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By Charles Rhodes, P.Eng., Ph.D.

Radio isotopes, if absorbed by the human body, can be highly toxic, even in minute quantities. Hence persons working with radio isotopes take extensive precautions to ensure that they do not inhale, ingest or absorb through their skin any radio isotopes. Persons working in the proximity of gamma ray emitting radio isotopes must also be protected by thick dense gamma ray absorbing shielding.

A major public safety issue in Ontario is that about 50,000 tons of spent CANDU fuel are presently stored at three locations, Bruce Nuclear Generating Station, Darlington Nuclear Generating Station and Pickering Nuclear Generating Station. These spent fuel storage locations are all adjacent to Lake Huron or Lake Ontario and are at elevations only a few feet above lake level. There is also the Western Waste Management Facility, located adjacent to the Bruce Nuclear Generating Station, which contains many thousands of cubic meters of Low Level and Intermediate Level nuclear waste.

A major earthquake, land subsidence, meteorite impact, tidal wave, volcanic erruption or explosion that brings this highly toxic material into contact with the adjacent lake water could cause an unparalled water pollution problem. An issue of high priority is moving the spent CANDU fuel and other nuclear waste to a secure high and dry storage location that provides inherent long term physical protection against future flood and like events outside human control. Based on the 100 m height of the Niagara escarpment, which is only 12,000 years old, and on the 80 m projected rise in sea level due to atmospheric thermal runaway, the elevation of this storage location should be more than 200 m above the surrounding water table.

The regulatory authorities have thus far simply refused to face and properly address such low probability but high consequence events. No matter what else is done with nuclear waste, the public will remain at risk from extreme natural events until the nuclear waste is moved to a safer location.

In the view of this author the existing political guidelines from both the government of Ontario and the government of Canada relating to nuclear waste transport, processing and disposal are wrong. These guidelines are founded in irrational public fears and in a political planning horizon that does not extend beyond the next election. It is a big mistake to let short term political considerations take precedence over scientific fact, good engineering, energy efficiency and public safety. The existing guidelines have the practical effect of placing the public at a much greater risk than would otherwise be the case.

If there is a problem with an irrational public response to nuclear power that problem is rooted in a failure in the public education curriculum and in the public's loss of confidence in the honesty and transparency of government and Ontario Power Generation. That confidence should be regained via honest public discourse, transparent public safety measures, fair allocation of risks and benefits and compulsory public education in the sciences. Current NWMO and OPG policies that are poorly thought with respect to future nuclear power requirements and potential long term potable water contamination do not help the situation. The public must be collectively convinced that nuclear energy is the best non-fossil energy source for Ontario. Incompetence and misrepresentaions by politicians and their lackeys are always eventually exposed and erode public confidence.

Low level waste (LLW) is nuclear waste consisting of isotopes with half lives of less than 30 years. From an engineering perspective LLW is simple to deal with. The LLW can be safely isolated from the environment in engineered containers that are stored for 300 years in a readily accessible, naturally dry, naturally ventillated and gravity drained depleted hard rock mine that is high above the local water table. Thus stored the LLW will spontaneously decay into safe stable isotopes.

High level waste (HLW) is nuclear waste consisting of isotopes with half lives greater than 30 years. Some low atomic weight HLW isotopes have half lives as great as 308,000 years. One category of HLW is low atomic weight isotopes with half lives greater than 100 years including the isotopes Be-10, C-14, Al-26, Si-32, Cl-36, Ar-39, Ca-41, Mn-53, Ni-59, Fe-60, Se-79, Kr-81, Zr-93/Nb-93m, Nb-94, Tc-97, Tc-98, Pd-107, Sn-126, I-129, Cs-135. Another category of HLW is low atomic weight isotopes with half lives in the range 30 years to 100 years including the isotopes Ti-44, Ni-63, Ba-133m.

