The Chernobyl Accident

The partial containment system had many rooms in one room, with a pressure capability of about 26 psi, was the reactor core. The steam drums were in two rooms four main recirculation pumps were in each of two rooms. 

The reactor had a loop for each half of the core. Each circuit had two steam drums and two. 500 MW(e) turbines. Water was circulated in each of the two circuits by four pumps in parallel (8 lotal), though usually each circuit had three in use and one on standby. The pumps supplied a t omplex of pipes under the reactor that fed water to the separate pressure tubes. 

The 211 control rods were moved in and out of the core by winches driven by electric motors. Power and neutron flux distribution were measured by in-core self-powered ion chambers, which were inaccurate at lower power. At low power, ion chambers in the graphite reflector were used. 

Below the Chernobyl reactor were water pools meant to capture and condense any steam released from a pipe break or any other failure in the containment rooms. A system of relief valves and ducts led from the containment rooms to these suppression pools. RBMK s were built in pairs  

Analyzing Nuclear Reactor Safety Systems reactor  

Unlike Western reactors that have negative feedback coefficients in all phases of operation, the RBMK reactors had a positive feedback coefficient under some circumstances. While the graphite provided the vast majority of neutron moderation, the cooling water also absorbed some neutrons. Under certain operating conditions, especially at lower power levels, an increase in power could turn some of the water in the core into steam. This increase in steam would then cause a reduction in neutron absorption because of the reduction in liquid water density. Thus, if power increased while at a low power state, the positive void feedback coefficient would drive the power higher, and potentially exacerbate an accident situation. 

The same coastdown test had been previously attempted at the Chernobyl site several years before. It was known to be a difficult test, in part because the RBMK reactor was relatively unstable at the low power levels needed for the test. The test conditions required the reactor power to be in the 700-1000 MWt (thermal) range, approximately 14—X its full power capacity (US NRC, 1987). Under these conditions reactor control became unstable because of the interplay between the coolant flow rate, energy transfer from the reactor to the coolant, and the effect of the large coolant density changes inherent to the boiling water design. 

The specific events that led to the Chernobyl accident began on Friday, April 25, 1986. Very early in the morning ( 1 am) control room operators at Unit 4 reactor began a planned power reduction that reduced core power by about 50 % down to 1600 MWt (US NRC, 1987). By the afternoon of the 25 , the Central Electricity Board requested that no further power reductions occur at the plant to ensure that local power needs were met. At approximately this same time, as part of the test, the emergency core cooling systems were shut down (US NRC, 1987). 

By approximately 11 00 pm Friday evening, the local electricity demand had subsided, and the load dispatcher gave permission for Chernobyl Unit 4 to continue with its power reduction. At this point, another step in a string of unfortunate events occurred when the automatic reactor control unit did not adjust for the new lower power level. The operators had reset the reactor monitoring system to the requested level, but had failed to reset the reactor automatic controller. The reactor s response was a dramatic drop in power, down to 30 MWt, 1 % of the normal operating level (US NRC, 1987). In effect, the automatic controller inserted control rods, which drove the overall core power dramatically lower than was intended. 

As the power of Chernobyl 4 reactor fell, xenon poisoning occurred. Xenon poisoning is a well-known phenomenon that occurs in reactors that are reducing power after they have been operating at high or full power (see Chap. 4). In the Chernobyl 4 reactor, xenon poisoning helped to decrease core power to the barely critical level of 30 MWt (US NRC, 1987). 

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X Bq of Pu has been released, mostiy from bum-up of the nuclear powered sateUite SNAP-9a and that 3.7 X 10 Bqof + ° Pu was released by the Chernobyl accident (167,168). Many studies have been done to determine the cumulative fallout on sods, plants, bodies of water, animals, and humans. For example, the cumulative Pu fallout ia forest and grasslands and ia the Hver of elderly humans ia Bavaria, Germany are approximately... 

A much more serious nuclear accident occurred at Chernobyl in the USSR on April 26, 1986, when one of the Chernobyl units experienced a full-core meltdown. The Chernobyl accident has been called the worse disaster of the industrial age. An area comprising more than 60,000 square miles in the Ukraine and Belarus was contaminated, and more than 160,000 people were evacuated. However, wind and water have spread the contamination, and many radiation-related illnesses, birth defects, and miscarriages have been attributed to the Chernobyl disaster. 

Chernobyl 241Am, in five samples of surface soil collected 1.5-15 km from the Chernobyl accident in July 1987, was predominantly in the 0.005-0.25 mm size fraction of soil, which comprised 65% of the mass (Berezhnoi et al. 1991). 

Knatko VA, Mayall A, Drugachenok MA, et al. 1993. Radiation doses in southern Byelorussia from the inhalation of specific radionuclides following the Chernobyl accident. Radiat Prot Dosim 48(2) 179-183. 

Paatero J, Jaakkola T. 1998. Transfer of plutonium, americium and curium from fallout into reindeer after the Chernobyl accident. Boreal Environment Research 3 181-189. 

