**2. Advantages of using lead enriched with lead-208 as coolant of FR and ADS**

### **2.1 Reducing of neutron absorption in cores of FRs and ADSs**

In Fig. 1 microscopic cross sections of radiation neutron capture, s(n, g), by the lead isotopes 204Pb, 206Pb, 207Pb, 208Pb and the natural mix of lead isotopes natPb in the ABBN-93 (Abagian-Bazaziants-Bondarenko-Nikolaev of 1993 year) system of 28 neutron energy groups [13] are given. The cross sections are cited on the basis of files of the evaluated nuclear data for the ENDF/B-VII.0 version.

As can be seen, the microscopic cross sections of radiation neutron capture by the lead isotope 208Pb for all of the 28 neutron energy groups of the ABBN-93 system are smaller than the cross sections of radiation neutron capture by the mix of lead isotopes natPb, and this difference is especially large, by 3-4 orders of magnitude, for intermediate and low energy neutrons, En <50 keV.

In Fig. 2 microscopic cross sections of radiation neutron capture, s(n, g), by the lead isotope 208Pb and the eutectic lead(45%) – bismuth (55%) in the ABBN-93 system of 28 neutron energy groups are given. The cross sections are cited on the basis of files of the evaluated nuclear data for the ENDF/B-VII.0 version.

The limiting factor of usage highly enriched 208Pb as the coolant is its high price in the world market. In the ISTC #2573 project [10], executed in the RF, the opportunity of creation of the plant for separation of lead isotopes using selective photoreactions was considered. The complex of calculations and theoretical works were carried out, the outline sketch of the separation installation was developed, and economic and technical estimations of industrial production of highly enriched 208Pb were made. Developers of the ISTC #2573 project expect that at the scale of manufacture equal to 150-300 kg of 208Pb per year its price will be of US \$200/kg [11]. But these theoretical predictions have not been confirmed experimentally yet. Presently lead isotopes are separated in gaseous centrifuges in using tetra methyl of lead Pb(CH3)4 as a working substance. According to estimations given in Ref. 12 the price of lead-208 with enrichment of 99.0% will be about 1000-2000 US \$/kg, which is relatively high for nuclear power plants. For comparison, another heavy metal coolant, Pb-Bi costs

Meanwhile, in nature besides lead of usual isotopic content: 1.48% Pb-204, 23.6% Pb-206, 22.6% Pb-207, 52.32% Pb-208, it can be found lead with higher enrichment of lead-208. Such type of lead can be found in ores and placers containing thorium. Lead-208 is a final product of decay the radioactive nucleus Th-232 and that is why such type of lead is called as radiogenic lead. The period of half decay of Th-232 nucleus is 1.41010 year. In ancient ores (~3109 year) the total content of thorium of 3-5 wt% is usual. In this case concentration of radiogenic lead reaches approximately 0.3 wt%. The enrichment of lead-208 in radiogenic lead is about 85-93%, depending on uranium content in ores and minerals. Uranium-238 produces in isotope mix the

As known, thorium containing ores and minerals can be found in India, Brazil, Australia, Ukraine, Russia and other countries. In Part 2 of this paper the possibility of reprocessing thorium containing ores and minerals for production of thorium-232 and lead-208 for

**2. Advantages of using lead enriched with lead-208 as coolant of FR and ADS** 

In Fig. 1 microscopic cross sections of radiation neutron capture, s(n, g), by the lead isotopes 204Pb, 206Pb, 207Pb, 208Pb and the natural mix of lead isotopes natPb in the ABBN-93 (Abagian-Bazaziants-Bondarenko-Nikolaev of 1993 year) system of 28 neutron energy groups [13] are given. The cross sections are cited on the basis of files of the evaluated nuclear data for the

As can be seen, the microscopic cross sections of radiation neutron capture by the lead isotope 208Pb for all of the 28 neutron energy groups of the ABBN-93 system are smaller than the cross sections of radiation neutron capture by the mix of lead isotopes natPb, and this difference is especially large, by 3-4 orders of magnitude, for intermediate and low energy

In Fig. 2 microscopic cross sections of radiation neutron capture, s(n, g), by the lead isotope 208Pb and the eutectic lead(45%) – bismuth (55%) in the ABBN-93 system of 28 neutron energy groups are given. The cross sections are cited on the basis of files of the evaluated

input of lead-206 which is product of uranium-238 radioactive decay.

