An important benefit of water and sodium coolants is their ability to scrub or retain fission products which would be released from the fuel and pass through these coolants in the event of a severe accident. This coolant characteristic would reduce the amount of fission products which might otherwise escape to the environment
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Table 1.6 Energy release reactions and fission product scrubbing in various coolants
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Water
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Helium
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Sodium1
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Lead/Lead — Bismuth
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Energy
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Zr-water/steam reactions:
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• Air reactions:
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Water reactions:
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Water reactions:
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release
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Zr(s) + 2H2O(I) ^
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C(s) + O2(g) ^ CO2(g)
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Na(l) + H2O(g) ^ NaOH(/) +
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Virtually no reaction
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ZrO2(s) + 2^(g) +
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393.15 kJ/(mol C)(798 K)
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&H2(g) + 160.1 kJ/(mol Na)
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with cold water or steam.
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537.8 kJ/(mol Zr)(500 K)
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C(s) + CO2 (g) ^ 2 CO(g) -171.4 kJ/(mol C) (798 K)
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(798 K)
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Air reaction:
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Zr(s) + 2H2O(g) ^
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Na(l) + NaOH(/) ^
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Results in Pb2O and then
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ZrO2(s) + 2H2(g) +
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CO(g) + Z2O2(g) ^
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Na2O(s) + Z2H2(g) +
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PbO. At the temperature
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583.6 kJ/(mol Zr)(1477 K)
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CO2(g)
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13.3 kJ/(mol Na)(798 K)
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of 450 °C the latter is
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+ 282.3 kJ/(mol CO)
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transformed to
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The hydrogen produced
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(798 K)
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The hydrogen produced
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Pb2O3, and then at
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can be oxidized as
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can be oxidized as
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450-470 °C to Pb3O4.
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H2(g) + ^(g) ^ HO(g) +
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All these unstable
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241.8 kJ/(mol H2)(298 K)
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Na(l) + Z2H2(g) ^NaH(s) +
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compositions dissociate
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57.3 kJ/(mol Na)(798 K)
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into PbO and O2
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Burning reaction, zone of small flames at the sodium-air interface.
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Air reaction:
Na2O oxide is produced which upon burning in air forms Na2O2. In the molten sodium only Na2O oxide is stable.
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Fission
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(1) Volatile FPs belonging to alkali
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None
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(1) Volatile FPs belonging to
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Same as sodium3
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product (FP)
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metals such as Cs, K, Rb will form
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alkali metals such as Cs, K,
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scrubbing in
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X-OH chemical compounds and will
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Rb have the same electronic
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primary
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remain in the water.
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structures as sodium (Na) atoms
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coolants2
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(2) Volatile FPs belonging to
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and dissolve in sodium, but they
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halogens such as I, Cl, Br will
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have very high vapour pressures
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dissolve in water in ionic form such
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and will evaporate with sodium
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as I(-1), Cl(-1), and Br(-1).
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during long accident times.
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(3) Non-volatile FPs such as Sr, Ba,
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(2) Volatile FPs belonging to
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Y, La, Zr, Nb, Mo, Tc and Rh do
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halogens such as I, Cl, Br will
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not dissolve significantly in water.
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form Na-X (NaI, NaCl, NaBr)
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(Sr and Ba will react with water to
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chemical compounds with
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form soluble oxides).
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sodium.
(3) Non-volatile FPs such as Sr, Ba, Y, La, Zr, Nb, Mo, Tc, and Rh do not dissolve significantly in sodium.
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1Endo et al. (1990).
2Pers. Comm. H. Endo (JNS Organization) to E. Baglietto (MIT), Jan 2013. 3Pers. Comm. G. Toshinsky (SSC IPPE) to N. Todreas (MIT), July 2013.
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if the containment were to be bypassed. As also detailed in Table 1.6, the various chemical-based fission products behave differently with regard to their retention in water and in sodium.
The conclusions which can be drawn are that relative scrubbing capabilities are (1) higher for water for alkali fission products, (2) higher for sodium for halogen fission products, (3) similar for non-volatile fission products, and (4) indeterminate due to lack of evidence for volatiles such as Sb and Te. Lead coolant behavior is similar to that of sodium. For gas reactors the coolant does not scrub fission products, but scrubbing occurs as plateout on cold surfaces.
