However, the use of R(0) is valid only for very low fluence and it should no longer be used, although one may come across its use in historic papers.

4.11.5.2 Equivalent Nickel Flux

Nickel foils were used to give a measure of the damage to graphite through the 58Ni(n, p)58Co reac­tion. This reaction has a mean cross-section of 0.107 barns and 58Co has a half-life of 71.5 days.

The change in graphite thermal resistivity was measured in the TE10 experimental hole in BEPO and the nickel flux was also measured at the same position. It was assumed that the graphite displace­ment rate f d was equal to the nickel flux f Ni at this position. For comparison, the change in graphite thermal resistivity was then measured at various other positions in BEPO, as given in Table 4

Later, the same exercise was repeated in PLUTO, the sister reactor to DIDO at Harwell, and the ratio compared to that at other positions. In this case, the ratio appeared to be largely invariant to position. Table 5 gives a few examples of the many measure­ments made.16

It was decided that the activity produced in nickel fNi could be related to the graphite damage rate by a

Modified from Bell, J.; Bridge, H.; Cottrell, A.; Greenough, G.; Reynolds, W.; Simmons, J. Philos. Trans. R. Soc. Lond. A Math. Phys. Sci. 1962, 254(1043), 361-395. b is a proportionality factor.

factor. However, care was still required with respect to the choice of reactor and irradiation location. Thus, a definition of damage based on a standard position in DIDO and a calculation route for equiva­lent DIDO nickel flux (EDNF) were devised.

It should be noted that there are difficulties related to a standard based on measurements made with nickel foils and the 58Ni(n, p,)58Co reaction because of the short half-life of 58Co and the interfering effect of the 58Co(n, g)59Co reaction. A method by Bell eta/.15 which went back to measuring activation of cobalt foils and the 59Co(n, g)60Co reaction, and then calcu­lating the ratio fNi/f Co, was used for a short while. This method used the following relationships:

115g fuel elements fNi/fco = 0.378 — 0.504b

150g fuel elements fNi/fco = 0.502 — 0.530b

where ‘b’ is the fuel burnup. However, this was not very satisfactory and it was clear that a validated calculation route was desirable, and is now becoming practicable through development in computer technology.