Method of reducing the volume of solid radioactive waste

Combustible, solid radioactive waste, such as paper, plastics, rubber, cloth and wood, are reduced in volume to an ash residue using pyrohydrolysis, a method which combines pyrolysis of the waste in a vessel at temperatures in the range of 500.degree. to 700.degree. C. and gasification of residual carbon with superheated steam. Pressures of 1.0 to 3.5 MPa are used with steam flows in the range 4 to 50 grams/second/cubic meter so that carbon containing components of the waste are removed as gaseous decomposition products in the form of carbon monoxide and hydrogen leaving an ash residue.

This invention relates to a method of reducing the volume of solid 
radioactive waste. 
Heavy-water-moderated, natural-uranium CANDU power-reactors as single-unit 
stations generate approximately five 45-gallon drums of noncompacted low 
level radioactive waste per day. This waste is primarily standard 
combustible garbage containing cellulose material (e.g. paper), plastics 
(e.g. disposable gloves, etc.), rubber, cloth and wood. At present, above 
ground storage of this waste in compacted form is the best cost option for 
handling. Ultimately, however, although the waste volumes are relatively 
small, 350 m.sup.3 /yr, further processing will be required to immobilize 
the radioactive waste. This is due to requirements for disposal as well as 
to keep storage costs low. Current technologies available for the 
reduction of combustible waste volume are complex and expensive. For 
example, present incineration technology requires a very sophisticated 
off-handling system due to the large volumes of particulate matter 
containing radionuclides. 
There is a need for a method of reducing the volume of solid radioactive 
waste wherein: 
(i) the off-gas handling is simple; 
(ii) the combustion process is endothermic for ease of temperature control; 
(iii) it is possible for the system to be contained by recirculating 
process water or steam; 
(iv) the capital investment is low; and 
(v) the method readily lends itself to automated operation. 
According to the present invention, there is provided a method of reducing 
the volume of solid radioactive waste, comprising: 
(a) pyrolyzing the radioactive waste in the interior of a vessel, while 
(b) passing superheated steam through the vessel at a temperature in the 
range 500.degree. to 700.degree. C., a pressure in the range 1.0 to 3.5 
MPa, and at a flow rate in the range 4 to 50 grams/second/cubic meter of 
the volume of the vessel interior, to cause pyrohydrolysis of the waste 
and to remove carbon-containing components of the pyrolyzed waste from the 
vessel, as gaseous decomposition products in the form of carbon monoxide 
and hydrogen leaving an ash residue in the vessel; 
(c) filtering any entrained particles present with the gaseous 
decomposition products, 
(d) removing any acidic vapours present with the gaseous decomposition 
products by solid sorbent, 
(e) condensing steam and any organic substances present with the gaseous 
decomposition products, and 
(f) removing the ash from the vessel. 
The radioactive waste may be deposited upon an upper screen in the vessel, 
so that at least a substantial portion of the pyrolysis of the radioactive 
waste takes place while the radioactive waste is on the upper screen, and 
pyrolyzed waste falls through the upper screen onto a lower screen, where 
at least a substantial portion of the pyrohydrolysis takes place, and the 
ash residue falls through the lower screen. 
In some embodiments of the present invention, the steam pressure in the 
vessel is in the range 1.4 to 2.8 MPa and the flow rate of the condensed 
steam is of the order of 16.7 grams/second/cubic meter of reation the 
vessel interior. 
In other embodiments of the present invention, the superheated steam is 
obtained by heating and recirculating the condensed steam. 
Organic liquid waste may be introduced into the vessel with the 
recirculated condensed steam.

In FIG. 1 there is generally shown, a reactor vessel 1, a superheated steam 
generating unit 2, filters 4 and 6, acid vapour absorption cells 8 and 10, 
a condenser 12, an off-gas pipe 13, an ash discharge vessel 14, and a 
vacuum line 15. 
The vessel 1 has an electrical heating coil 16 therearound and is fitted 
with two stainless steel screens 18 and 20, which extend thereacross at 
different heights in an intermediate portion of the vessel 1. A 
radioactive waste supply pipe 22, containing two ball valves 24 and 26, a 
gate valve 28, and a pressure gauge 29, is connected to an upper side of 
the vessel 1. A pressure gauge 32 is connected to the vessel 1 which has a 
gas outlet 33. 