A major sub-category of high atomic weight HLW is spent fuel from CANDU reactors which is highly radio toxic due to its plutonium and trans-uranium actinide isotope content. Most high atomic weight HLW can be converted into LLW by use of a liquid sodium cooled fast neutron reactor (FNR), as detailed in the Ottensmeyer Plan. The LLW can then be disposed of via 300 year isolated storage.

Intermediate level waste (ILW) consists of a mixture of LLW and HLW. If ILW is stored in isolation for 300 years the LLW component will spontaneoously decay away leaving behind the HLW component. The HLW can then separated into low and high atomic weight portions. The high atomic weight HLW can be converted into low atomic weight LLW using a FNR. The low atomic weight HLW is placed in long term deep geologic storage.

Metallic nuclear waste contains radio isotopes in metallic form. Metallic waste lends itself to future recycling through a fast neutron reactor but is a potential fire risk. Significant amounts of metallic nuclear waste are stored surrounded by argon as a fire protection/prevention measure. It is essential to do all necessary to ensure that metallic nuclear waste never burns in the atmosphere. The smoke from such a fire contains dangerous microscopic radioactive particles.

Oxide waste contains radioisotopes in metal-oxide form. From a storage perspective oxide waste is safer than metallic waste because oxide waste will not readily burn. However, oxide waste must be converted into metallic waste prior to recycling through a liquid sodium cooled fast neutron reactor. The uranium concentration in oxide reactor fuels is less than in metallic reactor fuels which significantly affects reactor and fuel design.

Part of the fission product mass is composed of inert gases which can be captured from FNR fuel tubes but which are difficult to store for long periods of time. The inert gas radio isotopes with half lives greater than 3 minutes are listed on the following table:
Ne-243.38 m
Ar-3735.1 d
Ar-39269 y
Ar-411.83 h
Ar-4233 y
Kr-7420 m
Kr-755.5 m
Kr-7614.8 h
Kr-771.19 h
Kr-7934.9 h
Kr-812.1 X 10^5 y
Kr-83m1.86 h
Kr-8510.76 y
Kr-85m4.4 h
Kr-8776 m
Kr-882.80 h
Kr-893.18 m
Xe-1186 m
Xe-1196 m
Xe-12040 m
Xe-12139 m
Xe-12220.1 h
Xe-1232.08 h
Xe-12516.8 h
Xe-12736.4 d
Xe-131 m11.8 d
Xe-1335.27 d
Xe-133 m2.26 d
Xe-1359.14 h
Xe-135 m15.6 m
Xe-1373.9 m
Xe-13817.5 m
Rn-2066.5 m
Rn-20711 m
Rn-20823 m
Rn-20930 m
Rn-2102.42 h
Rn-21115 h
Rn-21225 m
Rn-22125 m
Rn-2223.82 d
Rn-22343 m
Rn-2241.9 h

The major problem isotopes are: Ar-39, Ar-42, Kr-81, Kr-85. Ar-42 and Kr-85 concentrations will come to an equilibrium. Ar-39 and Kr-81 will keep accumulating in the atmosphere.

The basic problem with inert gas radio isotopes is thet they are expensive to contain. Inert gases do not chemically combine with other elements. The short lived inert gas radio isotopes naturally decay while they are trapped in FNR fuel tube plenums. However, the inert gs radio isotopes with long half lives. if not kept in cryogenic storage, will eventually leak into the atmosphere. In the atmosphere the inert gas isotopes with long half lives will gradually add to the background radiation level.

Almost all of the long lived high atomic weight isotopes can be converted into LLW via fissioning in a fast neutron reactor in accordance with the Ottensmeyer Plan. In isolated storage this fission product LLW naturally decays away in about 300 years. After interim storage for 300 years the principal radio isotopes that remain are Se-79 and Sn-126.

Another source of low atomic weight HLW is exposure of reactor component materials to the neutron flux. The resulting waste isotopes that are difficult to deal with over the long term are Be-10, C-14, Cl-36, Ca-41, Ni-59, Zr-93/Nb-93m, and Nb-94.