**2.1 Reducing of neutron absorption in cores of FRs and ADSs** 

approximately 50 US \$/kg.

nuclear engineering needs is discussed.

nuclear data for the ENDF/B-VII.0 version.

ENDF/B-VII.0 version.

neutrons, En <50 keV.

Fig. 1. Microscopic cross sections of radiation neutron capture s(n,g) by stable lead isotopes and by natural mix of lead isotopes taken from the ENDF/B-VII.0 library. Cross sections are represented in the ABBN-93 system of 28 neutron energy groups.

Fig. 2. Microscopic cross sections of radiation neutron capture s(n, g) by stable lead-208 isotope and by the eutectic Pb-nat(45%) – Bi (55%) taken from the ENDF/B-VII.0 library. Cross sections are represented in the ABBN-93 system of 28 neutron energy groups.

As can be seen, the microscopic cross sections of radiation neutron capture by the lead isotope 208Pb for all of the 28 neutron energy groups of the ABBN-93 system are smaller than the cross sections of radiation neutron capture by mix of lead natPb (45%) and bismuth, Bi

New Coolant from Lead Enriched with the Isotope Lead-208 and

0

1,00

1,25

1,50

1,75

2,00

Neutron Flux Ratio

2,25

2,50

2,75

Yn- total flux of neutrons in core 1.

**lead-208** 

5

10

Neutron Flux, %

15

20

Possibility of Its Acquisition from Thorium Ores and Minerals for Nuclear Energy Needs 61

 Cell=5, Pb-208, Yn=3.09e15, Cell=5, Pb-Bi, Yn=3.01e15

10-4 10-3 10-2 10-1 100 101

Fig. 3. Neutron spectra for the core1(small enrichment of fuel) of the reactor RBEC-M cooled

Fig. 4. The ratio of neutron fluxes in the core 1 of RBEC-M given in linear scale.The core is

10-6 1x10-5 1x10-4 10-3 10-2 10-1 100 101

Neutron Energy (MeV)

**2.3 Increasing effective neutron multiplication factor in FRs and ADSs cooled with** 

In the reactor RBEC-M in replacement its standard coolant to lead natural its effective neutron multiplication factor, Kef, decreases from its standard value, Кef=1.0096, to the value

cooled with lead-208 leading to increasing the mean neutron energy on 6.5%.

with its standard Pb-Bi coolant (dash line) and Pb-208 coolant (solid line).

Neutron Energy (MeV)

Pb-208/Pb-Bi, cell=5

(55%), and this difference is especially large, by 3-5 orders of magnitude, for intermediate and low energy neutrons, En <50 keV.

Share of neutrons with energies less than 50 keV, En<50 keV, usually is about 20-25% of all neutrons in FR or ADS cores and it increases in lateral and topical blankets of the core.

In Table 1 one-group cross sections of neutron radiation capture by two types of coolants - Pb-208 or the eutectic of Pb-Bi – in the lead-bismuth fast reactor project named as RBEC-M and designed in the Russian Kurchatov Institute [14] are given.


Table 1. One-group cross sections of radiation neutron capture by various coolants in the fast reactor RBEC-M core consisted from core 1, 2, and 3. Data are given for the standard lead-bismuth coolant, as it has been designed at the Kurchatov Institute, and for lead-208 coolants, proposed by authors of this paper.

Cross sections in millibarns are given.

From Table1 follows that the coolant from lead-208 is characterized with minimum onegroup cross section, about <s>=0.93-0.94 millibarns. In standard lead-bismuth coolant the value of the same one-group cross section is by ~4 times bigger, about <s>=3.62-3.71 millibarns. In lateral and topical blankets one-group cross sections for Pb-208 by ~6-7 times are less than for Pb-Bi. The small values of one-group sections in RBEC-M cooled with lead-208 and corresponding excess of neutrons can be used for minimization of fuel load of the core, increasing fuel breeding and transmutation of long-lived fission products in lateral and topical blankets.

