Post-closure Period Research Articles

The development and application of integrated procedures for post-closure assessment, based upon Monte Carlo simulation: the probabilistic systems assessment (PSA) approach

Abstract PSA seeks to define a comprehensive, integrated model of the disposal system and to explain the implications of the uncertainties and associated subjective judgements, under which forecasting about the long-term post-closure period must inevitably be conducted, by the use of Monte Carlo simulation. A general procedure for integrated assessment is described together with an overall representation of the evolving environmental system into which the repository and its wastes are emplaced. Following a brief history of the development of PSA within the OECD countries, during the past 15 years, some recent applications in the UK and the USA are discussed. Conclusions are drawn as to the likely developments that will occur in the next few years. In particular, the formal but imaginative treatment of uncertainty traced from after acquisitions, through the interpreter of geological site data, interpretation of interacting processes and the spatial and temporal variations of the system into justifiable estimates of radiological performance suited to particular regulatory interests. As a secondary issue, the article provides a basis for resolving current debates ove the use of ‘simulation’ and ‘scenario’ methods. Given the current level of perceived knowledge and articulation of the implications of uncertainties through PSA it is not yet clear how logically to decide on further site geological studies (for instance) and whether or not these can reliably improve the long-term performances estimates that result from further analyses.

Risk Assessment Methods for Establishing Clean Closure Levels

Establishment of clean closure concentrations for low levels of hazardous constituents in soils, ground water, and other environmental media by assessing risks to human health and the environment is a tool that is being used with increasing frequency. The four-step process sanctioned by the U.S. Environmental Protection Agency (EPA) includes: 1) hazard identification, 2) exposure assessment, 3) toxicity assessment, and 4) risk characterization. This paper describes the methodology and applies it to the closure of a hazardous waste surface impoundment in Florida. To date, the closure process has taken nearly five years and, pending the EPA's and Florida Department of Environmental Regulation's (FDER's) final approval, will result in clean closure of a small facility that has spent in excess of $450,000 U.S. during this period for removal of 516 cubic metres of sludge and other solids. If clean closure is approved, as is concluded to be protective of human health herein, the facility owner stands to save $275,000 U.S. that is estimated would be required to monitor ground water and to maintain an impermeable cap for the 30-year post closure period currently required under U.S. and Florida regulations. The risk assessment discussed in this paper began when soil samples collected after excavation of 516 cubic metres of sludges, soils, and other solids showed that the preliminary soil closure levels sanctioned by the FDER for chromium and nickel had not been achieved. Continued excavation would have cost a minimum of $50,000 to $100,000 U.S. (in 1988 dollars) and still may not have reached closure levels for these two metals. It is concluded in this paper that the maximum levels of chromium and nickel remaining onsite after remedial excavation (2.1 and 3.7 mg/kg, respectively) do not pose a risk to human health, and that the agency could raise the allowable closure level for these two metals from the current values of 1.1 and 0.70 mg/kg to these maximum values.

Mcc Corrosion Tests at Reference Testing Conditions for A27 Cast steel in Hanford Ground Water

AbstractThe Materials Characterization Center (MCC) at Pacific Northwest Lab- oratory is performing three kinds of corrosion tests for the Basalt Waste Isolation Project (BWIP) to establish the interlaboratory reproducibility and uncertainty of corrosion rates of container materials for high-level nuclear waste. The three types of corrosion tests were selected to address two distinct conditions that are expected in a repository constructed in basalt. An air/steam test is designed to address corrosion during the operational period and static pressure vessel and flowby tests are designed to address corrosion under conditions that bound the condi ring the post-closure period of the repository.The results of tests at reference testing conditions, which were defined to facilitate interlaboratory comparison of data, are presented. Data are reported for the BWIP/MCC-105.5 Air/Steam Test, BWIP/MCC-105.1 Static Pressure Vessel, and BWIP/MC-105.4 Flowby Test. In those cases where data are available from a second laboratory, a statistical analysis of interlaboratory results is reported and expected confidence intervals for mean corrosion rates are given. Other statistical treatment of data include analyses of the effects of vessel-to-vessel variations, test capsule variations for the flowby test, and oven-to-oven variations for air/steam tests.

Pattern of crypt cell proliferation in the pre- and post-closure ileum of the neonatal rat: effects of sympathectomy.

The present experiments were designed to investigate ileal crypt cell division before and after neonatal closure to macromolecular absorption by measurements of mitotic index, labelling index, colchicine-induced estimation of mitotic rate (mitoses/cell/hour) and crypt depth and villus height on postnatal days 15--23. In addition, the effects of the sympathetic nervous system on crypt cell proliferation were analyzed by guanethidine-induced sympathectomy. Guanethidine treatment resulted in at least a 70% reduction in superior cervical and coeliac perikarya at 15 days after birth. All cell proliferative indices demonstrated a rather constant rate of cell division in the ileal epithelium between 15 and 17 days after birth with a sudden burst of mitotic activity on day 18. The mitotic rate of control rats increased from day 18 to 23 with 3-week-old rat demonstrating a faster rate of ileal crypt cell proliferation than adult rats. The ileal crypt depth more than doubled between 15 and 23 days while the height of the villus column increased slowly but steadily (36%) during the period of this study. Cell division is inhibited by guanethidine-induced sympathectomy although there is still an acceleration of mitosis in the post-closure period. The relationship of sympathectomy and ileal crypt cell proliferation is discussed and compared to hormonal effects on closure and related developmental events.

Intestinal absorption and gastro-intestinal digestion of protein in the young rat during the normal and cortisone-induced post-closure period

Experiments were performed on 12-day-old cortisone acetate-treated rats, their litter mate controls, and 24-day-old normal rats. While orally administered 125I-labelled rat IgG passed freely into the circulation of 12-day-old rats, barely detectable amounts of this protein was transferred to the sera of 12-day-old cortisone-treated rats and normal 24-day-old rats. There is a large difference between the normal 12- and 24-day-old rat in respect of their peptic and tryptic activities, but the cortisone-treated 12-day-old rat is not significantly different from the normal animal of the same age. A comparison of the gel filtration results for the intestinal washes taken from normal and cortisone-treated 12-day-old rats implies an increased efficiency of the overall process of intra-luminal digestion in the latter animals, intragastric digestion of the administered rat IgG was complete in the 24-day-old rats. The recoveries of radioactivity from the stomachs and intestinal walls of rats fed 3 h previously with 125I-labelled rat IgG differed between representatives of each group.

A non-retreating Bunsen burner

There are two RBMK-1500 type reactors in Lithuania, which are under decommissioning now. Long-lived low and intermediate level radioactive metallic waste generated during operation and decommissioning of the nuclear power plant is foreseen for deep geological disposal (DGR). A preliminary assessment of the radionuclide release from metallic waste and migration through the engineered barriers of the generic DGR in crystalline rocks is presented in this study. The radionuclide flux to backfill and geosphere is investigated for three different container alternatives: (1) disposal of metallic waste in stainless steel containers, (2) disposal in carbon steel containers and (3) disposal in which the container is not a barrier for radionuclide migration. In addition, the impact of surface contamination of the activated waste on the radionuclide flux is investigated. Comparison of maximum fractional flux to backfill between Alternatives 1 and 2 revealed that the container material plays a noticeable role in the migration of long-lived low mobility radionuclides. The maximum fractional flux to the geosphere for all radionuclides increases from Alternative 1 to Alternative 3. Surface contamination is an important parameter when the maximum radionuclide flux to backfill is observed during the early post-closure period.