An important point to make is that there is no merit in physical separation of the LLW, ILW and HLW storage facilities. There is no reason why these different waste types cannot go in separate areas of the same high and dry Deep Geological Repository (DGR) facility. Such a facility will require a permanent staff of security personnel, radio chemists, etc. so there is no point in duplicating these overhead costs. When HLW is processed through a fast neutron reactor it becomes ILW. Fuel reprocessing is used to separate the ILW into HLW and LLW. In fuel reprocessing the important distinctions are the half life of the waste, the chemical composition of the waste, whether or not the waste can be converted into a shorter half life isotopes using fast neutrons and the neutron cross sections as a function of neutron energy.

The Joint Review Panel is charged with making decisions relating to disposal of Low Level Nuclear Waste (LLW) and Intermediate Level Nuclear Waste (ILW). The NWMO presently has responsibility for disposal of spent CANDU reactor fuel which is one type of High Level Waste (HLW). However, if a fast neutron reactor and spent fuel reprocessing are used to convert spent CANDU reactor fuel from HLW into LLW or ILW then it appears that the Joint Review Panel will also be responsible for disposal of that waste. The issue of who is responsible for HLW while it is in the process of being converted to fast neutron reactor fuel remains to be resolved.

The elements Be, C, Mg, Cl, Ca, Ni, Ru, I should be eliminated from the neutron flux region of a new Fast Neutron Reactor (FNR) designs because these elements lead to formation of long lived low atomic weight isotopes (HLW that cannot be treated via fast neutrons). Elimination of these elements indirectly constrains the maximum allowable FNR primary liquid sodium coolant operating temperature and the maximum allowable FNR fuel tube material stress.

Stable Be-9 absorbs neutrons to form Be-10 which has a half life of 2.5 X 10^6 y.

In nuclear reactors carbon occurs as a small component of steel and as a component of: hydrocarbon seals, electrical insulation, thermal insulation, vibration isolators and B4C neutron reflectors. Neutron absorption by the stable carbon isotope C-13 results in C-14 which has a half life of about 5730 years. Natural decay of C-14 to inconsequential levels takes over 50,000 years. C-14 decays into stable N-14 which is a nearly inert gas at room temperature. Hence if the C-14 containing material is in a sealed container the gas pressure inside the container may rise until the container fails. A further problem is that in the presence of water hydrocarbon compounds gradually deteriorate into carbon dioxide (CO2) gas and methane (CH4) gas. These gases, especially CO2, are difficult to contain. The CO2 gas goes into solution in surrounding water where it combines with any nearby calcium: oxide, hydroxide or carbonate to form Ca(HCO3)2 which is highly water soluble and which diffuses everywhere. The CH4 gas mixes with other natural sources of CH4 and becomes natural gas. In the atmosphere CH4 combines with O2 to form more CO2.

For the foreseeable future the C-14 problem can be mitigated by storing HLW containing carbon in containers in a dry, dark and low temperature environment so that the carbon remains chemically bound to other elements and does not react with air or ground water. These containers must have provision for internal gas pressure release. In the long term mankind will likely have to rely on careful containment to keep the local C-14 concentration at an acceptable level. The best solution to the C-14 problem is to not produce it in the first place.

A challenging problem in nuclear reactors is the use of graphite (C) or boron carbide (B4C) as a neutron moderator/reflector. This carbon is exposed to an intense neutron flux which will gradually produce C-14. The alternative is to make the reactor physically larger and rely upon a thick depleted uranium blanket and liquid sodium for peripheral neutron absorption.

This issue of C-14 formation might in the long term become a public health issue. In my view the best interim solution is to keep the carbon chemically bound in a stable chemical compound from which oxygen and water are carefully excluded.

If C-14 is placed in the OPG proposed Bruce DGR it will eventually escape into the environment. To minimize the environmental load carbon used in neutron reflector applications should be recycled. New FNR designs should use a thick U-238 blanket and a 2.8 m thickness of liquid sodium in preference to B4C to prevent neutron activation of containment materials.

