The choice of a coolant is defined by a number of practical considerations. Primarily, excellent heat transport properties are required, but, in addition, the coolant should have little chemical activity and be compatible with the containment material so that corrosion is minimized. It should be stable and not be subject to decomposition even under irradiated conditions; its melting temperature should be low to avoid having to preheat the system to obtain a liquid coolant, but its boiling temperature should be high to avoid having to impose high pressures to suppress boiling. In addition its nuclear properties should be such that it is a poor moderator, and its indu­ced radioactivity build-up should be low to avoid having excessive shielding around the primary system. Added to all these requirements is the final one, that the coolant should be inexpensive; at least its cost should be a small fraction of the total cost of the plant.

Table 4.1 lists coolant properties for 15 potential liquid-metal coolants for fast reactors together with three evaluation parameters with which the effectiveness and suitability of a coolant candidate may be judged (2).

(a) The pumping power. This factor is a measure of the efficiency of the coolant. For systems with the same geometry, the same heat output, and the same temperature rise in the coolant channel, the pumping power varies
as

{10ЛІО2С2/

In this expression, /л is the viscosity in lb/ft-hr, £ is the density in lb/ft3, and cv is the specific heat in Btu/lb-°F. The pumping power is the product of the coolant volume flow and the channel pressure drop which is propor­tional to the friction factor. The friction factor is proportional to Rё02.

A low value of the pumping power is required in order to avoid an econo­mic penalty in heat transport. Table 4.1 shows that lithium has the lowest pumping power factor followed by zinc and gallium. Sodium ranks fourth in this assessment.

(b) Induced activity. The buildup in induced reactivity increases the amount of shielding required for the primary system and it therefore should be kept as low as possible. The specific activity in Ci/gm is given by

s = — Jr £ ‘W1 — exp(-O.6930/0,)] (4.1)

where is the atomic number of the z’th coolant isotope; N0 is Avogadro’s number (0.60248-1024); 9у is the neutron flux in the y’th energy interval (?z/cm2-sec); ац is the microscopic cross section for the z’th isotope and the y’th energy interval (cm2/nucleus); Aw is the coolant atomic weight (gm/gm — mole); тг is the coolant residence time within the neutron flux for one coolant cycle (sec); r0 is the cycle time for the coolant (sec); 6 is the irradia­tion time (sec); is the half-life for the z’th isotope (sec); and К is 3.7-1010 disintegrations/Ci-sec.

The exponential term accounts for the loss of activity due to the fast decay. Note too that the two cycle times rr and r0 depend on the heat transfer properties.

Table 4.1 once again shows that lithium is the most suitable coolant based on this assessment, followed by lead and an alloy of lead and bismuth. Sodium ranks seventh in the list although the addition of potassium brings it into fourth position.

(c) A temperature range ratio. In order to obtain some quantitative assessment of the working range of the coolant the following ratio of tem­peratures provides a useful measure

(Tb — Tm)/Tw (4.2)

Tb, Tm, and Tw are, respectively, the boiling, melting, and outlet absolute temperatures of the coolant (°R).

Properties op Liquid Metals® Potassium Rubidium Sodium Tin Zinc Na(56%)- K(44%) Na(22%)- K(78%) Pb(44.5%)- Bi(55.5%) Sulfur Phosphorus 19 37 11 50 30 16 15 39.100 85.48 22.997 118.70 65.38 30.082 35.557 208.2 32.066 31 44.6 84.4 51.4 421 428 47.4 46.3 625.5 102.5d 84.9е 0.41 0.415 0.55 2.736 6.192 0.47 0.43 2.88 680** <1 0.0057c 0.0022^ 0.0101 0.0347 0.0535 0.0071c 0.0078c NA 0.00294 NA 48 60′ 29 55.54 35.22 61 71 129.85 173d NA 62 139 9 Neg. Neg. 26і 45і Neg. 310.2d 227.5e

146 102 208.1 449 787 66.2 12 257 246 111.5 1402 1270 1618 4118 1663 1518 1443 3038 832 536 0.182 0.0877 0.301 0.0639 0.1165 0.2485 0.209 0.035 0.248d NA 21.2 13.2 37 19.0 33.2 16.4 15.05 8.05 0.0952 NA 25.5 11.0 48.7 26.1 43.92 NA NA NA 25 8» 9.06» 853 363 1718 1031 755.1 NA NA NA 121.2 254» 0.0035 0.00276і 0.0044 0.0095 0.022 0.0071 0.0060 0.0125 1771 <1 2.41 2.5 2.5 2.6 6.9 2.5 2.5 0.0 NA NA

0.59 46.0 0.87 14.0 13.0 0.80 0.64 3.5 1.4*.* 3ft, i 4(3.7 MeV) NA <1 1 1 NA NA 0.58і NA NA 12.4 hr 19 days 15 hr 112 days None See K, Na See K, Na None None None 0.32, 1.1 2.775, 0.39 None NA NA None None None 1.51 0.0507 0.0234 1.368 0.0852 0.0169 0.0305 NA NA NA 0.0616 0.0637

S 3.66 S 390 SO.17 S 1.00 SO.13 S 0.60 S 0.80 See Pb, Bi S 0.08 S 0.09 304 SS 304 SS 304 SS Quartz Graphite 304 SS 304 SS Cr-Mo steel Graphite NA Slight1 Slight* Slight* SI. sol. SI. sol. Slight Slight SI. sol. Reacts Reacts Slight1 Slight* Slight* Alloys NA Slight Slight SI. sol. Reacts Reacts High Moderate High None None High High High Slight Moderate High High High Slight as dust Moderate High High Moderate as dust Slight High

A high value of this temperature ratio is desirable for a practical coolant, and Table 4.1 shows that tin, followed by gallium and an alloy of lead and bismuth head the merit table. This time lithium comes sixth while sodium comes eighth in the list. Other temperature factors could be used to assess working ranges but they give similar results.

Using these three evaluation parameters to choose a liquid-metal coolant would appear to lead to a choice of lithium followed by gallium.

However, lithium has a strong absorption cross section for fast neutrons. The main culprit is the eLi content and 7Li comprises 92.5% by weight of natural lithium, so the absorption can be reduced by enriching the lithium in its 7Li content. Lithium also has the disadvantage of having a low atomic number and it is therefore a strong moderator, three times as effective as sodium, which itself is a strong moderator. Strong moderation is undesirable because it degrades the spectrum and lowers the breeding ratio, and since breeding and neutron economy are most important to the success of the fast reactor, lithium cannot be used as a coolant. Gallium is far too expen­sive, even though the price could be expected to decrease if it were used in bulk.

Sodium is chosen as a coolant therefore, because it is a better-than — middle runner in all the assessment contests.