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Re: RADSAFE digest 2714
Hi Martin,
I occasionally forget radsafe doesn't take attachments as other lists
do.
I have a brief write-up from Dr. Seldin that I should comment on before
he sends, but just no time (also sent it to Hallisey - do you want it?)
Nothing on a schedule.
Regards, Jim
============
Martin Fraser wrote:
>
> Jim,
> Post attachments with caution (or never, as would be my preference!) ;)
>
> Have you heard of another KI meeting?
>
> regards
> Martin
>
> >------------------------------
> >
> >Date: Sun, 17 Oct 1999 14:25:11 -0400
> >From: Muckerheide <muckerheide@mediaone.net>
> >To: rad-sci-l@ans.ep.wisc.edu, ans-pie@nuke-ans.org,
> >Subject: Science -- Ewing 286 (5439): 415
> >Message-ID: <380A1454.63BFA04F@mediaone.net>
> >
> >This is a multi-part message in MIME format.
> >--------------F46C14A90C803149C7E2B003
> >Content-Type: text/plain; charset=us-ascii
> >Content-Transfer-Encoding: 7bit
> >
> >Group.
> >
> >Should the attached comparison radiotoxicity be for inhalation?
> >
> >Regards, Jim Muckerheide
> >muckerheide@mediaone.net
> >========================
> >
> >http://www.sciencemag.org/cgi/content/full/286/5439/415
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> > <A HREF=3D"/cgi/external_ref?access_num=3DEwing%2BRC&link_type=3DAUTH=
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> > <VARDEF ID=3DTEXT>
> ><!-- FULL -->
> ><H2 NAME=3DHEADLINE><FONT SIZE=3D-1>RADIOACTIVE WASTE:</FONT><BR>Less Geo=
> >logy in the Geological Disposal of Nuclear Waste</H2>
> ><STRONG>Rodney C. Ewing</STRONG><A HREF=3D"#affiliation"><SUP>*</SUP></A>=
> ><P>
> ><STRONG>A</STRONG>t the end of the 20th century, one of the most pressing=
> > problems, political and scientific, is the disposal and isolation of rad=
> >ioactive wastes (<A HREF=3D"#ref1">1</A>). There is now a worldwide conse=
> >nsus (<A HREF=3D"#ref2">2</A>) that effective long-term isolation for spe=
> >nt nuclear fuel and high-level radioactive or transuranic waste can be ac=
> >hieved by geologic emplacement. The chief elements of the geologic dispos=
> >al strategy are as follows: (i) it was to be deep, permanent, and long te=
> >rm, using to advantage the decrease in radioactivity, thermal output, and=
> > radiotoxicity over time; (ii) the geologic history of stable regions was=
> > taken as indicative of future stability and continued suitability; (iii)=
> > the passive hydrologic and geochemical properties of the geologic format=
> >ions were to be key to the isolation strategy. The large uncertainties in=
> > predicted future behavior were to be reduced by a system of independent,=
> > multiple barriers, geologic and engineered. However, the present approac=
> >h in the United States, as embodied in proposed rules and standards, move=
> >s away from the fundamental precepts of geological disposal.<P>
> >In the United States, two sites have been selected for geologic disposal.=
> > The Waste Isolation Pilot Plant (WIPP) in New Mexico now receives transu=
> >ranic wastes generated by defense programs. The Yucca Mountain site in Ne=
> >vada has been under active investigation for a quarter of a century as a =
> >site for the disposal of high-level wastes, more than 90% of which will b=
> >e spent fuel from commercial power plants. The Nuclear Regulatory Commiss=
> >ion (NRC) has recently proposed rules for implementing the standards for =
> >the disposal of high-level waste at Yucca Mountain, and the Environmental=
> > Protection Agency (EPA) has just released the radiation protection stand=
> >ards for Yucca Mountain. The EPA is responsible for developing "gene=
> >rally applicable standards" for the repository, and the NRC is respo=
> >nsible for promulgating rules or regulations, which if complied with, wil=
> >l ensure that EPA's standards are met. The new rules and standards rely o=
> >n a probabilistic performance assessment (PPA) of the repository to provi=
> >de a single quantitative measure of compliance. This approach, when combi=
> >ned with a proscribed compliance period that is short compared with the t=
