2.5.4.1.3 Results and findings

Lithium loop hazard: The most signifcant event to be considered is a major leak of the lithium, and the possibility of a fire if the lithium loop components were installed in an environment that supported combustion. Although the probability of such a leak, with subsequent combustion, is very low, and measures could be taken to control any releases to the environment, the loop has been designed nonetheless to completely eliminate the possibility of a lithium fire. The details of this prevention system are described in Section 2.5.4.3.

A vacuum environment in the test cell has also been provided to eliminate the possibility of combustion in the event of a backwall rupture. This arrangement also greatly reduces ingress of lithium into the HEBT during such an event. Further isolation of the HEBT and target is provided by a fast acting gate valve located in the beam line just outside of the test cell.

Target stability and backwall: Minor instability of the lithium flow resulting in a thinning of the liquid jet has been identified as a major hazard, with a likely probability of occurrence. A margin of approximately 4 mm in jet thickness, beyond that needed to absorb the deuteron beam, has been provided in part to compensate for this. It is also anticipated that experience gained in the initial stage of operation of the facility will permit target design optimization to further reduce the probability of such an occurrence. A major instability, with a complete loss of the jet has been identified as an unlikely event. Extensive instrumentation is provided to give an early indication of the onset of such an event. Moreover, complete loss of jet flow in a time scale less than several seconds, is not considered credible. In the event the backwall does rupture, it has been estimated that less than 1 liter of lithium will spill into the test cell.

No combustion in the vacuum environment of the test cell is possible and the cleanup of such a spill is considered simple.

Mobile radioactive nuclides, particularly tritium, are a potential major hazard for the environment in the case of an accident. Therefore, the IFMIF facility has been designed both to reduce the probability of such an event and, at the same time, to eliminate any subsequent release to the environment. In addition, control of site inventory of tritium is provided by frequent changeout of the cold trap in which the tritium accumulates. Measures are also been taken to reduce the flowing inventory to less than 5 g.

Multiple confinement of tritium and other radionuclides is provided by tertiary confinement of all loop components. The argon gas flowing in the secondary confinement is continuously monitored for any leakage of lithium and the presence of radionuclides. The argon gas in the lithium cells, the third confinement barrier, is also monitored. Isolation of the lithium cells is provided by a shut-off of argon circulation from the affected cell. For an off normal event involving the target, isolation of the test cell from the accelerator beam line is provided by a fast acting gate valve located in the beam line just outside the target cell.

Activation: Activation of the construction materials, equipment and solid materials in the IFMIF system induced by the scattered neutrons is not a hazard in the FMEA, because they are not specific to accidents. However, very high activity around the lithium target limits the access of workers for an emergency operation, quick repair, and any other efforts to minimize the damage in the accidents. Interlock, remote handling, and fail-safe design are extremely important.

Although the effects of neutrons that stray or are back-scattered are not estimated in the target system, activation and tritium production by these neutrons may not be negligible.

Beryllium: The chemical hazard of beryllium is expected to be negligible when compared to the hazard resulting from a leak of lithium. However, in the leaked lithium, the gamma activity of beryllium could possibly pose a significant hazard to personnel [8,9,10].

Beam line interface: Some of the events result in excess evaporation of lithium that would diffuse to the beam line vacuum. An acute effect is a poor vacuum that can be handled by pumps. Differential pumping will be effective to maintain a sufficient vacuum in the beam line. However, no known pump can handle a considerable amount of lithium vapor. Accumulation of lithium in a downstream beam line may cause contamination and problems such as fire and chemical hazards in maintenance that requires exposure to air. Here, the need for a vacuum pump that can handle lithium vapor and tritium is identified as a major technical issue.

External and area events: The external and area events identified as a significant hazard to take into account in the design are: Li fire (see Section 2.5.4.4), conventional fires and earthquakes. The screening analysis, that has allowed identifying these events are described in [23].

2.5.4.1.4 Recommendations

It is generally concluded that fabrication and safe operation of the IFMIF target system is feasible with the current technology. However some components designs require special measures to assure both acceptable and safe performance. For example, confinement of radionuclides is a major concern. If it is possible for a lithium fire to occur, it can not be immediately extinguished, but can only been limited - for example by a rapid injection of argon gas to the affected cell. It would then be necessary to clean and process products, including lithium aerosols by the use of filters and scrubbers. Therefore, complete prevention of combustion in the event of liquid spill or leak is strongly recommended for IFMIF and was adopted as a requirement in the design of the target system. This was achieved with the multiple confinement scheme described in Section 2.5.4.3.

Many of the technologies required for IFMIF are available from the fast breeder reactor programs. However, the combination of large quantities of tritium, a vacuum environment, a high energy beam and lithium are new and special attention has been given to these differences. Effects that may propagate from one system to another have also been considered, but will require continuing attention.

