In mid-September (1996) a brief story appeared in the U.S. media reporting that neutron monitors at the damaged Chernobyl Unit 4 had detected a factor-of-four increase in neutron level. Keep in mind that between 90 and 160 tons of solidified core melt are in the recesses beneath the Unit 4 reactor room. A search of the Internet located several Reuters reports. Herein we quote from those articles:
9/17/96 Officials at the station, 140 km (90 miles) north of Kiev, said on Tuesday that recorded neutron activity and radiation levels had climbed sharply twice in the past week inside the steel and concrete sarcophagus surrounding the reactor. Ukraine’s nuclear authority set up a commission to investigate whether the incidents on September 12 and 16, the third such set in the past decade, could lead to uncontrollable chain reactions.
“The latest increases were temporary. No conclusions were drawn from the earlier instances whether a chain reaction had occurred” , Valentyn Kupny, in charge of the sarcophagus, said by telephone. Kupny said three of 10 meters had shown increases in neutron radiation ... “Everything that is inside has not been fully studied or understood. We do not have a sufficiently complete control system.”
9/18/96 Yuri Kostenko, Ukraine’s Minister for Nuclear Safety, told Reuters during a conference in Vienna, that ... “there was an increase in neutron activity. If this were a nuclear reaction it would have to be accompanied by a rise in temperature and radiation, but these factors were not measured” ... contradicting remarks made by Chernobyl officials on Tuesday. Kostenko said his ministry had formed a special group to look into the incident, adding that the registered increase in neutron activity might be due to faulty measuring equipment.
Ukraine must come clean on safety problems related to its Chernobyl nuclear plant to allow potential aid donors to help minimize possible risks, the European Commission said on Wednesday. Even if the cause for the recurring incidents ... is not fully established, the incidents clearly demonstrate the need to bring the potential risks related to the sarcophagus under a satisfactory control ... we therefore urge the Ukrainian authorities to provide detailed information regarding the nature of the incidents reported, they said.
9/19/ 96 Viktor Baryakhtar, vice-president of Ukraine’s Academy of Sciences, told the agency that readings of neutron activity had risen by between five and 110 times this week This is a rather dangerous phenomenon ... , Baryakhtar, President Leonid Kuchma’s top nuclear adviser, told Ukrinform. The main reason is leakage under the sarcophagus of rainwater ... Baryakhtar described the latest incident as a chain reaction which had lasted between four and five hours on Monday.
9/24/96 Ukraine’s chief negotiator on closing the Chernobyl power plant said on Tuesday authorities were reviewing how to make the tomb around its ruined fourth reactor safe, after a chain reaction inside last week. Environment Minister Yuri Kostenko’s comments were the first admission that increased readings of neutron activity amounted to a limited nuclear chain reaction inside the reactor, 10 years after it exploded in the world’s worst nuclear accident. But he said data was insufficient to determine whether it posed a real threat. “What we are observing shows that we must resolve the problem of nuclear fuel inside. By some means or other we must remove as much fuel as possible to rule out the development of chain reactions inside the devastated reactor.”
9/25/96 Ukraine’s top security official on Wednesday denied a chain reaction had occurred in the Chernobyl power plant’s ruined fourth reactor ... Volodymyr Horbulyn, Secretary of the Security Council, said Ukraine’s Environment Minister had spoken out of turn by stating that increased readings of neutron activity last week indicated a nuclear chain reaction within the sarcophagus. I believe Mr. Kostenko is mistaken. He is no specialist in nuclear energy, Horbulyn told a news conference ... “Neither he nor I were there when all this occurred. I am inclined to be less pessimistic about what is happening in the sarcophagus.”
Viktor Baryakhtar, Vice-President of the Academy of Sciences, told Interfax Ukraine news agency: Increased neutron radiation was a periodic occurrence inside the structure. He said that the likelihood of a chain reaction occurring is virtually zero. “It’s hard to understand why there is such a panic now.”
Senior officials issued a flurry of statements discounting Environment Minister Yuri Kostenko’s contention on Tuesday that the increased levels last week had triggered a limited chain reaction within the cracking structure. Even Russia’s Deputy Nuclear Energy Minister, Lev Ryabev, issued a statement saying the readings were nothing unusual. Kostenko himself backtracked in a new statement saying news reports had exaggerated the dangers. Even if a further nuclear reaction occurred, he said, it would be limited. All reactors at the station are under the constant supervision of specialists and meet the strictest standards, the official Ukrinform news agency quoted him as saying. “Even if an emergency situation develops it would not occur beyond the limits of the Chernobyl (30-km exclusion) zone.”
9/26/96 Official attempts to ease public concern over the Chernobyl nuclear power plant appeared to have failed on Thursday as Ukrainian and Russian media speculated on whether the station’s ruined reactor could explode again. Residents questioned on Kiev streets all appeared resigned to the risks ... “but what are we supposed to believe with all these statements and denials?” Kuchma, speaking to reporters on Thursday, acknowledged that public opinion had been deeply affected by the recent reports on high neutron radiation readings. You must not upset people this way. Kuchma singled out for criticism Environment Minister Yuri Kostenko ... anyone making such statements, particularly a member of the government, must accept responsibility for them, he said. “No one knows what sort of reactions are occurring in the sarcophagus. There are no specialist on this.” The State Nuclear authority chairman, Viktor Chebrov said a commission set up to investigate the higher readings had found no proof of a chain reaction.
Is Unit 4 rising from its own ashes? It’s up to you to separate fact from fiction here ... we can’t. Stay tuned for future developments. The next issue of NUKEFACTS will discuss some technical aspects of this matter. For a neutron count increase due only to a change in subcritical reactivity, it is easily shown that the factor of increase in count is the same factor by which the subcritical reactivity value is reduced (divided). For example, an increase in neutron count by a factor of 110 would result from a reduction of an initial subcritical reactivity from rho = -0.1100 to rho = -0.0010.