CERN-PPE/97-14231 October 1997

Incineration with Fast Neutrons

Jean-Pierre Revol

Abstract

The management of nuclear waste is one of the major obstacles to the acceptability of

nuclear power as a main source of energy for the future. TARC, a new experiment at CERN, is

testing the practicality of Carlo Rubbia's idea to make use of Adiabatic Resonance Crossing to

transmute long-lived fission fragments into short-lived or stable nuclides. Spallation neutrons

produced in a large Lead assembly have a high probability to be captured at the energies of

cross-section resonances in elements such as 99Tc, 129I, etc.

An accelerator-driven sub-critical device using Thorium (Energy Amplifier) would be

very effective in eliminating TRansUranic elements which constitute the most dangerous part of

nuclear waste while producing from it large amounts of energy. In addition, such a system

could transform, at a high rate and little energetic cost, long-lived fission fragments into short-

lived elements.

Invited talk at theDeutschen Physikalischen Gesellschaft

61. Physikertagung 1997 MünchenMarch 19, 1997

EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH

Nuclear Waste Removal Using Particle Beams

–2–

I. Introduction

In the next few decades, Humankind will be facing a tremendouschallenge: produce the energy needed to sustain its growth without affectingthe environment in a dramatic and irreversible way. Strong Green House effects(CO2) and massive pollution of our planet (NOx, SOx, radioactive waste, etc.)could have uncontrollable economical and political consequences in addition toeffects on our health. Politicians and the general public in the Western worldare only slowly comprehending that the solution does not depend so much onwhat the so-called advanced countries will or will not do regarding energyproduction or savings, but rather on whether or not developing countries,which represent the bulk of people on our planet, will have at their disposalenvironmentally safe energy production facilities which they can afford. It isnot possible to predict the future in this domain, but there is no doubt that, if asolution is found, it will be through systematic fundamental R & D. It is in thisspirit that C. Rubbia and his team at CERN are investigating the concepts of anew type of system [1], able to produce safe, "clean" and economicallycompetitive energy, also able to destroy in an economically attractive way, theexisting stockpile of radioactive nuclear waste (civil and military).

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Air inlet

Airoutlet

Stack

Grade

RVACS flow paths

Containment dome

Spallation region Lead flow in target

Proton Beam

Heat exchangerSecondarycoolant

0.0 2.5 5.0 7.5 10 (m)

Seismicisolator

Single stage cyclotron(Ep = 200 MeV)

Cold airdowncomer

Hot air riser

Containmentvessel

Main silo

Mainvessel

Thermal insulatingwall

Heat exchanger

Normal coolant level

Cold trap

Bea

m p

ipe

Emergencybeam dumplevel

Figure 1: Sketch of the Energy Amplifier prototype facility.

Nuclear Fission Energy is a prime candidate to cope with the spectaculargrowth of electrical energy needs expected in the first half of the 21st century.However, public acceptability is dwindling because it is perceived as dangerousfor several reasons: (1) it is not absolutely safe – criticality or core melt-down

–3–

accidents did occur (Chernobyl 1986, Three Mile Island 1979, etc.); (2) themanagement of the waste produced (back-end of the fuel cycle) is anunresolved problem; and finally, (3) harnessing of nuclear power was firstdriven by the urge to build bombs, and nuclear bombs were used in a war,therefore the association with military applications is strongly present in thepublic mind, and the risk of nuclear weapon proliferation cannot be tolerated.

TOTAL

240Pu

239Pu

137Cs

238Pu

90Sr241Am

243Am

242Pu

237Np

238U

236U 99Tc

129I

210Pb

230Th226Ra

229Th 231PaCoal

ORNL DWG 95A-534

109

1011

1012

1010

108

107

106

105

105 106104103102101100

Decay Time After Discharge (yr)

Inge

stio

n To

xici

ty (

m3

H2O

/Mg

Hea

vy M

etal

)

Figure 2: Ingestive toxicity as a function of decay time for a number of nuclides in spentLight Water Reactor fuel [2], based on a burnup of 33 MWd(t)/Mg Heavy Metaland 3.2 % initial enrichment [Source: Oak Ridge National Laboratory (1995)].

In order to respond to these concerns, C. Rubbia has proposed the EnergyAmplifier (EA) (Figure 1) [1], which is the result of a cross-breeding betweenparticle accelerator and nuclear fission reactor technologies. It is a sub-criticaldevice, driven by a proton accelerator, using natural Thorium as fuel and Leadas neutron spallation target, moderator, container and heat-removal agent.

I would like to concentrate here on one aspect of the EA project: theelimination of nuclear waste. As K. Kugeler [3] admitted in his presentation, theworst unsolved problem of nuclear energy production is the accumulatingstockpile of nuclear waste. Transmutation products (mainly trans-uranicelements referred to as TRU) and Fission Fragments (FF) are the two majorcomponents of the radioactive waste generated in the nuclear fuel cycle. In atypical Pressurized Water Reactor (PWR), TRansUranic elements (Np, Pu, Am,Cm, Bk, etc.) are the result of neutron capture and subsequent decays. Even

–4–

though they only represent 1.1% of the spent fuel, they constitute the maincontribution to the long-term radiotoxicity of nuclear waste (Figure 2). The FF,which can reach about 4% of the mass of spent fuel, dominate the short-termradiotoxicity of the waste. In practice, TRU and FF are usually mixed withuranium oxide (95%), and packaged inside an appropriate cladding.

