Nuclear Instruments and Methods in Physics Research A 631 (2011) 105–110

Contents lists available at ScienceDirect

Nuclear Instruments and Methods inPhysics Research A

0168-90

doi:10.1

n Corr

E-m

journal homepage: www.elsevier.com/locate/nima

Neutronics design for the coupled para-hydrogen moderator for CSNS

Wen Yin n, T.J. Liang, Q.Z. Yu

Engineering Center of CSNS Target station and Instruments, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China

a r t i c l e i n f o

Article history:

Received 15 July 2010

Received in revised form

12 December 2010

Accepted 14 December 2010Available online 21 December 2010

Keywords:

Coupled para-hydrogen moderator

Neutron intensity

Decay power

Activity

Radiation damage

02/$ - see front matter & 2010 Elsevier B.V. A

016/j.nima.2010.12.130

esponding author. Tel.: +86 10 82649270; fax

ail address: wenyin@aphy.iphy.ac.cn (W. Yin)

a b s t r a c t

We performed a neutronics design for the coupled para-hydrogen moderator for the Chinese Spallation

Neutron Source (CSNS), which relied on the Monte-Carlo simulation code MCNPX. We chose a cylindrical

shape moderator with a diameter of 150 mm after considering the peak and integrated cold neutrons.

Under the cost limitations of the CSNS project, the target station may use light water rather than heavy

water as a target coolant during phase one. Using light water as the coolant decreased the cold neutron

intensity of various energies of the coupled moderator by about 15–20%. If the neutrons were extracted

from one side of the moderator instead of being extracted from both sides, the fluxes increased by about

5–10%. The decay power and radiation damages of the moderator vessel were calculated for 100 kW and

5000 h.

& 2010 Elsevier B.V. All rights reserved.

1. Introduction

In recent years spallation neutron sources, driven by protonaccelerators, have inspired great interest in many science andtechnology fields, becoming indispensable tools in these areas ofstudy. The Chinese Spallation Neutron Source (CSNS) project is ashort-pulse mode spallation neutron source. The power level is100 kW with its proton beam energy of 1.6 GeV and repetition rateof 25 Hz in phase one. This project’s progress depends on closecooperation between the Institute of Physics (IOP) and the Instituteof High Energy Physics (IHEP) [1]. When a proton beam with1.6 GeV of energy bombards a tungsten target, the neutrons thatescape from the target slow down in the moderator. They can thenbe extracted for neutron scattering experiments. Hydrogen is themost popular moderator material for providing cold neutrons inspallation sources. The CSNS target station uses para-hydrogen asthe moderator material, although para-hydrogen can be irradiatedby gamma-rays and converted into the ortho state [2]. Further-more, after considering the angular dependence of the neutronintensity presented in the J-PARC case [3], we chose a cylindricalshape for the para-hydrogen moderator.

For the purpose of this paper, we calculated the intensities of theneutrons that escaped from the coupled para-hydrogen modera-tors with different diameters. For cost savings, the CSNS targetstation may use light water as the target coolant in phase one. Wecompared the difference of the neutron intensity between lightwater and heavy water. We considered an asymmetric structure for

ll rights reserved.

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.

the moderator to increase the neutron flux and evaluated thosecases where the neutrons were extracted from one side. Wecalculated the decay power of the coupled hydrogen vessel andwe believed that the decay power was mainly due to nuclide 28Al.Finally, we estimated the moderator vessel’s damage. The decaypower, activity, and damage were calculated as follows: 100 kWproton beam and one operation year was equal to 5000 h. Allresults were obtained using the MCNPX2.5.0 [4] and the Cinder90[5] codes.

2. Neutron intensity and moderator dimensions

Making distinctions based on the cross-sections of para-hydro-gen and ortho-hydrogen is critical to determining the neutronicsperformance. The uniquely small total cross-section of para-hydrogen for low-energy neutrons means that these neutronscan easily leak out of the moderator. This leaky characteristic can becountered by increasing the thickness of the moderator and addinga pre-moderator [3].

