ORNL-TM-7207 - the Molten Salt Energy Technologies Web Site

74
because of the marked reactivity loss caused by admission of boron into
the fuel,
A small quantity of NaF-NaBF4 added to the DMSR fuel would allow
dissociation of the NaBF4 into NaF and BP3. The NaF would dissolve in
the fuel and remain as a minor parasitic neutron absorber. The BF3 is
relatively inssliub~e in the fuel92 993 and wopa~c~ be readily spargec~ with
the krypton and xenon into the off-gas system, A sufficiently small con-
tinuing leak could possibly be tolerated with some impairment in system
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performance. Given that the leaking tube could be plugged, infrequent
small leaks almost certainly ~ u l not d pose safety problems. Addition of
a sufficiently large quantity of WaF-MaBF4 could lead to formation of two
immiscible liquid phases.93
Such a leak (one capable of adding a few tens
of percents of coolant to fuel) seems incredible; the presence of the
large quantity of boron should certainly preclude reactivity accidents,
but the fuel would be ruined. ~eturning the fuel mixture to some secure
site for recovery would be necessary, and a most difficult cleanup and
repair of the reactor would be necessary, If possible.
Small leaks of coolant into the fuel system probably pose no safety
problems. However, additional study of the mixing of these fluids in
realistic geometries and in flowing systems is needed before we can be
certain that no potentially damaging situation eouEd arise as a conse-
quence of a sudden major failure of the heat exchanger. 20
The fluoroborate secondary coolant apparently will contain mall
quantities of oxygenated species and some species containing hydroxyl
ions.2o These would be capable of precipitating oxides from the fuel if
the coolant were mixed with fuel in large amounts, but the effects would
be trivial Compared With other effects noted previously. These SUbStanceS
in the secondary coolant, however, appear to have a nontrivial and bene-
ficial effect on DMSR perfomance.2 This beneficial effect is the appar-
ent ability of the secondary coolant to scavenge tritium and convert it
to a recoverable water-soluble fom.
As noted earlier, a DMSR must be expected to generate about I GBq/s
(2500 Ci/d) ~6 tritim, and most of this must be expected to diffuse
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However, the off-gas system would have to be designed to accomodate
the c~nsequences of BF3 admission.
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