It is for the long-lived elements that deep underground "geological"storage is seen as the only presently practical solution. However, in view of theuncertain long term consequences, it is obviously very important to considerthe alternative possibility of destroying them all together.

Destroying TRU and FF are two different problems: TRU are actinideswhich can only be destroyed by fission, thus producing a large amount ofenergy. On the contrary, FF can be destroyed by neutron capture (which willtransmute them into very short-lived or stable elements), however, this processwill potentially cost energy. To illustrate the magnitude of waste production,we note, for instance, that in a 1 GWe PWR, plutonium alone is produced at therate of 200 kg/year (Table 1), which corresponds to 12 tons for the yearlyproduction of France, and about 71 tons for the present total world annualnuclear electric energy production of 2130 TWe�hour.

Table 1: Typical nuclear waste production in PWR's (Uranium and TRU).

(1995) One PWR(1 GWe)

France(54 PWRs, 59 GWe)

World

Annual ElectricEnergy produced

6 TWe�h 359 TWe�h 2130 TWe�h

Uranium(0.9% 235U)

20 t 1200 t 7100 t

Pu 200 kg 12 t 71 t

Np 10.4 kg 620 kg 3690 kg

Am 9.8 kg (³ 5 yrs) 590 kg 3480 kg

Cm 0.8 kg 48 kg 285 kg

II. Incineration of TRU

There are several reasons why TRU cannot be incinerated in PWR's. In athermal neutron flux, there is a strong odd-even number of neutrons effect inthe fission probability (Figure 3). 233U, 235U, 239Pu, 241Pu, etc. have a highprobability to fission (³ 60%) while 234U, 236U, 238U, 238Pu, 240Pu, etc. have alow probability to fission (² 10%) and some of them do not fission at all.

–5–

229-

TH

230-

TH

232-

TH

231-

PA

232-

U23

3-U

234-

U23

5-U

236-

U23

8-U

236-

NP

237-

NP

238-

PU

239-

PU

240-

PU

241-

PU

242-

PU

244-

PU

241-

AM

242-

AM

*24

3-A

M24

3-C

M24

4-C

M24

5-C

M24

6-C

M24

7-C

M24

8-C

M25

0-C

M24

7-B

K24

8-B

K24

9-C

F25

0-C

F25

1-C

F

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

1.00

0.90

0.80

0.70

0.60

0.50

0.40

0.30

0.20

0.10

0.00

Fis

sion

Pro

babi

lity

Cap

ture

pro

babi

lity

Element

Figure 3: Fission and capture probabilities for Actinides in a PWR (Thermal) neutronspectrum (ORIGEN, ORNL–4628) [3]. Only elements with half-lives larger than 10years are shown.

229-

TH

230-

TH

232-

TH

231-

PA

232-

U23

3-U

234-

U23

5-U

236-

U23

8-U

236-

NP

237-

NP

238-

PU

239-

PU

240-

PU

241-

PU

242-

PU

244-

PU

241-

AM

242-

AM

*24

3-A

M24

3-C

M24

4-C

M24

5-C

M24

6-C

M24

7-C

M24

8-C

M25

0-C

M24

7-B

K24

8-B

K24

9-C

F25

0-C

F25

1-C

F

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

1.00

0.90

0.80

0.70

0.60

0.50

0.40

0.30

0.20

0.10

0.00

Fis

sion

Pro

babi

lity

Cap

ture

pro

babi

lity

Element

Figure 4: Fission and capture probabilities for Actinides in a Fast EA neutron spectrum [3].Only elements with half-lives larger than 10 years are shown.

Thus, for a large number of elements, neutron capture dominates, andelements with an even number of neutrons tend to accumulate, resulting in an

–6–

intolerable loss of neutron inventory through capture. The incineration of TRUis achieved quite naturally in the Energy Amplifier, by using a fast neutronenergy spectrum, in which all TRU undergo fission, most of them with a highprobability (Figure 4) (The possible choice of metal fuel makes the neutronenergy spectrum particularly hard, significantly harder than in standard LiquidMetal Fast Reactors). TRU incineration can in principle also take place in a fastbreeder reactor, although in a less favourable neutron energy spectrum, but inthat case, the high TRU contents lead to a much more unstable system, becausethe fraction of delayed neutrons allowing reactor control is very small. The0.65% of delayed neutrons in a PWR, becomes only 0.3% in CAPRA [5] and0.2 % in the Japanese version of the Actinide Burner at JAERI (Figure 5). Incontrast, the EA which achieves its control with the accelerator is much fartheraway from criticality. If k = 0.98, the margin is 3 times larger than that of a PWRand 10 times larger than that of Superphenix.

PWR

SPX

CAPRA

Ac BurnerJAERI

EAk=0.98

EAk=0.96

-0.050 -0.040 -0.030 -0.020 -0.010 0.000

Maximum distance from Prompt Criticality

Figure 5: Allowed operational safety margin comparison between the Energy Amplifier andvarious nuclear reactors (PWR and fast breeder systems) [3].

The EA works in a closed cycle in which, after reprocessing of the spentfuel, the TRU are re-injected into the system, together with the addition of theappropriate amount of fresh Thorium. The use of Thorium suppresses naturallythe rate of TRU production by about 4 orders of magnitude compared to a PWR(mainly because 5 successive neutron captures separate 233U from 238U). As aresult, TRU production in the EA cycle is extremely small (Figure 6): inprinciple TRU can be almost completely eliminated, down to a radiotoxicitylevel smaller than that produced by fusion, as it is claimed today [4].

