Re: Nuclear Waste,Science, & Politics: Regaining Virginity?

[Bernard Cohen <blc+@pitt.edu>]

I feel that I have provided the simple answer that the public can understand



John Jacobus wrote:



>While there may be lots of information, I doubt if the

>public understand the details.  Many want a simple

>answer.  Unfortunately, the "scientific" findings are

>filled with caveats.  The anti-nuclear side does not

>have to worry about anything but a single position,

>i.e., don't do what the government proposes.

>



    ---I feel that I have provided the simple answer that the public can 

understand. It is outlined below as an excerpt from my recent paper in 

Risk Analysis 23:909-915;2003". That paper was not written for the 

general public, so I would greatly appreciate suggestions for how I can 

present its essence to the general public. "PRA" means "probabilistic 

risk analysis"



AN ALTERNATIVE APPROACH



            In view of the many difficulties cited above, it is 

impossible to develop such a PRA for a specific site. However, there is 

a much easier approach to the problem. A PRA will be presented here for 

an average U.S. site, based on taking all properties of the site as the 

U.S. average. The result can be interpreted as the average result of 

PRAs for a large number of randomly selected sites. The reason why this 

is very much easier than a site-specific PRA is that all of the present 

geological characteristics and all of the unknowable future events 

mentioned above are actually occurring now at various places in the U.S. 

A wide variety of climates, rainfalls, and weather patterns are present, 

earthquakes and volcanoes are occurring, land uplifting is taking place 

and rivers are changing their courses, all sorts of animal and insect 

activities are encountered, there is a very wide variety of land use by 

humans, etc. All of these are therefore taken into account, with their 

appropriate probabilistic weighting, when doing a PRA for an average 

U.S. site. Moreover, any changes in national average properties are very 

much smaller than potential changes at a particular site.



But how does this PRA for an average U.S. site substitute for a PRA for 

a specific site? It seems intuitively obvious that by spending lots of 

effort and money on site selection, the experts should come up with a 

site that is at least as safe as a randomly selected site. This "leap of 

faith" would seem to be easily understandable and acceptable to the 

public. So effectively, we have a PRA for our specific site, or at least 

a conservative estimate of its health impacts. It should be much easier 

to convince the public on this simple "leap of faith" than on a "leap of 

faith" in an extremely complex and uncertain estimate of future 

conditions at a specific site



.



PRA FOR AN AVERAGE U. S. SITE



The PRA presented here [3] is for vitrified high level waste glass 

(HLW), following the technology used in most of the world. The reason 

for doing this rather than considering buried spent fuel will be 

discussed later. The analysis will use the linear-no threshold theory, 

which has been under strong attack as grossly over-estimating the health 

risk of low level radiation [4] such as might be encountered in leakage 

from a waste repository; this means that our PRA contains a measure of 

conservatism. The dominant health impact is deaths from cancer, and we 

assume no progress in curing that disease over the next many thousands 

of years, another major ingredient of conservatism. Still another 

element of conservatism is the fact that rainfall in the present era is 

much larger than the historical average [5].



Our PRA consists of two basic steps, first determining the number of 

cancer deaths expected if all of the HLW were ingested by people, and 

then estimating the fraction of this HLW that would enter human 

stomachs. The first step is a straightforward Health Physics 

calculation, presented in Appendix A, which results in Figure 1. This is 

a plot of the number of cancer deaths from the HLW produced per GWe-y of 

electricity if it were all fed to people vs the time after removal from 

the reactor at which this feeding takes place [6].



The second step is to estimate the probability per year for an atom of 

HLW to be dissolved out by groundwater and eventually enter a human 

stomach [7]. We do this by using natural rock as an analogue, and then 

assessing the differences between natural rock and HLW buried at the 

same depth, which we take to be 600 meters. For natural rock, the 

calculation is outlined and referenced in Appendix B. The result is the 

product of (1) the probability per year for an atom in the rock to be 

dissolved out by groundwater, and (2) the probability for an atom 

dissolved in groundwater to enter a human stomach.



