**1. Introduction**

56 Nuclear Reactors

Mercer, A. and Thompson.H., J. Br. Nucl. Energy Soc., 14, pp327-340 (Mercer, 1975)

Merzkirch,W., "Flow Visualization", Academic Press (Merzkirch, 1974)

Keulegan,G.H., U.S.N.B.S.Report 5831 (Keulegan, 1958) Kiso et.al.; JSME Annual MTG, pp.339-340 (Kiso, 1999)

In critical and subcritical fast reactors functional materials fulfill various tasks, including:


As such of materials molten heavy metals – mercury, lead, eutectic of lead (45%) and bismuth (55%) and others are using or to be used in future.

Heavy metals posses acceptable for FRs and ADSs neutron and physical characteristics while due to some their properties, for example chemical passivity to water, high boiling temperature, they are better as coolant in comparison to liquid light metal which is now used in sodium cooled FRs such as BN-600 and BOR-60 in Russia.

One of important parameters of functional material considered is a value of neutron absorption in coolant because it is desirable the neutron losses in the core of FR and ADS blanket have to be minimized.

Ways of minimization of neutron absorption in FR are well-known: it is offered to use wrapper less fuel assemblies, low neutron absorbing nitrogen isotope 15N in nitride fuel contents, structural materials with low cross section of neutron capture, etc.

The authors of this paper are pointing out on one more possibility of reducing the neutron losses in the core cooled with lead: it is connected with enrichment of lead isotope, lead-208, from its value in the natural lead isotope mix, equal to 52.3%, up to the value of 99.0% [1-9]. Lead-208 as a twice magic nucleus possesses a very low cross section of neutron radiation capture. This unique feature leads to economy of neutrons in the core and other profitable factors which are listed in the Part I of this paper.

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

10-1

10<sup>1</sup>

n,g mbarn

10<sup>3</sup>

Possibility of Its Acquisition from Thorium Ores and Minerals for Nuclear Energy Needs 59

Fig. 1. Microscopic cross sections of radiation neutron capture s(n,g) by stable lead isotopes

<sup>10</sup>-8 <sup>10</sup>-7 10-6 1x10-5 1x10-4 <sup>10</sup>-3 <sup>10</sup>-2 <sup>10</sup>-1 <sup>10</sup><sup>0</sup> <sup>101</sup> <sup>10</sup>-3

Neutron energy, MeV

Pb-208

Pb(45%)+Bi(55%)

 Pb-204 Pb-206 Pb-207 Pb-208 Pb-nat

<sup>10</sup>-8 <sup>10</sup>-7 <sup>10</sup>-6 <sup>10</sup>-5 <sup>10</sup>-4 <sup>10</sup>-3 <sup>10</sup>-2 <sup>10</sup>-1 <sup>10</sup><sup>0</sup> <sup>10</sup><sup>1</sup> <sup>10</sup>-3

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

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.

Neutron energy, MeV

Cross sections are represented in the ABBN-93 system of 28 neutron energy groups.

and by natural mix of lead isotopes taken from the ENDF/B-VII.0 library.

10-1

10<sup>1</sup>

n,g mbarn

10<sup>3</sup>

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 approximately 50 US \$/kg.

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

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 nuclear engineering needs is discussed.