The vessel 1 has an electrical heating coil 34 therearound, a superheated 
steam inlet pipe 36 thereto, connected to the superheated steam generating 
unit 2, a lower, ash collecting hopper portion 38 beneath the lowermost 
screen 20 and an ash discharge line 39. 
The superheated steam generating unit 2 has a water supply pipe 40, a 
pressure gauge 42, an electrical heating coil 44, and a superheated steam 
outlet 46 connected to the superheated steam inlet pipe 36 of the vessel 
1. 
The filters 4 and 6 are 0.5 micron mesh size, stainless steel, in-line 
filters. The filters 4 and 6 are connected to the gas outlet 33 of the 
vessel 1 by exit pipes 48 to 50 and valves 52 and 54. 
The acid vapour absorption cells 8 and 10 are connected by pipes 56 and 58, 
respectively, to the filters 4 and 6; pipes 60 to 62, valves 64 and 66, 
and steam control valve 68, to the steam condenser 12. Pipes 60 and 61 are 
connected to a pressure gauge 69. 
The steam condenser 12 is cooled by a water-cooled heat exchange coil 70 
and the condensate from the condenser 12 collects in a liquid collector 
72. The liquid collector 72 has a condensate stirrer 74, means 76 for 
adding a dispersement and a pH adjusting device 78. A pump 80 is provided 
for pumping condensate from the liquid collector 72 and recirculating it 
to the water supply tube 40 of the superheated steam generating unit 2. 
Gate valve 82 and ball valve 84 are provided for intermittently discharging 
ash from the vessel 1 into the vessel 14. 
Radioactive waste from, say, a heavy-water-moderated, natural uranium CANDU 
power-reactor typically includes paper, polyethylene, polyvinylchloride 
and cloth, and experiments have been carried out to pyrolyze these 
materials as a simulated waste in the vessel 1. 
In the experiments, these materials were fed on to the top screen 18 in the 
vessel 1 from the pipe 22, using the valves 24, 26 and 28 to more or less 
maintain the pressure within the vessel 1. A temperature not exceeding 
700.degree. C. was maintained in the vessel 1 using the heating coil 34, 
while superheated steam, generated in the unit 2 using the heating coil 
44, was fed to the vessel 1. 
Char product generated on the top screen 18, from the simulated waste, fell 
to the second screen 20, where the char is converted to ash and falls 
through the second screen 20 ready for discharge as ash to the vessel 14. 
Gases produced by pyrolysis of the simulated waste were found to undergo 
secondary reactions in both the vessel 1 and exit pipes 48 to 50 in the 
formation of heavy tars, char and a light gas component. Using 
pressurized, superheated steam produced a complete breakdown of the 
pyrolysis gas, with substantially no particulate entrainment therein with 
no evidence of char formation in the exit pipes 48 to 50, which was found 
to be present when pressurized, superheated steam was not used. This was 
because the pressurized, superheated steam enabled the endothermic water 
gas shift reaction to proceed, that is, char or fixed carbon was broken 
down to gaseous decomposition products in the form of carbon monoxide and 
hydrogen. This resulted in a high, overall volume reactions of as much as 
50:1. 
The use of fluid pressure in the reaction vessel 1 was found to provide two 
advantages. First, by pressurizing the reaction vessel 1, particulate 
release was minimized. Second, the fluid pressure increased the time that 
the pyrolysis gases were retained in the vessel 1, and increased the 
contact period between the steam and the gases. This allowed the water gas 
shift reaction to proceed more to completion and to elimination char 
formation and the release of heavy oils. 
In some tests, nitrogen was circulated through the vessel 1 and this was 
removed by the vacuum line 15. 
Any entrained ash particles were filtered from the gases by the filters 4 
and 6. 
HCL vapour in the off-gases was extracted therefrom by the absorption cells 
8 and 10 which contained CaO, Na.sub.2 CO.sub.3 or the like absorbent. The 
solid absorbent in the cells 8 and 10 was used to remove acidic vapours in 
preference to liquid scrubbers because less volume of waste was generated. 
