There is no operational experience with commercial lead-bismuth-cooled fast reactors in any country of the world. The Russian Federation is the only country that had used the technology of lead — bismuth eutectics coolant and produced and operated small marine propulsion reactors[28] with such coolant, gaining a cumulated 80 reactor-years experience of their operation in nuclear submarines. However, the lead-bismuth-cooled reactors in these Russian submarines were not fast reactors. A moderator (BeO) was used to soften the neutron spectrum.

A principal technical issue with the lead-bismuth eutectics is the corrosion of the fuel element claddings and structural materials in the coolant flow. Corrosion is temperature-dependent and, according to multiple studies performed worldwide [4.12], is easier to cope with at lower temperatures. In the Russian Federation the technology for reliable operation of stainless steel based structural materials in lead-bismuth eutectics was developed, allowing a reactor core continuous operation during seven to eight years within a moderate temperature range below ~500oC[29]. The technology includes chemical control of the coolant.

Another issue with the lead-bismuth eutectics is related to its relatively high melting point of 125oC, which requires continuous heating of the lead-bismuth coolant to prevent possible damage of the reactor internals due to coolant expansion in phase transition. In the Russian Federation they have developed and tested a safe freezing/unfreezing procedure for lead-bismuth cooled reactor cores based on the observance of a particular temperature-time curve.

One more issue with the lead-bismuth cooled reactors is related to the accumulation of volatile 210Po — a strong toxic alpha emitter. Polonium-210 is generated from 209Bi under irradiation and has a

half-life of about 138 days. In the Russian Federation, techniques to trap and remove 210Po have been developed. However, the presence of 210Po is by itself an incentive to consider complete factory fabrication and fuelling for a lead-bismuth cooled reactor.

Otherwise, lead-bismuth eutectics is chemically inert in air and water, has a very high boiling point of 1 670oC, a very high density and a large specific heat capacity which enable an effective heat removal. Also, owing to a freezing point of 125oC, lead-bismuth eutectics solidifies in ambient air contributing to the effective self-curing of cracks if they ever appear in the primary lead-bismuth coolant boundary.

For reasons mentioned above, a typical lead-bismuth cooled fast reactor design concept would be a two-circuit indirect cycle plant. Different from sodium, lead-bismuth cooled fast reactors do not use intermediate heat transport system.

The basic characteristics of the three lead-bismuth cooled SMR design concepts considered in this report are presented in Table 4.6[30]. Of the three SMRs, only the SVBR-100 has reached a degree of maturity with the detailed design development currently being in progress.

Table 4.6. Basic characteristics of advanced SMR designs — lead-bismuth cooled fast reactors

PASCAR

NUTRECK SNU, Republic of Korea [4.14]

All SMR designs are within 25-100 MWe range, with the New Hyperion Power Module being the minimum and SVBR-100 being the maximum. All designs are pool type reactors employing an indirect Rankine steam cycle for generating electricity. All designs are factory fabricated and fuelled reactors that are operated at very low, gravity defined primary pressures and are intended for 7-20 years of continuous operation without refuelling on site. Of the three, the Russian SVBR-100 has the shortest burn-up cycle duration of seven to eight years and does not rely on natural convection of the primary coolant in normal operation.

All lead-bismuth cooled fast SMRs are land-based reactors, although a barge-mounted option has been considered for the SVBR-100. Multi-module plant configurations are indicated for the SVBR-100 and the New Hyperion Power Module. For the SVBR-100, two concepts of such plants of a 400 MWe and a 1 600 MWe overall capacity have been elaborated at a design level [4.5].

The projected plants lifetimes are 50-60 years, and the targeted capacity factor is 95% or higher.

Owing to full factory fabrication and fuelling of the reactor modules, the targeted construction period is very short, 3.5 years for the SVBR-100 and 1.75 years for the New Hyperion Power Module.

When described, the reactor pressure vessels are compact, with the maximum dimension not exceeding 10 m, and in the case of the SVBR-100 — 7 m. External cooling of the reactor vessel by air is provided in the PASCAR, while the SVBR-100 and the New Hyperion Power Module are immersed in water pools. Safety implications of these and other safety design features of the lead — bismuth cooled SMRs are explained in Section 8.7.

The SVBR-100 and the New Hyperion Power module provide for a start-up fuel load based on the uranium of slightly less than 20% enrichment. PASCAR is being considered to operate with U-TRU fuel loads in a closed nuclear fuel cycle. The fuel burn-ups are reasonably high, 60-70 MWday/kg.