#### **2.2 Hardening of neutron spectra in FRs and ADSs cooled with lead-208**

In Fig.3 neutron spectra for the core 1(small enrichment of fuel) of the reactor RBEC-M cooled with its standard PB-Bi coolant and proposed Pb-208 coolant are given. Spectra were calculated for cases when their total fluxes were similar and the neutron multiplication factors were equal to 1 in using both of coolants. It can be seen that replacement of standard lead-bismuth coolant in RBEC-M leads to neutron hardening: the mean neutron energy increases from the value of 0.402 MeV to 0.428 NeV , i.e. on 6.5%.

In Fig.4 the ratio of neutron fluxes in the core 1 of RBEC-M in linear scale is represented. It is shown the increasing of share of fast neutrons (En>0.4 MeV) and the increasing of the very low share (less than 1%) of neutrons with energies En <100 eV in the core cooled with lead-208. In whole the mean neutron increases on 6.5% as it has been mentioned above.

(55%), and this difference is especially large, by 3-5 orders of magnitude, for intermediate

Share of neutrons with energies less than 50 keV, En<50 keV, usually is about 20-25% of all neutrons in FR or ADS cores and it increases in lateral and topical blankets of the core.

In Table 1 one-group cross sections of neutron radiation capture by two types of coolants - Pb-208 or the eutectic of Pb-Bi – in the lead-bismuth fast reactor project named as RBEC-M

**Pb-Bi 3.71190 3.62388 3.66404 4.82878 5.32383 5.22481 5.40967** 

**Pb-208 0.93296 0.94187 0.93931 0.86595 0.80867 0.81212 0.79005** 

Table 1. One-group cross sections of radiation neutron capture by various coolants in the fast reactor RBEC-M core consisted from core 1, 2, and 3. Data are given for the standard lead-bismuth coolant, as it has been designed at the Kurchatov Institute, and for lead-208

From Table1 follows that the coolant from lead-208 is characterized with minimum onegroup cross section, about <s>=0.93-0.94 millibarns. In standard lead-bismuth coolant the value of the same one-group cross section is by ~4 times bigger, about <s>=3.62-3.71 millibarns. In lateral and topical blankets one-group cross sections for Pb-208 by ~6-7 times are less than for Pb-Bi. The small values of one-group sections in RBEC-M cooled with lead-208 and corresponding excess of neutrons can be used for minimization of fuel load of the core, increasing fuel breeding and transmutation of long-lived fission products in lateral and

In Fig.3 neutron spectra for the core 1(small enrichment of fuel) of the reactor RBEC-M cooled with its standard PB-Bi coolant and proposed Pb-208 coolant are given. Spectra were calculated for cases when their total fluxes were similar and the neutron multiplication factors were equal to 1 in using both of coolants. It can be seen that replacement of standard lead-bismuth coolant in RBEC-M leads to neutron hardening: the mean neutron energy

In Fig.4 the ratio of neutron fluxes in the core 1 of RBEC-M in linear scale is represented. It is shown the increasing of share of fast neutrons (En>0.4 MeV) and the increasing of the very low share (less than 1%) of neutrons with energies En <100 eV in the core cooled with lead-

208. In whole the mean neutron increases on 6.5% as it has been mentioned above.

**2.2 Hardening of neutron spectra in FRs and ADSs cooled with lead-208** 

increases from the value of 0.402 MeV to 0.428 NeV , i.e. on 6.5%.

Lateral blanket

Topical blanket under core 1

Topical blanket under core 2

Topical blanket under core 3

Core 3 with large enrichment of fuel

and designed in the Russian Kurchatov Institute [14] are given.

Core 2 with middle enrichment of fuel

and low energy neutrons, En <50 keV.

Core 1 with small enrichment of fuel

coolants, proposed by authors of this paper. Cross sections in millibarns are given.

