cnd logo    Scottish CND     Safety of Trident

5. WARHEAD ACCIDENTS

This section looks at the inherent dangers of nuclear weapons and the consequences of an accident including the risk of a . nuclear explosion. Plutonium dispersal would endanger human health and present a long term environmental problem. The effects of a transport accident, warhead accident on a submarineand a combined reactor/warhead accident are considered.

5.1 SAFETY OF NUCLEAR WEAPONS

Relationship between US and British Trident warheads

The small number of British nuclear tests specific to Trident suggests that the warhead is not a completely British design. It is likely that it is modelled on an American design. There is close cooperation between British and American nuclear weapons designers. A former director of Lawrence Livermore Laboratory has said that the British Trident warhead is a copy of an American design. Britain will purchase the Mk4 RV for Trident. In the United States this is used for the W76 warhead which is a fission warhead boosted by tritium and deuterium.

The criteria adopted in the design of nuclear weapons in the United States was described in the Drell Report: "... a large nuclear weapon stockpile was built in the chilling environment of the Cold War. Modernisation and improvement programs gave priority to military requirements, such as achieving maximum yield to weight ratios for warheads and maximum payloads and ranges for missiles. Safety in general was not viewed with the same urgency".

Safety assessment in Britain

A nuclear weapon is complex and contains a wide range of exotic components. The way in which these will change and decay over the years is difficult to predict. The Director of Nuclear Policy and Security told the House of Commons Defence Committee - "Any warhead is subject to aging processes. These are not necessarily wholly predictable". Another senior MoD official at this meeting was concerned about Aldermaston's ability to assess the safety of the warheads in future years. He was worried about a possible reduction in nuclear testing - "With a restriction in testing in the future the competency of the relevant people at Aldermaston will decay, fall off, until the point where they are unable or unwilling to say 'This is still safe'". The ability to assess the safety of warheads is in doubt and would appear to rest on the continued destruction of the environment in the homeland of the Western Sheshone, the Nevada desert.

Nuclear warheads are designed to withstand the effects of reentry from space and are tested to simulate these effects. However these circumstances are not identical to those which might be encountered in a complex warhead accident.

Amounts of plutonium, tritium and HEU

The complete fission of 5.7 kg of plutonium will produce a yield of 100 kilotons. However 100 % fission will not be achieved even with tritium/deuterium boosting. A Trident warhead probably contains between 3 and 6 kg of plutonium, supplemented by HEU, which is more readily available. A reasonable estimate is 4 kg plutonium per warhead. Calculating the total US stockpile of tritium in the 1970s and dividing this by the number of nuclear weapons in the US arsenal produces a figure of 2.8 g of tritium per warhead. Many of the weapons in the US stockpile at the time did not contain tritium. The amount of tritium in a British Trident warhead is probably around 4 g. The amount of HEU is not known possibly 10 - 20 kg.

Combination of plutonium and high explosives

Trident nuclear weapons contain a sphere of approximately 4 kgs of plutonium, plus HEU, surrounded by at least 20 kg of high explosives. This combination is inherently dangerous. Guidelines issued by the MoD to local authorities state that in an explosion fragments could be propelled 600 metres from the scene. The explosives used will be of a high energy formula which is likely to be sensitive to shock and heat.

Sensitivity to heat

Temperatures only slightly above normal could be dangerous. Explosives may detonate because of heat. Alternatively they may melt. After they have melted they will detonate very easily. Many US nuclear weapons contain PBX explosives which have a plastic, polybutadiene base. The melting point of Trident warheads is not known. Conventional RDX explosives have a melting point of 203o C.

Sensitivity to shock and pressure

Warhead high explosive may detonate if it is subject to shock pressure of 20 kilobars. It may also detonate if there is an impact and the impact velocity is 100 mph. The precise response of explosives to shock is difficult to predict. One batch of explosives with its own history may not behave in exactly the same way as another batch of the same material. The US has in the past used at least one type of explosive in nuclear weapons which could easily be detonated. LX-09 was used in the W 68 warhead on Poseidon missiles. Safety tests were carried out after a fatal explosion on 30th March 1977 in which 3 workers who had been handling the explosives were killed. These tests established that LX-09 could detonate if a small weight was dropped from a height of 30 cms. In February 1959 two workers at Aldermaston were killed when unloading high explosives from a trolley. Insensitive High Explosives (IHE) have been developed which will not go off so easily but IHE is not used on the US W76 warhead and is almost certainly not used on the British Trident warhead.