Another unique feature of the lithium target system in IFMIF is its active function in maintaining safety. Normal and steady operation of the target are essential for target system safety, and some deviations or off-normal conditions of the system operation can lead to major accidents. Prevention of deviation events and operability of the sytstem in off-normal conditions are important. The HAZOP study described in the next section is to investigate the effects of the deviations of the target system from normal operating conditions on the total system safety.

At the 2nd IFMIF Design Integration Meeting held in May 1996 at JAERI, a Special Working Group was formed to address the safety related issues of IFMIF such as environmental impact, possible hazards to the public, and safety requirements for facility site selection. Because the IFMIF lithium system presents significant potential hazards to the environment, safety and health, it has been identified as one of the key issues to be addressed by the Special Working Group. It is understood that care must be excercised in the design and operation of the lithium system to achieve the highest confidence in system safety and operability. To that effect, major hazardous consequences related to system component failure have been discussed in the FMEA. A hazard prevention plan has been proposed, and presented in Section 2.5.4.3, to minimize the potential hazards that could accompany a major accident. Multiple containment of lithium carrying components is utilized in an argon gas atmosphere. The main focus of this HAZOP study is twofold: (1) to review the potential environment, safety and health hazards associated with the operation of the IFMIF target lithium system, and (2) to investigate the effects of deviations from normal operating conditions on the total system safety.

2.5.4.2.1 Description of hazards

The following potential environment, safety and health hazards associated with the operation of the IFMIF target lithium system have been identified:

Lithium liquid metal: The IFMIF lithium system contains approximately 21 m3 of liquid lithium. The inventory in the loop components has been shown in Table 2.5.2-2.

Lithium is an alkali metal; it is the lightest and least reactive of the alkali metal family. At room temperature, Li is a solid. The melting point is 180.6°C. Therefore, all Li containing components must be heated. Solid Li does not burn spontaneously in air, but liquid Li is (very) reactive with air, water, concrete, carbon dioxide, and nitrogen. Usually, these reactions go on until there is no Li left. In Li/water reaction, most of the hydrogen is bound to the liquid metal as LiH. The hydride is decomposed when temperature exceeds 600°C. Hydrogen liberation in air is very dangerous; it can be explosively ignited, causing harms to the surrounding objects and personnel. To eliminate the probabilty of lithium combustion in the event of a Li leak, a tertiary confinement scheme with argon atmosphere has been adopted, as described in the Section 2.5.4.3.

A very large database of experience exists for alkali metal loop systems, particularly sodium from the LMFBR experience. Much of this experience in loop design and component performance has been utilized in the design of the IFMIF lithium loop.

A more limited but significant experience base exists for lithium systems, particularly from work performed for FMIT. Several small lithium loops, of capacity less than 100 L, were/are being operated at Argonne National Laboratory, Westinghouse, the University of Colorado, the Tokyo Institute of Technology, the Osaka University and the ENEA-Brasimone. The primary objectives of these loops are to investigate material compatibility and liquid metal magnetohydrodynamic effects. In addition, a large FMIT prototypical loop was operated successfully at Westinghouse Hanford for over 14000 hours, under both argon cover gas and vacuum conditions. The loop piping and components were also similar in scale to that of IFMIF. No new safety related issues are expected to arise from the difference in scale. More recently, Argonne National Laboratory has been operating the ALEX facility, a 400-L capacity lithium loop, for the past two years in support of ITER advanced blanket development and DEMO self-cooled blanket development. This loop is currently in operation and is available to support IFMIF development efforts, particularly in the investigation of lithium jet hydraulic performance.

Radioactive materials: The two most hazardous radioactive materials related to the target system are tritium and Be-7. IFMIF will produce approximately 10 g/y of tritium (based on 75% duty factor). Under normal operating conditions, a few grams (~ 3-5 grams) of this tritium is confined in the loop components. The remainder of the tritium will accumulate in the cold or hot trap at the rate of about 10 g/y. The frequency with which the trap will be regenerated, and the tritium removed for disposal, will depend upon subsequent detailed accident analyses, and local licensing restrictions. Be-7 has a half life of 53 days, and for the 250-mA beam the equilibrium inventory is 0.331 g with an activity of 4.5 ´ 1015 Bq. If there were no removal, the circulating concentration would be 0.03 appm, or 2.2 ´ 1011 Bq/L. As indicated earlier, it is anticipated that cold trapping would reduce the circulating concentration by two orders of magnitude, so that under normal operating conditions the cold trap would contain most of the inventory of Be-7.

Chemicals: In addition to Li, potentially hazardous chemicals include:

All chemicals shall be used in compliance with the chemical hygiene plan. Protective equipment such as gloves, face shields, goggles and respirators are provided to personnel as required.

2.5.4.2.2 Safety system

In addition to the utilization of an argon atmosphere in the lithium cell and multiple confinement of lithium carrying components, the following safety features are available:

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