NEUTRON SPECTRUM HARDENING IN CRITICAL

AND SUBCRITICAL REACTORS COOLED WITH

LEAD-208

G.L. Khorasanov, A.I. Blokhin

State Scientific Centre of the Russian

Federation – Institute for Physics and

Power Engineering named after

A.I. Leypunsky (IPPE)

Paper presented by Dr. Georgii KHORASANOV

Introduction

As known, the spent nuclear fuel of light water reactors

contains approximately 1% of plutonium, 0.1% of neptunium,

americium and curium and 4% of long-lived products of

fission (technetium, cesium and others). The spent nuclear

fuel contains radio nuclides which have to be isolated from

the environment during a period more than 1000 years.

Among these nuclides the most dangerous from the radio

toxicity point of view are plutonium and americium-241.

Meanwhile plutonium can be used as fuel for future FRs, as

concerns the low fissile americium-241 it must be incinerated

or transmuted into other short-lived radio nuclides at future

ADSs or FRs.

As known, the minimum of Am-241 fission in the hard part

of neutron spectrum is around 0.1 MeV. In the intermediate

and thermal parts of neutron spectra the Am-241 fission

cross sections are great enough but at the same time

radiation neutron capture are much greater that impacts to

transmutation of americium into high order actinides rather

than to its fission. Am-241 fission microscopic cross section:

10-2

10-1

100

101

102

103

104

105

106

107

10-2

10-1

100

101

Cro

ss-s

ecti

on

, b

arn

En, eV

Am-241(n,f)

Radiation neutron capture cross section for Am-241

It can be seen that below En=0.1 MeV the radiation neutron

capture cross sections are equal to 1 - 1000 barns. In the

same region the fission cross sections are equal to 0.02 - 10 barns.

10-2

10-1

100

101

102

103

104

105

106

107

10-4

10-3

10-2

10-1

100

101

102

103

104

Am-241(n,g)

Cro

ss-s

ecti

on

, b

arn

En, eV

Usually in ADSs and FRs the mean neutron energy of

core does not exceed 0.5 MeV, while the mean energy

of fission neutrons emitted by uranium-235, for

example, is equal to 1.98 MeV.

10-4

10-3

10-2

10-1

100

101

102

10-4

10-3

10-2

10-1

100

12

Fn

[a.u

.]

En [MeV]

One of the ways to enhance the neutron spectra hardening

consists in using core materials – coolant, structural

element, etc, - having small neutron moderation.

As such a coolant the molten lead enriched with lead stable

isotope – lead-208 – was proposed by authors.

In the paper the possibility of neutron spectra hardening with

the aim to enhance Am-241 fission probability is analyzed.

Blanket of the ADS with thermal power of 80 MW designed by

authors, core and lateral blanket (LB) of the FR RBEC-M

with thermal power 900 MW designed at the National

Research Centre “Kurchatov Institute” are assumed.

Method of calculations

It was calculated the neutron spectra of the 80 MW ADS

blanket and of the reactor RBEC-M core and LB, and

then on the basis of spectra obtained the mean energies

of neutrons, one-group Am-241 fission and radiation

neutron capture cross sections were found.

For hardening neutron spectra a coolant from Pb-208

instead of Pb-nat in the 80 MW ADS and instead of Pb-Bi

in the RBEC-M were assumed.

Code MCNP5 and input data for RBEC-M were used

for determining the corresponding neutron spectra. Mean

energies of neutrons were calculated due to expression:

<Еn>=∑Еnφn/∑φn, were Еn – is the mean neutron energy in

the group g (number of groups g=28) of the ABBN-93

system, φn – is neutron fluxes into the group g,

summation ∑ is made with respect all groups where

neutron fluxes distinguishes from zero, practically.

Similarly Am-241 one-group fission and radiation neutron

capture cross sections, <σfis> and <σс> , were calculated.

The evaluated files of the library ENDF/B-VII.0 were used

for determining microscopic cross sections of Am-241

fission and radiation neutron capture.

Calculations and discussion

In Table 1 the mean energies of neutrons and probability

of Am-241 fission along the subcritical blanket (H=110

cm, outer D=124 cm) of the ADS with thermal power of

80 MW are given.

Blanket was homogeneously supplied with uranium-

plutonium nitride fuel in which the plutonium enrichment

was equal to 15%. Pb-208 and Pb-nat were used as

coolants.

In replacement of Pb-208 with Pb-nat coolant the

effective neutron multiplication factor Kef was

decreased approximately to 2%.