> >ime for geologic or climate change (10,000 years), a point of compliance =
> >that is far from the site (20 kilometers), and the elimination of perform=
> >ance standards on separate barriers, represents a significant deviation f=
> >rom the original concept of long-term geologic disposal.<P>
> ><A NAME=3D"image1"><!-- null --></A><P><HR WIDTH=3D60%>
> ><A HREF =3D"/cgi/content/full/286/5439/415/F1"><IMG ALT=3D"Figure 1" ALIG=
> >N=3DLEFT HSPACE=3D20 VSPACE=3D10 SRC=3D"/content/vol286/issue5439/images/=
> >small/415-1-thumb.gif"></A>
> >Relative radiotoxicity on inhalation of spent nuclear fuel with a burnup =
> >of 38 megawatt-days/kg uranium (<A HREF=3D"#ref12">12</A>). The radiotoxi=
> >city values are relative to the radiotoxicity (horizontal line) of the qu=
> >antity of uranium ore that was originally mined to produce the fuel (8 to=
> >ns of natural uranium yields one ton of enriched uranium, 3.5% 235U).
> ><P>
> ><FONT SIZE=3D-2>CREDIT: SKB (SVENSK KÄRNBRÄNSLEHANTERING AB, SW=
> >EDISH NUCLEAR FUEL AND WASTE MANAGEMENT CO.)</FONT><BR><HR WIDTH=3D60%><P=
> >>
> >The concept of geologic disposal began in the 1950s and over the next 30 =
> >years evolved with the publication of criteria for a geologic repository =
> >(<A HREF=3D"#ref3">3</A>) and an analysis of the types of information tha=
> >t would be required and the complexity and uncertainties inherent in a sa=
> >fety analysis of a geologic repository (<A HREF=3D"#ref4">4</A>).<P>
> >The Nuclear Waste Policy Act of 1982 represented a clear commitment to pe=
> >rmanent geologic disposal and provided for the investigation of three sit=
> >es that represented a variety of rock types (bedded salt, basalt, and vol=
> >canic tuff). However, the investigation of multiple sites proved to be ex=
> >pensive and time-consuming, and in 1987, Congress (in the Nuclear Waste P=
> >olicy Act Amendments) limited Department of Energy (DOE) focus to a singl=
> >e site, Yucca Mountain in Nevada. In 1992 (Energy Policy Act), Congress d=
> >irected that the EPA prepare standards specific to Yucca Mountain and tha=
> >t EPA arrange for an analysis by the National Academy of Sciences (NAS) o=
> >f the technical basis for proposed standards (<A HREF=3D"#ref5">5</A>). T=
> >hus, geologic disposal of high-level nuclear waste in the United States i=
> >s now discussed in the context of a single site and site-specific standar=
> >ds. Approximately 3 billion dollars have been spent on studies of the pro=
> >posed Yucca Mountain site, and considerable responsibility and pressure r=
> >ests on the shoulders of all involved federal agencies. In 1998, DOE comp=
> >leted a total system performance assessment-viability assessment (TSPA-VA=
> >) and has reported to Congress that there are no "showstoppers."=
> >; Work proceeds toward a license application as a repository in 2001.<P>
> ><FONT SIZE=3D+1><STRONG>Probabilistic performance assessment</STRONG></FO=
> >NT><BR>
> >During recent years, there has been a move toward "risk-informed, pe=
> >rformance-based regulation" (<A HREF=3D"#ref6">6</A>), and this appr=
> >oach combined with the analysis of total system performance is now the ke=
> >y to licensing a nuclear waste repository. The PPA methodology, as applie=
> >d to geologic disposal of nuclear waste, has developed from roots in prob=
> >abilistic risk assessments of nuclear reactors (<A HREF=3D"#ref7">7</A>).=
> > There is no doubt that PPA (for example, the recent TSPA-VA of Yucca Mou=
> >ntain) is an essential and highly informative method for analyzing comple=
> >x systems and focusing work and resources on their most important element=
> >s.<P>
> >Although PPA is useful, PPA modeling of the complex behavior of a geologi=
> >c system over extended periods of time has not been shown to be effective=
> >=2E The limitations of PPA are especially important when it is applied to=
> > natural systems for which there is a sensitive dependence of the final r=
> >esult on initial or bounding conditions (<A HREF=3D"#ref8">8</A>). In suc=
> >h "chaotic" systems, errors and uncertainty may increase expone=
