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.
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!)
| 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.
| 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 |
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 |
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.
Fukushima
This accident at the Fukushima power plant in north eastern Japan occured in March 2011 as a result of an earthquake and the tsunami that followed it. The damage done to the reactors at the released between 10-30% of the radiation produced in the Chernobyl accident. It occured mainly because of damage to the pumps which circulated cooling water through the reactors which then overheated.