–7–

Energy Amplifier

Magnetic FusionModel 1

Magnetic FusionModel 2

PWR (Origen)

PWR (Ref. [x])

No Incineration

With Incineration

Coal

10

10

10

10

10

10

10

1.0

0 100 200 300 400 500 600 700 800 900 1000

Inge

stiv

e R

adio

toxi

city

inde

x [R

elat

ive

units

]

Time after shutdown [years]

(1)

(3)

-7

-6

-5

-4

-3

-2

-1

Figure 6: Comparison of radioactive ingestive toxicities of spent fuels between a typicalPWR, the Energy Amplifier and two possible fusion systems: a conservativeapproach (model 1) and an advanced approach (model 2). The comparison is alsomade with the radiotoxicity of coal ashes also normalized to the same energyproduction

-20

0

20

40

60

80

100

120

140

-140

0

140

280

420

560

700

840

980

0 10 20 30 40 50 60

Net

Pu

cons

umpt

ion/

unit

Ene

rgy,

kg/

TW

he

Net

Pu

cons

umpt

ion/

unit

Ene

rgy,

kg/

GW

e/ye

ar

Percentage Plutonium in Mixture

Incineration

Breeding

Uranium-Plutonium Fast Breeder

Figure 7: Net plutonium consumption in CAPRA [5] as a function of the plutoniumenrichment of the fuel mixture (Source: CEA, unpublished).

One can even go one step further, and dedicate an EA to a most efficientdestruction of TRU. In any system where both neutron-induced fission andneutron capture are at work, under stable conditions, an equilibriumconcentration sets in, for any element, at a level where the rate at which theelement is produced through neutron capture is balanced by the rate at whichthe element is disappearing, either through decay or through fission. If for any

–8–

reason the equilibrium is perturbed, the system will act to restore it. This meansthat if one prepares, for example, a plutonium concentration smaller than theequilibrium value, the system will breed extra plutonium to increase itsconcentration, until the equilibrium is reached again: we have a plutoniumbreeder (this was the main reason behind building Superphenix, at a time whenit was thought that large amounts of plutonium were needed in view of thecoming shortage of natural uranium and perhaps also for military purpose). Ifon the contrary, the system is prepared with a plutonium concentration abovethe equilibrium value, then the system will restore the equilibrium by burningthe excess plutonium. This is the principle of an incinerator. The advantage ofthe EA is very clear: while for CAPRA or Superphenix the equilibriumconcentration of plutonium is about 15%, it is only of the order of 10–6 in an EA(Figure 8). Therefore, it is easy to have a plutonium concentration larger than10–6. On the other hand, burning plutonium at a high rate in CAPRA requiresconcentrations much beyond 15%; one must exceed 50% to obtain the highestpossible incineration rate of 80 kg/TWhe (Figure 7). The system becomesextremely dangerous (small bare critical mass, very little control margin, worsetemperature coefficient, increased void coefficient, etc.), and even under theseconditions, the effective incineration rate still remains modest because thereactivity decreases very rapidly as FF build up. One Superphenix type breederwould be required to cope with the TRU production of three PWR's. Francealone would need about 17 Superphenix type incinerators, solely to cope withthe present rate at which TRU are produced. In addition, one has anaccumulated stock corresponding to 20 to 30 years of massive nuclear power.

1x10-5

1x10-4

1x10-3

0.01

0.1

1

0 10 20 30 40 50 60 70 80 90 100 110 120 130

Rel

ativ

e co

ncen

trat

ion

Integrated Burn-Up, GW day/ton

Th-232

U-233

U-234

U-235

Pa-233

Pa-231

U-236 U-232

Th-230 Np-237

Pu-238 1x10-6

Figure 8: Equilibrium concentrations of the main elements in the Energy Amplifier.

The possibility to incinerate TRU has attracted the interest of Spain, and C.Rubbia has proposed [6] to build Energy Amplifiers to burn the entire stockpileof nuclear waste which will have been produced by Spain by the year 2029,when the present generation of 9 Spanish PWR's will have reached the end of

–9–

their life span. This alternative to vitrification and deep underground geologicalstorage is attractive because not only TRU are destroyed (only relativelymodest class A storage is required) but the operation is economically profitable.In this case, 8% of the present total energy need of Spain would be produced fora period of 37 years, while destroying the TRU stockpile. In addition, 233U isproduced which can be used either as fuel for other Energy Amplifiers, or forPWR's. Typically, 400 kg of TRU are destroyed per year and per EA.

0.001

0.01

0.1

1

1 6 11 16 21 26 31

Sur

vivi

ng F

ract

ion

of P

u R

eman

ents

Cycle Number

Incinerator Mode EA Mode

PWR's Plutonium

Military Plutonium

Figure 9: Residual stockpile of plutonium as a function of the number of fuel cycles [6]. Atcycle 16, in this example, there is no more addition of "external" plutonium and itbecomes possible to reduce the residual fraction to any small value desired.

