Sodium
Coolant Chemistry -
Experience, Impressions and Perspectives
G. Periaswami, MCD
Given
below is an account of my impressions and observations on selected issues in the
field of sodium chemistry gained through my personal work over a period of three
decades. These are only personal views and not the final word as they are
likely to be subjective.
A
small sodium bath that holds about 1.5 kg of sodium in liquid state
(150oC) was kept inside the box to indicate the purity. A shining surface
staying for about half an our without coverage by oxide layer was assumed to
indicate the sufficient purity. This sytem is currently in use at RCL.
A further advancement of this design would be the inclusion of an automated
regeneration cycle for the purification columns.
The
vacuum distillation process developed as the alternative where the sample was
heated using an RF induction ws found to be more reliable and convenient.
The RF generator used should be a medium frequency (500 kHz) one and not a
high frequency unit (2MHz) as otherwise the sodium vapour will ionize and flash.
The glass outer tube should be coated with sodium before taking the sample for
distillation. This will give a lower blank as contamination from surface sorbed
gases is minimized. Similarly the end point identification is better done
with a thermocouple from top dipping into molten sodium. Otherwise there
is a possibility of over heating and the sodium oxide residue decomposing.
The system currently in use at RCL has a thermocouple inserted from bottom.
This has the advantage of a lower contamination from outside as the O-ring seal
is avoided. But the end point detection should be done carefully.
4.Sodium
Sampling
Taking
a representative sodium sample from flowing sodium is very important. This
is because most of the impurities dissolved in sodium have a highly temperature
dependent solubility and can segregate very rapidly to the sampler wall and
deposit in an irreversible manner while cooling the hot sodium. (However, this
is not true for particulate carbon). This requires that the entire sample
held in a container should be analysed for getting the real value for oxygen and
dissolved trace metals. Among the most commonly used sampling methods,
only the overflow sampler meets the above requirement. It is
possible to use crucibles of different materials for getting samples for
different impurities e.g. nickel crucible for oxygen, quartz for carbon and
titanium for trace metals. The entire sample can be taken for an anlysis
thus avoiding problem of segregation. But the process for sampling using
this device is a bit tedious. Sodium flushing has to be done carefully and
the procedure has to be followed scrupulously, which is a big problem at the
operator level. Taking overflow samples from secondary loop of FBTR was a
challenge. For reasons not easily understood, there has been one problem
or the other in most occasions in FBTR
On
the otherhand, the tube sampler used in French reactors and in FBTR are
easy to operate. These samples are reliable only for analysis for
particulate carbon. For oxygen and trace metals there is possibility of
segregation and loss. However, the French are using only tube samples till
todate.
A
low volume and compact overflow sampler was designed at RCL and put to use in
FBTR secondary and EDG loops. PFBR is supposed to have overflow samplers.
5.Trace
metal analysis
Analysis
of FBTR samples for trace metals using AAS and ICPMS have given values within
specifications except for iron. The results for iron used to give
occasional high values. This is easily explained on the basis of
particulates circulating with sodium mostly in the form of gamma iron. This has
been observed in other reactors also.
6.Oxygen
Monitors
Monitoring for oxygen in sodium will help detect air leaks into primary system and steam leaks in secondary system. This will avoid incidents similar to the airleak that occurred into primary covergas in Super Phoenix which went undetected because there was no oxygen sensor in that reactor. These sensors use an oxide ion conducting solid electrolytes configured into a galvanic cell where sodium with dissolved oxygen forms the sample electrode. Owing to electronic conductivity consideration and compatibility with sodium at 400oC, Yttria Doped Thoria (YDT) happens to be the most suitable electrolyte for this purpose. YDT procured from Zircoa in the form of 6mm dia and 120mm long tubes were used as the first oxygen sensors built at RCL. The frequent breakage of these tubes at the freeze seal owing to the presence of high temperature gradients rendered this configuration unsuitable. Then YDT thimbles brazed to a low expansion alloy (Dilasil) obtained from GE were used and successfully tested in loops. They also exhibited high breakage tendencies due to the strain generated at the braze seal. More over YDT sensors are costly ($5000/piece) and are not available in the open market. This made us look for alternatives for YDT. Calcia stabilized zirconia (CSZ) earlier tried in US had been given up because CSZ reacts with sodium at 400oC and exhibits electronic conductivity on contact with sodium. At RCL, we felt that both these problems could be over come if CSZ is operated at lower temperatures. LEDB of CSZ were measured down to 200oC for the first time employing a method developed at RCL (Na/Mg electrode). The LEDB thus obtained showed that CSZ can be used for oxygen sensing in sodium if operated below 250oC. Sensors were constructed using CSZ tubes and K, K2O reference electrode and tested in benchtop sodium loops. This experiment showed that CSZ works well as oxygen sensor though with the short life times (~3 months). After this period the emf started increasing because of N2ZrO3 formation at the electrolyte surface.
