Sicherheitstechnische Untersuchungen zum Störfallverhalten des HTR-100 : Ergänzungsband mit vertiefenden Einzelbeiträgen zum Bericht Jül-Spez-477 (1991)

1. SCOPE AND OBJECTIVES The aim of investigations an the HTR-100 accident behaviour was to verify the safety concept of the plant for balance and to quantify the radiological risk to be expected in operating an HTR-100 double unit systeme Moreover, aspeets of the investment risk were considered. The spectrum of initiating events ranged from so-called transients to leaks in the primary circuit and steam generator andeven included earthquakes. Sosse of the event trees derived were highly complex and extensive due to the situation of the steamgenerator above the core and with regard to the double unit plant concept with increased possibilities of accident control, but also with respect to potential accident propagation. Correspondingly sophisticated analyses were required to identify riskerelevant event sequences, but ultimately only a few courses of events were concerned. 2. PLANT RISKAn HTR-100 plant produces 250 MW(th) per reactor. The limitation to this comparatively low output, a low power density and a core geometry with a relatively lange surface permit the omission of active core cooling without significantly affecting fission product retention in the fuel elements. The fission product decay heat is in this case transferred to a two-line natural circulation cooling water system via the surface of the reactor steel pressgare vessel so that the reactor concrete cell is cooled. Unlike larger reactors, the radiological risk is therefore not determined by active core cooling failure which, due to natural convection in the primary circuit of a pressurized reactor, is only possible in the event of a failure of the steam generator feed system, whereas blower failure is an additional cause in a depressurized primary loop. Although local temperatures of 1620°C are reached in a depressurized reactor, which ultimately also lead to the release of radiologically relevant fission products from the fuel elements in a small section of the core, cesium and strontium are retained praetically completely in the core region due to redeposition an colder graphite surfaces and only iodine and silver are reieased from the core as radioiogieaily relevant fission products. However, this only occurs at a time when the average gas temperatures in the primary circuit decrease again due to natural convection which also takes place in a pressureless reactor. The associated coolant contraction prevents fission product transport from the reactor pressure vessel. Another essential feature reducing the risk of cooling accidents is the fact that the reactor pressure vessel safety valves blow off into the gas store. Active core cooling failure in the pressurized reactor therefore does not lead to gas release into the environment, although the pressure increases to a level where one of the safety valves responds. Primary circuit depressurization with corresponding reactor core heatup can only develop if several engineered safeguards are assumed to fall in addition to active core cooling. A failure of active core cooling after accidenteinitiating leakage of the primary circuit is also highly improbable because the afterheat removal system is designed to cope with this event and possesses very high availability for reasons of plant availability and Investment protection. The radiological risk of the HTR-100 is essentially determined by accident sequenees initiated by heating tube leakage at the steam generator and causing release of pari of the fission products deposited an the steam generator surface during operation. The highest releases of noble gases and iodine are caused by a sequence of events due to failure of the steam generator relief system comprising the two pressure reducing stations so that the steam generator pressure increases due to continued natural convection heating until the main steam safety valves respond. If the valve in the defective leg then fails in the open position, there will be a direct connection between the primary loop and the environment causing slow depressurization of the primary system through the leak and open safety valve. Such depressurization temporarily leads to condensation at the steam generator due to rapid cooling of the primary circuit through the intact steam generator leg causing about 6 % of the metallic fission products deposited there to be detached. The radiologically relevant nuclides released near ground level comprise 78 GBq of cesium-137, 1.2 GBq of strontium-90 and 880 GBq ofiodine-131. More than 90 % of this iodine quantity is assumed to originate from "hydrolyzed:' fuel particles with defective [...]