Reactor Water Chemistry
Fuel Element Failure
During operation of a nuclear reactor facility an equilibrium level of fission products is
established in the reactor coolant. These fission products are the result of trace impurities of
fuel material contained in the cladding surfaces as either natural impurities or a result of the fuel
fabrication process. The mechanism by which the fission products enter the coolant is normally
by fission recoil. Weld porosity is another potential path for the fuel, but generally quality
control prevents this from occurring. During normal facility operation, these fission products
are minor contributors to the overall radioactivity of the reactor coolant system.
If a defect were present or a failure of a fuel element occurred, large amounts of fission
products would potentially have a path to the coolant system. If this happened, significant
changes would occur within the reactor coolant chemistry parameters. Because most facilities
analyze for gross coolant radioactivity either continuously or periodically, the analysis would
be likely to detect all but the most minute failures.
When routine gaseous radioactive levels are monitored, an increase in this parameter's value
would be seen. This is because many of the fission products are gaseous, and these gases are
more mobile than particles of exposed fuel (the exposed fuel generally undergoes a process of
erosion that washes the fuel into the coolant stream). The other parameter that may change is
the ion exchange efficiency (where utilized), because many of the fission products released have
a lower affinity for the exchange sites on the resin beads than the exchange anion or cation.
Accordingly, the ion exchanger would not effectively remove these fission products and effluent
radioactivity levels would increase significantly. Fission gases would also pass on through the
ion exchanger and contribute to effluent activity. In addition, because some of the fission gases
have relatively short half-lives, the amount of time they are held up in the ion exchanger is
sufficient for some of these gases to decay to a radioactive solid.
These solid particles would contribute to effluent samples that may be concentrated prior to
analysis. Some facilities monitor for specific fission product inventories in the reactor coolant
to provide base level information. If a defect or failure were to occur, these levels would
obviously increase to indicate the failure.
Because the potential for elevated temperatures exists during most conditions of facility
operation, we will summarize the results from the resin in an ion exchanger overheating.
Module 4 addresses resin in great detail, and the actual resin breakdown will be included there.
Basically the resin of an ion exchanger is an inert polystyrene structure with ion exchange sites
"loosely" attached. The basic structure is stable up to fairly high temperatures (approximately
300 F), but the active exchange sites are not. There are two types of exchange sites: anion and
cation. The anion resin begins to decompose slowly at about 140 F, and the decomposition