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NUCLEAR ACCIDENTS

One of the major worries about the development of nuclear energy is the fear of a nuclear accident. There have been five major accidents that the public has heard about, namely Windscale (1957) , Kyshtym (Chelyabinsk) (1957), Three Mile Island (1979), Chernobyl(1986) and Chalk River although some of them have come from research stations and only two, Chernobyl and Three Mile Island from commercial reactors.

We will consider just two - the accident at Chelyabinsk in 1957 and the Chernobyl disaster of 1986 - both in Russia.

THE SECRET CITY

In the southern Urals, nine hundred miles to the east of Moscow on the edge of Siberia lies Chelyabinsk. A city of some 80 000 people with beautiful buildings designed by German POWs and built by political prisoners in 1945. Known to the outside world as Kyshtym and to the local peasants simply as The City it was this that Gary Powers in the U2 spy plane was sent to photograph in 1958. What this city and its research station, Mayak (postal district Chelyabinsk 65) housed was the Russian counterpart of Hanford the US bomb factory. It was here that the USSR developed its first atomic bomb.

 

Waste from the factory was first stored in a lake - Lake Caratin which has now become the most radioactive place on Earth. The silt contains a deadly cocktail of strontium, caesium and plutonium. In 1966 there was a drought and the lake dried up, the following year winds blew the dust over 40 000 people!

They are now trying to fill the lake with rocks but it will take years. The drivers have twelve minutes to drop their loads, the cabs of the trucks are shielded with five tons of lead, and outside the radiation is so intense that a man would die in an hour!

Other waste from the plant was kept in stainless steel underground tanks, but in on September 29th 1957 at 16.20 local time tank 14 exploded with the force of a 1 kilo ton bomb. Eighty tons of nuclear waste were spewed out over the surrounding countryside.

Of the 20 M curies (7.4 x 1011 Bq) in the container 10% was sent into the air in a cloud one kilometre high and the rest was thrown out round the container. This was the worst nuclear accident before Chernobyl but it was thirty years before the rest of the world was told! The shattered lid is still intensely radioactive and even now (1992) the activity level in the decontaminated areas reaches 50 Bq. (about 100 times the background radiation in Great Britain!)

Composition of the accident discharge

Radionuclide Half life Radiation % contamination
89Sr 51 days beta, gamma traces
90Sr + 90Y 28.6 years beta 5.4
95Zr + 95Nb 65 days beta, gamma 24.9
106Ru + 106Rh 1 year beta, gamma 3.7
137Cs 30 years beta, gamma 0.036
144Ce + 144Pr 284 days beta, gamma 66.0
147Pm 2.6 years beta, gamma traces
155Eu 5 years beta, gamma traces
Pu (mixture of) 5 years alpha traces

The figures were mainly for the contamination by strontium 90 (90Sr) because this has a long half life (28 years) and therefore was most important in the long term irradiation of living organisms. The authorities decided that 74 GBqkm-2 was the safe limit for people to live , the background radiation in the area before the disaster being 0.05 GBqkm-2.

The crowns of the trees initially contained up to 90% of the fallout matter and the dose at a height of 1 m above the ground in the forests was two to three times that in exposed areas.

All parts of the environment were affected by the contamination and during the first few weeks after the accident scientists measured the increase in beta activity in various living things. Their results are shown below:

Material Relative increase in beta activity
Grass 100 - 2x105
Open reservoirs 1.5 - 2x104
Wheat 25 - 1000
Cows milk 10 - 2x103

In the spring of 1992 a team from the Channel Four programme Equinox visited the area to make a film, the title of which starts this section, the health Physicist was Dr Brian East, and to him and to the reporter Andy Beech I am grateful for much helpful information.

The scientists wore heavy boots and the film was made when there was snow on the ground. This protected them from the majority of the beta radiation from strontium 90 and caesium 137.