This was sought to be overcome by converting the surface into CaZrO3. This was done by reacting it with CaO slurry coated over it at 1400oC for 8 hours. The secret was the addition of trace quantities of Li2CO3 to CaCO3 as otherwise the coating generated had high resistance. The coated zirconia tubes were assembled into meters with both K, K2O reference electrode and In, In2O3 reference electrode. In, In2O3 electrode required a soaking at 500oC for a while before assembling the sensor. These sensors were tested in benchtop loop. They gave a higher emf value but with a theoretical slope for cold trap temperature variation. And they gave a better long term stability also. These sensors can be used for oxygen sensing in sodium but individual sensors have to be calibrated. The emf measuring system should have high input impedance as the cell resistnace is high. The high zero error was attributed to the asymmetric nature of the cell. There is a nagging doubt whether it is due to protonic conductivity of CaZrO3. The only way to test this hypothesis was to vary the hydrogen concentration without charging oxygen valves. We could not figure out an easy experiment to verify it.
The above are only arbitrary designs, not practically very sound. The most successful designs world wide are the GE design based on YDT cup brazed to metal with In, In2O3 reference and the glass soldered YDT pellet design from Rossendorf, Germany. The development of the isopressed YDT was taken up in collaboration with CGCRI in the X plan. YDT powder was produced at RCL based on a modified Pechini process where the combustion of the citrate is done in a controlled way. The sintering process conditions were also standardized at RCL. Isopressing the powder into crucibles as well as the final sintering are being done at CGCRI.
Yet
the most promising design for YDT based sensor should be based on YDT rod.
The rod is easy to isopress and sinter. This can also be easily glass
soldered to Al2O3. If an air reference electrode with RuO2 contact
can be used, this sensor would give a stable emf though not theoretical.
This is similar to the Westinghouse design without the prolems of cracking.
This deserves an examination in XI plan period.
7.Oxygen
in sodium
Oxygen
is present in sodium as its oxide. The
saturation solubility of oxygen in sodium has been well established and
the most reliable equations are that of 1) Noden and 2) Eichelbergor.
The oxygen dissolved in sodium is assumed to obey Henry’s Law as it is
a dilute solution. i.e ai = giCi. This has
not been experimentally confirmed till recetly. The availability of an
oxygen meter which can operate at 230oC and the possibility of changing the cold
trap from 120 to 2300 C provided us with an easy means of verifying this.
The activity can be obtained from the meter output and the concentration from
the cold trap temperature. The activity thus measured was expected to be
linear with respect to the oxygen concentration if Henry’s Law was obeyed.
But in actual measurements, the activity was found to deviate negatively
from Henry’s Law. This was the first such measurement ever
reported in literature. A formula for activity of oxygen in sodium with
oxygen concentrationwith pure oxygen as the standard state was worked out.
This has a significant influence on the calculated threshold oxygen
concentrations for the formation of ternary oxides in sodium. A Russian
model (Heterophase model) which gives the activity of oxygen “a” as equal to
(X/Xs)2 where X in the concentration and XS in the solubility was found to be
more accurate in giving the oxygen activity which is inagreement with our
measurements.
