No country with nuclear power today has solved the waste disposal problem. The preferred solution being sought today is to disperse the waste in repositories hundreds of meters below the earth’s surface. The (perceived) absence of success in this area is a dominant obstacles that the nuclear industry faces. Last Friday, I after a discussion of nuclear energy started, with a lot of half-remembered data on my side and in order to stop that feature of the conversation, I dug up on the net an authoritative report on the “future of nuclear energy.” These papers are in pdf form:
- The full document is here. This is a study by a group of MIT professors on the status of Nuclear power in the US and the world.
- The summary is here. This is a summary of the findings in the prior document.
- Finally, in 2009 (the original documents were written in 2003) an update of the current situation given the economic and political conditions is given here.
In the discussion last night (on this post) waste seemed the dominant topic. As noted, that post last night was a summary (of a summary). So I’m going to delve in to the report’s waste chapter for more grist.
The US EPA has a stringent requirement for a waste disposal facility. The exposure and individual living near a facility must not be exposed to more than 15 millirems per year (!) for the first 10,000 years after final disposition. 15 millirems is twenty times less than the average exposure received by an individual from background radiation. The authors take this standard without comment. This is the goal for a safe repository.
One recommendation they have is to store waste above ground and in temporary (decades in timespan) holding areas. This has the following benefits:
- So that the higher activity components (Strontium and Cesium) can “burn off” and the resultant waste is easier to handle and more can be put in the same repository. Str and Cs have a 30 year half life.
- This allows more flexibility in the logistics and movement into repositories.
- It allows a deliberate path and development route to the repository development, i.e., the timelines requirments for repository requirements would be understood in advance.
Currently fuel “burnup” is at LWR reactors is 33MWD/kg (Megawatts drawn per kg). Current industry estimates indicate that 75 or 100 MWd/kg is feasible which in turn would mean a threefold reduction in waste to be disposed. This would help. They also note in later years (21st century) this average has risen to almost 45 as plants do this in order to reduce downtime and increase plant capacity.
The authors of this study are proponents of a alternative disposal technique to the one noted above (several hundred meters down like at Yucca). Their appoach is:
An alternative to building geologic repositories a few hundred meters below the earth’s surface is to place waste canisters in boreholes drilled into stable crystalline rock several kilometers deep. Canisters containing spent fuel or high-level waste would be lowered into the bottom section of the borehole, and the upper section – several hundred meters or more in height – would be filled with sealant materials such as clay, asphalt, or concrete. At depths of several kilometers, vast areas of crystalline basement rock are known to be extremely stable, having experienced no tectonic, volcanic or seismic activity for billions of years.
The main advantages of the deep borehole concept relative to mined geologic repositories include: (a) a much longer migration pathway from the waste location to the biosphere; (b) the low water content, low porosity and low permeability of crystalline rock at multi-kilometer depths; (c) the typically very high salinity of any water that is present (because of its higher density, the saline water could not rise convectively into an overlying layer of fresh water even if heated); and (d) the ubiquity of potentially suitable sites.
Geologican conditions for such sites are common and can also be found offshore (where companies like Japan, Korea, and Taiwan would like to site reactors on man made islands … which could house the temporary storage facilities underground as well (as well as the boreholes for the deep repositories). Disadvantages are noted:
Implementing the deep borehole scheme would require the development of a new set of standards and regulations, a time-consuming and costly process. A major consideration would be the difficulty of retrieving waste from boreholes if a problem should develop (though the greater difficulty of recovering the plutonium in the waste might also be an advantage of the borehole scheme). Current U.S. regulatory guidelines for mined repositories require a period of several decades during which the high level waste should be retrievable. This would be difficult and expensive to ensure in the case of deep boreholes, though probably not impossible. Moreover, at the great depths involved, knowledge of in situ conditions (e.g., geochemistry, stress distributions, fracturing, water flow, and the corrosion behavior of different materials) will never be as comprehensive as in shallower mined repository environments. Recovery from accidents occurring during waste emplacement – for example, stuck canisters, or a collapse of the borehole wall – is also likely to be more difficult than for corresponding events in mined repositories. Finally, despite the order of magnitude increase in the depth of waste emplacement, it is difficult to predict the impact on public opinion of a shift in siting strategy from one large central repository to scores of widely dispersed boreholes.
A Swedish study cited estimates the cost of this sort of drilling of a 4-km deep borehole to be drilled and cased in 5 months at a cost of $5m.
It is noted that the legal limit set for the Yucca Mtn repository is set at 70k MTHM … expert observers set the real technical/engineering or physical limit at about twice that. In the prior post, there were notes on the requirements for how much waste repositories were required to run at the 1000 GW-year level at needed at 70k MTHM storage every 3-4 years, which puts the repository requirements in context.
The primary reason the authors are against reprocessing of waste, which entails separating the actinides and the cesium/strontium. The former to be returned to the reactor field for use as well as partitioning high energy waste material to be degraded by neutron activity and the latter with a 30 year half life, to allow to decay significantly before storage aboveground. Another reason for separation of the actinides is to remove the plutonium from the waste (so that could not be recovered from the waste in later generations for weapon use). This has the double edged sword of being then being around right now … for weapon use either by the government in question or stolen by third parties. The authors contend that the advantages of waste reprocessing, i.e., fuel recovery and less hot waste material is outweighed by the dangers and expense inherent in reprocessing as well as the greater (plutonium) proliferation risk.