Using the same method, military plutonium could also be destroyed [7]. Ithink that, both for political and economical reasons, effectively destroying the300 tons of bomb-grade plutonium owned mostly by Russia and by the UnitedStates of America is one of the most important and urgent duties of ourgeneration, to ensure that no nuclear bomb will ever be used again on ourplanet. Up to now, the only two possibilities considered by the US and Russiannegotiation teams is to either denature the plutonium before storing it in a deepgeological repository or to use it to manufacture MOX fuel for PWR's.However, the MOX solution only allows small amounts to be effectivelydestroyed since more TRU are regenerated in the process, and also theradiotoxicity of the resulting spent fuel is much worse than that of waste fromordinary fuel. It is known that it is partly economical issues which are actuallyhampering the negotiations with Russia. The EA is a new approach, which notonly can destroy completely military plutonium (Figure 9), but also offers asignificant economical bonus in the operation. When addition of external

–10–

plutonium stops, after a number of EA fuel cycles, the EA can be operated as astandard energy producing unit, where only Thorium is introduced. Theresidual plutonium concentration can essentially be reduced to any leveldesirable.

Table 2: Most offending FF activity and Toxicity after 1000 years of cool-down in a SecularRepository, for a typical PWR fuel having produced 1 GWe�year of energy.

Element Half-Life

(year)

Mass

(kg)

Activity

@1000y

(Ci)

Ingestive

Toxicity

(kSv)

Class ADilutionvolume

(m3)

129I 1.57�107 8.09 1.43 19.58 178.47

99Tc 2.111�105 16.61 284.29 27.67 947.65

126Sn 1.�105 1.187 33.79 3.20 9.65

135Cs 2.3�106 34.12 39.32 9.87 39.32

93Zr 1.53�106 26.11 65.64 2.38 18.75

79Se <6.5�105 0.30 2.06 0.745 0.59

III. Incineration of Fission Fragments

If TRU are destroyed, then, after 500 to 600 years, Long-Lived FissionFragments (LLFF) become the dominant type of nuclear waste. The mostoffending of these LLFF are by far 99Tc and 129I (Table 2).

The present global world annual production of 99Tc and 129I can beestimated at about 6 and 2 tons respectively. Both 99Tc (t1/2 ~ 2�105 years) and129I (t1/2 ~ 1.6�107 years) are of concern, because of their potential health risks:they are very long-lived, produced in significant amounts, generally solubleunder geological conditions they migrate relatively quickly under commonground water conditions and therefore, they can enter the food chain. Twoorders of magnitude increase of the 99Tc concentration is observed in the Oceanwaters, since the early 1960's, as a consequence of release by the nuclearindustry [8]. An increase of 99Tc concentration is observed off the coast ofGreenland [9], as a consequence of the Sellafield reprocessing plant releases,with a migration time of 7 years between the British coast and Greenland.Given the half-lives involved, since we do not know of a strong naturalmechanism which will remove 99Tc from sea water, we are dealing with anirreversible phenomenon on the human time scale. The long-term radiologicaland chemical effects are not well known, and if the present trends continue, itwill no longer be possible to eat lobsters, or Brown algae (Figure 10), if oneinsists on a 99Tc free diet. Under these conditions, it would seem reasonable to

–11–

consider an alternative to geological storage, or to direct dilution in the Oceanproposed by certain parties.

PHYTOPLANKTON

BROWN ALGAE

BRINE SHRIMP

EUPHAUSIID

SHRIMP

CRAB

ISOPOD

AMPHIPOD

LOBSTER

MUSSEL

OYSTER

ABALONE

POLYCHAETE

FISH

1 10 100 1000 10000

Log (Concentration Factor)

Org

anis

m

Figure 10: Concentration factors for 99TcO4–, in marine organism as determined in laboratory

experiments using 95mTcO4– (T.M. Beasley and H.V. Lorz [10]).

This justifies our interest in an original idea, put forward by C. Rubbia,which would allow, for the first time, an efficient destruction of large amountsof 99Tc, at almost no cost!

IV. Adiabatic Resonance Crossing

C. Rubbia's idea is making full use of the extraordinary properties of Lead,an element which plays several important roles, in the Energy Amplifier: aspallation target to produce neutrons from high-energy protons interactions, aneutron moderator and a neutron containment medium, and finally a heatextraction medium, by natural convection of the molten Lead. In the TARCexperiment [11] (Transmutation by Adiabatic Resonance Crossing) at CERN, itis mainly the moderator property of Lead which is being tested together withthe spallation process.

Lead is unique because it is the element with the lowest capture cross-section over a wide range of neutron energies. This stems mainly from thedoubly magic nature of 208Pb, present at a level of 52.4 % in natural Lead. Theother isotopes, only singly magic, still preserve a low neutron capture cross-section for natural Lead (Figure 11).

–12–

0 10 20 30 40 50 60 70 80 90 100

Z (Target)

100

101

102

103

104

σ effe

ctiv

e a

t 65

keV

Odd Z target nucleiEven Z target nuclei witheven-even component weighted bymultiplying by factor of 2.4

Pb

Na

Figure 11: 65 keV neutron capture cross-section, as a function of the number of protons (Z) inthe nucleus of various elements. Black dots refer to even Z nuclei, and white dots toodd Z nuclei.