Our calculations were performed using the geometrical layoutshown in Fig. 1. This MCNPX model included a tungsten target, acoupled hydrogen moderator, a decoupled water moderator, and aberyllium reflector. The target was bombarded by a uniform1.6 GeV proton beam with a width of 120 mm and a height of40 mm. The repetition rate was 25 Hz. The target was 400 mmthick, 150 mm wide, and 50 mm high. There were twelve waterchannels, each of which was 1.5 mm thick, between the targetplates. The width of the in/out coolant pipe was 50 mm. Thethickness of the SS316 target vessel was 10 mm, with an exceptionof its window, which was reduced to 2 mm to reduce thermal

20

reflector

premoderator premoderator

moderator

vessel

vessel

target10

2010

0

0

-10

-10-20

20

10

0

-10

-20-20 20100-10-20

Fig. 1. Geometrical layout of the moderator and the target: (a) horizontal view and (b) vertical view.

0.00E+0001.00E+0152.00E+0153.00E+0154.00E+0155.00E+0156.00E+0157.00E+015

En=2meV

Neu

tron

Inte

nsity

(n/c

m2 /s

/sr/e

V/p

ulse

/10

0kW

at 2

5 H

z)

Neu

tron

Inte

nsity

(n/c

m2 /s

/sr/e

V/p

ulse

/10

0kW

at 2

5 H

z)

Neu

tron

Inte

nsity

(n/c

m2 /s

/sr/e

V/p

ulse

/10

0kW

at 2

5 H

z)

0.00E+000

2.00E+015

4.00E+015

6.00E+015

8.00E+015

1.00E+016 En=5meV

Neu

tron

Inte

nsity

(n/c

m2 /s

/sr/e

V/p

ulse

/10

0kW

at 2

5 H

z)

Neu

tron

Inte

nsity

(n/c

m2 /s

/sr/e

V/p

ulse

/10

0kW

at 2

5 H

z)

Emission time (μs)

0.00E+000

2.00E+015

4.00E+015

6.00E+015

8.00E+015

1.00E+016 En=10meV

0.00E+000

1.00E+015

2.00E+015

3.00E+015

4.00E+015 En=20meV

0 200 400 600 800 1000 0 200 400 600 800 1000

0 0

00.00E+000

2.00E+014

4.00E+014

6.00E+014

8.00E+014

1.00E+015 En=50meV

Emission time (μs)

λ=6.395 angstrom λ=4.045 angstrom

λ=2.860 angstrom

Emission time (μs)100 200 300 400 500

Emission time (μs)50 100 150 200 250 300

λ=2.022 angstrom

Emission time (μs)10080604020

λ=1.279 angstrom

Fig. 2. Pulse shapes from para-hydrogen moderators with the diameter of 150 mm. (a) En¼2 meV, (b) En¼5 meV, (c) En¼10 meV, (d) En¼20 meV, and (e) En¼50 meV.

W. Yin et al. / Nuclear Instruments and Methods in Physics Research A 631 (2011) 105–110106

stress. The height of the coupled hydrogen moderator was 100 mm,which was equal to the height of the surface viewed by theinstruments after considering the enhanced effect of the water

pre-moderator [6]. The aluminum alloy A6061 vessel of thecoupled hydrogen moderator was 6 mm thick. The diameter ofthe beryllium reflector was 700 mm. The diameter of the stainless

W. Yin et al. / Nuclear Instruments and Methods in Physics Research A 631 (2011) 105–110 107

steel reflector was 1000 mm. We assumed that the thickness of thepre-moderator near the target side was 20 mm and the thickness ofthe pre-moderator far from the target side was 10 mm. In CSNStarget station design, the coupled para-hydrogen moderator fed8 beam-lines. The viewed moderator area was 100 mm high and100 mm wide with an extraction channel having an opening angleof 661.

Fig. 2 shows the pulse shapes of the para-hydrogen moderatorwith a diameter of 150 mm. The method of calculating the neutronperformance of the moderator was outlined by Iverson et al. [7]. Tooptimize the coupled moderator, we considered five moderatordiameter, 100, 125, 150, 175, and 200 mm. The peak intensities ofthese moderators are shown in Table. 1. The moderator withdiameter 125 mm gives the highest peak intensity of the coldneutrons (En¼2, 5 meV). As an example, Fig. 3 gives the pulsewidth (FWHM) of the cold neutrons (En¼5 meV). The moderatorwith a diameter of 125 mm has the highest peak intensity and anarrower pulse width. In order to find the optimal diameter, wealso considered the integrated intensity over the energies below5, 10, and 20 meV (see Table 2). The integrated intensity below5 meV is almost saturating when the diameter of the moderator isabove 150 mm. After considering the peak and integrated inten-sities of the cold neutrons, we chose the moderator with a diameterof 150 mm as the para-hydrogen moderator. We compared thepeak neutrons intensity with J-PARC results. As an example, forEn¼10 meV our results give 30% higher neutron intensity than that

Table 1Peak intensity of the moderators with the different diameters.