In a fast neutron flux high energy neutron decay produces protons which can be absorbed by Mg-26 to form Al-26 and lower energy neutrons. Al-26 has a half life of 7.4 X 10^5 y and decays by positron emission to form stable Mg-26. However, the Al readily chemically combines with oxygen to form Al2O3 which is insoluble in water. Containment of the Al-26 is relatively simple.

In a CANDU reactor chlorine occurs as a component of chlorinated hydrocarbons used in sealing and insulating materials and as impurity chlorides in zirconium pressure tubes and moderator tubes. Neutron absorption by the stable isotope Cl-35 results in Cl-36, which has a half life of 308,000 years and decays into Ar-36 which is a stable inert gas. Fortunately, as compared to the masses of nickel and calcium, the chlorine content of a CANDU reactor is relatively small. However, chlorine has the chemical property that it forms water soluble salts with a large number of common elements. The best that we can do with respect to existing Cl-36 is to chemically bind it to sodium or lithium and then encase that salt in a sealed container with a pressure relief vent that is engineered to last as long as possible. At some time in the distant future someone will likely have to repackage the stored Cl-36. The only alternate disposal methodology is dilution which will occur if uncontained Cl-36 is buried in the proposed Bruce DGR.

In the future the Cl-36 formation problem can be minimized by changing from CANDU reactors to liquid metal cooled fast neutron reactors that do not use chlorinated materials anywhere near the neutron flux. However, there would still be some impurity chlorine chemically bound to the metallic zirconium and uranium. In this respect it is essential to do all necessary to ensure that in new Canadian nuclear reactors the amount of Cl-35 exposed to the neutron flux is minimized.

One of the consequences of eliminating the isotope Cl-35 from the neutron flux is that practical realization of a molten salt nuclear reactors (MSRs) using a molten salt instead of liquid sodium as the primary coolant becomes much more difficult. Use of a chloride salt requires a high degree of separation of the isotopes Cl-35 and Cl-37. The alternative fluoride salts have very high melting points. While the aforementioned isotope separation is theoretically possible the financial cost is likely prohibitive.

Calcium is a substantial component of concrete and mortar. The isotope Ca-41, which has a half life of 80,000 years and decays via positron emission into stable K-40, arises as a result of neutron absorption by the stable isotope Ca-40. In the presence of water and carbon dioxide calcium forms water soluble Ca(HCO3)2. Isolating radioactive calcium from the environment for a million years means excluding it from water and carbon dioxide for that period. That is a daunting task.

Unless extreme care is used over a protracted period ultimately Ca-41 will find its way into the environment. With respect to the existing Ca-41 the best that we can do for now is to make suitably engineered containers that, if undisturbed and stored in a naturally dry location will last as long as possible. However, at some point in the distant future someone will likely have to repackage the stored Ca-41. Right now the only alternate solution to the Ca-41 problem is dilution. That in effect is what will happen if the Ca-41 is buried in the OPG proposed Bruce DGR.

New reactors should be designed to avoid producion of Ca-41. That means that new nuclear reactor designs should not rely on concrete for peripheral neutron absorption. Adding more non-concrete neutron shielding such as boron or liquid sodium will likely increase the initial cost of new nuclear reactors, but so be it. The Joint Review Panel should recommend that the CNSC do all necessary to ensure that Ca-41 formation is negligible in new Canadian nuclear reactor designs.

Nickel is an essential and relatively expensive component of all steels that have useful strength at high temperatures. Nickel is a relatively rare element. It constitutes about 10% of common stainless steel alloys and constitutes as much as 70% of alloys used in construction of steam generators. When steel is recycled a primary objective is recovery of the nickel content. A nuclear generating station contains many tons of nickel, which accounts for a significant fraction of the total facility cost. The isotopes Ni-59 and Ni-63 arise as a result of neutron absorption by the stable nickel isotopes Ni-58 and Ni-62. Ni-59 has a half life of 80,000 years. Ni-63 has a tabulated half life of 92 years.