The elastic scattering cross-section in Lead, on the contrary, is very highand uniform over a large range of energies (Figure 12), the typical scatteringlength is 3 cm. Neutrons produced in Lead can live a very long time before theyare eventually captured. The migration length of neutrons, in an infinite Leadmedium, is found to be 1.2 m (distance between the production point and thecapture point). It is easy to imagine that, because of the long random walknature of neutron travel in pure Lead, a given neutron can contribute severaltimes to the local flux, Lead acting thus effectively as a neutron flux amplifier.

10-4

10-3

10-2

10-1

1

10

10 2

10-5

10-3

10-1

10 103

105

107

Elastic

Capture

Neutron Energy (ev)

Cro

ss-S

ecti

on

(b

arn

s)

Natural Lead

Figure 12: Neutron elastic and capture cross-sections in natural Lead, as a function of neutronkinetic energy.

–13–

More precisely, it is the smallness of the diffusion coefficient in Lead (D ~3.01 cm) and the long capture length which can result in an increase by an orderof magnitude of the local flux, around a source in Lead as compared to thesame source in vacuum [6].

The simulation of neutrons produced by a single 3.5 GeV/c proton hittingthe large Lead volume used in the TARC experiment illustrates very clearly ourpoint (Figure 13). A given neutron can cross several times the same elementarysurface thereby, contributing several times to the flux through that surface.

Figure 13: Example of the neutron cloud produced by a single 3.5 GeV/c proton in the3�3.3�3.3 m3 Lead volume of the TARC experiment. At such a proton momentum,about 100 neutrons are produced on average per incident proton. In this particularexample about 55000 elastic collisions took place.

Another parameter of importance is the large mass of the Lead nucleus,which makes the elastic scattering process almost isotropic at low energies andwith extremely small neutron kinetic energy changes between collisions. Theratio of neutron energies after and before collision is:

E2E1

=mn

2 + mPb2

mn +mPb( )2+

2mnmPb cosθ

mn + mPb( )2

–14–

where mn and mPb are respectively the neutron and the Lead nucleusmasses, and � is the scattering angle. A consequence of this simple kinematics isthat �E1 ² E2 ² E1 where:

α ≡mPb − mn( )2

mn +mPb( )2≈ 0.98

This means, that a neutron loses at most 2% of its energy in an elasticcollision with a Lead nucleus. On average, the energy loss is about 1%. Anotherimportant characteristic of this peculiar kinematics is the fact that the ratiobetween E2 and E1 is independent of neutron energy. It is convenient tointroduce here the concept of Lethargy change between collisions:

∆u ≡ − lnE2E1

≈

∆EE

; if∆EE

«1

The average Lethargy change is given by:

ξ ≡ ∆u = 1+α

1− αln α( ) ≈ 9.6 ×10−3

The special kinematics in Lead results in a flat neutron Lethargydistribution in the regime where captures can be neglected, together with avery "slow" moderation of neutrons, providing a harder neutron energyspectrum, best suited for burning actinides as already explained. A spallationneutron, surviving the Lead capture resonances, cannot miss a given resonanceenergy such as, for instance, 5.6 eV in 99Tc. In its "adiabatic" slowing downprocess, a neutron will sweep all energies with very small steps, until it iscaptured with high probability on an "impurity" with a high resonance in itscapture cross-section, as in the case for 99Tc (Figure 14).

A neutron capture on 99Tc produces 100Tc, an isotope of Technetium withonly 15.8 seconds half-life, whose �-decay produces 100Ru, a stable element.Therefore, through neutron capture, the long-lived (2�105 years) 99Tc iseffectively transmuted into a stable element, eliminating totally the initialradioactivity. In addition, Nature is kind enough to provide a relatively smallcapture cross-section for 100Ru. Therefore, given that 101Ru and 102Ru are alsostable, the neutron activation of 100Ru plays no significant role. At the 5.6 eVresonance, the neutron capture cross-section of 99Tc is more than two orders ofmagnitude larger than in a PWR (Thermal) flux, and, as we will see later, it ismoreover possible to transmute large amounts of 99Tc in an EA withoutperturbing its operation, which is not the case for a PWR.

–15–

Neutron Energy (eV)

10–5 10–3 10–1 101 103 105 107

Cro

ss-s

ectio

n (b

arns

)

10–4

10–3

10–2

10–1

100

101

102

103

104174µs 92µs

Thermal

Adiabatic Slowing Down

Figure 14: 99Tc neutron capture cross-section, as a function of neutron kinetic energy.

(To Scale / Side view)

0 10

meter

1

Scintillators #1&2

Scintillator #3 and #4

QDE04 QFD05BHZ03

DVT01

DVT02Vacuum pipe

MWPC #2

Beam Transformer #1(old from FEAT)

0 1 2

meter

Concrete Roof

0.44 m

He bag

MWPC #1

1.9 m

2.09 m

Embeco

Iron rafter

1.28 m

Concrete Floor

3.74 m

Beam Transformer #2(new this year)

3.2 m

LEAD

99Tc

Figure 15: Schematic view of the PS211 beam line, showing how protons are injected into theLead assembly. The approximately cylindrical volume of lead has a 1.65 m radiusand is 3 m long, for a total weight of 334 tons. The diameter of the beam hole is77.2 mm and that of the instrumentation holes 62 mm.