Diameter

(mm)

Peak intensity of neutrons (1015 n/cm2/sr/eV/pulse/100 kW 25 Hz)

En¼2 meV En¼5 meV En¼10 meV En¼20 meV En¼50 meV

100 4.81 7.20 6.98 3.16 1.19

125 6.18 9.11 8.55 3.94 1.09

150 5.94 8.82 8.37 3.43 0.92

175 5.71 8.57 7.96 3.10 0.81

200 5.27 8.10 7.48 2.80 0.73

100

150

200

250

En=5meV

Puls

e w

idth

, FW

HM

(μs)

Diameter of the moderators (mm)100 120 140 160 180 200

Fig. 3. Pulse width (FWHM) of the cold neutrons as the function of the moderator

diameter.

Table 2Integrated intensities of the moderators with the different diameters.

Diameter Integrated intensity of neutrons (10�4 n/cm2/sr/proton)

100 mm 125 mm 150 mm 175 mm 200 mm

Eno5 meV 4.33 6.18 6.68 6.76 6.65

Eno10 meV 8.97 12.76 13.76 13.98 13.82

Eno20 meV 13.54 18.71 19.71 19.76 19.37

of J-PARC para-hydrogen moderator [3,6,8] when the results arenormalized to the same power. We thought the differences weredue to the following reasons: Firstly, CSNS use a thinner tungstentarget, whose height is 50 mm compared to 80 mm in J-PARC case,which increases the coupling between the target and the mod-erator. Secondly, the energy of the proton beam in CSNS is 1.6 GeVcompared to 3.0 GeV in J-PARC case. The cost energy required toproduce one neutron increases with the proton energy when theproton energy is above 1.1 GeV, which means the effective neutronyield in CSNS case is higher than that in J-PARC case. Of course, theproton energy must be determined taking into account the sourcepower. Thirdly, the models between our calculation and J-PARCcalculation are different, such as the thickness of the vessel. Herewe did not compare our results with SNS case because the SNScoupled moderator was not located at the optimum position. In theabove calculation, the thickness of the pre-moderator near thetarget side was fixed to 20 mm by the reduced heat deposition inthe moderator (see Fig. 4), though it was not the optimal thicknessfor the para-hydrogen moderator (see Fig. 5).

Given the cost limitations of the CSNS project, the target station mayuse light water rather than heavy water as a target coolant during phaseone. A large loss of neutrons is unavoidable in this case because thehydrogen in the light water thermalized neutrons in the tungstentarget area. Because tungsten has a large capture cross-section forthermal neutrons, thermalization of the neutrons close to the targetincreases loss due to absorption. Fig. 6 shows that using light water asthe coolant decreases the cold neutron intensity of the various energiesof the coupled moderator by about 15–20%.

We also considered how the asymmetric structure of thecoupled moderator increased the neutron flux. Fig. 7 was a specialexample in which the neutrons were extracted from one side. Inthis case the reflector and the pre-moderator immediately behind

0 4 8 12 16 200.8

1.0

1.2

1.4

1.6

1.8

Hea

t dep

ositi

on in

mod

erat

or(a

rb.u

nit)

Premoderator thickness (mm)

Fig. 4. Heat depositions in the para-hydrogen moderators having the different thick

pre-moderators.

0 5 10 15 200.8

0.9

1.0

1.1

1.2

En<10meV

Neu

tron

rela

tive

inte

nsity

(arb

.uni

t)

Premoderator thickness (mm)

Fig. 5. Integrated intensities of neutrons (Eno10 meV) from the para-hydrogen

moderators having the different thick pre-moderators.