8.Sensor
for Hydrogen in Sodium
Monitoring
hydrogen in sodium is very crucial for detecting steam generator leaks. The most
used device for this purpose is the ion pump based hydrogen detector. This uses
a nickel membrane at about 450oC with one side exposed to sodium and the
other side to the high vacuum generated by an ion pump. The ion-current of
the ion pump is proportional to the hudrogen flux which in turn can be
correlated to the hydrogen concentration in sodium. This is a device with quick
response time and very good sensitivity. But there is a problem with
respect to the ion pump. Special pumps with titanium anodes have to be
used as otherwise the activation of the anode by the hydrogen pumping can lead
to oscillation of the pump capacity and the ion-current. Moreover the task
of measuring a small current across high voltages can lead to discharge like
peaks which makes them unreliable. Even the gasket leaks can
cause change in current and so frequent calibration becomes necessary.
The
system used at FBTR makes use of a mass spectrometer to measure the dynamic
pressure in addition to the ion-pump current. This takes care of all the
problems mentioned earlier. But the whole system becomes a bit complicated
and costly with the addition of MS. There were many operational problems
at FBTR with this kind of system.
As
an alternative, a galvanic cell based hydrogen sensor was developed at RCL.
This was based on a sensor reported by CG Smith of Berkelay Labs,
The
sensor is configured such that the electrolyte is held in an iron cup into which
another iron thimble containing Li, LiH is inserted. The electrolyte is
melted at ~800oC and the inner electrode is lowered into it. The
electrolyte is then allowed to freeze. Because of the near zero volume
change during freezing the electrolyte solidifies without leaving a void.
The most important precaution to be taken is that the electrodes are
electrically shorted while freezing. Otherwise the electrolyte will
solidify which an electrochemical gradient leading to a large zero error and
drift. The diaphragm thickness is also kept. uniform at ~0.5 mm to get a
quick response. The meter was constructed and tested in table top
sodium loops first. Only the meters that gave stable and near
theoretical values were taken for further use. Others were rejected.
About
four such meters were incorporated in the reactor, FBTR. The first meter
incorporated in the east loop performed very well for well over two years always
giving immediate and reproducible response to cold trap changes and hydrogen
injections. The rest of the meters gave good outputs initially but drifted
and became sluggish in response in a few months of operation. As a result more
meters were constructed and tested in the lab and then placed in the reactor.
All these meters had varying outputs after some time though they gave good emf
values initially. The various reasons attributed to this were:
1)
Formation of carbide in CaCl2 from carbon from iron and increased ionic
conductivity.
2)
Diffusion of lithium through the iron thimble.
3)
Dissolution of iron into the electrolyte while casting the electrolyte while
casting the electrolyte at ~800oC.
4)
Introduction of oxygen into electrolyte
during fabrication.
In order to reduce the possibility of carbide formation precautions were taken by decarburizing the steel by hydrogen firing and then also by equilibrating in sodium containing calcium.
The
lithium problem was sought to be overcame by taking a mixture MgO and CaH2 as
the reference electrode. This gives a slightly higher hydrogen pressure
compared to Li, LiH and it is an original idea from Dr. R. Sridharan.
The
third problem of dissolved iron in CaCl2 was tackled by reducing the casting
temperature from 800oC to 700oC by taking a mixture of CaCl2-LiCl which has a
lower eutectic temperature. The alternative route of reducing the
temperature as well as increasing the conductivity was tacked by taking
CaBr2-CaH2 as the electrolyte which has a lower eutectic point and better
conductivity.
Finally
the problem was identified to be due to the oxygen contamination of the
electrolyte while inserting the reference. The development of a reference
electrode introduction tool which helps move the reference electrode smoothly
without loosening the teflon gland much. Only after using this device, the
meters constructed were found to give consistently good results. To arrive
at this state, nearly a hundred meters had to be constructed with
different variables.
