nb-97 1.23E+00 3.23E-07 5.00E-01 1.88E+05 1-132 2.28E+00 9.11 E-08 4.30E-01 4.57E+04 Sr-92 2.71 E+00 5.94E-02 4.22E-06 2.54E-08 5.60E-01 1.19E+04 Y-92 3.54E+00 1.92E-07 9.02E+04 Pr-145 5.98E+00 3.93E-02 1.27E-06 1.25E-07 Tc-99m 6.00E+00 1.24E-07 1-135 6.57E+00 6.28E-02 1.84E-06 2.23E-07 2.20E-01 5.72E+04 Sr-91 9.50E+00 5.83E-02 1.18E-06 2.74E-07 1.72E+05 5.60E-01 1.72E+05 Y-93 1.02E+01 6.35E-02 1.20E-06 3.08E-07 5.40E+04 Zr-97 1.68E+01 5.98E-02 6.85E-07 2.99E-07 5.10E-01 1.78E+05 1-133 2.08E+01 6.70E-02 6.20E-07 3.18E-07 2.20E-01 8.16E+04 Ce-143 3.31 E+01 5.96E-02 3.47E-07 2.28E-07 1.28E+05 4.90E-01 1.28E+05 La-140 4.03E+01 1.11 E-08 7.60E-01 1.29E+04 Pm-149 5.30E+01 1.08E-02 3.92 E-08 3.02E-08 Y-90 6.41 E+01 8.49E-12 5.77E+00 Mo-99 6.60E+01 6.11E-02 1.78E-07 1.44E-07 Te-132 7.80E+01 4.30E-02 1.06E-07 8.87E-08 4.45E+04 1-131 1.93E+02 2.89E-02 2.88E-08 2.68E-08 3.20E-01 1.00E+04 Nd-147 2.64E+02 2.25E-02 1.64E-08 1.56E-08 Ba-140 3.07E+02 6.21 E-02 3.89E-08 3.72E-08 4.34E+04 1.00E+00 4.34E+04 Pr-143 3.26E+02 1.18E-08 Ce-141 7.80E+02 5.85E-02 1.44E-08 1.41 E-08 4.90E-01 8.06E+03 Nb-95 8.40E+02 1.32E-10 9.31 E+01 Ru-103 9.43E+02 3.03E-02 6.19E-09 6.10E-09 5.35E+03 8.20E-01 5.35E+03 Sr-89 1.21 E+03 4.73E-02 7.51 E-09 7.51 E-09 5.60E-01 4.71 E+03 Y-91 1.40E+03 6.08E-09 3.82E+03 Zr-95 1.54E+03 6.50E-02 8.15E-09 8.08E-09 5.70E+03 6.60E-01 5.70E+03

25 FOI-R--0054--SE

Ce-144 6.84E+03 5.50E-02 1.55E-09 1.55E-09 4.90E-01 8.86E+02 Sr-90 2.55E+05 5.78E-02 4.37E-11 4.37E-11 5.60E-01 2.97E+01 Cs-137 2.65E+05 6.19E-02 4.51 E-11 4.51 E-11

Nuclides with no data in the table were not possible to analyse by available methods. Their total yield relative Ba-140 may only be roughly estimated. In order to produce a complete table, a decent assumption would be that their yields are around 0.5 and that would lead to the following table, Table 10, which may be considered as the final data for the standard solution, referred to the time t = 20 h after the imaginary nuclear weapons explosion.

Table 10 A complete table of the assumed nuclide content referred to the time t = 20 h after the imaginary nuclear weapons explosion.

Nuclide Half-life Activity at TV*, h t=20h Bq/ml

Pr-144 2.88E-01 8.86E+02 Nb-97 1.23E+00 1.88E+05 1-132 2.28E+00 4.57E+04 Sr-92 2.71 E+00 1.19E+04 Y-92 3.54E+00 4.51 E+04 Pr-145 5.98E+00 7.30E+04 Tc-99m 6.00E+00 5.02E+04 1-135 6.57E+00 5.72E+04 Sr-91 9.50E+00 1.72E+05 Y-93 1.02E+01 2.70E+04 Zr-97 1.68E+01 1.78E+05 1-133 2.08E+01 8.16E+04 Ce-143 3.31 E+01 1.28E+05 La-140 4.03E+01 6.40E+03 Pm-149 5.30E+01 2.29E+04 Y-90 6.41 E+01 2.88E+00 Mo-99 6.60E+01 8.42E+04 Te-132 7.80E+01 5.18E+04 1-131 1.93E+02 1.00E+04 Nd-147 2.64E+02 9.75E+03 Ba-140 3.07E+02 4.34E+04 Pr-143 3.26E+02 2.73E+03 Ce-141 7.80E+02 8.06E+03 Nb-95 8.40E+02 9.31 E+01 Ru-103 9.43E+02 5.35E+03 Sr-89 1.21E+03 4.71 E+03 Y-91 1.40E+03 3.82E+03 Zr-95 1.54E+03 5.70E+03 Ce-144 6.84E+03 8.86E+02 Sr-90 2.55E+05 2.97E+01 Cs-137 2.65E+05 2.55E+01