Other properties of warhead components

Small splinters of plutonium metal can spontaneously ignite in air. HEU in a dust form is also a fire hazard and a moderate explosive hazard. Beryllium is a moderate fire hazard and a slight explosives hazard. The depleted uranium in the weapon is also a fire hazard in splinter or dust form. The warhead will contain lithium which has a melting point of 186o C and reacts with water. Plutonium melts at 639o C, uranium at 1132oC and beryllium at 1278oC. The warheads will also contain plastics and components which may melt at relatively low temperatures. The way in which a nuclear warhead responds to heat will be complex. If these reactions result in a very small chemical explosion, equivalent to one thousandth of an ounce of TNT, this could detonate the main high explosive.

5.2 NUCLEAR EXPLOSION

The MoD have said that the British Trident warhead was proved to be one-point safe on 13 March 1992. "One-point safe" means that in a nuclear weapons accident if the explosive detonates at any one point the probability of a nuclear yield of more than 4 lbs TNT equivalent is less than 1 in a million. There are two concerns about this information. The MoD Chief Scientific Adviser earlier expressed doubt that the safety of the Trident warhead could ever be properly assessed, given that the calculations would only be carried out at one installation, Aldermaston. The Americans have argued that they need to have two establishments, Los Alamos and Lawrence Livermore, so that results can be cross checked. The second concern is that this approach does not address the situation where part of a missile explodes and fragments impact on the warhead from various directions.

5.3 PLUTONIUM DISPERSAL

The more likely outcome of a serious nuclear weapons accident is the dispersal of plutonium dust over a wide area. This could occur during a fire or a conventional explosion. A conventional explosion is regarded as 100 times more serious than a fire, because the plutonium would be in the form of fine particles, which are more dangerous.

Absorption of plutonium into the human body

When plutonium has been dispersed into the atmosphere, the main way in which it is likely to enter the body in the short term is by the inhalation of dust particles. Small particles are particularly dangerous as they can remain in the lung and be transferred to other parts of the body where they remain for many years. Particle size is very important. Particles larger than 10 microns are likely to be cleared from nasal airways and swallowed. If the particles are smaller than 10 microns then the smaller they are the greater the proportion deposited in the lung. Large amounts of plutonium which are taken into the body can have serious toxic effects in addition to the effects of radiation.

If 100 particles of plutonium, each 1 - 10 microns in size, are inhaled then 8 of these particles are expected to reach the bone structure and remain there for many years. After 50 years, half of the particles would still be in the skeleton. Also of the 100, 7 more particles would be retained in various parts of the body for between 6 months and 40 years or more. The remaining particles would be removed from the body more quickly.

Alpha radiation is emitted from plutonium for thousands of years. While there is a risk from inhalation of plutonium, there is also a risk, particularly in the long term that a significant proportion will find its way into the food chain. The proportion of ingested plutonium which remains in the body is small. For plutonium dioxide, 0.1% of the amount ingested will remain in the body for a significant length of time. Higher amounts will be absorbed if the plutonium is in a soluble form or if the particles are extremely small.

Biological effects of plutonium

The alpha radiation from plutonium can only travel a very short distance and cannot penetrate the dead layer of skin around the body. However a particle emitting alpha radiation which is inhaled and absorbed can have a considerable effect on live cells in its immediate vicinity. The relative biological effectiveness of alpha radiation is greater than gamma or beta radiation by a factor of 20. A particle of plutonium can damage cells which are between 10 and 50 microns from it.

Cells have an ability to divide to create more cells. There are natural mechanisms to prevent this division when it is not needed, however the damage caused by plutonium particles and other forms of radiation, can have the effect of disrupting these mechanisms. This results in cells dividing and redividing when they should not. In this way, the inhalation of plutonium dust can result in cancer in the bones, lungs and other parts of the body.

There are also risks of genetic damage. In a male, 3 out of 1000 particles are likely to reach the gonads. In a female the proportion is 1 out 1000. These particles are likely to remain there for many years. They can damage reproductive cells in their immediate vicinity and break the DNA chains within cells. If these breaks are complex, the cell may be unable to repair the damage. This can result in genetic abnormalities in the offspring of the person who is exposed to radiation.

Assessing the probability of cancers and of genetic damage is very difficult and not very reliable. Damage to one cell from a small dose of radiation can result in cancer. While the chance of this happening is very small, the damage caused by exposure to a small dose is as great as for a larger dose. However, the larger the dose, the more likely it is that cancer will occur. The same is true of genetic damage.

Casualties

It has been calculated that if 8 x 10-5 g of weapons grade plutonium is inhaled and 15% is retained in the lung there would be a 100% risk of death from lung cancer. Assuming that one Trident warhead contains 4 kg of plutonium then this is equivalent to 50 million lethal doses. If 2 warheads in a vehicle were to explode the total amount of plutonium involved could be equivalent to 100 million lethal doses.

