6) A news release about Fukushima stated that “Neutron Beams” were observed 1.5 km from the reactors at levels of 0.01-0.02 microSieverts per hour. Neutrons can be released by low level spontaneous fission of plutonium in the spent fuel rods (about 1% of the fuel) or by “recriticalization” of the spent fuel under conditions of exposure and water loss-the latter would mean ongoing fission chain reactions with much higher neutron releases. The point of this problem is to try to assess which is more likely to be the scenario.

Let us work backwards to make an estimate of the neutron flux back at the reactors.

a) For definiteness assume the neutrons have an energy of 1 keV. Given the neutron mass, estimate their speed from the Newtonian formula for kinetic energy. Are they likely to decay in the 1.5 km traverse?

b) Assume the neutrons are produced at a rate R at the source (neutrons/sec). At a distance of 1.5 km, on a detector of area 0.5m2 , how many neutrons per unit second in terms of R would you expect? (The neutrons are emitted spherically from the source)

c) Next assume the detector measuring the neutrons is 0.5m2 in area, 0.2 m in thickness, has a density of 1 g/cc (like water), and has an N factor for Sieverts of 0.12 like bone marrow. The neutrons have a Q factor for Sieverts of 5. Use this information to work backwards and estimate R at the source.

d) Focus on plant 4. Neutrons from Fukushima can come from two sources: spontaneous fission of 240Pu present in spent fuel rods (making up about .2% of the spent fuel) which releases neutrons at 1000/g-sec, OR fission of “recriticalized” spent or live rods. Of particular concern are the air exposed live rods from Reactor 4 (the core assembly was removed and put in storage prior to the earthquake and tsunami). From what you estimate in c) can you assess which scenario is a more likely source of the observed “neutron beams”?

Using your estimate of the neutrons produced per second from parts b,c, you are to compare to the spontaneous fission rate of 240Pu versus ongoing live fission of 235U and 239Pu in the spent/active fuel rods. If you estimate an R value much higher than the spontaneous fission rate of the estimated amount of 240Pu, you will have to assume the neutrons are due to the far more dangerous “recriticalization” of the spent/active fuel. Accurately estimating the number of neutrons associated with that is far more complicated than the spontaneous fission of the rarer 240Pu.

There are 1000 fuel assemblies there with 63 rods per assembly, each having about 0.2% 240Pu by mass