premise: On March 11, a total of 37 workers were exposed to low-level radiation following the fire and blast at the facility, in what was labeled Japan's worst nuclear accident in terms of people affected.
  At the facility, liquid nuclear waste is immobilized by dehydration and mixed with asphalt. The treated substance is then packed into drums.
  A press report said the reprocessing plant would remain shut until at least 2001 as Donen needs time to incorporate new reprocessing technology for radioactive waste, the Kyodo news agency said.
  The long-term closure of the facility deals a heavy blow to Donen because reprocessing spent nuclear fuel is a major source of income for the state-run company.
   What does Japan's worst nuclear accident refer to?
hypothesis: Donen needs time to incorporate new reprocessing technology