The large volume of liquid waste from scrubbing would require a lot more 
processing than the solid absorbent. A further advantage is that the solid 
absorbent can be handled using a similar or the same system to that used 
to immobilize the ash discharged from vessel 1. The pressure of the 
off-gases was then reduced to atmospheric pressure using the valves 64, 66 
and 68. 
A condensible liquid fraction comprising water from steam injection and 
light organics from incomplete cracking of the off-gases from the vessel 1 
were condensed in the condenser 12. 
Off-gases were removed by pipe 13 and passed through a filter (not shown). 
The condensate from the condenser 12 collects in the collector 72 where the 
pH was adjusted by control 78 while a dispersant was added by means 76 and 
mixed with the condensate by stirrer 74 to form an emulsion which was 
recycled to the superheated steam generating unit 2 by pump 80. 
The experiments were carried out at elevated pressures and the simulated 
waste was added in discrete quantities (batch mode) to the vessel 1. Using 
a gas pressure in the vessel 1 of the inert gas fed thereto, or by 
generated pyrolysis gas, in the range 1.0 to 3.5 MPa and a temperature in 
the range 500.degree. to 700.degree. C. substantially avoided particulate 
entrainment in the off-gases. 
The pyrolysis of simulated waste product, under inert gas pressure or 
generated pyrolysis gas pressure, using the apparatus shown in FIG. 1, 
gave an overall volume reduction of at least 20:1 from a charge intially 
compacted 5:1 by volume. The pyrolysis gases were found to undergo 
secondary reactions in both the vessel 1 and the pipes 48 to 50 resulting 
in the formation of heavy tars, char and a light gas component. Tests 
without pressurized steam produced excessive char build-up throughout the 
system. Tests carried out using pressurized steam produced a substantially 
total breakdown of the pyrolysis gases, substantially no particulate 
entrainment, and substantially no evidence of char formation. Using 
superheated steam was found to enable the endothermic water gas shift 
reaction to proceed; that is, char or fixed carbon was broken down to 
gaseous decomposition products in the form of carbon monoxide and hydrogen 
so that high overall volume reductions of the order of 50:1 were achieved. 
In FIG. 2, similar parts to those shown in FIG. 1 are designated by the 
same reference numerals and the previous description is relied upon to 
describe them. 
Apparatus based on the flow diagram shown in FIG. 2 was used for 
experiments wherein the apparatus was operated on a semi-continuous basis. 
In FIG. 2, the valves 52 and 54 are situated in pipelines 86 and 88, 
respectively, which may also contain cyclone separators 90 and 92. 
The filters 4 and 6 are provided with nitrogen backflow pipes 94 and 96, 
respectively, to assist filter cleaning. Bleeds 98 and 100 are provided to 
allow replacement of the absorbents after they become exhausted. 
A filter 102, having an air inlet 104 and an air outlet 106 is connected to 
the pipe 13. 
The collector 72 has an organic liquid waste charging pipe 108 and a water 
make-up pipe 110. 
The pipe 36 has a pressure gauge 112. 
The ash discharge vessel 14 has a pneumatic transfer pipe 114 for 
delivering the ash to an immobilization device, such as ribbon blender 116 
provided with a bitumen feed 118. 
In FIG. 3, similar parts to those shown in FIG. 2 are designated by the 
same reference numerals and the previous description is relied upon to 
describe them. 
In FIG. 3, the cyclone separator 90 has a pipeline 120, containing valves 
122 and 84, and a vacuum branch pipe 15 for nitrogen flushing the system, 
connected to the ash discharge vessel 14. 
The cyclone separator 92 is connected to the ash discharge pot 14 in the 
same manner as the cyclone separator 90, is shown connected thereto in 
FIG. 3. 
Organic liquid wastes generated during nuclear reactor operations include 
heavy oils, which are released from hydraulic and lubricating systems, and 
scintillation liquids, which are used in the analysis of tritium. It was 
found that these wastes could be converted to carbon monoxide and hydrogen 
by introducing them to the collector 72 through pipe 108 where they are 
mixed with the water, fed back through the superheated steam generating 
unit 2 by pump 80, and then introduced into the vessel 1. The organic 
liquids are then subjected to the same processes as the solid wastes and 
are decomposed to gaseous oxides and hydrogen. 
