Readers letters
I have a number of problems with "Nuclear reactors could power cargo vessels" article. These may in part be caused by PE only reporting a portion of the statements made by Babcock’s Mr Dobson and Professor Carlton. Some of the problems mentioned have been addressed before but it is difficult to see how they could be aware of the work and whether they could still access it. This comment might also apply to the item on the previous page, coincidentally, called “Learn past nuclear lessons”.
I spent five years (1963-68) in UKAEA’s BPWR design office and twenty seven years (71-98) in the Safety and Reliability Directorate’s section which acted as Independent Nuclear Safety Advisor to MoD for its nuclear submarines. The note below is based on memory since I did not keep diaries of the work. The comments are mostly based on the earlier working period and therefore should avoid anything classified from the latter one.
Presumably cargo vessel reactors would have to be commercially viable and despite all the experience in nuclear warships and submarines the type of core they use would probably be too expensive for this role. It is inevitable that there will be great emphasis on achieving the longest core life possible to reduce the high costs of refuelling together with the loss of availability whilst this takes place. There have been two principal ways of doing this, firstly, put sufficient U235 in the core to achieve the desired core life and offset the excess reactivity this creates by placing burnable poison in the core. The poison can be designed to burn up at approximately the same rate as the fuel so that the core reactivity stays within the capacity of the control rods. This was the approach used in the Burnable poison PWR mentioned above. Secondly, thorium can be added to the core, which converts to U233 as the U235 burns away thereby maintaining the activity of the core. Thorium was a significant topic in the Nucleonics Week magazine in 1960 but all references to it disappeared during 1961 probably due to security rules. The UKAEA’s studies of High Temperature Reactors (HTR) in 1961 and 1962 did include thorium in the core design. A later HTR team tried to reduce the size of the core by increasing the power density to 20 KW per litre but this reduced the conversion of the thorium to U233 and instead produced proto actinium - a poison.
The specification for BPWR was not based on a particular application. I believe the largest tanker at that time carried 30,000 tons of oil and someone attempted to future proof this by assuming this could rise to 70,000 tons. The design office was asked to design a core to support a 40,000shp propulsion system and the ship’s electrical requirements. This resulted in a 117MW uranium oxide core which had an active core length of 54 inches with a similar diameter. This makes Professor Carlton’s dustbin sized core a little improbable. In any case, it would be difficult to fit sufficient control rod drive mechanisms in such a small pressure vessel head, and although the core would not be subject to tight noise requirements which would permit both high pumping power and high mass flow through the core, anyone familiar with departure from nucleate boiling (DNB) calculations would recognise that this could be a problem for such a small core. It is unlikely that anyone would attempt a fast reactor for this application even though core power densities of up to 60KW/litre could be achievable.
One assumption that the BPWR initially took on board was that an oil tanker was the most suitable application. Eventually the oil company involved explained that 90% of its stored oil was in the tankers at sea. In addition, its tankers operate like beads on a chain and if demand rises the whole chain is speeded up. One could not therefore have one nuclear tanker going twice as fast as the remaining tankers and arriving just as the land based storage had been filled by the preceding ship.
One major difference between submarines and surface ships is the fact that surface ships can capsize. Most land based reactors rely on natural circulation to remove the decay heat which continues to be produced after a reactor shuts down. In a capsized ship natural circulation would not work as the heat source would be at the highest point in the circuit. It would be interesting to see how current surface ships have addressed this problem. Early attempts to address this included an auxiliary pump driven by a seawater battery which would flood and be activated if the ship turned over. The intent should always be to prevent core damage and the escape of the fission product inventory to the environment. This would be a significant problem in shallow waters. The first US Navy submarine to be lost sank in 6000 feet of water. At the time many questions were asked about fission products in the sea and whether these had been detected. A US Navy statement confirmed they hadn’t and also said that the total fission product inventory was equal to the potassium activity in 9 cubic miles of seawater. It is difficult to imagine the primary circuit remaining intact after the collapse of the pressure hull and at this depth the sea pressure would totally suppress any boiling in the remains of the core.
A second problem created by capsizing is the need to ensure that the control rods do not drop out of the core. BPWR used springs to hold the control rods in the core. This eliminates the need to guarantee that any locking mechanism used would work reliably for an extended core life. Dividing the spring into four sections was intended to reduce the effect of a spring breaking and the two pieces interlocking to shorten the spring length.
The world has changed since the 1960s and the ability to cope with sabotage and aircraft strike have been added to the design specification. A difficult problem will be the need to define the maximum shock load for the design of the core. As is said in the PE article there will need to be significant international collaboration on this and other aspects of these designs.
If a long core life became achievable but one which still required the ship to be refuelled during its working life, it will be necessary to prove that the core remains dimensionally stable and makes it possible to remove the old core and install a new one. Thus the research programme will not be a short one.
Michael Pugh, Warrington, Cheshire