Reactor and its coolant

**RBEC-M,** 

**RBEC-M,** 

topical blankets.

Fig. 3. Neutron spectra for the core1(small enrichment of fuel) of the reactor RBEC-M cooled with its standard Pb-Bi coolant (dash line) and Pb-208 coolant (solid line). Yn- total flux of neutrons in core 1.

Fig. 4. The ratio of neutron fluxes in the core 1 of RBEC-M given in linear scale.The core is cooled with lead-208 leading to increasing the mean neutron energy on 6.5%.

#### **2.3 Increasing effective neutron multiplication factor in FRs and ADSs cooled with lead-208**

In the reactor RBEC-M in replacement its standard coolant to lead natural its effective neutron multiplication factor, Kef, decreases from its standard value, Кef=1.0096, to the value

New Coolant from Lead Enriched with the Isotope Lead-208 and

large enough.

Cross sections in barns are given.

0

20

40

60

Mass accumulation (gram/kg U-238)

cooled with lead natural.

80

100

cooled with lead-208.

given.

Possibility of Its Acquisition from Thorium Ores and Minerals for Nuclear Energy Needs 63

From Fig.5 it can be seen that at neutron energies near to En=5-10 eV these cross sections have maximum equal to 170 barns. That is why if the neutron spectra contains an increased share of neutrons of small and intermediate energies the corresponding one-group will be

In table 2 the one-group cross sections of radiation neutron capture by U-238 averaged over neutron spectra of the 80 MW ADS and various FRs (BREST, BN-600 and RBEC-M) are

ADS-80 MW th. Pb-208 **0.6393**  ADS-80 MW th. Pb-nat **0.4053**  BREST-300 MW el. Pb-nat **0.3089**  BN-600 MW el. Na-23 **0.2965**  RBEC-M -340 MW el. Pb-208 **0.1874**  RBEC-M-340 MW el. Pb-Bi **0.1886**  Table 2. One-group cross sections of radiation neutron capture by U-238 averaged over neutron spectra of the 80 MW ADS and various FRs (BREST, BN-600 and RBEC-M) cores.

It can be noted that one-group cross section of neutron capture by uranium-238 in ADS spectrum is by 2.15 times bigger than for sodium reactor BN-600 spectrum and this fact indicates to the possibility of enhancing the breeding gain in the blanket of ADS 80 MW

> Pu-239, ADS+Pb208, cell=3 Pu-239, ADS+Pbnat, cell=3

0 2 4 6 8 10

Irradiation time, years

Fig. 6. Mass accumulation of Pu-239 in the ADS 80 MW subcritical blanket in inserting 1 kg of U-238. in the cell 3, near the blanket's far margin. The solid curve corresponds to the case, when the blanket is cooled with lead-208, the dash curve – to the case, when the blanket is

Reactor Coolant One-group cross sections in barns

Кef=0.9815. But replacement of Pb-Bi to lead-208 leads to the value Кef=1.0246, i.e. Кef increases approximately on 1.5%. For reducing this increased value to the standard value, Кef=1.0096, the plutonium enrichment must be decreased from its initial value equal to 13.7% as designed in lead-bismuth RBEC-M project to the value equal to 13.0%. It means that initial plutonium fuel loading must be decreased from 3595 kg to 3380 kg, i.e. on 215 kg. Thus, it means that economy of plutonium will be of 650 kg per 1 GW electrical power in using lead-208 as coolant instead of lead-bismuth in RBEC-M type reactors. It can be noted, that this quantity of power grade plutonium is comparable with the annual value of plutonium quantity, about 1 tone, which is now obtaining after reprocessing the spent fuel of Russian NPPs – VVER-440 and BN-600.

In the ADS with subcritical blanket of 80 MW thermal power [5] Кef increases approximately on 1.7% in replacement lead natural as coolant to lead-208, from its value of Кef=0.95289 for lead natural to Кef=0.96997 for lead-208. In this case to liberate the nominal 80 MW thermal power in the blanket the power of the proton beam can be reduced from 2.59 MW to 1.68 MW, i.e. by 1.5 times.