> >ntially with time. A typical PPA of a repository contains hundreds of sub=
> >system models (often highly simplified descriptions of the physical and c=
> >hemical phenomena), requires thousands of input variables (fixed and samp=
> >led over ranges, many based only on expert opinion), and often does not a=
> >ccount for nonlinear coupling between important elements of the system. I=
> >f the PPA is to be the single quantitative criterion, then it is essentia=
> >l to distinguish between a result that is driven mainly by assumptions ab=
> >out boundary conditions (for example, climate change and assumed rates of=
> > human intrusion) versus the actual properties and behavior of the reposi=
> >tory (such as sorptive capacity). If the results of the PPA depend mainly=
> > on assumed boundary conditions that are often probabilistic, then the ge=
> >ologic properties of the site have little bearing on the analysis.<P>
> >Actual experience with specific subsystem models of hydrologic and geoche=
> >mical processes has shown how disappointingly unpredictive they can be (<=
> >A HREF=3D"#ref7">7</A>). The uncertainties in the fundamental database, k=
> >nowledge of the site, conceptual models, expert opinions, probabilities a=
> >ssigned to initiating or bounding events, and knowledge of the effects of=
> > coupled phenomena on the total system performance pose serious obstacles=
> > to a convincing demonstration of compliance (<A HREF=3D"#ref7">7</A>). E=
> >ach of these uncertainties will propagate through the PPA, and although t=
> >he different types of uncertainty can be described mathematically (<A HRE=
> >F=3D"#ref9">9</A>), little has been done to test the methodology against =
> >the behavior of actual geologic systems. The large uncertainties could ob=
> >scure the analysis of the performance and mask the actual environmental a=
> >nd health impacts of the repository.<P>
> ><FONT SIZE=3D+1><STRONG>Multiple barriers</STRONG></FONT><BR>
> >One early tenet of geologic disposal was that uncertainties in the assess=
> >ment would be large, but could be reduced by requirements placed on indiv=
> >idual geologic or engineered barriers (such as release rates of radionucl=
> >ides, groundwater travel times). The present NRC rulemakings substantiall=
> >y reduce the importance of multiple barriers by eliminating specific perf=
> >ormance standards. This is partly because it is difficult to identify tru=
> >ly independent barriers in a repository system. Another argument against =
> >performance criteria for individual barriers has been that performance sp=
> >ecifications for subsystems can lead to less than optimal design and perf=
> >ormance of the total system. In the extreme, such an argument is inherent=
> >ly illogical. An inert waste form that does not release radionuclides wou=
> >ld improve total system performance. The absence of groundwater flow in t=
> >he unsaturated zone (above the water table) would not allow release of wa=
> >ter-borne nuclides. In real life, however, there is no "silver bulle=
> >t," and each subsystem may fail to some degree over extended time. N=
> >o subsystem can (or should) account for the total system performance; how=
> >ever, a system of arguably independent barriers (for example, the corrosi=
> >on rate of a canister in the unsaturated zone is not closely related to t=
> >he flow-rate in the saturated zone) can reduce uncertainty and increase c=
> >onfidence. More important, in a complicated system, components are more e=
> >asily analyzed than the whole. If the components cannot be analyzed, then=
> > the analysis of the total system is not made more tractable or useful by=
> > combining the subsystem models in order to obtain a single quantitative =
> >measure of successful performance.<P>
> >Most important, understandable and clear subsystem requirements may contr=
> >ibute to increased public acceptance. A geologic repository for which tra=
> >vel times to the accessible environment are less than 1000 years or waste=
> > package release rates are in excess of reasonable materials science perf=