Subzone 1

En=0.4254/0.4406

Fis=13.9982/11.3604

Subzone 2

En=0.4438/0.3408

Fis=13.6897/9.8404

Subzone 3

En=0.3346/0.2362

Fis=6.2326/3.8327

Target-

source of neutrons

Subzone 4

En=0.4820/0.5576

Fis=18.9701/21.5634

Subzone 5

En=0.5377/0.4929

Fis=21.7887/17.6977

Subzone 6

En=0.3723/0.3754

Fis=9.1580/11.9034

Subzone 7

En=0.3365/0.2554

Fis=8.6446/8.3228

Subzone 8

En=0.3731/0.3811

Fis=11.2360/12.4424

Subzone 9

En=0.3182/0.3285

Fis=6.0852/6.7810

The mean neutron energy averaged over the blanket is

equal to 0.4026 MeV in using Pb-208 as coolant and

0.3787 MeV in using natural lead, Pb-nat.

Thus, the coolant replacement leads to neutron

spectrum hardening on 6.3%.

Correspondingly on 5.8% increases the averaged over

the blanket probability of Am-241 fission which is

determined as the ratio Fis=<σfis>/(<σfis>+<σс>), where

<σfis> - is one-group fission cross section and <σс> - is

one-group radiation neutron capture cross section.

Probability of Am-241 fission in the central parts of

blanket reaches 22% while at the periphery of blanket it

falls down to 6%.

Along 80 MW ADS blanket one-group Am-241 fission cross

sections are of 0.1170–0.3724 barns while maximum cross

section reaches at the central parts of blanket and

minimum cross section falls down at the periphery of

blanket . This dependence is in a good correlation with the

dependence of mean neutron energy along the blanket.

In the reactor RBEC-M the replacement of its standard lead-

bismuth coolant with Pb-208 coolant leads to hardening of

neutron spectra of sub cores and lateral blanket on 6.4%

and 6.1%, respectively.

In the next slide the neutron and physical parameters of the

900 MW thermal reactor RBEC-M are given. Reactor is

cooled by its standard coolant from Pb-Bi (thin lettering)

and by coolant from Pb-208 (bold lettering) as it was

proposed by authors of this presentation.

Parameters Inner core Middle core Outer core Lateral

blanket

Mean energy

<En>, МeV,0.4246/0.3992 0.4408/0.4209 0.4433/0.4307 0.2662/0.2509

Relative increasing

of <En>, %6.3627 4.7280 2.8790 6.0980

Coolant volume

share, %62.5 57.3 44.6 56.5

Fuel volume

share,%

23.3 27.6 38.2 -

Fuel plutonium

enrichment, %

13.59 13.59 13.59 -

Am-241 cross

section <σfis>, barns0.2882/0.2629 0.2975/0.2779 0.2950/0.2829 0.1671/0.1521

Relative increasing

of <σfis>, %9.6234 7.0529 4.2771 9.8619

Am-241 capture

cross section <σс>,

barns

1.5816/1.5967 1.5306/1.5366 1.5632/1.5627 2.4249/2.4958

Probability of Am-

241 fission, %15.4134/14.1374 16.2737/15.3155 15.8755/15.3283 6.4468/5.7442

For comparison: Am-241 fission probability in the

spectrum of Pu-239 fission neutrons:

The mean energy of neutrons for Pu-239 spectrum of

fission neutrons: <En>= 2.0345 MeV

One-group Am-241 fission cross section in this spectrum

of neutrons: <σfis> = 1.3676 barns

One-group Am-241 radiation neutron capture cross

section in this spectrum of neutrons: <σc> = 0.2736 barns

Probability of Am-241 fission: Fis= <σfis>/<σfis>+ <σc>=

83.3%

Conclusion

It might be concluded that replacement of lead or lead-

bismuth coolant with lead-208 coolant in installations with

fast neutrons leads to neutron spectrum hardening up to

6.3-6.4%. Under these conditions one-group Am-241

fission cross sections are increasing on 8-10%.

It was shown that in the ADS annular blanket (H=110 cm,

outer D=124 cm) the Am-241 fission probability reaches

22% in the central parts of the blanket while at its

periphery it falls down to 6%.

The probability of Am-241 fission in the lateral blanket of the

fast reactor RBEC-M does not exceed 5.7-6.4%, while in the

sub cores of this reactor the probability of Am-241 fission is

dramatically higher, 15-16%.

It might be mentioned once more that Am-241 fission in

relatively hard neutron spectra is more preferable than its

transmutation via neutron capture in the intermediate and

thermal neutron spectra which leads to accumulation

of curium and californium.

To pick up the 80 MW ADS blanket heat by means of the

Pb-208 coolant about 60 tones (~6 m3) of this lead isotope are

required.