It
is obvious that the present design having a Teflon gland for insulating
the reference and sample electrodes needs improvement. A better choice
would be a ceramic to metal seal for separating the reference electrode from the
body. The reference capsule should have a conical bottom to help insert
the reference into the electrolyte powder before welding the ceramic to metal
seal. This also demands that the sensor electrode also should have a conical
bottom to keep the electrolyte thickness uniform. With all these
modifications ECHM would have reached a stage where it can be used in PFBR.
CaCl2-CaH2 still remains the best electrolyte because of its zero volume change
during freezing.
9.Carbon
Meter
The
electro chemical carbon meter (ECCM) for monitoring carbon in sodium is also a
galvanic cell with a eutectic liquid of Li2CO3-Na2O3 as the electrolyte and pure
graphite as reference electrode. This is based on a sensor developed by Hobdell
of CEGB. The improvement made here is in the reference electrode. Since
the graphite used in CEGB meter disintegrates with time and shortens the
cell, a nickel sleeve was introduced in the carbon meter developed by Rajendran
Pillai. This also had long term stability problems. Since the nickel
got decarburized much faster than the graphite dissolving into it.
This problem was later overcome at RCL by including a wetting electrolyte
between the graphite and the iron sleeve. A small hole was drilled near
about the top in the sleeve so that the electrolyte can get in and fill
the interface. This concept gave better results. But still a basic
problem with this meter is the iron needles that form over a period of time.
Also the meter can not be kept connected to the read out meter continuously
since even a small measuring current can polarize the electrodes because of the
low diffusivity of carbon. One important thing learnt in this meter is the
need for keeping the oxygen leakage through the Teflon gland very low.
Here also they will have to go for a CM seal. A simple test for
performance testing of this meter is to getter the carbon in sodium using
ss foil and noting the maximum emf developed.
10.Carbon
in sodium
One important experiment done using this meter was the measurement of the carbon activity in sodium for different temperatures while keeping the carbon concentration constant. The activity variation with temperature for the same carbon concentration was more in agreement with the postulate that the carbon exists as a diatomic species in sodium at the high carbon activity of about 0.8 as obtained with FBTR sodium. This is at variance with the British results which showed that carbon exists in sodium as methides. That may be true at low carbon activities as their measurement were done with low carbon sodium.
One question that interests fast reactor operators is whether carburization of structural materials by any oil leak is serious enough to alter the mechanical strength. Available data from the reactor operating experience and the experimental results reported indicate that it is not serious. The carburized layer formed discourages further diffusions and there is no deterioration in mechanical strength in the operating temperatures of the reactor.
One
problem with Indian sodium is the high carbon activity and the carbon meter’s
utility for detecting any oil leakage is limited since the change in
voltage output is low.
11.Reaction
of oil with sodium
It
is quite interesting to know the way that coolant oil reacts with high
temperature sodium (3~00oC)as oil leak through shaft seal is a definite
possibility in a fast reactor. Oil reacts with sodium to give amorphous carbon
which is voluminous. About 10% of carbon is released as methane in the
initial cracking. The amorphous carbon produced by oil-sodium reaction
is sticky and can clog any filter down stream. With passage of time
the graphitization of carbon increases and ends as graphitic carbon.
So the best way to detect oil leakage in FBTR sodium is to look for methane in
covergas.
12.
Graphite reaction with sodium
Carbon
solubility in sodium is low. Graphite is stable in sodium. So we
expect graphite rods exposed to sodium at high temperatures be
stable. To verify this, experiments were done at RCL. It was found
that graphite swells well even at 500oC due to the intercalation of Na2O into
sodium. So the mechanical integrity of graphite exposed to sodium is
suspect. This means that any graphite to be used in sodium system
should be doubly clad with steel as otherwise it can swell in the event of a
leak. A proposal to use graphite blocks between the core and the neutron
detectors in PFBR was discouraged owing to this problem.
13.Carbon
meter for PFBR
Since
the PFBR sodium will most probably be having lower carbon level since sodium is
going to be produced HWB using quality carbon anodes. Monitoring PFBR using
electrochemical meter would would appear to be more meaningful. Since the
main source of hydrocarbon is the shaft coolant for the pump, the seal design
for PFBR would be more uptodate. Since it is a pool type reactor
monitoring the covergas for methane using a fast cycle GC would be sufficient
for detecting oil leaks. Putting a carbon meter in the purification line
may not be useful and necessary.
