What is a serious radiation dose?


by Peter Thomson

Amongst all the current hysteria over the Japanese nuclear power plants and worldwide panic over the release of radioactive material from the Fukoshima nuclear plants, there is a need to bring a little bit of scientific reality into the mayhem.

First, there is no analysis of what radioactive chemicals were released in any of the plumes.  At Chernobyl, because there was an actual fire open to the outside, long-lasting radioactive particles such as caesium and strontium along with short term radioactive ions such as iodine were released in a large volume.  The spread was made worse because the thermal fire in the graphite rods drove these particles high into the air, into the jet stream.  The problems were compounded by the Soviet state’s insecurity, deep sense for the need for secrecy and failure to seek support to deal with the emergency.  The delay in evacuating locals from the Chernobyl area added to the disaster and loss of life – a mistake the Japanese are not repeating.

The Japanese plants did survive the earthquake intact and shut down as per their original design.  The problem is that the cooling systems needed to cool the core have failed due to the impact of the tsunami.  One of the best ways to imagine the damaged reactor vessel is as a pressure cooker running out of water with the safety valve jammed.  The pressure inside builds, the steam becomes superheated and as the pressure rises the physics says that the temperature can only rise further.  In a normal boiler you would forcibly break the safety valve to release the pressure, but in a reactor vessel of the boiling kettle sort (standard design in the late 60s) it is not that easy.  Under high pressure the superheated steam reacts with the exposed fuel rod containment metal to produce hydrogen and a metal oxide.  The explosion of reactor one at the Japanese plant was a direct result of this pressure release and the ignition of the hydrogen gas.  If you watch the first blast carefully you can see the blast wave as a layer of condensation going out ahead of the destruction.  There is little muck and dust.

This result then left the engineers with a problem in number 3 reactor.  If they tried the same approach how much worse would the explosion be?

This time they appear to have attempted to pump sea water into the vessel.  However in order to get in to the vessel the pumped water would have to be at high pressure.  The volume of water being pumped in would, perversely, increase the pressure in the vessel – unless there was a controlled release of pressure.  This could not happen because the safety valve had locked shut.  Something had to give, first it was the cooling system ‘doughnut ring’ under the vessel.  Then it appears the containment vessel failed.  The difference in the second explosion is clear as you see the narrow funnel of dirty cloud in a near vertical column.

In reators two and four there is evidence of cooling failures with a fire in reactor four which was already shut down prior to core removal then refuelling.  In reactor two there are unconfirmed reports of small explosions and a possible breach of the reactor vessel.

The problem in any attempt to assess the real risk from radioactive contamination, from this distance, is that none of the reporters appear to have any understanding of the units of radioactivity being bandy about by experts or the importance of total exposure.

In my combat role as an RN Triage officer I had to have a detailed knowledge of radiation exposure time, accumulation of dose and inhalation or ingestion rates before I could even start to assess the risk to any individual.  In my day we were still working in Rems, Rads and Rontgens and not Sieverts, so while I am in the right ball park an up to date radiophysicist will probably take me to task.

The risk has to be assessed against the dose rate per hour because as that is what determines the overall chances of any individual starting to succumb to radiation sickness.  So a dose rate of 10 millisieverts an hour (0.01 Sieverts an hour) means there is very little effect as the annual dose for non-radiation workers is 11 Sieverts, whereas a 1 Sievert an hour is a serious exposure (1000 millisieverts an hour).  Now radiation workers’ safe annual maximum dose is set at 20 Sieverts.  In medical terms this is usually set at 25% of the dose that is medically assessed which starts to trigger the symptoms of radiation sickness.  To give you some sort of comparison a full body CAT Scan will expose you to 6900 microsieverts an hour (0.69 sieverts)

According to the Japanese physicists the highest recorded figure has been 860 microsieverts an hour (0.086 Sieverts) on the reactor site.  However they claimed that this rapidly decayed back to 460 microsieverts (0.046 Sieverts), and then continued to reduce down to 100 microsieverts an hour (0.001 Sieverts).  The problem is there has been no timescale given in any of the press information that allows me estimate the total dose exposure at the power plant in reaching the peak level and then dropping back.

Next: direct dose exposure reduces exponentially over distance so a 15 kilometre safety zone actually means the risk of direct exposure starts at 7 Km.

The imponderable is the effect of any radioactive plume, but it appears that the main wind direction appears to be offshore with nothing for umpteen thousand miles.  It also appears, currently, there is no evidence of any of the radioactive plume rising to over 25,000 feet and into the jet stream.  So the sort of spread of caesium seen after the Chernobyl disaster, when places as far away as Scotland were directly affected, does not yet seem to be a threat to South Asia or the North West Coast of Canada and the USA.

The low level plume does mean that radioactive particles will be included in the low level rain clouds.  The Japanese have intimate knowledge of this effect as after Hiroshima it is estimated that the ‘black rain’ that fell caused as many long term radiation effects through ingestion as the known fall out.  Again the Japanese are telling people within a 50km radius to stay indoors when it rains and to leave their umbrellas and other gear outside their homes.

This is a serious nuclear accident, of that there can be no doubt.  However depsite the reactors’ age, out-dated design and the fact that they are nearing the end of their operational life, the design did do what it said on the tin.  It resisted an earthquake in excess of Richter magnitude 8.  Just imagine the mess if they had not met their basic design requirement.  Chernobyl would have been made to look like a children’s tea party and vast areas of South Asia and the West Coast of America would be in serious trouble, a large chunk of Honshu (Japan’s main island) would have been left uninhabitable.

Would I have a nuclear reactor next door to me?  Not on your nelly.  Do I think that a major nuclear disaster has been, so far, avoided?  Most certainly.  So let us applaud the efforts the Japanese nuclear engineers are undertaking to prevent any further problems and get the cores cooled and safe, at great risk to their own health.  We in Scotland are blessed with plenty of alternatives to nuclear power from coal, oil and gas through hydro to tidal power but in Japan nuclear remains key to their industrial capacity whether they want it or not.

The time for navel gazing is when this is all done and dusted but one thing for sure is, if we do not need nuclear for Scotland’s future – let’s not build any more.