In experiments using the arrangement shown in FIG. 2, the superheated steam 
generating unit 2 was supplied with steam from two autoclaves (not shown) 
connected in parallel and valved to permit continuous steam generation. 
One of the autoclaves was 4 L in capacity and was a primary steam 
generator. The other autoclave was a back-up steam generator for use when 
the primary generator was cooling down, being refilled with water and 
warmed up for steam generation. 
The superheated steam generator 2 was a coiled, 3/8 inch (9.52 mm), 
stainless steel tube with a parallel winding of electrical heating 
elements. This generator operated at .about.900.degree. C. and .about.600 
psi (4.1 MPa) yielding a steam temperature at the vessel 1 of 
.about.600.degree. to 700.degree. C., the operating temperature required. 
The samples used for semi-continuous trials were 1 g to 8 g compressed 
charges of cylindrical shape and contained UO.sub.2 for evaluating 
particulate entrainment in the system. The sample charge distribution was 
32 w/o paper, 8 w/o PVC, 36 w/o plastic, 12 w/o rubber, 4 w/o cloth and 8 
w/o wood. 
Normal sample loading involved the following operation sequence: 
(i) a cylindrically shaped, compacted charge was dropped between the two 
ball valves 24 and 26, 
(ii) with both of the two ball valves 24 and 26 closed, the volume between 
them was pressurized with N.sub.2, from a source not shown, to slightly 
above the operational pressure, 
(iii) the gate valve 28 was then opened and then, immediately following, 
the ball valve 26 was opened, and 
(iv) the charge then dropped on to the first screen 18 in the vessel 1 and 
then the gate valve 28 was closed. Both of the ball valves 24 and 26 were 
opened for visual inspection to ensure that the charge had been introduced 
properly into the vessel 1. 
Product discharge was tested after four day trials. The reactor was cooled 
to .about.100.degree. C. and pressurized to 400 psi with N.sub.2. The gate 
valve 18 in the ash discharge line 39 was opened followed by opening the 
ball valve 84 so that the ash discharged into the evacuated vessel 14. 
Two types of tests were conducted. In the first case, the operating 
variables of temperature, pressure and steam flow were pre-set. A summary 
of the tests completed and the results achieved are given in Table 1. The 
actual experimental design was of a factorial type where temperature 
ranged from 500.degree. to 650.degree. C., steam pressure ranged from 0 to 
400 psi (0 to 2.8 MPa) and steam flow ranged from 1.0 to 4.0 cc/min. 
(condensed steam). By choosing high and low point combinations, an 
efficient optimization of operating parameters was obtained. 
In the second type of tests, variation of one or more operating parameters 
during the experiment was attempted. The purpose of these tests was to 
assess the influence of small operating parameter changes. Steam leaks 
were detected in some cases, however, data obtained prior to leakage 
remains valid. An overall summary of these tests is given in Table 2. Data 
abstracted from experiments C-11 to C-19 gave valuable information on the 
interplay of temperature, pressure and steam flow. These interactions have 
been summarized in Tables 3, 4, 5, 6 and 7. 
The semicontinuous trials were also performed to gather further information 
about the process. The vessel 1 was kept hot and pressurized and 
approximately every 3 to 5 hours, a similar waste package to that 
previously described was placed into the vessel 1 using valves 24, 26 and 
28 on the feed line 22. Trial operations for periods of up to 96 hours 
were carried out with further variations in temperature, pressure and 
steam flow and these were found to generate volume reductions of 25:1 and 
weight reductions of 93%. The results of the semicontinuous trials are 
summarized in Table 7. 
At the conclusion of these tests, an analysis of batch versus 
semi-continuous processes was made. Table 8 outlines the advantages and 
disadvantages of batch and semi-continuous pyrohydrolysis systems. 
TABLE 1 
__________________________________________________________________________ 
BATCH PYROHYDROLYSIS TESTS - TYPE 1 
Retention time 
Condensed 
Temperature Pressure 
at T = 500.degree. C. 
steam flow 
Reduction 
Experiment 
.degree.C. 
psi 
MPa 
hours cc/min 
weight % 
volume % 
__________________________________________________________________________ 
C-1 650 400 
2.8 
101/4 2.0 80 81 
C-3 650 400 
2.8 
7 2.0 85 85 
C-4 650 400 
2.8 
12 1.5 80 79 
C-5 650 200 
1.4 
12 2.2 72 82 
C-6 600 100 
0.7 
13 1.6 .sup. 79.sup.2 
.sup. 78.sup.2 
C-8 500 0 0 12 2.2 .sup. 78.sup.2 
.sup. 82.sup.2 
C-9 500 200 
1.4 
12 4.0 74 79 
C-10 650 400 
2.8 
12 3.75 75 81 
__________________________________________________________________________ 
NOTES: 
.sup.1 C7 and C2 failed due to leaking gasket. 