14.Cold
Trap Regeneration
The
main source of hydrogen for sodium is the corrosion generated hydrogen in the
steam generator. This enters the sodium almost quantitatively in the
steam generator. This ends up in the secondary cold trap and the cold trap
capacity gets used up fast.. As a result the secondary cold traps require
periodic regeneration. There are many approaches to this job. The
American approach was to raise the temperature to (>420oC) and convert it
into hydroxide by reacting the hydrogen and oxygen. The NaOH generated is
in liquid form and sink as it has a higher density. They use a catch pot
to collect this at the bottom of the cold trap. There is also the Japanese
approach where argon flushing over the sodium after raising the sodium
temperature. Standardising the procedure for PFBR cold trap generation was
taken up at RCL during X plan. Based on the basic studies done on the
Na-O-H system a reliable procedure for secondary cold trap generation must
become available at the end of the plan. This work should be continued and
a regeneration procedure for PFBR cold traps should be evolved with utmost
priority.
15.Limitations
of hydrogen sensor for steam generator leak detection
In-sodium
hydrogen sensors can measure the hydrogen dissolved in sodium as sodium hydride.
Almost 50% of the hydrogen contribution in the event of a steam leak is in the
form of hydrogen bubbles. The dissolution of this gas into sodium is
temperature dependent. The average size of these bubbles produced in the
region of 2mm or so and good fraction of it can escape into the covergas
depending on the temperature. This is because of the slow kinetics of
hydrogen gas dissolution in sodium at low temperatures. Under these
conditions there is a fear that the in-sodium hydrogen sensors may not be able
to detect steam generator leaks at low sodium temperatures as
obtained during reactor startup. In order to verify this, small amounts of
water taken in a specially designed capsules equipped with fusible seals which
melt at low temperatures (~250o C) and injected into sodium in RASCL. The
in-sodium meters gave a good response (~75%) since 50% of the hydrogen content
of the steam leak ends up a NaOH. NaOH is highly soluble in sodium and
helps the assimilation of the hydrogen by the liquid metal. So there
is no possibility of in-sodium meter missing out any steam leak through the
signal is not quantitative at low sodium temperatures
(T<250oC).
16.Cover
gas hydrogen meter
For
unambiguous and positive detection of any steam generator leak at sodium
temperatures below 3000C (as in reactor start up) it is helpful to measure
hydrogen in cover gas. The cover gas hydrogen meter developed
by RCL for use in FBTR is based on the diffusion of hydrogen into a nickel
capillary tube at 400o C. The capillary is swept by argon carrier
gas at a low flow rate (30 ml/min) and the argon is monitored for hydrogen using
a thermal conductivity detector (TCD). The length of the tube was
optimized at 6 M so that the signal is maximum and flow independent. This
concept was chosen for FBTR since it was suitable to incorporate it as an add on
and avoids problem introduced by sodium aerosols. The TCD is a proven
detector and commercially available. The electronics and measuring system
can be located away from sodium loops. This device was extensively checked at
RCL and then at EDG loops for its reliable performance before incorporating it
in the reactor. It has performed well in the reactor, always responding
quantitatively for hydrogen injections.
One
drawback with this equipment is the possible reduction in sensitivity with the
passage of time since the nickel can recrystallise. But the degradation is
less than the in-sodium meters of the same type as in PFR where the additional
pick up of impurities from sodium by the nickel makes the deterioration a bit
faster.
The
cover gas hydrogen meter based on a nickel crucible and an ion-pump can give a
faster response. The system is also simpler. But there are some
practical problems such as heating the entire area of the nickel crucible
uniformly which is difficult in the gas phase. The ion-pump also has
problems of activation by hydrogen and change in the capacity leading to base
line shifts.