V. The TARC experiment

In order to test the concept of incineration of 99Tc or other Long-LivedFission Fragments, an experiment known as PS211 [11] was setup on a PS beamline at CERN, with important financial support from the European Union. Theset-up (Figure 15) consists of a large volume of pure (99.99%) Lead, where a 3.5GeV/c proton beam penetrates a 1.2 metre hole before hitting the Lead toproduce a neutron shower approximately centred on the geometrical centre ofthe Lead volume. A few small holes, parallel to the beam line allow to introducesamples to be studied, as well as various instruments to measure the neutronflux. The size of the Lead volume was chosen so that only about 25% of allneutrons escape its boundaries. Care was taken to minimize neutron reflectionsfrom the surrounding cave, by having walls and ceiling as far away as possible,

–16–

and by incorporating B4C in the cement (Embeco) used on top of steel I-beamsto absorb reflections from the floor.

As a result of the special kinematics of neutrons in pure Lead, a strongcorrelation exists between the energy of a neutron and the time at which it isdetected (Figure 16) (counted from the time is was created by spallation in thelead). In practice, the beam pulse (~ 20 ns wide) provides the T-zero for timemeasurements:

t =2λel.

ξ1v

−1v0

where �el. is the elastic scattering length, � the average Lethargy changedefined above and v the neutron velocity. For times sufficiently long therelation can be simply written as:

t ≈2ξ

λel.v

the factor between brackets is the time it takes for the neutron to travelone scattering length at the velocity at which it is detected.

0.1 µs 1 ms100 µs10 µs1 µs 10 ms

1 eV

100 eV

10 eV

1 KeV

100 KeV

10 KeV

1 MeV

0.1 eV

0.01 eV

10 MeV

Figure 16: Relation between capture time and energy for the spallation neutrons in TARC.

By measuring the time of detection of a neutron one can determine itsenergy. This is the technique used in the TARC experiment to measure the

–17–

neutron flux, for neutron energies up to about 100 keV, corresponding to timeslarger than about 1 �s. Note that it takes about 3 ms (a very long time), for a 10MeV neutron to thermalize through, on average:

n =1ξ

lnE0E

≈ 2000 collisions .

During its random walk, such a neutron will have travelled 60 metres inthe Lead, in search for impurities on which to be captured. On the contrary, inhydrogen, thermalization takes only about 20 collisions (� = 1).

The advantage of this so-called "slowing down" calorimeter alreadydescribed in the 1950's [12] is that one can make very high statisticsmeasurements, with a reasonable energy resolution:

∆EE

= 2 ×ξ3

+kT4E

12

valid for E ≤ 10 keV

The first term arises from the diffusion process, the second term, due tothermal motion, becomes important at energies below about 1 eV. The resultingenergy resolution between 11% and 25 % in the domain of interest, is quiteadequate for us, because of the very high statistics of neutrons at our disposal,which is the clear advantage of this technique over time-of-flightmeasurements. Our first task in the TARC experiment was to calibrate thetime–energy relation, by observing the time distribution of prompt �'s emittedin the capture on elements with strong and known capture resonances, such as181Ta, 55Mn, 238U, 99Tc, etc. This was done by measuring the time distributionof the scintillation light produced by the prompt �'s, in a CeF3 crystal, mountedin front of a photomultiplier tube. The result is:

E eV( ) =173000

t µs( )+ 0.4( )2 in excellent agreement with the EA simulation.

Equipped with this relation we measured the neutron flux at variouslocations in the Lead assembly with two different techniques:

(a) the detection with a photomultiplier of the light emitted in the reaction:n + 3He 3H + p + 764 KeV. The positive Q value allows detection of neutronsfrom vanishingly small energies up to about 100 keV.

(b) the detection in a Si-diode detector of the products of the reaction: n +6Li � + 3H + 4.8 MeV.

In this case, an absolute calibration is provided with a 233U target, fromwhich we can both detect �'s from the decay of 233U and FF from neutroninduced fission of 233U:

–18–

233U 229Th + � + 4.86 MeV

n + 233U FF + 200 MeV

At higher energies, where the time-energy relation cannot be used mainlybecause of the lack of knowledge of the v0 term, one measures directly theneutron energy in a 3He ionization chamber, where the products of thereaction: n + 3He 3H + p + 764 KeV are both detected. This detector extendsthe flux measurement up to neutron energies of about 2 MeV, a very importantregion, since it covers the capture resonances of Lead.

With electronics measurements, we have measured in TARC the neutronflux at ~ 100 locations inside the Lead volume each from thermal to MeVneutrons, over about 7 orders of magnitudes (Figure 17). The preliminaryanalysis indicates that there is an excellent understanding of the properties ofthe neutron flux, both in energy and in space within the Lead volume. Thisconstitutes a precious precision validation of the general EA simulation code.

10-1

100

101

102

103

104

105

106

107

10-2 10-1 100 101 102 103 104 105 106 107

Flu

x (

n/cm

2 /eV

/109 p

roto

ns )

Neutron Energy (eV)

3-He Ionization Chamber

Si Detector (233-U)

PS211PRELIMINARY

Figure 17: Example of measurement of the neutron flux versus neutron energy in TARC.

