Especialización del sentido

As discussed earlier, the radioactive species that comprise the spent fuel to be disposed of at Forsmark are

of the following three major types:

 Fission products: these are the radionuclides generated when the fissile U-235, Pu-239 and Pu-241

atoms split during the fission reactions. Many of these fission products undergo further decay into

other radioisotopes, with each decay releasing more radiation in the form of alpha particles, beta

particles, neutrons, and x- and gamma-rays.

 Actinides: some heavy isotopes absorb neutrons and convert to even heavier forms, such as

uranium, plutonium, and curium – all of which are radioactive.

 Activation products: these are caused by the absorption of neutrons from lighter weight elements

found in assembly hardware and cladding. The most important activation product in terms of

radiation is Co-60 caused by the activation of nickel found primarily in assembly hardware.

The relative radioactivity of a particular radionuclide is dependent on:

 The relative abundance of the radionuclide in the waste;



The ―half-life‖, or the amount of time it takes for half of the radionuclide to decay away; and

 The type of radioactivity decay that occurs, include how much energy is released.

For a particular radionuclide to contribute significantly to the overall dose assessment, the radionuclide

must have the following properties:

 Is of sufficient abundance in the spent fuel

 Has a relatively high solubility in groundwater

 Has a relatively short travel time through the rock fractures to the biosphere. This is dependent on

the sorption characteristics of the radionuclide onto the rock.

 Has a relatively high LDF

 Will be able to escape from the repository before it decays significantly. This is dependent upon:

o The location of the radionuclide in the spent fuel. Since the spent fuel dissolution rate is

slow, the shorter-lived radionuclides must also be found in locations of the spent fuel

where it can be dissolved more quickly than the bulk of the spent fuel, such as in the

fuel/cladding gap or along fuel particle grain boundaries.

 If it is relatively short, the radionuclide may only provide a significant contribution

to dose at earlier times. Even then, it can only contribute to dose at earlier times if

the buffer and canister have both failed at earlier times, the radionuclide is able to

escape from the spent fuel quickly, and the geology does not provide much retention

of the radionuclide as it travels to the biosphere.

 If the half-life is longer, then it could contribute at times all the way out to one

million years. There are some radionuclides with half-lives in excess of one million

years.

The vast majority of the radionuclides in the waste to be disposed of in a geologic repository will have

decayed away well before they even have a chance to escape from the waste container after the container

fails. Once the waste container fails, the cladding must also fail before the fuel is exposed to groundwater.

Then the fuel must dissolve in the groundwater, pass by the clay buffer and through the rock fractures

before the remaining radionuclides can enter the biosphere. Thus, with the exception of cases where both

the buffer and canister fail at early times, the doses to humans from radioactive waste in deep geologic

disposal will be from very long half-life, soluble, and mobile radionuclides in the waste.

4.7.6 Evaluation of the Total System Performance Assessment Results

The IRT has reviewed the performance assessment results presented in TR-11-01, including the sensitivity

studies, and choice of fixed and uncertain variables included in the probabilistic performance assessment.

The primary conclusions from the total system performance assessment (TSPA) are:

 For the reference scenario, the mean number of deposition holes for which the buffer would lose its

function (due to advective conditions leading to buffer erosion) is 23 out of 6000 in one million

years, or 0.4%.

 For the reference scenario, even assuming 100% of the deposition holes suffered advective

conditions from the very start, the mean number of canister failures is 0.17 out of 6000, or

0.003%.

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 The peak dose to an individual in the most exposed group for the canister failure due to corrosion

scenario is:

o Negligible for times up to approximately 50,000 to 100,000 years after repository closure;

o About two orders of magnitude below the regulatory limit at 100,000 years

19

; and

o About one order of magnitude below the regulatory limit at 1,000,000 years.