26 FOI-R--0054--SE

The remaining nuclides

Thermal neutron irradiation of uranium may lead to splitting of uranium nuclei, which implies creation of fission products, or to activation of the nuclei by neutron capture which produces transuranium elements, i e U-239, Np-237, Pu-239, etc. Fission takes place in the uranium isotope U-235 while neutron capture dominates in U-238. A U-238-atom that captures a neutron in its nucleus is transformed to a U-239-atom, which in turn disintegrate to Np-239, a beta-gamma emitting nuclide with rather low gamma-ray energies. Np-239 disintegrates to the long-lived, alpha emitting transuranium element Pu-239.

Np-239 is created in nuclear weapons explosions as well and that is why it should be present also in the simulated set of radioactive materia for the exercise. The amount of Np-239 that is produced during an irradiation in a nuclear reactor may be estimated from appropriate reaction data, but in the nuclear weapons case this is not that simple. The irradiated uranyl nitrate in this exercise contained 20 h after irradiation about 1.5E9 Bq/g fission products while the Np-239-activity amounted to around 1.8E9 Bq/g, in other words roughly equal specific activity.

Even Pu-239 is with a high probability present in the fallout from nuclear weapons explosions and in the exercise, there was also some Pu-239 in the irradiated uranyl nitrate that was irradiated. The activity relation was around 0.1 for Pu-239/U-238 and Pu-239 as well as U-234 and U-238 could easily be detected with an alpha spectrometer.

Conclusion

It has been shown that it is possible to produce a realistic nuclear weapons explosion exercise scenario by using fission products from thermal neutron irradiation of uranyl nitrate. The difference in nuclide composition and activity between the expected fission products from a nuclear weapons explosion and those produced by irradiation in the nuclear reactor is significant only during a few hours after the fictitious explosion. This time may be of the same order as the time it will take to set the preparedness group in action. Therefore, when the analyses of samples from the exercise area can start, the nuclear reactor created fission products are very good substitutes for those expected from a nuclear weapons explosion. The amount of activity that should be added to various samples is given by the MARS function.

This method for creating an exercise scenario also offers unique possibilities regarding development and test of all appropriate radiochemical and radiometric analysis methods, since all expected radionuclides are present in the samples and their activity are of a realistic magnitude. It also offers an excellent opportunity for development of fast analysis methods for long-lived radionuclides, e g Sr-90, which may be determined by gamma-ray spectrometric analysis of short-lived Sr-isotopes.

27 FOI-R-0054--SE

References

1. H.Tovedal, The Nuclear Weapons Fallout Preparedness Exercise LOTTA, FOA-report FOA-R—00-01619-861—SE, October 2000

2. H. Tovedal, The MARS Simulation of the Nuclear Weapons Preparedness Exercise LOTTA Scenario, FOI-report FOA-R--0053-SE, March 2001.

3. H. Tovedal, MAthematical Radiac Simulation, MARS, A Nuclear Weapons Case Application, manuscript FOI-report Umea, 2002.

4. L. Thaning, Application of the Atmospheric Transport Model PELLO in the Preparedness Exercise LOTTA, manuscript FOA-report Umea, 2001.

5. K. Lidstrom, Evaluation of Doserates and Doses During Preparedness Exercise LOTTA, manuscript FOI-report Umea, 2001.

6. U. Nygren, Determination of Radioactive Strontium During Preparedness Exercise LOTTA, FOA-report FOA-R-99-01045-861--SE, May 1999

7. T. R. England, B. F. Rider, Fission Product Yields per 100 Fissions for 235U Thermal Induced Fission Decay, LA-UR-94-3106, ENDF-349, 1993

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