17.Sodium
leak detector for SGTF
The
oil heated sodium carrying tubes in SGTF
facility can develop cracks and the
sodium leaks have to be detected early before they can expand. The logical
way to detect these leaks is to look for Na2O aerosol in the flue gas. A
leak detection device based on a flame photometer detector was designed and
developed by RCL. The flue gas is sampled and let into a flame photometer
and the sodium emission lines are monitored using a wave length filter and a
photo tube. This detector is based on a proven principle and so it is suitable
for plant use. The detector was extensively tested at the laboratory by
aspirating sodium containing solutions. This was finally incorporated in
SGTF and tested by carrying out sodium fires in the oil burner area.
However, over the time, some problems were encountered in the signals as
the aerosols started depositing on the sampler walls. Thus there is a need
to keep the length of the sampler line as small as possible. This
modification in the sampler line is being carried out by EDG.
18.Activity
Transport
Radionuclides are produced in the reactor core 1) by activation of reactor structural materials and 2) fission of the fuel. These radionuclides can get released into sodium, get transported to components outside the core and deposit there leading to radiation build up in areas and components requiring repair and maintenance. This will lead to prolonged shutdown and increased exposure to personnel during repairs. Thus there is a need to prevent such build ups either by minimizing their transport or by continuously removing the radionuclides from sodium. Among the activation products, Mn-54 has been found to be the major nuclide that gets transported to cold legs forming almost 90% of the radioactivity burden. This is because Mn-54 is produced by (n, p) reaction of Fe-54 which is a component of the structural materials. Mn-54 has a high diffusion coefficient in steel and a higher solubility in sodium. If Mn-54 problem is solved, the activity transport problem is almost solved in fast reactors. Other activation products like Co-60, Cr-59, etc., deposit immediately downstream from the core and do not migrate much. It is true that the activity transport problem is less acute in pool type reactors. However, the contamination of IHX, and the pump still remain.
Among the fission products that get released into the coolant on fuel-failure, Cs-137 is the only nuclide that gets quantitatively released. Since this is highly soluble in sodium it gets mixed with the entire primary sodium. In the hot leg regions Cs-137 can diffuse into the grain boundaries and complicate decontamination of components for repairs. Cs-137 can also evaporate preferentially and get fixed in the sodium frosts in the primary covergas space. This is found in the above sodium regions of the pump shaft as well. Other fission products like Ce-144, Y-90, Ba/La-140 etc., are released only by a small fraction into sodium and deposit immediately downstream from the failure location. So, Cs-137 is the only fission product that should b dealt with.
Among
the impurities in sodium that can get activated Zn is the most important.
Zn-65 has been observed in almost all the reactors. Ag-110m is another such
nuclide. Sn-113 also has been found. All these are highly soluble
and are found throughout sodium. But their levels never rise beyond µCi
levels as the amounts of these impurities is small.
19.Radionuclide
traps
Brehm
and Coworkers in
Reticulated
Vitreous Carbon in the form a porous foam supplied by Ergenics Inc of
There
are two designs of CS-137 trap. In out of core concept used in western
reactors a small vessel filled with RVC is kept at 200oC in a purification
loop outside the core. This is usable all the time even when the
reactor is in operation. The vessel should be shielded with lead and will
be the last component to be removed from the reactor during
decommissioning. The amount of RVC required is quite small and the sodium
cleanliness can be kept very good.
The
alternative in-core caesium trap involves a subassembly incorporating a known
and adequate amount of RVC which is inserted into the ore during reactor
shutdown and when sodium temperature is low. After the clean up this has
to be removed and disposed off. This is the concept used in Russion
reactors. There is no extra pipeline or shielding required except that RVC
used once can not be used again. This calls for a larger amount of RVC to
be stocked. This is more desirable when the anticipated fuel pin failures
are low. This is the concept chosen for PFBR.
Other
radionuclides that are amenable for trap purification by traps are Zn-65,
Ag-110m and Hg-203. These are highly soluble and can be removed by
exposing sodium to copper turnings at a low temperature (~250o C) in a
purification loop.