The starting point for (1) is the amount of material dissolved out of 

U.S. (lower 48 states) rock and soil and carried into the oceans each 

year. This is well known from analyses of water emerging from the mouths 

of the Mississippi, St. Lawrence, Columbia, Colorado, Hudson, and a few 

other rivers. Dividing this by the area of U.S. then gives the average 

meters of rock depth dissolved; combining this with estimates by 

hydrologists of the fraction derived from ground water (vs. river 

water), we find the material dissolved out of rock by groundwater to be 

3.6 E-6 meters of depth per year. We next need to estimate the fraction 

of this material that is dissolved from 1 meter of depth at 600 meters, 

i.e. from between 599 and 600 m depth. It is obviously less than 1/600 

which would be the case if aquifer flow were uniform with depth down to 

600 meters and zero below that; analysis of hydrological data gives this 

fraction to be 1/4000 [8]. Multiplying this by 3.6 x 10-6 gives our 

result  that about 1 E-9 m of depth is dissolved from this 1 meter. This 

means that the probability per year for dissolution of an atom of 

average rook at this depth is 1 E-9. An alternative completely 

independent calculation of this probability is outlined and referenced 

in Appendix B.



An estimate for (2), the transfer probability from groundwater to human 

stomachs, is obtained by assuming that the probability for an atom 

dissolved in groundwater to enter a human stomach is the same as that 

probability for a molecule of the groundwater itself. This is calculable 

from hydrological information as illustrated in Appendix B, where this 

probability is estimated to be 4 E-4. Thus the probability per year for 

an atom of average rock from 600 m depth to enter a human stomach is (1 

E-9 x 4 E-4 ) =4 E-13.



But how does this apply to high level waste glass? There are ways in 

which this HLW is less secure than average rock. The HLW is connected to 

the surface by shafts and boreholes used in site selection and 

construction of the repository, but expert opinion seems to be that 

these can be sealed to be at least as secure as undisturbed rock [8]. 

The temperature of the HLW is elevated due to radioactivity heat for the 

first few hundred years, which can cause accelerated leach rates and 

rock cracking. But the high temperature problem is eliminated by 

enclosing the HLW in a casing that will prohibit contact with 

groundwater for a thousand years or more, and the rock cracking problem 

can be avoided by spacing the HLW packages far enough apart to keep 

temperatures well below the cracking threshold. Another difference 

between HLW and average rock is that the former is about three times 

less resistant to leaching as determined by analysis of leach rate tests 

[9]. This means that the above calculated probability should be 

multiplied by three to give the probability per year for an atom of HLW 

to enter a human stomach as about 1 E-12.



It should be noted that there are also ways in which the HLW is more 

secure than average rock. The leach resistant casing, the backfill 

material (bentonite clay) which swells when wet to seal against water 

intrusion and which also strongly adsorbs potentially escaping 

radioactive materials, the fact that the site is carefully selected by 

geology and hydrology experts rather than being randomly selected, the 

ability to easily detect escaping material and take protective action 

long before it becomes a health menace, etc are examples of this 

improved security, but we conservatively take no credit for them here. 

Similarly, we take no credit for the very substantial time delays 

(typically a thousand years) for movement of groundwater from deep 

underground to the surface, and the retardation relative to groundwater 

flow velocity by factors of hundreds or thousands in transport of 

radioactive materials by various rock adsorption processes [10]; these 

allow most of the radioactivity to decay away, as can be seen in Fig. 1, 

before reaching human stomachs. It is therefore with substantial 

conservatism that we adopt the above 1 E-12 result.



The number of expected cancer deaths per year from buried HLW is then 

the curve in Figure 1 multiplied by 1 E-12, which is easily obtained by 

simply reading that curve with the scale on the right side. Since this 

is the number of deaths per year, the total number of eventual cancer 

deaths is calculated by summing it (i.e. integrating) over millions of 

years -- the end point of this summation is explained in Reference [6]. 

The final result is that we may expect about 0.02 eventual cancer deaths 

per GWe-y of electricity generation [7]. That is the result of our PRA.



Once we have a PRA result, we are in a position to judge whether HLW is 

an acceptable risk. 'To do this we may make comparisons with the wastes 

from coal burning which is our principal alternative. Coal burning is 

estimated to cause about 30 deaths per GWe-y from air pollution [11], 

plus similar numbers from carcinogenic chemicals released into the 

ground [12], and similar numbers from uranium released into the ground 

[13] to serve as a source of future radon exposures - nuclear power 

avoids future radon exposures by mining uranium out of the ground. Thus 

the HLW is thousands of times ([30 + 30 + 30] / 0.02) less harmful to 

human health than the wastes from coal burning, which surely makes its 

risks acceptable. Our PRA has served its principal purpose.



Therefore, a rational regulatory requirement would be that the selected 

site be at least as favorable, judging by readily obtainable 

information, as a randomly selected site. That should be cheap and easy 

to establish with a reasonable degree of confidence. It should also be 

much more understandable to the public than the present extremely 

complex and somewhat arbitrary system for judging the safety of a HLW 

repository.