Other techniques are used to complement this measurement, such asactivation foils [Au (4.9 eV), W (18 eV), In (1.5 eV), etc.] to measure the flux atone specific neutron energy (that of a huge capture resonance of the elementconsidered). A sandwich of three foils of carefully chosen thickness is arranged

–19–

in such a way that the outer foils are opaque to neutrons at the captureresonance. The comparison of activations between the central foil and the twoouter foils provides a measurement of the flux at the energy of the resonance.This is a standard technique used in activation studies. Integral fluxmeasurements were also performed. Below 0.5 eV, the integral of the flux wasobtained by standard dosimetric methods, measuring the thermoluminescencelight from sandwiches of 6Li/7Li samples, activated with and without aCadmium cover. The difference in light output from 6Li and 7Li, during heatingof the sample, provides the neutron dose, hence an integral measurement of theneutron flux. The comparison between the naked samples and the samplescovered with Cadmium, provides the component of the flux below 0.5 eV. Athigh energies, we used chemical etching of Lexan foils, to detect tracksproduced in the Lexan by FF from various fissile materials used as neutrontargets. 237Np and and 232Th with a fission threshold at 1.5 MeV, with a fissionthreshold at 2 MeV, were used to provide both an integral measurement of thehigh-energy component of the flux, and also a measurement of the spaceevolution of that component, showing that it dies away, over a distance of onlyabout 50 cm from the centre of the beam shower. This is a feature very relevantto the EA design, since it determines the choice of the distance between thespallation region and the fuel, in order to minimize radiation damage by fastneutrons on the various structural materials.

VI. 99Tc incineration by Adiabatic Resonance Crossing

After having verified that the neutron flux is well understood over theentire Lead volume, we measured the capture rate of neutrons on samples of99Tc. A complete mapping of the capture rate was performed, as well assystematics studies with samples of various nature (Metallic 99Tc, 99TcO4K) andvarious amounts (0.2 and 0.4 g 99TcO4K). The experiment consists of detectingthe �-decay �'s of 100Tc produced by neutron capture on 99Tc. Since the half-lifeof 100Tc is only 15.8 s, it was necessary to install a pneumatic system (Rabbit), inorder to allow fast extraction of the sample between beam shots spaced by 14.4s, to count the �'s on high resolution Ge-Li counters. The technique allows aprecise positioning of the sample in the Lead volume. About 0.5 s after thebeam shot, the sample is brought 20 meters away between two Ge-Li countersfor a 9 seconds counting, it is then sent back to its irradiation position shortlybefore the next beam shot arrives. This scenario takes advantage of the CERNPS machine cycle which matches well the half-life of 100Tc, and allows us tomake high statistics measurements. Typically, at each position we collected ofthe order of 20000 counts in the 539 or 590 KeV lines of the 100Tc � spectrum(Figure 18). The Ge-Li detectors were placed sufficiently far away from theLead assembly as to have a negligible neutron background, and inside a

–20–

compact Pb-Cu-Al shielding, in order to have a negligible � background fromthe environment.

100

101

102

103

104

105

0 200 400 600 800 1000 1200 1400 1600 1800 2000

Cou

nts

/ 0.2

7 ke

V /

50 m

in

652

keV

(98

Tc)

661

keV

(13

7Cs)

745

keV

(98

Tc)

1130

keV

(10

0Tc)

822.

5 ke

V (

100T

c)

689.

4 ke

V

590

keV

(10

0Tc)

539

keV

(10

0Tc)

Pb x-rays

99T

c B

rem

sstr

ahlu

ng

1024

keV

(10

0Tc)

1512

keV

(10

0Tc)

1201

keV

(10

0Tc)

1362

.1 k

eV (

100T

c)

1847

keV

(10

0Tc)

0x100

1x103

2x103

3x103

530 540 550

Cou

nts/

0.27

keV

/50

min

Energy (keV)

Resolution@ 539 keV

2 keV

Photon Energy (keV)

100Tc γ Spectrum from PS211

Figure 18: Typical � energy spectrum obtained with the Ge-Li counters, showing all thecharacteristic lines of 100Tc, with very low background.

Systematic studies were performed by using 27Al and 107Ag samples withthe same Rabbit technique. The availability of two Ge-Li counters, and ofvarious � lines in the 100Tc decay spectrum, offers many checks of thesystematic errors, providing high confidence in the measurements. The absoluteerror in the number of neutron captures is dominated by the uncertainty ofabout 15% in the � lines branching ratios, the uncertainty of 7% on thedetermination of the system dead time and the 5% calibration error on the beamintensity measurement.

0.0×10-4

0.5×10-4

1.0×10-4

1.5×10-4

2.0×10-4

2.5×10-4

3.0×10-4

3.5×10-4

-150 -100 -50 0 50 100 150

Cap

ture

s/pr

oton

Z (cm)

hole#3

hole#6

hole#12

Figure 19: Captures per incident proton in 0.2 g of 99Tc, for 3 different measuring holesparallel to the beam line, and for various positions along the holes. Error barsinclude uncertainties from statistics, beam intensity (~ 5%) and dead time (~ 7%).

An example of the scan along three different measuring holes is shown inFigure 19. The EA simulation reproduces well within the overall error marginthe behaviour of the number of captures per incident proton. This confirms that

–21–

the spallation process, the neutron diffusion process in Lead and the captureprocess on 99Tc are all very well understood (including self-shielding in thesample). Similar results were obtained with the activation of a sample of ~ 0.1 gof 129I. In this case, the capture can produce two states: 130mI, an isomer with 9mn half-life and 130gI the ground state with 12.36 h half-life. Fortunately, thevarious � lines produced allow to measure each of the two states, and thereforedetermine the number of captures. In the case of 129I, the end product is 130Xewhich is stable. These results have important consequences concerning theincineration of Long-Lived Fission Fragments.

