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8 Management of MSRE Hazards Several hazards associated with the fissile and toxic content of the inactive Molten Salt Reactor Experiment (MSRE) facility have been recognized (Liebenthal et al., 1994; Peretz, 1996c). Some of the hazards listed here are hypothetical, not confirmed by direct observations or measurements and not necessarily significant risks at this time. in part, steps have already been taken that reduce the possibility or likelihood of occurrence of some hazards by substantial safety factors. Other hazards (such as melting effects) become possible risks only when that step is taken. Still others are likely to be proven hypothetical and can be removed from further consideration when better information is developed. Essentially all of these hazards, both real and hypothetical, appear to be readily open to better definition of their significance or nonsignificance as risks by means of types of measurements available for assessment of condition and tests for process behavior. CURRENT HAZARDS The hazards2 identified, some of which come from Peretz (1996c), include the following: An important distinction is drawn between "hazards" and "risks." For this section, "hazards" are defined and identified as events that could or might go wrong, so as to produce damaging consequences to people, environment, facilities, or the ability to complete the desired remedial actions as planned, initially without regard to the likelihood or importance (or size) of the consequences. "Risks" are defined as the product of the likelihood of a hazard scenario occurring and a measure of the importance (or size) of the possible consequences (i.e., risk equals probability times consequence). In this report, the main evident hazard scenarios are identified, and preliminary judgments of their manageability are discussed. These judgments are provided with the caveat that normal good safety practices and certain noted risk-limiting precautions be implemented rigorously. 2Some hazards have been identified for the auxiliary charcoal bed, which is outside the scope of this report and for which a remediation plan is already in effect. These hazards 72
il MANAGEMENT OF MSRE HAZARDS 73 I. the alpha and gamma radiolysis-induced reduction of fluorides, probably including the tetrafluorides of uranium and plutonium (UF4 and PuF4), with the emission of fluorine and the formation of uranium hexafluoride (UF6~; 2. the low solubility of the radiolytically produced reducer! components in the salt, causing possible precipitation of segregants on melting and resulting in a possible criticality hazard (thought to be of low probability, as detailed in Chapter 61; 3. a possible critical configuration in the salt tanks due to ntrusion of water into the salt drain tanks; 4. the potential for radiotoxicity and chemical toxicity hazards in the event of system leakage (e.g., radon-220 t220Rn]~; 5. the potential for radiation and chemical exposures of personnel in connection with monitoring, characterization, or remediation activities; 6. the potential for chemical or physical reactions of the salt with the containment, piping, and proposed processing systems, which could excessively delay or preclude orderly completion of a selected plan for remediation (e.g., system plugging by nonvolatiles involving significant inventories of uranium); 7. the potential behavior and criticality hazard of uranium deposited on tank freeboards or headspace (the void above the solidified salts) in the form of radiolytically generated fluorides of uranium that are reduced to oxidation states of less than vr; and 8. the migration of uranium as UFO from the salt, driven by the temperature differential maintained between the interior of the drain tanks and the ambient temperature of connected piping and hardware. are a possible criticality configuration for uranium accumulated on the carbon absorption bed if water were to enter the vessel, the potential for a vigorous exothermic reaction of fluorine with the carbon bed due to the stored chemical or Wigner effect energy, which could result in the dispersal of uranium and fluorine (ammonia treatment may eliminate these concerns of chemical reactivity); and the potential for reactions of UFO with the carbon bed to form UF4, solid CFX, C(F2) (i.e., F2 absorbed on an activated carbon surface), and ultimately gaseous CF4, with the release of a large amount of energy.
74 AN EVALUATION OF DOE ALTERNATIVES FOR MSRE HAZARD MANAGEMENT ACTIVITIES A number of important remedial hazard control operations3 have been completed or are well under way, including 1. measures to reduce the potential for flooding of the drain tank cells; 2. installation of new pressure sensors at several points in the off-gas system; and 3. plans for steps to reduce pressure and to clear known plugs in the system piping. This list is not exhaustive; other measures have been taken or are in progress (ORAL, 1995; ORNE, 1996b). The panel assumes that standard safety precautions for handling fluorine compounds and fissile material will be followed. Considerable experience with these materials is available not only at the Oak Ridge Reservation, but also (for fluorine, hydrogen fluoridetHtF], fluorinating agents, and other fluoride compounds) in industrial and chemical manufacturing applications. The quantitative evaluation of such well- known hazards lies outside the scope (see Box I.~) of the panel's Statement of Task. However, the pane! perceives no obstacles to attaining safety standards equivalent to those required by law for all licensed facilities. CRUCIAL ROLE OF FURTHER ACTIVITIES FOR CONDITION AND PROCESS ASSESSMENT An important part of the evaluation of remediation alternatives involves the extent to which significant known hazards of the alternative process steps may differ and some estimation of the relative degree of 3The list excludes operations associated with the auxiliary charcoal bed (ACB) since this hazard is outside the scope of this report and remediation measures are under way. Regarding the ACE, its isolation from the rest of the system, measures to reduce the potential for waterflooding, the installation of a restraining ring and reinforcements on the cell plug, and an intensive analysis of the hazards associated with it have been done.
