Safety issues in a reprocessing facility

Among the NFCFs other than nuclear reactors, reprocessing facilities are the most complex with respect to safety. The potential safety hazards to which fuel reprocessing facilities are prone include criticality excursions, radiation exposure, chemical reactions, fire and explosion.

Safety issues related to the storage of spent nuclear fuel at a reprocessing facility before the reprocessing starts are discussed in the following Section 10.

9.3.1. Criticality

Criticality control is a dominant safety issue for reprocessing plants due to the large amount of fissile materials treated and the presence of water, a moderator, in many parts of the plant. To prevent criticality relevant parameters such as the mass, volume, concentration of fissile

material have to be controlled and geometry of components that contain fissile material has to be designed accordingly.

The following areas in a reprocessing facility have a significant risk of criticality [31]: Shear pack (due to accumulation of spent fuel powder), storage of hulls (due to residual fuel), dissolver, solvent extraction process, and purification of U and Pu, and conversion to oxide.

Use of neutron poisons (in liquids or in casings of equipment) can allow larger sized components and reduce the possibility of criticality.

9.3.2. Radiation exposure

Dispersion of (highly) radioactive material may result in exposure of personnel. Thus, several barriers are necessary to be installed to avoid dispersion leading to contact of radioactive material with the workers. Two types of containment are used as barriers against dispersion:

static and dynamic containments. The first static containment (or barrier) is composed of the walls of process equipment, i.e. such as tanks and pipes. Almost all process equipment is contained within (hot) cells. Thus, the second one consists of the enclosures of the (hot) cells, i.e. typically thick concrete walls. The third containment is formed of the external walls of the building. An additional dynamic containment function is achieved by adequate ventilation of the process equipment, vessels and cells, plus the working areas. The IAEA safety standard [36]

states:

“4.28. In a reprocessing facility (for most areas), three barriers (or more, as required by the safety analysis) should be provided, in accordance with a graded approach. The first static barrier normally consists of process equipment, vessels and pipes, or gloveboxes. The second static barrier normally consists of cells around process equipment or, when gloveboxes are the first containment barrier, the rooms around the glovebox(es). The final static barrier is the building itself. The design of the static containment system should take into account openings between different confinement zones (e.g. doors, mechanisms, instruments and pipe penetrations)”.

To minimize external radiation exposure, the working areas need to be far away from radioactive material, adequate shielding can be established adapted to nature and energy of the radiation, and the exposure time of workers needs to be limited.

Radiation exposure of the public and the environment is discussed in the INPRO methodology manual on environmental impact of stressors [5].

9.3.3. Fire and explosion

A large fire spreading through a reprocessing plant or an explosion might be one of the main causes of dispersion of radioactive material into the environment, notably in the event of ventilation system failure, and damage to engineered controls. Reprocessing plants use flammable solvents (e.g. kerosene or dodecane), certain pyrophoric materials (e.g. zirconium powder, TBP), and agents (e.g. hydrogen, hydrazine) with oxidising and reducing properties.

For example, the fragmentation of LWR fuel assemblies by shearing during head-end processing produces powder of Zircaloy that present a potential for ignition or explosion.

Hydrogen gas probably represents the greatest potential cause of explosion, due to its rapid rate of diffusion, its low ignition energy input, and the wide range of concentration limits that may give rise to an explosion.

The risk of fire can be reduced by eliminating sources of ignition and hot points and by installing fire detection and extinction systems.

In the following an example is presented of an explosion in a reprocessing facility, namely, the so-called ‘red oil’ explosion. Red oil is the name of a substance of non-specific composition

formed when an organic phase consisting of tri-butyl-phosphate (TBP) and diluents in contact with concentrated nitric acid is heated above 120°C under reflux. At temperatures above 130°C, the degradation of TBP, diluents, and nitric acid proceeds at rates fast enough to generate heat and voluminous amounts of detonable vapour. The generated heat further increases the temperature of the liquid, which in turn increases the rate of reaction (i.e. a runaway or autocatalytic reaction). For a comprehensive review, see Refs [124–126].

