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The HTR research The THTR incident in the 'Spiegel'

This article is from "Der Spiegel" Week 24 1986 -
Pages 28, 29 and 30

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CORE POWER

Sparkling eyes

The Hammer reactor type was considered to have a promising future

- until the incident in early May. The operators hushed up the breakdown that the NRW Social Democrats have, in addition to Kalkar, another problematic pile.

FThe shift started quietly on May 4th for the technician on duty in the heart of the Hammer high-temperature reactor (THTR). The kiln wasn't even running at half speed.

Shortly after 15 p.m. on this May Sunday, the man in the control room received an order. He was supposed to maneuver 41 "absorber elements", graphite spheres filled with boron, which serve to moderate the atomic chain reaction, via a pipe into the so-called charging system - a normal process, with one exception.

The automatic system can only feed exactly 60 balls, no less. Why there should be only 41 balls this time was unclear until the end of last week. In any case, the odd number of the "control room driver", as the technician on duty is called in THTR jargon, required special precautions. He had to switch the system to manual mode. This is only allowed in emergencies and must then be done by a specialist engineer.

Because manual operation is complicated. In order to place a ball, more than twenty rules have to be followed exactly. Every control handle has to be right if the ball is to reach the reactor core, which is cooled with the noble gas helium. One of the absorber elements introduced (diameter: six centimeters) got stuck halfway in the pipe system. "Bullet not acknowledged," reported the computer.

The control desk driver switched back to automatic mode and tried to blow away the blocking ball with high gas pressure. What he did not notice at first: Contaminated helium had flown into the airlock and then, through a valve that had been opened by mistake, escaped into the chimney to the outside.

The control clerk informed:

"Aerosol activity concentration in the chimney high." Alarm horns went on, the computer sent more than twenty alarm messages. The men in the reactor, the shift supervisor was within shouting distance of the technician, apparently did not consider this serious. The technician, who tried his hand at the device for six hours, shot twenty or thirty absorber balls to get the clogged pipe free again. He switched several times between manual and automatic operation - all without success.

The pressure was so great from the opening and closing of the locks that all balls pushed after were shattered. At 21.40:XNUMX p.m., the end of the shift, the man stopped work. Colleagues, as the shift book shows, continued playing the ball. But now the arm that carried the round elements could no longer be moved. The system was switched off during the night.

What happened in the reactor building in Hamm-Uentrop at the beginning of May is reminiscent of Charlie Chaplin's slapstick scenes in "Modern Times": A man struggles with the pitfalls of technology and loses. But what Chaplin's clever gags were in Hamm was a chain of Breakdowns that no engineer could have imagined.

More than 100 tons of paper have been labeled in 15 years for the approval of the THTR - with operating instructions, TÜV documents and production plans. An incident like the one on May 4th does not occur in it.

What came out of the chimney, an estimated 90 million Becquerel, was just a radioactive cloud - no comparison to Chernobyl. What made the incident a scandal was the company's attempt to cover up everything.

When the reactor breakdown became known bit by bit last week and the North Rhine-Westphalian Minister of Economic Affairs Reimut Jochimsen shut down the nuclear power plant, a preferred argument of West German nuclear politicians was devalued: that the domestic reactors are the safest in the world. The accident hit a very German development, the so-called pebble-bed reactor, which nuclear power technicians and politicians alike viewed as promising for the future * (see below "Box on page 29"). Especially since the ruling Social Democrats in North Rhine-Westphalia had relied on this type of reactor. The preference was founded in the hope that the Hammer Atomfabrik could one day supply process heat for coal gasification - an essential for the coal and industrial state of North Rhine-Westphalia.

Leading Social Democrats also remember that the former head of the mining industry union, Adolf Schmidt, "always got sparkling eyes" "when there was talk of the THTR".

NRW Economics Minister Jochimsen recently described the reactor development as a "milestone" in the foreword to a work book. And last week, Prime Minister Johannes Rau, the SPD's candidate for chancellor, said he still considered the high-temperature reactor to be the "safest reactor line He was "convinced" of this. The environmental protection propagandist Jo Leinen, today the Saarland's SPD environment minister, called the Düsseldorf comrades' favorite kiln years ago "the state reactor of the North Rhine-Westphalian state government". In the SPD parliamentary group, fears arose last week that Rau and his friends would, if not necessary, help to further blur the SPD's already unclear position on the exit from nuclear energy.

In the decision to withdraw from the federal executive committee on May 26th, it says: "The share of nuclear energy in electricity generation will be gradually reduced." Rau agreed to this. However, he only let his parliamentary group in Düsseldorf pass the non-binding phrase: "That is why it (nuclear energy) is only responsible for a transitional period." In the NRW Energy Commission, State Economics Minister Jochimsen explained the difference as follows: "It makes a difference whether you are in the opposition in Bonn or whether you hold politics responsible for a country."

The Düsseldorf SPD sees itself in a doubly tricky situation after the accident in Hamm. After the fast breeder in Kalkar, the THTR has now also become a problem. "We shouldn't get stuck here," said a cabinet member. "

Environmental expert Volker Hauff said that the shutdown reactor could only be back on the grid when all safety issues had been thoroughly examined and "a broad public discussion has taken place". Presidium member Herta Däubler-Gmelin explained: Test for the party's credibility. "

The most recent nuclear accident might have been completely covered by the Chernobyl cloud had not a THTR employee provided an anonymous tip. The stranger, probably a senior employee, has been providing targeted information about dangers in the plant for months.

Apparently a wink also reached the employees of the alternative Darmstadt Eco-Institute. At the beginning of May, its experts determined during measurements near the reactor that three quarters of the radiation - a total of 35000 Becquerel per square meter - came from the THTR itself, only the rest from Chernobyl winds.