The risk of contracting cancer would be roughly in proportion with the amount inhaled. The proportion of plutonium which was inhaled as very small particles would be particularly significant. The risk factor for fatal cancer is given by the ICRP as 0.05 per Sv. The risk factor for all cancers is approximately to be 0.075 per Sv. The risk factor for hereditary effects for all future generations is given by the ICRP as 0.01 per Sv for all ages and 0.006 per Sv for people of a working age.

Long term behaviour of plutonium in the environment

Plutonium which has been dispersed into the atmosphere will continue to be a significant radiation hazard for thousands of years. Studies have been carried out to assess likely ways in which plutonium which has been discharged can reach the human foodchain and these give some indication of the way in which plutonium behaves in the environment. In sea water a large proportion of plutonium is expected to accumulate in sediments. The levels of plutonium found in phyto plankton is likely to be high. From plankton the plutonium may then be passed along the food chain to zooplankton, plantiverous fish and pisciverous fish. Plutonium is also likely to be found in shrimps, crabs, lobsters, mussels and gastropod molluscs - algae have a particular ability to concentrate plutonium. Plutonium may also be taken in as food by sea mammals and birds feeding on sea life and feeding between the high and low water marks. In the case of plutonium deposited on the soil the rate at which it is moved downward will depend on the soil type, for example there is less downward migration in clay soil. Downward migration in soils varies between 0.1 and 1 cm per year. Plutonium may be moved upwards by mycelium and subsequently dispersed into the air in airborne spores. It may be absorbed by microorganisms which are food for protocoa and nematodes. These may then move the plutonium upwards towards the surface. While downward movement will be the main factor there are also a number of ways in which plutonium will move upwards in the soil.

The uptake of plutonium by plant roots is likely to be small, less than one ten thousandth per year, although the proportion will be higher for plutonium in soluble compounds. The proportion is greater if there are more organic ligands and micro organisms present. Plutonium can also be deposited on plants from the resuspension of material on the surface of the soil. A high proportion of material which is resuspended will be very close to the ground and so could be deposited on plants. Particular plants show greater ability to concentrate plutonium, eg the lichen which is eaten by reindeer in Northern Scandinavia.

Animals are likely to inhale some plutonium through inhalation of resuspended particles. The primary intake for herbivores is likely to be through food. Of the plutonium taken in as food, a proportion of 1 in 10,000 is likely to be subsequently deposited in the bones and liver. Relatively large amounts of plutonium will cause cancers in a range of animals which have been used in experiments.

Response of Emergency Services

Fire fighting

If there is a fire in the immediate vicinity of the warhead then fire crews could be in great danger. MoD guidelines indicate that if the weapon is jetting, hoses should be tied down onto the warhead before fire crews withdraw. Elsewhere the guidelines say that if the weapon is jetting the high explosive may be about to detonate. By remaining in the area to fix hoses fire crews may reduce the chance of a major nuclear accident, but they will be at great personal risk if the warhead was to explode, both from the explosion and from radiation. In addition fire crews may not be able to tell whether it is the high explosive or other materials which are on fire. Contrary to the information given in the guidelines to local authorities, training manuals used at the Naval College in Greenwich say that it will probably not be possible to distinguish between the flames from high explosives and those from other materials.

Detection of alpha radiation

Detection of alpha radiation would be a major problem. Local authorities would have to rely on detection equipment provided by the MoD much of which would not arrive until 24 hours after the incident. The immediate response would be based on computer predictions from Aldermaston rather than on readings taken on the ground. The actual dispersion would vary from the computer model. It would also be difficult to assess how much plutonium people have inhaled. It is not possible to detect a particle of plutonium from outside the body. Plutonium intake is normally assessed by measuring the amount of plutonium which is removed from the body in urine.

Immediate area

The MoD guidelines contain contradictory information about what should be done in the immediate area around a nuclear weapons accident. The explosives hazard and the immediate radiation hazard demand that the area be evacuated as quickly as possible. However there is also concern that people and vehicles leaving the area would be taking with them plutonium dust. Decontamination facilities would be set up to process everyone and all vehicles before they left the area. The demands of immediate evacuation and of contamination are not easily reconciled.

5.4 EXAMPLE OF A WARHEAD TRANSPORT ACCIDENT

Effect of a nuclear weapons accident in Glasgow city centre

If an accident took place while nuclear weapons were being transported over Kingston Bridge in the centre of Glasgow, and there was a conventional explosion then plutonium would be dispersed downwind. Calculations have been carried out to determine the effect that a nuclear weapons accident on a naval vessel could have on the city of New York. This has been used as the basis for this example with some modifications. It is assumed that the wind is from the South West, the prevailing direction, with a wind speed of 4 m/s, around 9 mph, and average air stability, Category D. It is assumed that a total of 2 kg of the plutonium is dispersed in particles small enough that they are likely to be retained in the body.

