XYLENE POWER LTD.

NUCLEAR WASTE CATEGORIES

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

NUCLEAR WASTE CATEGORIES:
The Joint Review Panel is presently 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 High Level Waste (HLW), which in Canada is spent CANDU reactor fuel. However, if a fast neutron reactor is used to convert spent CANDU reactor fuel into LLW and 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.
 

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

HLW:
High level waste (HLW), which in Canada is spent fuel from CANDU reactors, is highly radio toxic due to its plutonium and trans-uranium actinide content. The NWMO currently plans to bury untreated HLW in copper containers.

In my view the present NWMO plan is complete foolishness. Using proven technology HLW can be converted into fuel for liquid sodium cooled fast neutron breeder reactors. Such reactors fission the HLW atoms so that they become LLW atoms which are simple to deal with from a disposal perspective. Furthermore, fast neutron reactors multiply by 100 fold the useful energy obtainable per kg of natural uranium as compared to a CANDU reactor.

The existing inventory of HLW could be interim stored at Jersey Emerald and then chemically processed at Trail, BC which is about 40 km away. Trail has a long history of bulk chemical processing of highly toxic materials.
 

ILW:
All nuclear power technologies produce some intermediate level waste (ILW). The main source of this ILW is exposure of reactor component materials to the intense particle and radiation fluxes present inside a nuclear reactor. This ILW is the most difficult nuclear waste to deal with in the long term and deserves attention on an element by element basis. The ILW problem isotopes are Ni-59, Ni-63, Cl-36, Ca-41 Zr-93/Nb-93m, Nb-94, and C-14.
 

WASTE TYPES:
An important point to make is that there is no merit in physically separating 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 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 part of it becomes ILW and part becomes LLW. These artificial waste distinctions are from a science perspective, nonsense. 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.
 

NICKEL:
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 typically contains hundreds of 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 tabulated 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. From a nickel conservation perspective it makes little sense to irradiate fresh nickel and then 60 years later to permanently bury that irradiated nickel. It makes much more sense to recycle irradiated nickel into future nuclear reactors. Such recycling may require a dedicated steel mill facility. However, the major point is that metal alloys with significant radioactive nickel content should be interim stored in a safe, accessible, high and dry location, such as Jersey Emerald or another comparable naturally dry mine, until the inventory of these irradiated alloys is sufficient to justify the dedicated steel mill facility required to process these alloys into new nuclear reactor components.
 

CALCIUM:
Calcium is a substantial component of concrete and mortar. The isotope Ca-41, which has a half life of 80,000 years, 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, such as Jersey Emerald, will likely last over 10,000 years. However, at some point in the distant future someone is going to have to deal with 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 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 will likely increase the initial cost of new nuclear reactors, but so be it. The Joint Review Panel should recommend that the CNSC ensure that Ca-41 formation is negligible in new Canadian nuclear reactor designs.
 

CHLORINE:
In a CANDU reactor chlorine occurs as a component of chlorinated hydrocarbons used to in sealing and insulating materials. Neutron absorption by the stable isotope Cl-35 results in Cl-36, which has a half life of 308,000 years. 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 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 that is engineered to last over 10,000 years. At some time in the distant future someone will have to deal with the stored Cl-36. The only alternate disposal methodology is dilution which will occur if the 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. In this respect it would be helpful for the Joint Review Panel to recommend that the CNSC ensure that in new Canadian nuclear reactors there is no chlorine in the proximity of the neutron flux.
 

ZIRCONIUM:
Zirconium is extensively used for fuel tubes and moderator isolation tubes in CANDU reactors due to its relatively low neutron absorption cross section. Zirconium has many stable and short lived isotopes. However, 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.

The real issue with zirconium is that it is an essential alloy component of fuel for liquid sodium cooled fast neutron reactors. 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, such as Jersey Emerald, 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 irradiated zirconium be stored or buried where it is not easily accessible.
 

NIOBIUM:
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.
 

CARBON:
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 neutron reflectors. Neutron absorption by the stable 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. A basic problem is that in the presence of water carbon containing compounds gradually deteriorate into carbon dioxide (CO2) gas and methane (CH4) gas. These gases 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 carbon containing ILW in containers in a dry, dark and low temperature environment, such as Jersey Emerald, so that the carbon remains chemically bound to other elements and does not react with air or ground water. In the long term mankind will likely have to rely on careful containment to keep the local C-14 concentration at an acceptable level.

A challenging problem in nuclear reactors is the use of graphite (C) or boron carbide (B4C) as a neutron reflector. The carbon in the neutron reflector is exposed to an intense neutron flux which will gradually produce C-14. The alternative is to make the reactor physically twice as large and rely upon a uranium blanket 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 recycle the irradiated carbon so that the total amount of irradiated carbon is minimized and the C-14 remains chemically bound in a stable compound such as B4C from which oxygen and water are carefully excluded.

If C-14 is placed in the Bruce DGR it will eventually dilute into the environment. To minimize the environmental load carbon used in neutron reflector applications should be recycled.
 

This web page last updated July 20, 2014