When Minister Jochimsen asked the operators at noon on May 7th, they were weighed down. He got to hear: "Nonsense, there is nothing to it."

The answer was wrong. Because those responsible had already found out in their own measurements that morning that some of the radioactivity was actually home-made. The company's information policy took on almost Soviet features, it was bricked up and covered up. On May 12th, the operating company announced by express mail to all members of the Düsseldorf state parliament that the rumors about problems with the THTR were not true - they "lacked" any "basis": the THTR "works properly".

At this point in time there was already a lot going on in Hamm. The debris from the balls was vacuumed off and the defective loading system repaired. The reactor was made up like a Potemkin village.

When a group of FDP members of the state parliament came to visit in mid-May, the politicians were shown not an accident report, but an advertising film that, with the voice of Tagesschau spokeswoman Dagmar Berghoff, extolled the advantages of the pebble bed reactor. The Bielefelder Zeitung "Neue Westfälische" on the visit: "Radiant in - clean out again."

The Düsseldorf Ministry of Economic Affairs was already following up on the first indications of radioactive emissions. The operator's false reports did not get stuck in the bureaucracy, as initially assumed, but were initially checked too slackly. Only when the Öko-Institut informed the public was a government commission sent to Hamm, and Jochimsen declared on television that it was an "unbelievable cover-up" - which prompted the head of the United Electricity Works, Klaus Knizia, to take legal action The scandal divided the licensing authority and the operator, and the Düsseldorf Social Democrats finally had to recognize that their THTR is also a perfectly normal and therefore vulnerable nuclear reactor.

Maybe not even that. At the end of last week, a paper from the Kraftwerk-Union (KWU), a Siemens subsidiary, was circulating among the Düsseldorf Social Democrats. It says that the high-temperature reactor has been given particularly high safety properties. However, according to the KWU managers, they "did not come to fruition in the expected manner" with the large Hammer Meiler. And: an improvement in safety was "not achievable".


"Environmentally friendly in metropolitan areas"

Hopes and failures in the high-temperature reactor

ZGroping with hopeful anticipation, television viewers watch every week as the black numbered plastic balls plop down from the glass drum through a conveyor channel into the seven tubes.

Something like that, only with almost kilometer-long conveyor lines like a pneumatic tube and all in all 675 000 graphite spheres the size of tennis balls, one has to imagine the supply and discharge system of the 300 megawatt thorium high-temperature reactor (THTR 300) in Hamm-Uentrop. A kind of traffic jam occurred in one of the helium-filled supply pipes to the reactor core - the triggering factor for the incident.

Graphite spheres instead of the steel-coated uranium fuel rods usually used in atomic reactors - that was the core idea for the high-temperature reactor designed three decades ago by the German physicist and Heisenberg student Rudolf Schulten. The idea promised a whole range of economic and safety advantages compared to the otherwise widely used light water reactors:

While conventional reactors have to be shut down regularly to replace the spent fuel elements, the high-temperature reactor can be operated continuously; The atomic fuel (highly enriched uranium and / or thorium) trapped in the graphite spheres is in a constant cycle through the reactor, spent fuel elements are constantly being replaced by unused ones (see graphic).

The heat generated by the atomic chain reaction is not dissipated with water, but with the noble gas helium, which is heated up to almost 1000 degrees and then passes its heat on to the turbine via a secondary water-steam cycle - with a far higher degree of efficiency than in conventional nuclear power plants.

In the event of a failure of the heat-dissipating helium gas flow, the reactor core does not theoretically overheat, but the reactor output automatically falls to about five hundredth of the nominal output; According to the operators, melting of the fuel elements is therefore hardly possible at the THTR.

In 1987, the first mini reactor (15 MW) based on the pebble bed principle went online at the Jülich nuclear research facility. It ran satisfactorily for several years until an unexpected accident occurred in 1978: 25 tons of water broke into the test reactor, a first indication that this type of reactor was not fail-safe either.

Atomic electricity from the high-temperature reactor should cost 1,5 pfennigs per kilowatt hour, calculated its inventor Schulten at the end of the XNUMXs - far less than electricity from coal and about as much as atomic electricity from light water reactors. But the THTR should, again due to its different technical principle, be superior to the common atomic piles in two respects:

The helium gas, which is brought to such high temperatures in the reactor, can be used, unlike the coolant water, not only to generate electricity, but also as so-called process heat - for example for coal liquefaction or other energy-intensive processes in the chemical industry.

Relatively small pebble bed reactors should not only deliver electricity in urban areas, but also district heating.

Its advocates praised the pebble bed reactor as "particularly safe and environmentally friendly"; its advantages, it said in 1972 in a "project information" from the Uentrop operator, "could be used when choosing future reactor locations in densely populated areas".

The test reactor built in Jülich initially seemed to confirm the presumed safety advantages. However, when trying to build a reactor of the same type with 20 times the capacity as a large-scale plant, problems and costs increased. Instead of the estimated five years of construction, it was almost 15; instead of the originally estimated 690 million marks, the reactor ultimately devoured more than four billion marks in construction costs.

So far the reactor has stood still 21 times, sometimes an emergency generator failed, sometimes an exhaust fan, or defective sensors reported "too high temperatures" in the reactor hall.

There were also technical problems with the loading of the reactor. The graphite spheres - among which there are always empty tubes and several boron-filled "absorber elements" to moderate the Meiler fire - had proven to be tough in the prototype Pushing balls, this became the weak point: Contrary to all forecasts, the balls break in heaps - since September 675 there have been 000 breaks.

(Note: up to the decommissioning there were 8000 fuel element balls that broke!)

THTR 300 Thorium high temperature reactor, pebble bed reactor
I would like to express my thanks to "Spiegel" for the copies of the articles on the THTR topic.

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