In Anderson centrre 400 m from the accident the resulting fatal cancer risk would be 1 in 29
In Sauchiehall Street 1.2 km away it would be 1 in 91
In Springburn 4.1 km away it would be 1 in 444
In Stirling 34 km away it would be 1 in 8697

300 mSv is the NRPB upper ERL above which evacuation is recommended. 30 mSv is the lower ERL at which evacuation should be considered and the upper ERL at which sheltering is recommended. 3 mSv is the lower ERL at which sheltering should be considered. The risk of fatal cancer in particular places is shown, estimated on the basis of the ICRP risk factor. In this example, sheltering should be considered at 28 km from the accident and evacuation should be considered at 5 km.

Other accident models

An example of the effect of an accident is contained in an official US model (NARP). The dose levels and response at given distances are shown :

At 0 - 2 kms the dose would be 250 - 750 mSv, and the response would be to evacuate
At 2 - 10 kms the dose would be 50 - 250 mSv and the response would be to take shelter & consider evacuation
At 10 - 35 kms the dose would be 5 - 50 mSv and the response would be to consider shelter

Further information is available from details of Exercise Pantograph which was carried out by the MoD from 9th to 13th May 1988. The exercise simulated the response to a nuclear weapons accident involving an air crash. Evacuation was recommended 3.5 km from the scene on a 180 o arc; the MoD EAGL for evacuation is 100 mSv. Sheltering was recommended 10 kms from the scene for 36 hours; the MoD EAGL for sheltering appears to be 30 mSv.

The dose levels estimated in the example of an accident in Glasgow city centre are less than those in the US model above. They also appear to be compatible with the calculations from Exercise Pantograph. All three examples, the US model, the MoD exercise and the one used for this report, show that the effects of a nuclear weapons accident could be more serious than indicated in guidelines issued by the MoD to local authorities. These guidelines suggested that evacuation would only be required up to 600 m from the site and sheltering up to 5 km.

Ground concentrations of plutonium

The map in figure 14 gives radiation dose estimates from inhalation of plutonium dust in the initial cloud. The dust in the cloud will settle, in decreasing concentrations from the site. This presents a hazard as a proportion of the dust will be resuspended and could be inhaled. The resuspension factor is measured as air concentration (Bq/ m3) / ground concentration (Bq/m2). An average factor for newly deposited material is around 10-6 however this can be 10-5 on hard surfaces and higher where there is traffic. Naval manuals recommend evacuation within 12 hours if ground concentration is higher than 2 x 107 Bq/m2 and evacuation within 7 days if ground concentration is higher than 2 x 106 Bq/m2. While these levels might only be exceeded generally within 1 km of the site and at hot spots further afield, lower levels would present a long term hazard over a wide area.

5.5 NUCLEAR WARHEAD ACCIDENT ON A SUBMARINE

There is the potential for a very major nuclear warhead accident on a submarine. One Trident submarine could be armed with as many as 96 nuclear warheads in a small area without adequate blast protection between the warheads or from the missiles. In the event of a series of explosions there could be two very serious effects.

Nuclear explosion. The risk of a nuclear yield becomes significant in a complex explosion on a submarine. Blast fragments could impact on warheads from a variety of directions which would substantially increase the chance that a detonation might produce a nuclear yield. Even a very small nuclear yield, equivalent to a few kg of TNT would present a serious problem because of the generation of fission products and the effects of the intense heat on other radioactive substances and explosives.

Plutonium dispersal. The second risk is the dispersal of a very large amount of plutonium. If each warhead contains 4 kg of plutonium and there are 96 warheads on a submarine then the total amount of plutonium carried is around 0.4 tonnes. This is equivalent to 4,800 million lethal doses. Patterns of dispersal would be determined by the weather. The size of particles dispersed would affect the degree of risk to the general public. Plutonium would be dispersed over a wide area which would lead to long term agricultural restrictions and also to the relocation the local population.

Figure 16. This is the Explosives Handling Jetty at Coulport where nuclear warheads will be placed on Trident submarines. This is the most likely scene of a major accident which could involve both a large number of warheads and the submarine's reactor. Strong winds blowing up Loch Long caused problems when HMS Vanguard berthed here for the first time on 22nd January 1994.

5.6 COMBINED REACTOR AND NUCLEAR WARHEAD ACCIDENT

The worst form of accident would be one in which not only was there a major release of fission products from the reactor, but also the dispersal of plutonium from nuclear warheads, possibly from the detonation of one or more missiles. Detailed information is not available on the effects which such an accident would have, however the result could combine the worst of a containment failure accident and a multiple warhead accident. In addition to the immediate risk to the general public from the reactor fission products and from inhaling plutonium dust, there would be long term risks from plutonium dispersal.

Scottish CND     Safety of Trident