Reactor Coolants.

As Lead-Bismuth Eutectic or LBE is a eutectic alloy of lead (44.5%) and Bismuth (5 % Lead-Bismuth (55.5%)) It maybe used as a coolant in some nuclear reactors and is a proposed coolant. As it appears wise to accelerate the development of fast reactors. With the efficient reprocessing fuel technology. Here with some of these papers, as coolants are made secure its been proposed as inefficient technologies become hazardous and dated. These components are based on two types of small fast reactors, on and off systems. If you like non hazards safety these are here been discussed – the sodium-cooled fast reactor, which has already been built and can be further improved. The lead-cooled fast reactor that could be developed relatively soon. An accelerated development of the latter is possible due to the sizeable experience on lead-bismuth alloy coolant, as in some alpha-class submarine reactors. With the research efforts on accelerator driven systems in the EU and other countries. As a preference something that remains a fluid that doesn't boil on impact? Some forms of silica are in use with a new range of subatomic particles. Also some coolant maybe applicable for small casings such as in heat ex changers, these can be use as inherent gas, for example helium or ‘radon’ maybe a better alternative, here with an open discussion at a later date.

The first, comparative calculations on critical masses, fissile enrichment's, and burn-up swings of mid-sized SFRs and LFRs (600 MWe) are presented. Monte Carlo transport and burn-up codes were used in the analyses. Moreover, local Doppler, coolant temperature and axial fuel expansion reactivity coefficients were also evaluated with MCNP and subsequently used in the European Accident Code-2 to calculate reactivity transients and unprotected Loss-of flow accidents (ULOF). Further, unprotected Loss-of-Flow as well as decay heat removal (total Loss-of-Power, TLOP) were calculated with STAR-CD CFD code for both systems. The tight pin lattice SFRs (P/D=1.2) showed to have a better neutron economy than wide channel LFRs (P/D=1.8), resulting in larger BOL actinides inventories and lower burn-up swings for LFR. The reactivity burn-up swing of LFR self breeder could be limited to 3$ in 3 years. 

The calculations revealed that LFRs have an advantage over SFRs in coping with the investigated severe accident initiators (ULOF, TLOP). The reason is better natural circulation behaviour of LFR system and much higher boiling temperature of lead. An unprotected Loss-of flow accident in LFR leads to only a 250 K coolant outlet temperature increase whereas in SFR coolant would boil. Regarding the economics, the LFR seems to have an advantage since it does not require an intermediate coolant circuit. However, it was also proposed to avoid an intermediate coolant circuit in an SFR by using a super critical CO2 Brayton cycle.

Sodium has superior thermal hydraulic properties, allowing for tight pin lattices. There is a large (but not always positive) experience with operation of sodium-cooled fast reactors. While several power reactors have been shut down, BOR-60, JOYO, Phénix, and BN-600 are still operating, the latter being in quasi-commercial operation since 1982. New sodium-cooled reactors are under construction in Russia, China and India. Sodium features a reasonably low melting temperature, but also a low boiling point (1156 K), which raises safety concerns regarding unprotected transients leading to a coolant heat up. Sodium exhibits high chemical activity with water, water vapour and air - a limited sodium leak and fire has stopped the operation of the Japanese MONJU reactor since 1995. 

The choice of lead and lead-alloys as coolants is motivated on the one hand by their high boiling temperatures, which avoids the risk of coolant boiling. On the other hand, lead and lead-alloys are compatible with air, steam, CO2, and water, and, thus, no intermediate coolant loop is needed as in the sodium-cooled system. Lead-bismuth eutectic provides a low melting point (398 K) limiting problems with freezing in the system and features a low chemical activity with water and air excluding the possibility for fire or explosions. A drawback connected with lead/bismuth 'is the accumulated radioactivity in lead/bismuth' (mainly due to the α-emitter 210Po, T1/2 = 138 days), which could pose difficulties during fuel reloading or repair work on the primary circuit. However, IPPE Obninsk staff has developed methods to cope with the polonium during refuelling and maintenance.

Lead is considered as a more attractive coolant option than lead/bismuth mainly due to its higher availability, lower price and lower amount of induced polonium activity (by a factor of 104 ), as given in a publication about BREST- 300 LFR reactor design (Adamov, 2001). Pure lead has a melting temperature of 601 K, which narrows in the reactor’s operational interval to about 680-870 K. However, after more research, higher outlet temperatures will eventually be possible. Redundant electrical heaters are proposed to be introduced in order to avoid problems with freezing and blockages in fresh cores. Lead-alloy coolant velocities are limited by erosion concerns of protective oxide layers to about 2.5-3 m/s (Novikova, 1999). 

Typical sodium velocities are up to 10 m/s, hence lead has, in practise, a lower heat removal capacity, which require higher pin pitch-to-diameter ratios to stay below cladding temperature limits. However, as shown later in this paper these high pitch-to-diameter ratios enhance the natural circulation capability of the coolant, and thus, the safety performance of LFRs. Corrosion resistance of the structural material can be achieved through controlling oxygen content in lead or lead-alloy. This technology has been used in the alpha class submarines and its effectiveness up to 820 K has been confirmed by the EU ADS research. The surface alloying by the so-called GESA method enhances corrosion resistance of the structural material further, at least up to 870 K (Wider, 2003). It should also be noted that pure lead shows to be less corrosive than lead/bismuth eutectic at the same temperature (Wider, 2003). Fast creep of the reactor vessel during coolant heat-up transients is another important issue to be considered. It occurs significantly below the lead boiling point, ∼1170 K for SS- 316, 1250 K for NIMONIC alloys and possibly higher for ODS steels. These values refer to an 11 m tall vessel. although this reactor appears very safe the sump has its limitations. 

Neutronically, the lead and lead/bismuth energy loss due to the elastic scattering is significantly smaller than that for sodium. However, due to the presence of several thresholds for inelastic scattering in the energy interval from 0.57 to 2 MeV, the neutron energy loss in inelastic scattering is notably larger than for sodium. Therefore, the neutron spectrum of lead and lead/bismuth cooled reactors will be decreased for energies above 1 MeV. On the other hand, the magnitude of the neutron flux for sodium-cooled reactor is decreased in the energy interval of 0.7-1.5 MeV, where contributions to the neutron slowing down from elastic and inelastic scattering reactions are nearly equal. Additionally, the neutron mean free path in sodium is larger than that of lead or lead/bismuth. 

Therefore, the leakage of neutrons and their contribution to overall neutron balance in the system is more significant for sodium. Further, higher scattering in lead and lead/bismuth without increasing the moderation for neutrons below 0.5 MeV prevents the neutrons from escaping from the internal parts of the lead-alloy cooled cores and, at the same time, provide an excellent reflecting capability for the neutrons, which escape the core. Hence, we can also infer that the neutron economy of the lead-alloy cooled systems would be better than for sodium-cooled counterparts having the same geometry. E.g., lead-alloy cooled, (U,Pu)O2 fuelled systems require smaller plutonium enrichments than sodium counterparts to reach critical.