THTR-300

From Wikipedia, the free encyclopedia

The THTR-300 was a thorium high-temperature nuclear reactor rated at 300 MW electric (THTR-300). The German state of North Rhine Westphalia, in the Federal Republic of Germany, and Hochtemperatur-Kernkraftwerk GmbH (HKG) financed the THTR-300’s construction [1]. Operations started on the plant in Hamm-Uentrop, Germany in 1983, and it was shut down September 1st, 1989 [2]. The THTR was synchronized to the grid for the first time in 1985 and started full power operation in February 1987 [2]. The THTR-300 served as a prototype for high-temperature reactors (HTR) and was the first to use a pebble bed design and TRISO fuel. The THTR-300 cost €2.05 billion and is predicted to cost an additional €425 million until December 2009 in decommissioning and other associated costs.

1 History
2 Design
3 Accident
4 Decommissioning
5 References
6 See also
7 External links
- 7.1 General
- 7.2 IAEA technical documents

[edit] History

The electrical generation part of the THTR-300 was finished late due to ever-newer editions and licensing procedures. It was constructed in Hamm-Uentrop from 1970 to 1983 by Hochtemperatur-Kernkraftwerk GmbH (HKG) [1]. Dr. Heinz Riesenhuber, Federal Secretary of Research at that time, inaugurated it, and it first went critical on September 13, 1983. It started generating electricity on April 9th, 1985, however it did not receive permission from the atomic legal authorizing agency to feed electricity to the grid until November 16, 1985.

[edit] Design

The THTR-300 was a high-temperature reactor with a pebble bed core, consisting of approximately 670000 spherical fuel compacts each 6 cm in diameter with Uranium-235 and Thorium-232 fuel and core walls consisting of graphite and was helium cooled. The pressure vessel that contained the pebbles was pre-stressed concrete (This was the first time this had been used instead of a steel pressure vessel). The THTR-300's power conversion system was similar to the Fort St. Vrain reactor in the USA, in that the reactor coolant transferred the reactor core's heat to water. The thermal output of the core was 750 megawatts, this heat was transferred to the helium coolant which then transported its heat to water which then was used to generate electricity via a Rankine cycle. Because this system used a Rankine cycle, water could occasionally ingress into the helium circuit. The electric conversion system produced 307 megawatts of electricity. The waste heat from the THTR-300 was exhausted using a dry cooling tower.

[edit] Accident

On May 4th, 1986 it released a small amount of radioactive material into the surrounding environment from a dislodged pebble [1] [2]

Fuel replenishment at the THTR at Hamm/Uentrop was supposed to proceed automatically - at certain intervals, precisely 60 new pellets were to be automatically fed in at the top, and 60 "old" pellets were supposed to be discharged from the bottom. On the evening of 4th May 1986, however, the exceptional occurred and only 41 pellets were to be added (who knows why?). Since the fuelling mechanism could not cater for exceptions in automatic mode, it was switched to manual mode.

One of the pellets - apparently the very first one - got stuck in the tubing of the fuelling device, so that it could move neither forward nor backward. The technician then applied gas pressure to force the pellet out of the tube and into the reactor, but the pressure alone was not strong enough. Then the man at the controls sent the remaining 40 pellets down the tube, one after the other, perhaps in the hope of building up pressure this way.

The result of these efforts: 41 broken pellets and one damaged gas lock. The function of this lock was to prevent the cooling agent, helium, from leaving the reactor; now it failed to close, probably kept ajar by remnants of the broken pellets. In this way, the dust from pulverised pellets and any amount of helium entered the atmosphere surrounding the reactor. As fate would have it, one measuring instrument happened to be switched off at the time, so that nobody could tell precisely how much radiation escaped.

Initially the accident was described as a "routine cleaning operation" and then as an "unforeseeable misfortune". Only several weeks later did the company admit that there had indeed been a breakdown, accompanied by the release of considerable quantities of radiation.

[edit] Decommissioning

The THTR dry cooling tower

On September 1st, 1989 the THTR-300 was deactivated due to its cost and increased public scrutiny following both the Chernobyl accident and the THTR-300 fuel pellet accident. On October 10th, 1991, the 180 meter high dry cooling tower, which at one time was the highest cooling tower in the world, was explosively dismantled and from October 22nd, 1993 to April 1995 the remaining plant was decommissioned.

From 1985 to 1989 the THTR-300 registered 16410 operation hours and generated 2891000 MWh according to a full-load working time of 423 days. By 1982 it was planned by a group of firms to proceed with construction of a HTR-500, the successor of the HTHR-300, but up-rated to a thermal output of 1250 megawatts and an electrical output of 500 megawatts.