However,
Mn-54 trap and Cs-137 trap are sufficient for reducing the activity of a fast
reactor sodium.
20.Fuel-coolant
reaction
The
mixed oxide fuel can react with sodium coolant in the event of fuel-sodium
contact when the clad looses its tightness. The reaction product is a low
density material and its formation leads to increase in fuel volume. This
results in expansion of the defect and exchange of sodium in and out of
the fuel. Ultimately this can lead to release of fuel into flowing sodium which
is not desirable. Also understanding of the fuel-coolant reaction is
essential in operating the reactor till the failed fuel pin is located and
removed.
The
mixed oxide reacts with sodium to give a compound Na3(U, Pu) O4 where the ratio
of U/Pu remain sense in the fuel and the product. At equilibrium the
oxide, Na3MO4 and the sodium are coexisting. As per phase rule, the
three phases would have same oxygen potential for all the three phases.
The reaction is an oxygen consuming reaction and can proceed forward only till
the fuel can contribute oxygen, since the amount of sodium entering the
fuel pin initially is limited and so cannot contribute much oxygen. The
equilibrium O/M in the fuel and the oxygen in sodium have been measured
experimentally. This could also be calculated from the free energy of
formation of Na3MO4. These values are in agreement with each other.
Using this equilibrium oxygen potential it is possible to calculate the extent
of the volume increase for the fuel of a given initial O/M.
However,
the presence of a temperature gradient gives rise to an O/M ratio distribution
in the fuel from the centre to the periphery. Also the Na3MO4 compound is
not stable above ~1200oC giving rise to a dissociation isotherm. As a
result, the formation of the reaction product is limited to the outer 200µ of
the fuel. Moreover, the poor thermal conductivity of the reaction product
and the reduced thermal conductivity of the reduced O/M of the fuel results in a
higher temperature gradient. This pushes the reaction product boundary to
still further outside.
21.Evolution
of fuel failure
The
DND signal as a function of time gives a measure of the fuel surface exposed to
sodium. RBCB experiments in EBR II and experiments in SILOE reactor
indicate that the evolution in general tapers off after reaching a shoulder.
This indicates that fuel-sodium reaction product acts as a barrier for further
fuel reaction. This factor ensures that the fuel-coolant reaction is not
catastrophic in nature and will not lead to extensive release of fuel.
Only in case of a few disturbed operations the fuel-failure tends to increase
monotonically.
The
time available for the reactor operator from DND signal is noticed to
fuel-removal DND threshold varies from 10 mm to a few hours depending on the
burnup and clad swelling. This is sufficient for locating the failed fuel
and shutdown the reactor for its removal. This calls for reliable fission
gas detector, delayed neutron detector and failed fuel detection system.
22.
High plutonium fuels
The
oxygen available for the reaction increases with the plutonium content of the
fuel as plutonium is less oxygen binding. But this factor is counteracted
by the poor thermal conductivity of the fuel which pushes the reaction front
still further outside and limits the reaction product formation.
Moreover the buffering action of the clad and the fuel-fission product system
which always limits to O/M at the outer periphery to 1.98 also helps in reducing
the severity of the reaction. Thus in practical terms it is still possible
to operate the reactor with high plutonium fuels even with slightly high O/M
without any great risk.
23.
Failure at the storage positioner
The
fuels that fail in the core and have undergone the initial reaction before they
are removed and moved over to storage position at the outer rings of the core
behave very benignly. There is no further opening of the fuel or release
of the particulates. But in the case of the fuels that fail after they are
moved to the storage position the reaction induced swelling is severe since the
fuel cannot creep. This has resulted in the break up of some fuels
into pieces. This factor again points to the importance of the plasticity
of the fuel on power and its role in accommodating the volume increase due to
fuel-sodium reaction.
24.
Sodium removal from reactor components
Components
taken out of the sodium system for repair and maintenance have to be cleaned off
sodium as a first step. That includes fuel pins removal from the reactor
for reprocessing. Different techniques have to be adopted for different
components as the process requirements are slightly different.
25.