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 The peak dose to an individual in the most exposed group for the canister failure due to shear

scenario is:

18 _{The buffer and canister failure rates are the means of a probabilistic distribution in which the number of failures is} calculated for many different input values. Each, single calculation using a set of input values sampled from a distribution is called a ―realization‖. For the canister mean failures, the number is less than one because many of the results from individual realizations had zero canister failures in one million years while a few of the realizations resulted in estimates of one or more canister failures in one million years. Averaging over all realizations results in the mean value of 0.17 canister failures. The same process was used to derive the mean number of buffer failures of 23 in one million years.

19 _{Note that the IRT is in no way providing an assessment of whether or not SKB has complied with the}

regulations. That assessment is the responsibility of SSM and other national organizations, and involves much more than a simple comparison of the calculated dose rates against the regulatory criterion.

20

The IRT is aware that the dose or health risk criterion in the SSM regulations does not apply out to one million years.

o About three orders of magnitude below the regulatory limit at 1000 to 10,000 years after

repository closure; and

o About two orders of magnitude below the regulatory limit at 1,000,000 years.

 The single radionuclide contributing the most to the dose estimate is Ra-226, except for the dose

estimates for the canister shear failure scenario at times prior to about 50,000 years for which C-14

and Nb-94 dominate the dose estimate.

With the exception of the relevant issues affecting overall repository performance discussed earlier in this

report, and those identified by SKB itself, the IRT finds SKB‘s approach to estimating repository to be

―conservative‖ – meaning SKB is likely to have overestimated the number of buffer failures, the number of

canister failures, and the assessed dose rates to a human in the most exposed group. Many of the properties

of the geology and repository design are uncertain over the long time periods required for SKB to conduct

an assessment. For many of these properties – but not all – SKB has assumed ―bounding‖ property values,

i.e., those property values that would maximize the estimated number of buffer and/or canister failures.

While the goal is to characterize the uncertainties well enough to be able to use a reasonable distribution of

geology and repository properties, it is standard international practice to use bounding, or near-bounding

properties when the actual distribution or uncertainty range of geology and repository properties are not

known. This does not mean the IRT is certain SKB has overestimated the buffer and canister failure rates

or the dose rates to humans. Both the IRT and, more importantly, SKB itself recognize more work need to

be done to improve confidence of the repository system performance.

A few examples of properties for which the uncertainty should be reduced are:

 Canister properties at the time of production – particularly the reliability of both the welding

process and the ability to detect welding flaws of sufficient size to jeopardize the ability of the

canister to provide its required safety function. The IRT understands that at the time the TR-11-01

report was produced, testing of only eight tubes and 20 lids formed the basis for SKB‘s estimates

of container failures. IRT recommends and SKB is already planning to increase the number of

tests of these canister components.

 Similarly, the buffer initial density distribution estimate SKB uses is based on just 25 blocks (10

ring-shaped, and 15 solid). Yet Table 5-12 in TR-11-01 provides buffer densities out to the 99.9%

confidence interval.

Regarding the radionuclides that dominate estimated doses, the IRT finds the dominant radionuclides

sensible. For the cases in which SKB determines both the buffer and canister fail at shorter times, both C-

14 and Nb-94 have higher LDFs, are in sufficient abundance, are soluble, are present in the fuel gaps

and/or between fuel grains, and are not well sorbed onto rock surfaces. Their half-lives, however, are not

long enough to contribute to dose estimates at very long times.

The IRT also finds that Ra-226 contributes to long-term dose estimates reasonable. There are, however,

other long-lived radionuclides that could contribute to long-term dose rate estimates. The IRT is not fully

convinced that the dominance of Ra-226 is ―real‖ in the sense that the SKB dose estimates, which are

based on many assumptions about radionuclide solubilities, travel times, and LDFs, may be dependent on

the particular choice of SKB‘s assumptions. SKB did provide a response to the IRT‘s question about this

particular issue, however (SKB document 1334122), although the response does not fully put to rest IRT‘s

concerns.

4.8 Performance confirmation

In document Idiomas - Lenguaje de Signos (página 93-99)