#### **2.4 Increasing the fuel breeding gain in FRs and ADSs cooled with lead-208**

The excess of neutrons due to their small absorption in lead-208 can be used for fuel breeding and transmutation of long-lived radiotoxic fission products. Here, as an example, we assume the radiation capture of neutrons by uranium-238 leading to creation of plutonium-239. The affectivity of this process will be as large as the value of one-group cross section of radiation neutron capture by uranium-238 nucleus is large. In Fig.5 microscopic cross sections of radiation neutron capture by U-238 taken from ENDF/B-VII.0 library are given.

Fig. 5. Microscopic cross sections of radiation neutron capture by uranium-238 taken from ENDF/B-VII.0 library.

Кef=0.9815. But replacement of Pb-Bi to lead-208 leads to the value Кef=1.0246, i.e. Кef increases approximately on 1.5%. For reducing this increased value to the standard value, Кef=1.0096, the plutonium enrichment must be decreased from its initial value equal to 13.7% as designed in lead-bismuth RBEC-M project to the value equal to 13.0%. It means that initial plutonium fuel loading must be decreased from 3595 kg to 3380 kg, i.e. on 215 kg. Thus, it means that economy of plutonium will be of 650 kg per 1 GW electrical power in using lead-208 as coolant instead of lead-bismuth in RBEC-M type reactors. It can be noted, that this quantity of power grade plutonium is comparable with the annual value of plutonium quantity, about 1 tone, which is now obtaining after reprocessing the spent fuel

In the ADS with subcritical blanket of 80 MW thermal power [5] Кef increases approximately on 1.7% in replacement lead natural as coolant to lead-208, from its value of Кef=0.95289 for lead natural to Кef=0.96997 for lead-208. In this case to liberate the nominal 80 MW thermal power in the blanket the power of the proton beam can be reduced from 2.59 MW to 1.68

The excess of neutrons due to their small absorption in lead-208 can be used for fuel breeding and transmutation of long-lived radiotoxic fission products. Here, as an example, we assume the radiation capture of neutrons by uranium-238 leading to creation of plutonium-239. The affectivity of this process will be as large as the value of one-group cross section of radiation neutron capture by uranium-238 nucleus is large. In Fig.5 microscopic cross sections of radiation neutron capture by U-238 taken from ENDF/B-VII.0 library are

U-238(n,g)

10-2 10-1 10<sup>0</sup> 10<sup>1</sup> 10<sup>2</sup> 10<sup>3</sup> 10<sup>4</sup> 10<sup>5</sup> 10<sup>6</sup> 10<sup>7</sup>

Fig. 5. Microscopic cross sections of radiation neutron capture by uranium-238 taken from

En, eV

**2.4 Increasing the fuel breeding gain in FRs and ADSs cooled with lead-208** 

of Russian NPPs – VVER-440 and BN-600.

MW, i.e. by 1.5 times.

10-3

ENDF/B-VII.0 library.

10-2

10-1

10<sup>0</sup>

Cross-section, barn

10<sup>1</sup>

10<sup>2</sup>

given.

From Fig.5 it can be seen that at neutron energies near to En=5-10 eV these cross sections have maximum equal to 170 barns. That is why if the neutron spectra contains an increased share of neutrons of small and intermediate energies the corresponding one-group will be large enough.

In table 2 the one-group cross sections of radiation neutron capture by U-238 averaged over neutron spectra of the 80 MW ADS and various FRs (BREST, BN-600 and RBEC-M) are given.


Table 2. One-group cross sections of radiation neutron capture by U-238 averaged over neutron spectra of the 80 MW ADS and various FRs (BREST, BN-600 and RBEC-M) cores. Cross sections in barns are given.

It can be noted that one-group cross section of neutron capture by uranium-238 in ADS spectrum is by 2.15 times bigger than for sodium reactor BN-600 spectrum and this fact indicates to the possibility of enhancing the breeding gain in the blanket of ADS 80 MW cooled with lead-208.

