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Tuesday, July 16, 2019
Andrei Ivanov and Judith Perera
MOSCOW, Sep 25 1996 (IPS) - Ukrainian environment chief Yuri Kostenko is pressing for a new sarcophagus around Chernobyl’s crippled Reactor IV after monitors traced sub-atomic activity inside the present shell that could indicate the start of a nuclear chain reaction.
Environment and nuclear safety minister Kostenko has reports on his desk detailing two neutron fluxes recently recorded inside the existing structure, of the kind that would be recorded in the event of a chain reaction.
If a reaction took place, there might not be an explosion, but it could release substantial radioactivity into the atmosphere.
Plans to build a new sarcophagus, based on a French-British design and estimated to cost 1.2 billion dollars, were finalised in 1993 as part of a series of studies financed by the European Union’s executive Commission.
However no finance was available for the construction and the plans were effectively shelved despite Ukraine’s insistence that they should be a priority for all European states.
Now the EU has revised the design criteria and the winner of a new tender to build a cheaper version should be announced in October. However, once again, the source of finance for this project has still to be decided.
There are up to 200 tonnes of partly burnt nuclear fuel inside the present sarcophagus, hastily built around Reactor IV immediately after the April 1986 accident.
Around 180 tonnes of this was from the reactor core and 20 tonnes from the spent fuel pond. The fuel is in various forms mostly referred to as fuel-containing masses (FCM). These include fragments of active reactor core, lava (melted fuel), fuel dust and waste bearing dissolved uranium.
It was generally considered after the first sarcophagus was built that none of this debris could become “critical” and start a chain reaction. However views have changed as studies of the inside of the shell show a significant build up of water from condensation and rain seepage.
A ‘limited distribution’ working document produced by the International Atomic Energy Agency (IAEA) last year estimated that by the end of 1994 there were 3,000 cubic metres of water inside the shelter.
It noted that this increased the danger of a chain reaction starting because “water gathering around accumulations of FCM can act as a moderator and cause increased criticality of the system”.
In addition enriched uranium dissolved in water can move from place to place. Water has also cause disintegration of some of the lava masses which originally were hard black glass but are now crumbling — and releasing radioactive dust.
According to Valentin Kupny, deputy director of the agency in charge of the sarcophagus, each of the two recent incidents lasted about 90 minutes. On both occasions the reactor area was evacuated and radiation levels were measured. No rise in gamma radiation or in short-lived radionuclides was reported, he said.
But Kupny said the readings could be misleading. Twice before such an increase in neutron flow was recorded, once in 1990 and again in 1995. Commissions were formed to study the incidents, but reached no final conclusions, with many analysts doubting that an increase had actually occurred.
However the IAEA document, referring to the 1990 incident noted that experts could come to no simple or indisputable conclusion. “One possibility left under suspicion was the beginning of a self- maintaining nuclear chain reaction”.
Kostenko too had initially attributed the latest reports of increased radiation levels to faulty equipment but subsequently accepted that they were valid readings.
Kupny notes that in the latest incidents, “if the indicators are accurate, then it is definitely dangerous for those who are working nearby.”
He admitted, however that the reasons for the neutron fluxes were not understood. “Everything that is inside has not been fully studied or understood. We do not have a sufficiently complete control system.”
According to David Kidd of the IAEA a certain change in radiation may be possible as a result of an increase in humidity.
In any case, Kupny noted that a chain reaction “would not be an explosion or some huge noisy process. But it could involve a big release of radiation into the atmosphere.”
Various measures have been taken to reduce the danger of criticality including the depositing of boron, which suppresses such reactions and a special dust suppression system.
Ukrainian officials are emphasising once again that rebuilding the sarcophagus is the top priority in making safe and closing down the Chernobyl plant, where two reactors still operate.
“As we are observing a chain reaction in the ruined reactor, we have to review our strategy and take decisions to make the sarcophagus safer,” says Kostenko. “We must resolve the problem of nuclear fuel inside. By some means or other we must remove as much fuel as possible.”
Ukraine is pressing for prompt payment of aid and loans promised by the West.
The IAEA document concluded last year that the “absolute safety” of the shelter was no longer certain because of the deterioration of the fuel-containing masses.
“Under the influence of water considerable destruction of the lava has taken place and the glass-like material is no longer solid nor considered waterproof,” it concluded.
“A specific danger is water penetration into the mixture of lava and fragments of active zone (core) which could cause criticality of the FCM.”
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