To pick up the 900 MW RBEC-M core heat about 680 tones

(~70 m3) of this lead isotope are required.

At last I would like to announce a new book titled:

“Application of stable lead isotope Pb-208 in nuclear power

engineering and its acquisition techniques”.

Editor G.L. Khorasanov. New-York: NOVA publishers, 2013,

194 p., (ISBN: 978-1-62417-653-1).

Table of Contents:

Preface pp. i-x

Chapter 1. Some Advantages in Using Lead-208 as Coolant For Fast Reactors and Accelerator Driven Systems

(Georgy L. Khorasanov and Anatoly I. Blokhin, State Scientific Center of the Russian Federation – Institute for Physics and

Power Engineering named after A.I. Leypunsky (SSC IPPE), Obninsk, Russian Federation)pp. 1-20

Chapter 2. Introductions of 208Pb Coolant to Innovative Fast Reactors

(Hiroshi Sekimoto, Tokyo Institute of Technology, Tokyo, Japan)pp. 21-42

Chapter 3. Radiogenic Lead with Dominant Content of 208Pb: New Coolant, Neutron Moderator and Reflector for Innovative

Nuclear Facilities

(A.N. Shmelev, G.G. Kulikov, V.A. Apse, A.A. Chekin and E.G. Kulikov, National Research Nuclear University “MEPhI”, Moscow,

Russia)pp. 43-98

Chapter 4. Photochemical Laser Separation of Lead Isotopes for Safe Nuclear Power Reactors

(P.A. Bokhan, N.V. Fateev, V.A. Kim and Dm. E. Zakrevsky, A.V. Rzhanov Institute of Semiconductor Physics, Siberian Branch of

the Russian Academy of Sciences, Novosibirsk, Russia)pp. 99-124

Chapter 5. Assessment of Specific Cost of Highly Enriched Lead-208 Isotope by Gas Centrifuges Using Various Raw Materials

(V.D. Borisevich, A. Yu. Smirnov and G.A. Sulaberidze, National Research Nuclear University "MEPhI", Moscow, Russia)pp.

125-136

Chapter 6. Method for Obtaining Isotopically Enriched Metal Lead from Monoisotopic Tetramethyllead and its Purification

(Dmitry V. Akimov, Oleg S. Andrienko, Nikolay B. Egorov, Ivan I. Zherin, Denis V. Indyk and Mishik A. Kazaryan, Tomsk

Polytechnic University, Tomsk, Russian Federation and others)pp. 137-176

Series:

Nuclear Materials and Disaster Research

Binding: Hardcover

Pub. Date: 2013- February

Pages: 194, 6x9 - (NBC-C)

ISBN: 978-1-62417-653-1

Status: AV

THANK YOU FOR YOUR ATTENTION!

References

• D.A. Blokhin, E.F. Mitenkova, G.L. Khorasanov, E.A. Zemskov, A.I.

Blokhin. Evolution of fast reactor core spectra in changing a heavy liquid

metal coolant by molten Pb-208. In CD-ROM Proceedings of the

International Conference PHYSOR 2012 – Advances in Reactor Physics –

Linking Research, Industry, and Education, Knoxville, Tennessee, USA,

April 15-20, 2012, Paper #219.

• G.L. Khorasanov, A.I. Blokhin. One-group fission cross sections for

plutonium and minor actinides inserted in calculated neutron spectra of fast

reactor cooled with lead-208 or lead-bismuth eutectic. In CD-ROM

Proceedings of the International Conference PHYSOR 2012 – Advances in

Reactor Physics – Linking Research, Industry, and Education, Knoxville,

Tennessee, USA, April 15-20, 2012, Paper #106.

• D.A. Blokhin, E.F. Mitenkova, A.I. Blokhin. Generation of point wise

nuclear data library on the basis of ENDF/B-VII.0, JEFF-3.1.1, JENDL-4.0.

Preprint IBRAE-2011-08, 2011, Nuclear Safety Institute of RAS, Moscow.

The advantages of the photochemical method of lead

isotope separation consist in one- or two- photon

excitation of atoms, a possibility of using commercially

available highly effective semiconductor lasers and high

efficiency of lead isotope separation in a reaction chamber.

The technique of selective photoreactions makes use of

such working substance as lead vapor and does not require

conversion of lead into a volatile substance and its back

transformation into the target product which is the case with

separating gas centrifuges.

All this gives good ground to expect in the years to

come the production of 208Pb (with 99.0%

enrichment) in large quantities (tones) for

acceptable price, about $200/kg.