Future displacement of fossil fuels with nuclear power will require much more nickel for pipe and tank uses outside the neutron flux. If these pipes and tanks are located in areas not normally accessible to humans, from a nickel conservation perspective it makes sense to recycle weakly radioactive nickel. Such recycling may require a dedicated steel mill facility. The major point is that stainless steel with significant radioactive nickel content should be interim stored in a safe, accessible, high and dry location until the inventory of the stainless steel is sufficient to justify the dedicated steel mill facility run required to process this stainless steel into new nuclear power station components.

The Ru concentration in nuclear reactor components is usually very small, so neutron activation of Ru-96 into Tc-97 (half life = 2.6 X 10^6 years) and Tc-98 (half life 1.5 X 10^6 years) is seldom a significant problem.

Zirconium is extensively used for fuel tubes, calendria tubes tubes and fuel cladding in CANDU reactors due to its relatively low neutron absorption cross section. Fuel tubes are ~ 4 inch diameter X 4 mm wall thickness. Calendria tubes are typically ~5 inch diameter X 1.4 mm wall thickness. Fuel cladding is typically 0.4 mm thickness. Zirconium has many stable and short lived isotopes. From a nuclear waste perspective the troublesome isotope is Zr-93, which has a half life of about 1,500,000 years. Zr-93 arises both as a result of neutron absorption by the stable isotope Zr-92 and as a fission product. The decay product of Zr-93 is Nb-93m, which has a half life of 13.6 years. Its decay product is stable Nb-93.

An important issue with zirconium is that it is an essential ~ 10% alloy component of fuel for liquid sodium cooled fast neutron reactors (FNRs). The zirconium prevents formation of a low melting temperature plutonium-iron eutectic. In a fast neutron flux Zr-93 becomes Zr-94 which is a stable isotope.

For this reason neutron irradiated zirconium should not be buried. It should be stored in a safe accessible high and dry location until it is required as a fuel alloy component for fast neutron reactors. That date may be only a few years hence. Under no circumstances should neutron activated zirconium be stored or buried where it is not easily accessible.

In CANDU reactors a small fraction of the fuel tube weight is niobium. Neutron absorption by the stable isotope Nb-93 results in Nb-94 which has a half life of about 20,000 years. The simplest way to deal with Nb-94 is to leave it alloyed with its host zirconium and to use it as a component of fast neutron reactor fuel. In a fast neutron flux Nb-94 becomes Nb-95, which has a half life of 35 days and decays into stable Mo-95.

Si-32650 yarises from neutron absorption by Si-30
Ar-39269 yarises from neutron absorption by Ar-38
Ti-4448 yarises from proton irradiation of Sc-45
Mn-531.9 X 10^6 yarises from proton irradiation of Cr-52, Cr-53
Fe-603 X 10^5 yarises from protons irradiation of Cu
Se-796.5 X 10^4 yfission product
Kr-817.1 X 10^5 yarises from neutron absorption by fission product Kr-80
Tc-972.6 X 10^6 yarises from neutron absorption by Ru-96
Tc-981.5 X 10^6 yarises from neutron absorption by Ru-96
Pd-1077 X 10^6 yarises from Pd-106
Sn-12610^5 yfission product
I-1291.7 X 10^7 yfission product
Cs-1353.0 X 10^6 yfission product daughter of Xe-135
Cs-13730 yfission product

Of these isotopes the two biggest residual radio toxicity problems in processed spent reactor fuel are the fission products Se-79 (half life = 65,000 years) and Sn-126 (half life = 100,000 years). The shorter half life isotopes Ti-44 and Cs-137 will decay away during a few centuries of isolated storage. The concentrations of the isotopes Si-32 and Ar-39 are negligibly small in fission products. The longer half life isotopes decay sufficiently slowly that they are much less radio toxic. In order to safely recycle processed fission products into unrestricted non-nuclear applications it may be necessary to selectively remove Se and Sn from the stored fission products after these fission products have been in isolated storage for several centuries.

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Overview on Recent Trends and Developments in Radioactive Liquid Waste Treatment

Liquid Radioactive Wastes Treatment - A Review

This web page last updated January 9, 2022.

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