0.00E+0001.00E+0152.00E+0153.00E+0154.00E+0155.00E+0156.00E+0157.00E+015

Heavy water water

En=2meV

Neu

tron

Inte

nsity

(n/c

m2 /s

/sr/e

V/p

ulse

/10

0kW

at 2

5 H

z)

Emission time (μs)

0.00E+0001.00E+0152.00E+0153.00E+0154.00E+0155.00E+0156.00E+0157.00E+0158.00E+0159.00E+0151.00E+016

Heavy water water

En=5meV

0

00.00E+000

2.00E+014

4.00E+014

6.00E+014

8.00E+014

1.00E+015

Heavy water water

En=50meV

λ=1.279 angstrom

Neu

tron

Inte

nsity

(n/c

m2 /s

/sr/e

V/p

ulse

/10

0kW

at 2

5 H

z)

200 400 600 800 1000

Emission time (μs)0 200 400 600 800 1000

λ=6.395 angstrom

λ=4.045 angstrom

Emission time (μs)10080604020

Neu

tron

Inte

nsity

(n

/cm

2 /s/s

r/eV

/pul

se/

100k

W a

t 25

Hz)

Fig. 6. Neutron intensities with the different coolants: (a) En¼2 meV,

(b) En¼5 meV, and (c) En¼50 meV.

Fig. 7. Geometrical layout of the asymmetric moderator.

0.00E+0001.00E+0152.00E+0153.00E+0154.00E+0155.00E+0156.00E+0157.00E+015

symmetric reflector asymmetric reflector

En=2meVλ=6.395 angstrom

Neu

tron

Inte

nsity

(n/c

m2 /

s/sr

/eV

/pul

se/

100k

W a

t 25

Hz)

0.00E+000

2.00E+015

4.00E+015

6.00E+015

8.00E+015

1.00E+016

symmetric reflector asymmetric reflector

En=5meVλ=4.045 angstrom

Neu

tron

Inte

nsity

(n

/cm

2 /s/

sr/e

V/p

ulse

/10

0kW

at 2

5 H

z)

0

0

00.00E+000

2.00E+014

4.00E+014

6.00E+014

8.00E+014

1.00E+015 symmetric reflector asymmetric reflector

En=50meVλ=1.279 angstrom

Neu

tron

Inte

nsity

(n/c

m2 /

s/sr

/eV

/pul

se/

100k

W a

t 25

Hz)

Emission time (μs)10080604020

Emission time (μs)200 400 600 800 1000

Emission time (μs)200 400 600 800 1000

Fig. 8. Differences in neutron intensity between the moderator with an asymme-

trical structure (where neutrons were extracted from one side) and that with a

symmetrical structure (where neutrons were extracted from both sides):

(a) En¼2 meV, (b) En¼5 meV, and (c) En¼50 meV.

W. Yin et al. / Nuclear Instruments and Methods in Physics Research A 631 (2011) 105–110108

the moderator contributed more to the neutron flux; however, if weadopt this structure, the number of beam-lines taken out from themoderator will fall to one-half. The results show that this structureincreased the peak intensity of the cold neutrons (En¼2, 5 meV) byabout 5–10% (see Fig. 8).

3. Decay power and activity of the moderator vessel

Next, we calculated the decay power and activity of the coupledhydrogen moderator’s vessel, which provided the basic dataneeded to design the cryogenic system and a disposal methodfor the irradiated components. Here, MCNPX was used to calculatethe neutron fluxes, as well as the production rates of neutrons andother particles. These results were then used to calculate theradionuclide inventory via the CINDER90 code. In this calculation,the secondary particles such as gamma-rays are tracked in theA6061 vessel. It is very important because if the gamma-rays arenot tracked in the calculation, they will be deposited at the point ofthe interaction. This will bring meaningful inaccuracies to thecalculation for this thin vessel [9]. The main contents of A6061calculated in this paper includes Si 0.5 %wt, Fe 0.7 %wt, Cu 0.25 %wt,Mn 0.15 %wt, Mg 1.0 %wt, Zn 0.25 %wt, Ti 0.15%wt, and Al97.00 %wt. The decay power of the vessel decreased to about0.1 W, which is a relatively low value, when cooled down over aten-day period. However, because of the production of the nuclide28Al, which has a short half-life of 2.241 min, the decay power of thevessel, once the station had been operating for one hour, wasapproximately 27 W, which is almost 20% of the prompt heatenergy. The decay power was up to 30 W after the station had beenoperating for one day due to 28Al and 56Mn. The intensity of this

1E-3 0.1 10 1000 1000001E-7

1E-5

1E-3

0.1

10 Al-28 Mn-56 Na-24 Cu-64 Zn-65 Na-22 Total

Dec

ay p

ower

(W)

Cooling time (d)

1E-3 0.1 10 1000 100000100000

1E7

1E9

1E11

1E13

Al-28 Mn-56 Cu-64 Zn-65 Na-24 H-3 Na-22 Total

Activ

ity (B

q)

Cooling time (d)

Fig. 9. (a) Decay power and (b) activity of the vessel of the coupled para-hydrogen

moderator.

