Near miss at a nuclear material production and storage facility

A potentially explosive situation was reported by safety inspectors at a nuclear waste tank. The waste slurry had formed a thick crust which was trapping hydrogen which was being continuously generated underneath. It was not known Chemistry and contents of the solid crust was unknown.
According to the New Scientist article four million litres of waste "have been behaving mysteriously inside the underground storage tank. Every few months the tank ‘burps’ releasing a burst of hydrogen and nitrous oxide."

Calculation estimated a potential for an explosion equivalent to 230 kilos of TNT. Up to 66 tanks at Hanford were believed to be leaking, 22 were accumulating hydrogen and a further 22 were potentially explosive.
Periodically, the crust broke and collapsed, releasing the gases. When this was happening, researchers could measure concentrations of 4% vol. of hydrogen at the exhaust stack of the tank when the crust breaks, which could imply very high hydrogen concentration under the crust.
For years, no satisfying solution was available, and only various mitigation measures could be attempted, to keep the wate systems under control.

Radiation in the waste caused the remaining water to break down, producing hydrogen by radiolysis. Additional hydrogen and nitrous oxide resulted from the breakdown of organic chemicals that were added to the waste. A waste slurry had formed a thick crust which was trapping the hydrogen generated underneath.

Nuclear waste tank

According to the New Scientist article (see references) efforts were made in the 1970s to reduce the volume of nuclear waste. In one tank liquid waste was concentrated into a thick slurry by evaporating off water and adding chemicals that caused other liquids to form solid crystals. The slurry is about the consistency of peanut butter.

The reported event is a near miss. The sources available do not report any escalating event.

According to the New Scientist report (see reference), experiments with the synthetic version of the crust show that its combination of nitrates and organic chemicals will ignite if temperatures exceed 500 Degree C. This is not a danger in normal operation, since wastes in the tank are kept near 50 Degree C. But burning hydrogen could reach sufficient temperatures to set off a much more powerful explosion in the crust.

It was also calculated the hydrogen produced could have a explosion potential equivalent to 230 kilos of TNT could occur. Up to 66 tanks at waste storage facility are believed to be leaking, 22 are accumulating hydrogen and a further 22 are potentially explosive.

Near miss, but the source do not report solutions which eliminated completely the problem.

According to the New Scientist report, various methods were tried to prevent hydrogen from building up under the crust. "From 1983 to 1986, crust was broken up by jets of water, but the operation added too much additional water to the tank. Later, pressurised air was used in the same way. A review in 1989 concluded that neither technique worked well and often made the problem worse. "

A historical summary of the plant operation can be found in Gephard 2010 (see reference). However, the report does not report specifically the hydrogen case.

That many years later the radiolysis problem was not yet completely solved can be deduced from a RT news report of 2013, which mentioned a similar explosion potential from hydrogen generated at the walls, especially those with double wall. This source quotes the DOE, stating that all affected areas "...are actively ventilated, which means they have blowers and fans to prevent hydrogen gas build-up... These ventilation systems are monitored to ensure they are operating as intended".