Of course as time goes by the effects of the disaster will become weaker thanks to radioactive decay and the distribution of the radioactive material by various means including agriculture. The following table gives the actual and predicted changes during the first seventy five years after the accident. Notice that in twenty five years the overall activity has fallen to 3% of the original value while that due to strontium 90 alone has only dropped to a half.

CHANGE IN THE ACTIVITY AFTER THE MAYAK ACCIDENT

        Concentration  of  radionuclides  (%) 
Time after the
accident (years)
Density of contamination
at 1m (Rh-1)
Power of exposed
gamma dose
Total beta
Sr 90
Grass Grain Milk Leaves
0 100 100 150 100 100 100 100
1 34 96 8.7 10 20 10 3
5 5.7 89 0.33 1 1 1 1
10 4.3 78 0.15 0.4 0.8 0.4 0.75
25 3 52 0.053 0.05 0.3 0.06 0.1
75 0.88 16 0.017 0.01 0.1 0.01 0.05


Problems
1. Plot a graph showing the decrease of contamination due to the total beta radiation and the strontium 90, use the same axes.
2. Draw a graph showing the decay of contamination in grass, grain and milk.


THE CHERNOBYL ACCIDENT

On 26th April 1986 - almost thirty years after the accident at Chelyabinsk 65 another nuclear disaster happened in Russia. At Chernobyl, a then virtually unknown nuclear complex to the south east of Kiev the reactor at Unit Four exploded. Up to the present this has been the world's worst nuclear disaster and unlike Chelyabinsk the wind was blowing from the south east and so the fall out from the reactor was blown across a large area of western Europe from Lapland to Scotland.

The irony of this accident was that the engineers were attempting a series of designed to improve reactor safety!

They had planned to reduce the reactor power from 3200 MW to 700 MW and had disconnected one turbine and were going to use the energy from this as it ran down to power the cooling pumps. Unfortunately they let the power level drop too low, the Chernobyl reactor was a type RBMK 1000 which was designed to have a minimum power load of 300 MW, what actually happened was a drop from 3200 MW to just 30 MW. At this power the reactor became unstable; in an attempt to power up the reactor they switched off the cooling water and withdrew many of the control rods. The minimum number for this type of reactor was 47, at the height of the experiment just eight were left in! It was then that the reactor started to get out of control and the engineers could not replace the control rods fast enough. One of the improvements made to the other reactors on the site after the accident was to reduce the time needed to lower the rods into the reactor from the 18 s of Unit Four to just 2.5 s.

The temperature rose! The water in the reactor core of the RBMK 1000 also acted as cooling water and this began to boil. The zirconium tubes burst and the uranium fuel disintegrated. Now steam is much less efficient at absorbing neutrons than water - so less water and more steam meant more neutrons. More steam also meant less cooling water, less cooling water meant more power which meant even more steam and at 01:23.40 local time the pressure finally reached a critical value, the power rose to one hundred times the safe level and there were two explosions. The first due to high- pressure steam blew off the 2000 ton steel and concrete lid, the second was probably caused by the ignition of emitted hydrogen. It is important to realise that this was not a nuclear explosion as in an atomic bomb. It happened because of high-pressure steam - in just the same way that the lid of a kettle could be blown off if something got stuck in the spout!

The graphite in the reactor core caught fire and nine tons of radioactive fuel poured out into the atmosphere. About a ton fell on the site itself while the remainder was blown by the wind towards the northwest across Europe.

Five years later it also became clear that the explosion had not only blown the top off the reactor but had also forced its base downwards over four metres. Under the intense heat a lot of the uranium fuel had melted and had combined with the sand used in the reactor shielding to form a highly radioactive lava which flowed into the rooms below the reactor floor. The lava flow formed weird shapes, one known as the 'Elephants foot' had radiation levels of up to 10000 R/ hour on its surface three years after the explosion.

It turned out that 50% of the fuel turned into lava.