.sup.2 Blowout of material observed yielding higher weight losses. 
.sup.3 Steam flow is in cc/min of water overflow from impingers (condense 
steam). 
TABLE 2 
__________________________________________________________________________ 
PYROHYDROLYSIS TESTS - TYPE 2 
Retention time 
Condensed 
Temperature Pressure at T = 500.degree. C. 
steam flow 
Reduction 
Experiment 
.degree.C. 
psi MPa hours cc/min 
weight % 
volume % 
__________________________________________________________________________ 
C-11 500-700 
200-400 
1.4-2.8 
12h 3.5-4.5 
86 87 
C-13 500-700 
200-400 
1.4-2.8 
12h 2.2 (avg) 
90 -- 
C-14 700 400 2.8 12h 2.4 (avg) 
83 -- 
C-15 650 400 2.8 5h 1.5 75 -- 
C-16 650 200-400 
1.4-2.8 
12h 2.0 75 -- 
C-17 650 200-400 
1.4-2.8 
12h 1.8-6.5 
83 -- 
C-18 650 200-400 
1.4-2.8 
12h 1.0-4.0 
81 83 
C-19 650 200-400 
1.4-2.8 
12h 0-4.0 
82 -- 
__________________________________________________________________________ 
NOTES: 
.sup.1 Experimate C12 failed, and C13 to 16 showed steam leaks. 
.sup.2 C18 & 19 used an aluminumasbestos gasket. 
.sup.3 C17 showed a hydrogen leak. 
.sup.4 Ranges quoted for operating parameters are controlled variations t 
assess relationships between each parameter and the reaction rate as 
determined by product gas flowmeter readings. 
TABLE 3 
______________________________________ 
VARIANCE OF FLOW 
WITH PRESSURE FOR A 4.5 h PERIOD - EXP. C-16 
Condensed Rotameter 
Temperature Pressure Steam Flow Gas Flow 
Time .degree.C. psi MPa (cc/min) (cc/min) 
______________________________________ 
7 hr 
650 300 2.1 2.0 69.0.sup.1 
.dwnarw. 
650 200 1.4 2.0 26.5 
.dwnarw. 
650 300 2.1 2.0 23.0 
.dwnarw. 
650 200 1.4 2.0 24.0 
11.5 hr 
650 400 2.8 2.0 15.0.sup.2 
______________________________________ 
Note: 
.sup.1 High initial flow due to insufficient purging of pyrolysis gases 
from first phase. 
.sup.2 System leak had developed. 
TABLE 4 
______________________________________ 
VARIANCE OF FLOW 
WITH PRESSURE OVER A 9 h PERIOD - EXP. C-18 
Temper- 
ature Pressure Condensed Steam 
Rotameter Gas 
Time .degree.C. 
psi MPa Flow (cc/min) 
Flow (cc/min) 
______________________________________ 
7 hr 
655 400 2.8 4 35 
.dwnarw. 
656 320 2.2 4 30 
.dwnarw. 
650 200 1.4 4 25 
.dwnarw. 
650 400 2.8 4 37.5 
.dwnarw. 
650 300 2.1 4 35 
.dwnarw. 
650 400 2.8 4 40 
.dwnarw. 
650 200 1.4 3 22 
16 hr 
650 200 1.4 2 18 
______________________________________ 
Note: 
.sup.1 Shows decrease in water gas flow with decreasing pressure and stea 
flow rate. 