Three red oil events have occurred in the U.S. Department of Energy’s defence nuclear facilities complex at the Hanford Site in 1953, and at the Savannah River Site in 1953 and 1975 [127, 128]. A red oil explosion also occurred in 1993 at the Tomsk-7 facility in Seversk, Russian Federation. More recent fire and explosion incident occurred in 1997 at the Bituminization Demonstration Facility of the Tokai reprocessing plant in Japan.

A report by U.S. Defence Nuclear Facilities Safety Board highlights the conditions under which red oil formation and explosion can take place and means to avoid such incidents [129].

9.3.4. Containment of radioactive material and/or toxic chemicals

Due to the use of highly aggressive acids and other chemicals in a reprocessing facility corrosion in process equipment can cause leaks of radioactive material and/or toxic chemicals.

In the following an example is presented of a leakage of radioactive material that occurred on 21 April 2005 in the feed clarification cell of the THORP plant at Sellafield in Great Britain.

The THORP plant is designed to reprocess irradiated fuels produced by advanced gas-cooled reactors (AGR) and light water reactors. Reprocessing campaigns have been carried out with uranium oxide spent fuel originally enriched in ²³⁵U by up to 4.8 %. The plant had already reprocessed around 5700 tonnes since its commissioning in 1994. During the incident about 83 m³ of clarified radioactive fluid leaked into one of the recovery pans and was discovered during a camera inspection of the main feed clarification cell. This cell is closed off to personnel at all times and its walls guarantee the radiological protection of the adjacent premises. The toxic fluid present in the recovery pan contained uranium and plutonium that were yet to be separated from the fission products and estimated to represent about 20 tonnes and 200 kg, respectively. The plant was shut down as soon as the incident was discovered. The main cause of the leakage was from a failed pipe running between two accountancy tanks. Only the first barrier consisting of the transfer pipe failed during this incident. The static and dynamic integrity of the two remaining containment barriers remained intact. The operator emphasized that the leak posed no danger to the workers or the environment. In particular, no abnormal activity around the plant’s stack has been detected. The operator also underlined the absence of any risk of criticality, which was corroborated by the British safety regulator.

A leak in the pipes and tanks of reprocessing facilities, if undetected, can create a hazard of internal flooding. Examples of fluids causing flooding are: cooling water, heating water, treated water, chemical solutions, fire-fighting water, etc. In a reprocessing plant, the most important hazardous consequences generated by flooding are: criticality, damage to equipment fulfilling safety functions, and dispersion into the environment of radioactive material transported by the fluid involved.

To prevent a release of radioactive material and/or toxic chemical to the outside of the plant several barriers (combinations of static and dynamic containments) are necessary: The first barrier is the casing of the equipment (e.g. wall of a vessel or pipe), the second barrier is a glove box or hot cell, and the last one is the building of the facility. Additionally, dynamic containments are to be provided by ventilation systems in process equipment, hot cells and glove boxes but also in the working area of the facility.

9.3.5. External hazards

Reprocessing facilities are expected to be designed against all credible external hazards (see Section 4.2.1 and 4.2.6). The IAEA Safety Standard [17] provides a list of selected external postulated initiating events including natural phenomena and human induced phenomena:

“(a) Earthquakes, volcanoes and surface faulting;

(b) Meteorological events, including extreme values of meteorological phenomena and rare events such as lightning, tornadoes and tropical cyclones;

(c) Floods, including water waves induced by earthquakes or other geological phenomena or floods and waves caused by failure of water control structures;

(d) Geotechnical hazards, including slope instability, collapse, subsidence or uplift of the site surface, and soil liquefaction;

(e) External human induced events, including transport events such as aircraft crashes and accidents at surrounding activities such as chemical explosions”.

9.4. ADAPTATION OF THE INPRO METHODOLOGY TO A REPROCESSING