Fig. 6. Mass accumulation of Pu-239 in the ADS 80 MW subcritical blanket in inserting 1 kg of U-238. in the cell 3, near the blanket's far margin. The solid curve corresponds to the case, when the blanket is cooled with lead-208, the dash curve – to the case, when the blanket is cooled with lead natural.

New Coolant from Lead Enriched with the Isotope Lead-208 and

Possibility of Its Acquisition from Thorium Ores and Minerals for Nuclear Energy Needs 65

As it is shown in Ref. 16, the main source of thorium in Russia is the Lovozerskoe deposit at Kola Peninsula. Estimations show that in reprocessing 2 mln tones of loparit ore per year 500-600 thousand tones of Ln2O3 and TiO2, 100 thousand tones of Nb2O5, 10 thousand tones of Ta2O5, 13 thousand tones of ThO2 and 65 tones of radiogenic lead can be produced. In Ref 16 the conclusion was made that is possible to extract in near future large quantities of

The separate problem is the level of lead-208 enrichment of lead-208 in various deposits. It can be strongly different. For example, in Brazil monazites radiogenic lead is enriched by lead-208 up to 88.34% [17]. For FRs and ADSs it can be desirable the following isotopic composition of radiogenic lead: lead-208-93% and lead-206-6% with minimum content of lead-207 – the isotope with large cross section of neutron capture. In Ref. 18 the data concerning thorium-containing ores and monazites in the world scale are given. The authors of this paper pointed out that as a rule radiogenic lead contains very small quantities of

It can be noted that the advantages of lead-208 can be used, besides nuclear power plants, in other branches of nuclear science and technology. It seems that lead-208 as low moderating material will be preferable in the lead slowing down neutron spectrometers [19] and also in the spallation neutron sources to have the harder neutron spectra under interaction of high

**3.2 Prospects of ancient monazite from placers and bed-rock's deposits of Ukraine as** 

Monazite is the phosphate containing mainly ceric rare earths ((Ce, La, Nd …, Тh) PO4) and is the main natural concentrator of thorium. It is widely spread (though usually in small amounts) in rocks and some types of ores. Owing to chemical and mechanical durability

The crystal structure of monazite can be presented as three-dimensional construction of oxygen nine apex polyhedron with rare-earth center atoms and oxygen tetrahedrons with the central atom of phosphorus. Nine-fold coordination allows a wide occurrence of relatively large ions of the light rare earths and thorium in mineral structure. The total content of thorium in a mineral can reach 28 wt%, and concentration of 5-7 wt% is usual. Though there are no experimental data about the form of radiogenic lead presented in the monazite structure, the numerous data, summarized for example in work [21], argued for its good stability in a monazite crystal matrix that allows monazite to be used for isotope

In Ukraine monazite contains in developed fine-grained titanium-zirconium placers. By the explored easily enriched titanium-zirconium ores Ukraine comes to the forefront in Europe and in the CIS. The resources of zirconium in Ukraine make more than 10% of world ones. Now the largest Malyshevsky (Samotkansky) placer is developed and the working off of the

Owing to the marked paramagnetism monazite at existing capacity of mines can be taken in passing by working out of placers in quantity of about 100 tons per year that corresponds

thorium from the progress of industry and as co-product of rear metal raw.

lead-204 and lead-207–isotopes with large cross sections of neutron capture.

energy protons with liquid proton target from lead-208 [2, 20].

**the raw materials to produce highly enriched 208Pb** 

monazite is accumulated in placers.

Volchansky placer has been started.

dating.

As an illustration, in Fig. 6 and 7 the results of burning 1 kg of uranium-238 placed in the one part of ADS 80 MW subcritical blanket (cell 3 near the blanket far margin) and corresponding accumulation of plutonium-239 are given. Calculations have been performed on the basis of code ACDAM [15] developed at the IPPE Centre of nuclear data.

Fig. 7. Mass burning of 1 kg of U-238 in the neutron spectra of 80 MW ADS blanket, in the cell 3 which is near far blanket's margin. The solid curve corresponds to the case, when the blanket is cooled with lead-208, the dash curve – to the case, when the blanket is cooled with lead natural.