Sodium removal from fuel-subassembly
The
subassembly removed from the core has a large decay heat. So this has to
be continuously cooled by blowing nitrogen. This gas jet is able to remove
a good amount of sodium sticking to the fuel in between the fuel pins.
After the initial nitrogen flushing period steam is injected into the
nitrogen stream while continuously monitoring the hydrogen content of the exit
gas. This process is quite fast and highly suitable for fuel pins.
However, this process can not be used with components that are reusable.
26.
Water vapour nitrogen process
Components
such as pumps, 1HX, valves etc., have to be repaired and reused. The
temperature of the components has to be kept below ~100oC during sodium removal
in order to prevent caustic stress corrosion cracking. This is done by
passing nitrogen containing water vapour over the components kept at a low
enough temperature in a closed vessel. The water vapour/steam injection is
done in a controlled manner. The process is slow since the NaOH layer
formed at the surface slows down the water supply to the sodium surface.
27.
Water vapour-CO2 process
Here
CO2 moistened by passing through water kept at a slightly elevated temperature
but below 90oC is passed over the component kept in a vessel. The CO2 keep
the pH of the layer low and prevents caustic stress corrosion cracking.
The NaHCO3 layer formed at the surface is porous and permits the sodium-water
reaction to proceed without hindrance. The reaction is monitored by
measuring the hydrogen in the exhaust gas. There is no fear of NaOH
droplets dripping from the components and releasing NaOH aerosols and fouling
the hydrogen sensor. This is the process recommended for use in PFBR.
Other
processes like water-vacuum process, autoclave process, and alcohol process have
specific limited applications.
28.
Sodium disposal
When
there is a need to dispose off large quantities of sodium the process of choice
is the NOAH process. Here the liquid sodium is pumped into 10 M NaOH where
the sodium reacts with water in a smooth and a controlled manner. Of
course the hydrogen generated has to be exhausted out well. The trick here
is the design of the nozzle ejecting sodium into fine droplets. NaOH
solution is pumped around the nozzle to take the sodium away from the nozzle.
This prevents the attack of the nozzle metal by the hot alkali produced at the
tip. Of course the selection of the nozzle material is also to be
carefully done. The alkali has to be cooled using a heat exchanger.
29.
Sodium reuse
Reuse
and disposal of primary and secondary sodium when the reactor is decommissioned
is important. The secondary sodium can be cold trapped for sufficient
length of time to remove any tritium and allowed to cool for sufficient time (10
half lives) so that Na-24, if any can completely diedown. Then it can be
used for any chemical process.
The
primary sodium can be first polished using 1) RVC for removing Cs-137 and Cs-134
and 2) copper trap for removing any activation products (Hg-203, Sb 113, In 129,
Ag-110 m etc.,). Then it can be allowed to cool for about 6 years so that
Na-22 levels come down. Then it can be transported in shielded tanks for
other reactors within the campus. If it has to be transported to far away
places then a greater cooling time would be required (~12 years). Anyhow
it is not desirable to convert the Na to NaOH as the environmental impact is
severe.
30.
Suggestion for the chemistry lab of PFBR
PFBR
should have an in-place chemistry laboratory which can carry out chemical
quality control of sodium, water and cover gas. There should be an
instrument room, radiochemistry room and a counting room sufficiently manned to
carry out all the routine analysis. An Atomic Absorption Spectrophotometer, pH
meter, spectrophotometer, ion-selective electrodes and a quick cycle gas
chromatograph are the essential instruments. Specific high sensitivity
analysis like ICP-MS can be done at Chemistry Group, IGCAR. A sodium box,
sodium distillation set up, active hoods are the facilities required at the
Radiochemistry room. Three chemists (from training school) should be
recruited in advance and trained at Chemistry Group to get their doctorate
eventually after posting at PFBR so that chemistry problem in PFBR could be
tackled.
2)
An indigeneous alternative to RVC should be developed.
3)
A sodium compatible high alumina concrete should be developed.
4)
Mn54 trap should be tested in FBTR.
5)
YDT based oxygen meter should be
developed for use in PFBR.