1 101

10

100

1000

Neutron Proton

Hel

ium

Pro

duct

ion

Cro

ss s

ectio

n (m

b)

Energy (meV)

1000100

Fig. 10. Neutron- and proton-induced helium production cross-sections, via ENDF/

B-VI (Eno20 meV), LA150 (20 meVoEno150 meV, 1 meVoEpo150 meV), and

Bertini-Julich (En4150 meV, Ep4150 meV).

0.1 1 10 100 10001E-9

1E-7

1E-5

1E-3

0.1

10

Neutron Proton

Inte

nsity

(n/c

m2 /m

eV/p

roto

n)Energy (meV)

Fig. 11. Neutron and proton flux of the top of the vessel.

W. Yin et al. / Nuclear Instruments and Methods in Physics Research A 631 (2011) 105–110 109

heat energy cannot be ignored when we consider the total heat ofthe vessel and the hydrogen moderator, which must be removedvia a cryogenic system. After cooling for ten days, the vessel’sactivity was still above 1011 Bq, due to the long half-life of thenuclides 65Zn (243.6 days), 3H (12.33 years), and 22Na (2.602 years)(see Fig. 9).

4. Radiation damage of the vessel

To quantify the radiation damage to the vessel, we consideredthe following quantities: (i) radiation damage in terms of thenumber of displacements per atom (dpa) and (ii) the total heliumand hydrogen productions. We had calculated the maximum dpa ofthe vessel [10]. In this paper, we focused on the helium productionof the vessel because it would lead to swelling of the material.

The cross-sections of the displacement per atom and gas productionwere often different, due to the various physics models and codes. Inthis study, the physics model was used for the particles, whose energieswere higher than 150 meV. The LA150n/LA150h library was used forthe particles, whose energies were between 20 and 150 meV. Forneutrons with energy below 20 meV, we used the Evaluated NuclearData File (ENDF/B-VI). The same physics model used for neutrontransport was selected for proton transport. As recommended inRef. [11], the Bertini model, together with the appropriate level densityJulich, gave the closest agreement for aluminum in the heliumproduction calculation. Fig. 10 shows the helium production cross-section of aluminum.

We calculated the maximum helium production of the vessel. Thetop of the vessel (see Fig. 1) was the area most heavily exposed to thehigh-energy particle flux. Therefore, we focused on the radiation

damage there. Fig. 11 shows the neutron and proton fluxes of thetop of the vessel. By folding the neutron and proton fluxes into thedamage cross-sections, we calculated the helium production to be4.9 appm/5000 h/100 kW. Combining the previous damage results,which showed that the displacement rate of the top of the vessel was0.8 dpa/year [10], we found the ratio of the helium production to thedisplacement rate of the moderator vessel was about 6.1 appm He/dpa.

5. Conclusions

We compared the neutron fluxes for different dimensions of themoderator. For the moderator, we chose a cylindrical shape mod-erator with its diameter of 150 mm from the point of the peak andintegrated neutron intensities. Using light water as the coolantresulted in large losses of neutrons. This is because the hydrogenin the light water thermalizes some neutrons in the tungsten targetarea, and tungsten has a large capture cross-section for thermalneutrons. We calculated the decay power of the moderator vessel. Thedecay power was approximately 20% of the prompt heat energy,which should be considered when choosing a cryogenic system. Wealso calculated the helium production of the moderator vessel. Theresults showed that the maximum helium produced was 4.9 appm/year. These calculations were based on one 100 kW proton beam andone operation year consisting of 5000 h.

Acknowledgements

We would like to thank our colleagues in SNS and J-PARC forvery helpful suggestions for the CSNS target station neutronicsdesign. This work was supported by the National Basic ResearchProgram of China (973 Program; Grant no. 2010CB833102).

W. Yin et al. / Nuclear Instruments and Methods in Physics Research A 631 (2011) 105–110110

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