The Russians were then left with the terrifying task of trying to make the damaged reactor safe. Helicopters flew over the burning core trying to bomb it with neutron absorbing material with little success in fact several helicopter pilots later died of the radiation they received. It is thought that thirty-seven people died either in the explosion or from radiation burns. Robots were used to try and clean up the roofs of surrounding buildings which were littered with pieces of uranium fuel rods and chunks of graphite blocks with masses up to 50 kg. The robots failed and so between 3000 and 4000 members of the Russian army were sent to shovel them back into the pit in the centre of the reactor. The radiation levels they experienced were awesome, it could kill within the hour and so they were allowed only 1 minutes exposure but still received up to 20 R, the dose for a British nuclear worker for a year! There was always the additional fear of another explosion or a self-sustaining chain reaction.

On the 6th May the emissions stopped, something had happened inside the reactor.
It was decided that the reactor must be enclosed rapidly before rain washed radioactive dust away and caused further problems with steam on the red hot core. Some 250 000 people built a huge concrete and steel sarcophagus round the reactor.

The nearest town was Pripiat, at the time of the accident it was home to over 60 000 people, it is now deserted, too dangerous for people to live there but too expensive to pull down. In 1989, three years after the accident the radiation level on the ground in and around the city ranged from 80Bq to 2000 Bq. Trees in a nearby forest had suffered mutations from radiation levels that in one day would give a man fifty times as much radiation as the permitted maximum for a year for a British nuclear worker!

Apart from the damage caused to the reactor and the illness and death suffered by those who tried to make it safe the cloud of radioactive dust spread north west over Europe. The reindeer meat of Lapland became radioactive as did Welsh mountain sheep and their sake for human consumption was forbidden for some years after the accident.

Humans were also affected although the levels were not thought to be damaging. As an example of this the following table shows the values of radio caesium (134 (half life 2.06 years) and 137 (half life 30.2 years)) in members of the Scottish population after the Chernobyl reactor accident measured at the Scottish Universities Research and reactor Centre.

Volunteer number Days 134Cs (Bq) 137Cs (Bq)   Volunteer number Days 134Cs (Bq) 137Cs (Bq)
1 5 <23 <26   16 14 442 599
1 12 <23 64   16 148 518 1073
1 34 78 156   16 509 165 423
1 71 161 294   16 1189 <23 81
1 148 172 328   16 1406 <23 58
1 183 174 347   122 301 3774 8110
1 235 191 347   122 1218 165 776
1 265 194 362   122 1422 109 577
5 7 41 <26   86 153 175 399
5 21 119 91   86 272 173 411
5 28 137 206   86 377 181 466
5 35 171 263   86 482 126 368
5 46 177 288   86 580 107 272
5 50 186 309   86 664 118 471
5 55 269 350   202 348 307 735
5 100 189 306   202 533 169 356
5 132 178 329   202 707 82 254
5 166 134 286   202 1259 27 74
5 207 141 273   260 376 326 686
5 252 110 236   237 587 104 312


The concentration of radioactivity in volunteers who ate venison and goat meat was found to be much higher than those with a "normal" diet. Volunteer 122 was one such; he changed his eating habits after finding out the results of the first set of tests!
It was thought (1991) that levels of 137Cs in the population of Scotland will have fallen back to pre-accident levels by early 1992.


Student Activity
Plot a graph of radiation levels against time for volunteers one and five. From the lines work out the half life for the radiation they have absorbed.


There were still problems in 1992 - scientists are testing the roof of the sarcophagus for dust wafting out of the damaged core - much of this is plutonium.
The real fear now is that the lid may fall back inside the reactor. The shock of this would release a huge cloud of radioactive dust and because the sarcophagus is not airtight, nor was it ever designed to be, this would escape to cause another catastrophe in the surrounding countryside.
There appear to be three choices:
(a) build a second airtight sarcophagus around the first to allow work to go on
(b) cover the old sarcophagus with sand
(c) cover it with concrete
both of these two later possibilities would prevent any work on the damaged reactor.

 
 
 
© Keith Gibbs 2010