> >ormance standards is certainly not acceptable, regardless of the positive=
> > results of a "quantitative" PPA.<P>
> ><FONT SIZE=3D+1><STRONG>Compliance period</STRONG></FONT><BR>
> >The compliance period of 10,000 years is based on three considerations: t=
> >he decay of short-lived fission products will substantially reduce the ac=
> >tivity; the period is long enough to capture the essential performance fe=
> >atures of the repository; extrapolation of models beyond 10,000 years is =
> >unrealistic. Although there will be a substantial reduction in radioactiv=
> >ity during the first 10,000 years, the repository will still contain subs=
> >tantial quantities of long-lived fission products and actinides that cont=
> >inue to contribute to elevated exposures (see graph on page 415). The rec=
> >ent TSPA-VA of Yucca Mountain showed that the highest levels of exposure =
> >due to <SUP>237</SUP>Np, <SUP>239</SUP>Pu, <SUP>99</SUP>Tc, and <SUP>129<=
> >/SUP>I occurred well after 10,000 years. The NAS committee that examined =
> >the technical basis for the Yucca Mountain standards recommended that the=
> > compliance assessment be extended to the time when the highest risk occu=
> >rs (<A HREF=3D"#ref5">5</A>). This is typically well beyond 10,000 years.=
> > Selecting such a short period, with no analysis extending beyond 10,000 =
> >years, has the effect of eliminating from consideration the effects of ev=
> >ents, seismic and volcanic, that occur at low probabilities.<P>
> ><FONT SIZE=3D+1><STRONG>Point of compliance</STRONG></FONT><BR>
> >The total system performance of a repository is taken as a measure of dos=
> >e to an individual or critical population at some distance from the repos=
> >itory; in the case of Yucca Mountain, this distance has been set at appro=
> >ximately 20 km. Such a calculation is appropriate and reasonable in the e=
> >valuation of risk, but it has little to do with the performance of the un=
> >derground facility. The calculation of risk is qualitative, again because=
> > of the large inherent uncertainties. In order to effectively compare dif=
> >ferent repository designs or disposal strategies at a single site, the ca=
> >lculation of radionuclide release should be made over a much shorter dist=
> >ance in order to emphasize the properties and performance of the reposito=
> >ry. The attractiveness of the Yucca Mountain site was, at least initially=
> >, based on its location in an arid environment and the absence of flowing=
> > groundwater in the unsaturated zone. On the basis of the present approac=
> >h, the site may now be judged to be acceptable because of retardation, di=
> >spersion, and dilution effects during transport over the 20-km path to th=
> >e point of compliance.<P>
> >The sole reliance on PPA to provide a quantitative criterion, in conjunct=
> >ion with the elimination of performance standards for individual barriers=
> >, the geologically short compliance period, and the extended distance of =
> >the point of compliance all combine to reduce substantially the role of t=
> >he geologic properties of the repository in the waste containment strateg=
> >y. Such a shift in approach does little to build public confidence in the=
> > repository site; however, as others have noted, the "U.S. geologic =
> >disposal program is making a last stand at Yucca Mountain" (<A HREF=3D=
> >"#ref10">10</A>). This last stand now relies heavily on engineered barrie=
> >rs (for example, zircaloy cladding and extended waste package lifetime) a=
> >nd the probabilistic analysis of their behavior during the first 10,000 y=
> >ears (<A HREF=3D"#ref11">11</A>). The conclusion that there are no insurm=
> >ountable obstacles in the present strategy and analysis begs the question=
> > of whether Yucca Mountain provides effective geologic barriers to radion=
> >uclide release and whether the present analysis provides the resolution r=
> >equired to recognize a showstopper.<P>
> ><STRONG><STRONG>References</STRONG></STRONG><BR>
>
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