Residual activity of a lead target after 1 year irradiation with a high power, 0.8GeV*30mA, proton beam is studied. It is concluded that the main radiotoxicity of irradiated lead is connected with bismuth isotope, Bi-207, which is produced in natural lead, mix of several stable isotopes, via (p,2n) reaction with Pb-208 nuclei. It is proposed to use, as a target material, lead enriched with another stable isotope, Pb-206, in order to reduce producing Bi-207 and Po-210. Estimation of charges for obtaining large quantities of lead-206 is also given.
KEYWORDS: Target – Radwaste – Lead Enrichment.
Presently, lead is considered as one of candidate materials for a target of the ADS. Lead is a low cost material, it is a good liquid coolant and proton-to-neutron converter. Meanwhile, when lead natural (Pb-nat) with isotopic composition: %, Pb-208/Pb-207/Pb-206/Pb-204 =52.35/22.08/24.14/1.42 is irradiated with fast protons the few hazardous long-lived nuclides are produced. The purpose of this paper is in discussing the low activation target material proposed firstly in the Ref. 1.
Calculation for lead target activity was performed using CASCADE/SNT code /2/. In these calculations the neutron flux was taken as 21015 n/cm2/sec, irradiation time was fixed at 1 year, proton and neutron spectra were corresponded to dissipation of a 0.8 GeV, 30 mA proton beam in a 0.5 m diameter and 1.0 m length lead target.
Many types of nuclides are produced in a such target due to spallation, fission, (n,), (n,xn) and other nuclear reactions. The alpha- and hard gamma-active long-lived nuclides are the most hazardous among them. A clearance level of 104 Bq/kg was established for activity of these radiotoxic isotopes. Materials with activity above this clearance level must be kept under radiation control. The alpha-active polonium isotopes, Po-210,209,208, and hard gamma emitting bismuth isotopes, Bi-208,207,206,205, are known as very hazardous and relatively long-lived radiowaste. Calculated total residual activity produced in a target of the full-scaled ADS is represented in Fig. 1. It can be seen that activity of the Pb-nat based target exceeds the clearance level by many orders of magnitude. The main contribution to this target radiotoxicity brings bismuth isotope, Bi-207 (Fig.2). Its activity is of 5109 Bq/kg during 100 years of decay time, its mean energy of gamma and X-rays is hard enough, about 1.5 MeV, and the equivalent dose rate is above the remote recycling level (10mSv/h) up to 103 years of decay time. It seems that in a hard proton spectrum the main channel for Bi-207 producing is the 208Pb(p,2n)207Bi reaction. This conclusion can be confirmed by calculation results given in Fig. 3. Activity induced by summarized proton and neutron interaction with nuclei and activity induced by neutron-nuclei interaction only are represented in this figure. These values of activity differ by two orders of magnitude indicating that the (p,2n) reaction is the main mechanism for Bi-207 production in the Pb-nat target.
It must be pointed out that several isotopes, products of nuclear reactions, can be exempted from radiation control. It concerns, for example, long-lived lead radioisotope, Pb-205, which is beta emitter with energy of particles less than 5 keV and consequently is released from control.
In order to reduce accumulation of Bi-207 in lead a new forthcoming technique of isotopic tailoring can be used. It is clear from Fig. 2 that activity of Bi-207 arising from lead isotope, Pb-206, used as a target, decreases to the level of 105 Bq/kg under the same irradiation conditions as from Pb-nat.
Correspondingly, activity of other heavy hazardous nuclides, such as Po-210, Po-209, Po-208, can be suppressed essentially when Pb-206 is used instead of Pb-nat in ADS targets. As it follows from Fig. 4, in this case production of radiotoxic alpha-active isotopes of polonium can be practically excluded.
Thus, although isotopic tailoring option requires tremendous technical efforts, it is still the attractive option which provides a low-activation target material for future ADSs. As follows from the analysis performed the maximum acceptable price of lead-206 should not be higher than 200 USD/kg, then the cost of such coolant should not exceed 20% of the blanket cost. In Ref.3 it is declared that the use of gas centrifuges makes it possible to produce the large quantities of isotopes for which a suitable process gas exists. In the RNC "Kurchatov Institute" several grams of Pb-206 enriched up to 95 percent were produced using a laboratory centrifuge cascade and tetrametyl of lead as a process gas.
Fig.1. Total activity of lead target irradiated with proton beam 0.8 GeV and 30 mA.
Fig.2. Residual activity of Bi-208, Bi-207, Bi-206 and Bi-205 produced in Pb-nat and Pb-206 after irradiation with proton beam Ep=0.8 GeV, Ip=30 mA, and Tirr=1 year.
Bi-207. Spallation spectrum.
Tirr=1 year, Ep=0.8 GeV, Ip=30 mA.
Fig.3. Activity of Bi-207 produced in Pb-nat and Pb-206 after irradiation with proton beam Ep=0.8 GeV, Ip=30 mA, and Tirr=1 year.
Fig.4. Residual activity of Po-210, Po-209 and Po-208 produced in Pb-nat and Pb-206 after irradiation with proton beam Ep=0.8 GeV, Ip=30 mA, and Tirr=1 year.
Accumulation of hazardous radionuclides, Bi-207, Bi-208, and Po-210, in natural lead to be used as a coolant in future fast reactors and accelerator driven reactors is predicted.
In accelerator driven systems a large portion of Bi-207 can be produced via Pb-208(p,2n)Bi-207 reaction in a target of natural lead (Pb-208/Pb-207/Pb-206/Pb-204=52.35/22.08/24.14/1.42 % ).
A new isotopically tailored coolant-converter for ADS consisting of lead isotope, Pb-206, is proposed. By using this material, it is possible to reduce essentially the production of the most radiotoxic isotopes of Bi and Po and to avoid disposing the large amounts of lead.
To provide the future fast reactors and accelerator driven systems with low-activation coolant – converter, the new technology of obtaining the large amounts of natural lead enriched with lead isotope, Pb-206, should be developed.
This investigation is performed at financial support of the Russian Foundation for Basic Research and Administration of Kaluga region (project # 01-02-96009).
1. G.L.Khorasanov et al. Lead and tin targets for reducing polonium waste. In: Proc. of the Int. Conf. on Accelerator Driven Technology and Applications ADTTA'99, June 7-11, 1999, Praha, Czech Republiñ. Proc. of Abstracts, p.46, (1999).
2. Yu.A.Korovin, A.Yu.Konobeyev, P.E.Pereslavtsev. The Code for Calculation of Isotope Concentration and Induced Activity of Irradiated Materials. Transactions of Higher Schools, Ser.: Nuclear Constants, 1992, No.3-4, p.117-121 (in Russian)
3.W.L.Roberts.Gas Centrifugation of Research Isotopes.NIM,1989,A282,p.271-279.