TABLE 5 
______________________________________ 
VARYING PRESSURE 
AND STEAM OVER A 4 h PERIOD - EXP. C-19 
Tem- Condensed 
pera- Steam Rotameter 
ture Pressure Flow Gas Flow 
Time .degree.C. 
psi MPa (cc/min) 
(cc/min) 
______________________________________ 
.dwnarw. 655 200 1.4 3 22.5 
.dwnarw. 655 220 1.5 4 23.5 
.dwnarw. 654 220 1.5 3 22.5 
.dwnarw. 651 220 1.5 4 25.0 
.dwnarw. 652 320 2.2 4 27.5 
.dwnarw. 651 400 2.8 4 30.5 
.dwnarw. 655 300 2.1 4 27.5 
Readings taken at 
655 220 1.5 4 22.5 
1/2 hour intervals 
after first 1/2 hour 
of operation. 
______________________________________ 
Note: 
.sup.1 Sampling occurring over 15-30 min intervals 
.sup.2 Results shown for all tests are with increasing time 
.sup.3 Decreasing flow and pressure confirms results of C18 
TABLE 6 
______________________________________ 
DEPENDENCE OF 
GAS FLOW RATE ON TEMPERATURE - EXP. C-11 
Tem- Condensed 
pera- Steam Rotameter 
ture Pressure Flow Gas Flow 
Time .degree.C. 
psi MPa (cc/min) 
Flow (cc/min) 
______________________________________ 
.dwnarw. 600 400 2.8 4 2.5 
.dwnarw. 625 400 2.8 4 12.5 
.dwnarw. 650 400 2.8 4 22.0 
.dwnarw. 675 400 2.8 4 57.5 
.dwnarw. 683 400 2.8 4 63.0 
.dwnarw. 680 200 1.4 4 47.5 
.dwnarw. 695 200 1.4 4 52.5 
.dwnarw. 691 400 2.8 4.5 65.0 
Readings taken at 
700 400 2.8 5.0 72.0 
1/2 hour intervals 
after first 1/2 hour 
of operation. 
______________________________________ 
Note: 
.sup.1 Substantial increase of water gas with increasing temperature 
TABLE 7 
__________________________________________________________________________ 
SEMI-CONTINUOUS TRIALS 
Reactor 
Operating Conditions 
Charge.sup.1 Steam 
Trial 
Total Wt. 
No. of 
Avg. Sampling 
Duration 
Temp. 
Pressure 
flow 
Charge Reductions 
No. 
(g) Charges 
weight (g) 
(in hr.) 
(.degree.C.) 
(MPa) 
cc H.sub.2 O 
Weight.sup.2 
Vol..sup.3 
Vol..sup.4 
__________________________________________________________________________ 
I 55 25 2.2 80 650 1.5 1.6 87 -- -- 
(1.1 to 6.8) 
II 173 40 4.8 96 650 1.5 1.2 93 90 96 
(2.2 to 8.6) 
III 
122 25 4.9 66 650 1.5 0.9 84 -- -- 
(4.6 to 6.1) 
__________________________________________________________________________ 
.sup.1 Charge consisted of 32% paper, 8% PVC, 34% plastic, 12% rubber, 4% 
cloth and 8% wood; 0.125 g UO.sub.2 added per gram charge. 
.sup.2 This value is calculated after removing the weight of UO.sub.2 and 
stainless steel corrosion product. 
.sup. 3 Based on 10 ton compression with rebound. Ratio of compacted 
charge volumes before and after test. 
.sup.4 Recalculated with initial charge not compacted. 
TABLE 8 
______________________________________ 
COMISON OF BATCH vs SEMI-CONTINUOUS 
PYROHYDROLYSIS OPTIONS 
Batch Semi-Continuous 
______________________________________ 
Advantages Advantages 
______________________________________ 
1. Simple system 1. Higher volume reduction 
(longer retention per charge) 
2. Accepts either loose or 
2. Significant reduction in 
compacted charges of capital cost (small volume 
waste reactor vessel) 
3. Clean off-gas (no particle 
3. No special treatment of 
entrainment) off-gases (no after burner 
required) 
4. Less maintenance (thermal 
cycling reduced) 
______________________________________ 
Disadvantages Disadvantages 
______________________________________ 
1. Capital intensive (requires 
1. Requires a pressurized feed 
a large pressure vessel) 
system 
2. High maintenance costs 
associated with thermal 
cycling 
3. Further treatment of 
off-gases 
with an afterburner 
4. High operating costs 
(gasket replacement & 
heating large vessel) 
______________________________________