A~1NAGEMENT OF MSRE HAZARDS 75 predictability and manageability of these hazards that can result in different levels of risks. The state of knowledge of the salt system and the hardware systems is incomplete in several important characteristics, which limits the evaluation of some of the hazards. The pane} believes that qualitative judgments of the manageability of many of the hazards (and thereby, the likely relative risks) are possible at this time. These judgments are contingent on further definition of specific process parameters as well as on expected improvements in equipment (or hardware) assessment. However, improved knowledge of the system condition and of process behaviors may uncover additional hazards that are unrecognized at present. MAJOR RECOMMENDATION CONCERNING HAZARDS Accordingly, the panel's principal recommendation for the management of hazards4 is to focus on available means for measurements and tests to reduce the present uncertainties in basic physical and chemical conditions and to better define the physical and chemical behavior for the major remediation alternatives. The pane! supports a process of information-gathering activities that would define hazards better, reduce unknowns or uncertainties, and confirm (or refute) assumptions (e.g., that the salt can be remelted successfully) that guide present thinking and are testable prior to large-scale remediation operations. An illustration of this concept is provided in Appendix E, where a course of actions consistent with the preferred approach of Chapter 4 is revisited to show how such a course manages hazards. The level of technical detail is greater than in Chapter 4, with explicit emphasis on how such actions provide for hazard mitigation and control. The pane} notes the developing nature of the remediation program at the MSRE and believes that the present narrow focus on efforts to define hazards by condition assessment, testing, and the use of available resources could be broadened by planned feasibility studies, 4The third issue in the Statement of Task addresses the hazards associated with the technical alternatives for salt removal.
76 AN EVALUATION OF DOE ALTERNATIVES FOR MSRE safety plans, risk analyses, and process activities in the near future. Given adequate condition assessment and planning for the remediation campaign, the panel's opinion is that the hazards of any of the process alternatives reported so far do not appear to exhibit sufficient attributes that preclude bringing them to the status of well-controlled risks by using available techniques. However, control measures for the different alternatives may be found to differ substantially in their required costs and times. DETAILED RECOMMENDATIONS REGARDING HAZARDS The likelihood of unexpected off-normal conditions that can transform hazards to actual damaging event scenarios and consequences is still subject to a range of uncertainties. These uncertainties can be reduced by available and practical diagnostic measurements to overcome inadequate information on some system conditions and on the behavior of some materials under desired process conditions. Accordingly, the principal and urgent mitigation activities recommended for consideration at this time are near-term actions5 focused on improving the state of knowledge of the controlling uncertainties. These include (in a partial listing) the following: 1. better data on the physical and chemical conditions of the drain tanks, cells, and piping, to limit the present unknowns; and 2. a structured campaign of bench- and pilot-scale laboratory testing with salt mixtures (using irradiated surrogates or actual drain tank samples) to limit unknowns in compositions, reaction rates, and physical behaviors. This campaign could seek to extend existing data on the following: Regulatory procedures of the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) allow these information-gathering activities to be accomplished as removal actions, whether time critical or non-time critical, if it is even necessary to invoke CERCLA requirements (due to any potential for releases) (Ed Carreras, Environmental Protection Agency Region IV, private communication with Thomas Kiess). The panel offers no further discussion of the regulatory framework, noting that flexibility is available to implement an appropriate strategy.
MANAGEMENT OF MSRE HAZARDS 77 · the composition of actual salt samples (or irradiated surrogates) along with their critical physical and chemical properties, and the likely degree and types of inhomogeneities (with particular attention to the degree to which the radiation-caused particulates or phases are amenable to redissolution); . conditions; . the corrosion rates for alternative practical fluorination any enhancements to corrosion due to the history of radiation breakdown of the salts (this could be mocked up by using irradiated samples from the High Flux Isotope Reactor tHFIR] facility); · the established diffusion and reaction rates for fluorination at melting and annealing temperatures, by using various fluoridating agents; · initial testing with simulated nonradioactive salt to maximize the rates of learning and explore optimum ranges for process parameters; confirmatory bench-scale process testing using actual salt samples; and · possible confirmation of overall process behavior using the flush tanks for pilot runs with added natural or depleted uranium and pretreatment to simulate the reduced condition in the drain tanks. . Longer-term recommendations include the preparation of well- modeled hazard scenarios for the processes selected in the form of sequence-of-event tree diagrams, supplemented by success path sequences for the development of procedures. This type of effort would provide the orderly assurance that all foreseeable and suspected hazards have been identified and that means for their control or mitigation have been defined. The pane! recommends that ongoing reevaluation be part of the planning process and be applied periodically as additional measurements and test results become available. This is also desirable during the implementation and monitoring of selected procedures. The pane! also recommends that hold point and contingency plans and emergency response plan revisions be defined at appropriate times. This recommendation results from the judgment that a final inflexible plan for a remediation campaign cannot be defined Wily in advance, even after a preferred approach is selected and ratified. This uncertainty results from the unavoidable presence of several first-of-a- kind situations in the alternatives considered so far.
78 AN EVALUATION OF DOE ALTERNATIVES FOR MSRE Finally, the pane! recommends, as a long-term measure, that cleanup strategies for the MSRE project provide one or more sets of logical approaches for the removal, processing, and interim storage of fuel and flush salts in a safe manner, by taking into account the need for alternative and backup strategies. During this strategy development, all pertinent factors should be considered, including criticality potential, remediation effectiveness and implementability, risk management, uncertainties, trade-offs, and duration of actions. Of equal importance is the need to include full consideration of possible process perturbations, failures, contingencies, resource requirements, and other factors warranting backup support or alternative approaches to offset possible factors that might deter success. Because the final resolution of disposal may take considerable time, the panel suggests that the Department of Energy use a phased decision approach strategy focused on interim storage, with the flexibility it provides, rather than try to select a final disposition alternative in the near future. It appears that final resolution of ultimate site criteria and characteristics lies beyond the time horizon of the MSRE cleanup project.