In the February 1995 issue of the Nuclear Safety Newsletter, the Nuclear Safety Division of the United Kingdom (UK) Health & Safety Executive (HSE) reported an event involving a plutonium nitrate leak in a plutonium evaporator at British Nuclear Fuels (BNFL) Sellafield works. All the plutonium was contained in the evaporator cell and there was no release to the environment. However, site personnel were concerned about the potential for a criticality if the leak had remained undetected for a longer period. In reviewing the article and portions of the HSE investigation report, NFS identified lessons learned that may be beneficial to DOE facilities operating or planning to operate plutonium evaporators. (HSE Report "Leakage into the B205 Plutonium Evaporator Cell at Sellafield", May 1994)

The leak occurred in a plutonium evaporator at a plant that separates spent nuclear fuel rods into component parts of uranium, plutonium, and fission products. The outer ladding is removed from the fuel rods and they are dissolved in nitric acid. The dissolved uranium, plutonium, and fission products are then split into different liquid streams by solvent extraction, which separates the constituents of a mixture by means of a selective solubility in a solvent. The streams are fed to other parts of the plant for further treatment and, ultimately, conversion to uranium trioxide, plutonium dioxide, and vitrified high-level waste. The plutonium evaporator, where the leak occurred, receives the dilute plutonium nitrate stream from the solvent extraction process.

BNFL determined that 28 kilograms of plutonium leaked over a period of five-to-six hours. Although the likelihood of a criticality was small, the consequences could have been extremely serious, if people were in the area at the time. Very high radiation doses (possibly fatal) to persons in the immediate vicinity were possible. BNFL classified the event as Level 3 on the International Event Scale. A Level 3 event is defined as a serious incident due to degradation of defense-in-depth. HSE conducted an investigation and decided that, because the lessons from this incident may have wide applicability to other nuclear plants, the facts of the incident should be made public. Following are some of the key lessons learned from the event.

- Leak detection provisions for the plutonium evaporator were not adequate because the plutonium nitrate did not behave as expected when it leaked into the cell. Designers and operators of similar plants should reconsider the adequacy of detection methods for a loss of primary confinement of hot plutonium nitrate solution, taking into account the possibility that the bulk of the material may solidify.

- The operator's assumptions about the behavior of plutonium nitrate under certain conditions were not correct.

- Metallurgical evidence indicated that an initial failure of an 18/8/1 titanium-stabilized stainless-steel weld may grow very quickly in evaporator conditions (i.e., high temperatures and corrosive environment). A substantial leak rate may develop rapidly, even when the site of the failure is above the liquid level. The frequency and method of inspections should consider this possibility.

- The incident casts doubt on the suitability of alpha-in-duct monitors for detecting a loss of primary containment of hot plutonium nitrate solutions because high levels of airborne activity in the cell extract duct may not necessarily occur in the short term.

- Operators of similar plants should consider the benefits of installing "safe-by-shape" devices adjacent to plant equipment to mitigate the consequences of a loss of primary containment of fissile material. There should be adequate means to detect an initiating event which could lead to a serious incident at an early stage. There should also be appropriate defense-in-depth, methods including diversity and redundancy of safeguards.

- The success of the recovery operation, which proceeded without incident, illustrates the value of careful planning for the whole job and the division of the job into discrete steps, with a thorough assessment of the technical and safety implications at each stage.

The HSE is transmitting the full investigation report to NFS. Personnel interested in obtaining additional information on this incident can contact Dick Trevillian, Office of Operating Experience Analysis and Feedback, at (301) 903-3074.