Although the Chernobyl accident was very serious, the defects in design and operating procedures that led to it were so egregious that the accident has tittle relevance to current reactors outside the former Soviet Union. However, it serves as a reminder of the need for rigorous care in the design, construction, and operation of nuclear power plants. 

Fig. 4. Change in (he "Te/137Cs activity ratio with time. Two values in 1986 represent those before and after the Chernobyl accident. (Reprinted with permission from Ref. 16. Copyright (1993) Elsevier Science Ltd)...

Regional total effective human dose-equivalent commitment from the Chernobyl accident... 

Today, the most important environmentally damaging anthropogenic radiation comes from atmospheric testing of nuclear weapons conducted 20 to 30 years ago, authorized discharges to the sea from nuclear reprocessing plants, and from the Chernobyl accident in 1986 (Aarkrog 1990). 

Table 32.17 Regional Total Effective Human Dose Equivalent Commitment from the Chernobyl Accident...

Acute biological effects of the Chernobyl accident on local natural resources were documented by Sokolov et al. (1990). They concluded that the most sensitive ecosystems affected at Chernobyl were the soil fauna and pine forest communities and that the bulk of the terrestrial vertebrate community was not adversely affected by released ionizing radiation. Pine forests seemed to be the most sensitive ecosystem. One stand of 400 ha of Pirns silvestris died and probably received a dose of 80 to 100 Gy other stands experienced heavy mortality of 10- to 12-year-old trees and up to 95% necrotization of young shoots. These pines received an estimated dose of 8 to 10 Gy. Abnormal top shoots developed in some Pirns, and these probably received 3 to 4 Gy. In contrast, leafed trees such as birch, oak, and aspen in the Chernobyl Atomic Power Station zone survived undamaged, probably because they are about 10 times more radioresistant than pines. There was no increase in the mutation rate of the spiderwort, (Arabidopsis thaliana) a radiosensitive plant, suggesting that the dose rate was less than 0.05 Gy/h in the Chernobyl locale. 

After the Chernobyl accident, radiocesium isotopes were also elevated in trees and lichens bordering an alpine lake in Scandinavia and in lake sediments, invertebrates, and fishes (Table 32.18). Radiocesium levels in muscle of resident brown trout (Salmo trutta) remained elevated for at least 2 years (Brittain etal. 1991). People consuming food near this alpine lake derived about 90% of their effective dose equivalent from the consumption of freshwater fish, reindeer meat, and milk. The average effective dose equivalent of this group during the next 50 years is estimated at 6 to 9 mSv with a changed diet and 8 to 12 mSv without any dietary changes (Brittain et al. 1991). 

Swedish Lapland reindeer herders have experienced a variety of sociocultural problems as a result of the Chernobyl accident. The variability of contamination has been compounded by the variability of expert statements about risk, the change in national limits of Bq concentrations set for meat marketability, and the variability of the compensation policy for slaughtered reindeer. These concerns may result in fewer Lapps becoming herders and a general decline in reindeer husbandry (Beach 1990). 

Caribou in northern Quebec contained up to 1129 Bq 137Cs/kg muscle FW in 1986/87, but only 10 to 15% of this amount originated from Chernobyl the remainder is attributed to fallout from earlier atmospheric nuclear tests (Crete et al. 1990). The maximum concentration of 137Cs in meat of caribou (Rangifer tarandus granti) from the Alaskan Porcupine herd after the Chernobyl accident did not exceed 232 Bq/kg FW, and this is substantially below the recommended level of 2260 Bq 137Cs/kg FW (Allaye-Chan et al. 1990). Radiocesium transfer in an Alaskan lichen-reindeer-wolf (Canis lupus) food chain has been estimated. If reindeer forage contained 100 Bq/kg DW in lichens and 5 Bq/kg DW in vascular plants, the maximum winter concentrations — at an effective half-life of 8.2 years in lichens and 2.0 years in vascular plants — were estimated at 20 Bq/kg FW in reindeer-caribou skeletal muscle and 24 Bq/kg FW in wolf muscle (Holleman et al. 1990). 

Ahman, G., B. Ahman, andA. Rydberg. 1990b. Consequences of the Chernobyl accident for reindeer husbandry in Sweden. Rangifer, Spec. Issue No. 3 83-88. 

Cristaldi, M., E. D Arcangelo, L.A. Ieradi, D. Mascanzoni, T. Mattei, and I.V.A. Castelli. 1990. 137Cs determination and mutagenicity tests in wild Mus musculus domesticus before and after the Chernobyl accident. Environ. Pollut. 64 1-9. 

Daburon, F., Y. Archimbaud, J. Cousi, G. Fayart, D. Hoffschir, I. Chevallereau, and H. Le Creff. 1991. Radiocaesium transfer to ewes fed contaminated hay after the Chernobyl accident effect of vermiculite and AFCF (ammonium ferricyanoferrate) as countermeasures. Jour. Environ. Radioactivity 14 73-84. 

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