1

Activation problems

S.Agosteo(1), M.Magistris(1,2), Th.Otto(2), M.Silari(2)

(1) Politecnico di Milano;

(2) CERN

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Introduction

Problems of material activation:• in the target system and its surroundings (for

Neutrino Superbeam and BetaBeams)• in the machines for ion acceleration and in the

decay ring (for BetaBeams only)

An estimation of the production of residual nuclei in the target station has been performed with FLUKA

3

FLUKA simulations

• A compromise between CPU time and precision:

A simplified geometryDEFAULTS SHIELDIN, conceived for

calculations for proton accelerators

The new evaporation module is activated (EVAPORAT)

The pure EM cascade has been disabled

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MicroShield

• A program which analyzes shielding and estimates exposure from gamma radiation

• Input:DimensionsMaterial information and build-up factorsSource strengthIntegration parameters

5

The target station

The facility consists of a target, two horns and a decay tunnel. It is shielded by 50 cm thick walls of concrete and is embedded in the rock.

Top view

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Target and horns

A 2.2 GeV, 4 MW is sent onto the mercury target, inserted in two concentric magnetic horns for pion collection and focusing.

Proton beam

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Decay tunnel

The decay tunnel consists of a steel pipe filled with He (1 atm), embedded in a 50 cm thick layer of concrete

60 m long

Inner diameter of 2 m

Thickness of 16 mm

Cooling system (6 water pipes)

Front view

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Surroundings

The whole structure (target, horn and decay tunnel) is embedded in the rock, which has been divided into 100 regions for scoring the inelastic interaction distribution

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Activation of mercury

Assumptions: 0.5 m3 of liquid circulating in the systemthe mercury is uniformly irradiatedit will circulate in pipes (2 cm radius) and

be stored in a spherical tank10 years of operation and 1 month cooling

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Dose rates due to the mercury

Dose equivalent rate at:• 50 cm from a 1 m long pipe, filled with Hg:

320 mSv h-1

• 5 m from the tank, without shielding:

68 mSv h-1

• 10 cm from a droplet (1 mg Hg):

1 Sv h-1

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Horn

• Material: ANTICORODAL 110 alloy (Al 96.1%)

• Irradiation time: six weeks

• Specific activity (MBq/g) at different cooling times

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Horn, after 6 weeks of irradiation

1E-3 0.01 0.1 1 10 100 10001E-5

1E-4

1E-3

0.01

0.1

1

10

100

1000

10000

1000 years

2 years

100 years

50 years

10 years

30 days

1 day

inner part of the horn outer part of the horn

Sp

efic

ic a

ctiv

ity

(MB

q g

-1)

time of cooling (years)

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Dose rates due to the horn

• At one metre from the horn, after six weeks of irradiation and one day of decay:

Dose equivalent rate: ~10 Sv h-1

• Equipment for the remote handling of the magnetic horns will be mandatory.

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Steel pipe

• Material: steel P355NH (Fe 96.78%)

• 60 m long• Filled with Helium• 10 years of operation• Operational year of 6

months (1.57*107 s/y)

Steel pipe

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Steel pipe, power density crossing the inner surface

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Steel pipe, after 10 years of operation

1 year of cooling

0 10 20 30 40 50 60

10

100

29 MBq g-1

Sp

ecif

ic a

ctiv

ity

(MB

q/g

)

decay tunnel [m]

average value

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Dose rates in the decay tunnel

• 89% of the dose rate comes from the steel

• The dose rate does not depend on the radial position

After ten years of operation, one month of cooling

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Earth, after 10 years of operation

1E-3 0.01 0.1 1 10 100 10001E-3

0.01

0.1

1

10

100

1000

10000

100000

1000000

depth

10 m from the target

Sp

ecif

ic a

ctiv

ity

(Bq

/g)

years of cooling

0-1 m 1-2 m 2-3 m 3-4 m 4-5 m 5-6 m

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Earth, after 10 years of operation

10-3 10-2 10-1 100 101 102 10310-3

10-2

10-1

100

101

102

103

104

105

10-3

10-2

10-1

100

101

102

103

104

10510m from the target

Swiss exemption limit

about 20 years

50 years

5 years6 months

10 days1 day

Fra

cti

on

of

exem

pti

on

lim

its

Sp

ecif

ic a

cti

vit

y (

Bq

/g)

years of cooling

2-3 m from the concrete

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Radioactivity in molasse

• There is the risk that the radioactivity in the earth may leach into the ground water.

• Radionuclides to be considered:In a soluble chemical formWith half-lives longer than 10 h

22Na, 3H

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Radioactivity in molasse

• The radioactivity induced in the rock may leach into the ground water.

• Two possible risks:

1) Contamination of surface water

(limits on the Bq/year produced)

• 2) Contamination of public water supplies

(limits on the concentration Bq/l released)

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Contamination of public water supplies

• Severe constraints for the concentration (Bq l-1) of activity induced in the ground water

• The estimation of the concentration of 3H and 22Na requires a hydro-geological study of the construction site

• No evaluation can be done, before the site of the facility has been chosen

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Contamination of surface water 50 cm thick

concrete wallsAnnual release (Bq per year)

Constraint (*)

22Na 4.6·1012 4.2·1011

260 cm thick concrete walls

Annual release (Bq per year)

Constraint (*)

22Na 3.2·1010 4.2·1011

3H 7.8·1011 3.1·1015

(*) Max dose to the critical group: 0.3 mSv per year,

release constraints valid for CERN Meyrin site only

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BetaBeams: induced radioactivity

• A large portion of the initial beam will decay during acceleration, and all injected beam is essentially lost in the decay ring

• Losses in the decay ring:~8.9 W m-1 (6He, 139 GeV/u) (*)~0.6 W m-1 (18Ne, 55 GeV/u) (*)

(*) M. Lindroos et al., Neutrino Factory Note 121

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BetaBeams: induced radioactivityLack of data on induced radioactivity from ions

• Possible ways of estimating the material activation:

1) For high-energy particles, an A-nucleus can be approximated by A single protons (It is the easiest way to obtain a first estimation)

2) At GSI, people are working on the implementation of a code, which deals with transport and fragmentation of heavy ions

3) A new version of FLUKA is being implemented

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Conclusions

• Even if it is not correct to simply scale the induced radioactivity produced in the decay tunnel (~kW/m) to that produced in the decay ring (~W/m), the latter is expected to be much lower than the former.

• A good estimation of the induced radioactivity in the decay ring requires a detailed study, possibly using both the simplified model and a Monte Carlo code, if available.