The full EA simulation, the same simulation used for the TARCexperiment, indicates that there exist regions, inside the EA lead volume, wherethe neutron flux is Lethargy driven (it is almost flat in Lethargy bins). Theseregions are outside the core, where neutrons have a small probability to returnto the fuel and be useful in terms of producing fissions. Therefore, in theseregions, one can, in principle, introduce large amounts of "impurities" such as99Tc or 129I, without affecting the operation of the EA in a significant way.

One could envisage to destroy this way more 99Tc than is produced by theEA core. This means that, in addition to its own production of LLFF (mainly99Tc and 129I), the EA could also destroy the stockpile from the presentgeneration of nuclear reactors in a parasitic mode, hence, almost at littleadditional cost.

VII. Conclusion

There is fundamental R & D related to the field of nuclear energy, whichindicates that an alternative solution exists, to expensive and potentiallydangerous geological storage of the huge amounts of radiotoxic wasteproduced by nuclear industry, as well as of military plutonium.

The TARC experiment, which has to a large extent been funded by theEuropean Union and which is currently being carried out at CERN indicatesalready that the phenomenology of spallation neutrons in pure Lead is wellunderstood and that Adiabatic Resonance Crossing can be used to increase byseveral orders of magnitude the efficiency with which large amounts of LLFFsuch as 99Tc and 129I can be destroyed.

At the same time, the advanced simulation technique developed for theEA is validated with precision in the neutron spallation/moderation area,following a previous validation (the FEAT experiment [13]) of the energy-producing domain.

–22–

VIII. Acknowledgements

I would like to thank especially Carlo Rubbia for inducing me to exploresome domains of nuclear physics which have brought to me a new vision of thefield of energy production and for sharing with me some of his most recentideas.

I would like also to thank the organisers of the Deutschen PhysikalischenGesellschaft in Munich, in particular Walter Blum, for the kind invitation totheir annual meeting.

I am also very pleased to acknowledge the help of Federico Carminati,José Galvez, Enrique Gonzalez-Romero, Yacine Kadi and Robert Klapisch,specifically for the preparation of this talk, and I would like to thank all mycollaborators in the TARC experiment for their enthusiastic support.

My gratitude goes also to Susannah Tracy for her precious help with thetext.

IX. References

[1] C. Rubbia et al., "Conceptual Design of a Fast Neutron Operated HighPower Energy Amplifier", CERN/AT/95-44 (ET), 29th September, 1995; seealso C. Rubbia, "A High Gain Energy Amplifier Operated with fast Neutrons",AIP Conference Proceedings 346, International Conference on Accelerator-Driven Transmutation Technologies and Applications, Las Vegas, 1994.

[2] "Nuclear Wastes (Technologies for Separation and Transmutation),published by National Academic Press, Washington DC (1996), Chapter 2.

[3] C. Rubbia's hearing at the French Assemblée Nationale, November 22,1996, and M. Claude Birraux, "Rapport sur le Contrôle de la sûreté et de lasécurité des installations nucléaires", Office Parlementaire d'Evaluation desChoix Scientifiques et Technologiques, No 3491 (Assemblée Nationale), No 300(Sénat).

[4] C. Rubbia, "A Comparison of the Safety and EnvironmentalAdvantages of the Energy Amplifier and of Magnetic Confinement Fusion",CERN/AT/95-58 (ET), 29th December, 1995.

[5] J. Rouault et al., "Physics of Pu Burning in Fast Reactors: Impact onBurner Core Design", Proc. ANS Topical Meeting on Advances in ReactorPhysics, Knoxville, 11-15 Apr. 1994.

[6] C. Rubbia et al., "Fast Neutron Incineration in the Energy Amplifier asAlternative to Geological Storage: the Case of Spain", CERN/LHC/97-01 (EET),17th September, 1997.

–23–

[7] C. Rubbia et al., "A Realistic Plutonium Elimination Scheme with FastEnergy Amplifiers and Thorium-Plutonium Fuel", CERN/AT/95-53 (ET), 12thDecember, 1995.

[8] E. Holm, J. Rioseco and S. Mattsson, "Technetium in the Baltic Sea", inProc. of Technetium in the Environment, edited by G. Desmet & C. Myttenaere,Elsevier (1984).

[9] A. Aarkrog et al., "Time trend of 99Tc in Seaweed from Greenlandwaters", in Proc. of Technetium in the Environment, edited by G. Desmet & C.Myttenaere, Elsevier (1984).

[10] T. M. Beasley and H. V. Lorz, "A Review of the Biological andGeochemical Behaviour of Technetium in the Marine Environment", in Proc. ofTechnetium in the Environment, edited by G. Desmet & C. Myttenaere, Elsevier(1984).

[11] "Experimental Study of the Phenomenology of Spallation Neutrons ina Large Lead Block", CERN-SPSLC Proposal, (1995).

[12] A. A. Bergman, A. I. Isakov, I. D. Murin, F. L. Shapiro, I. V. Stranikh,M. V. Kazarnovsky, "Lead Slowing-Down Neutron Spectrometry", Proc. 1st Int.Conf. on Peaceful Uses of Atomic Energy, v. 4, 1955, p 135.

[13] S. Andriamonje et al., "Experimental Determination of the EnergyGenerated in Nuclear Cascades by a High Energy Beam", Phys. Lett. B348(1995) 697-709.