The HGCR-1, a Design Study of a Nuclear Power Station Employing a High-Temperature Gas-Cooled Reactor with Graphite-UO₂ Fuel Elements Page: 70

                                
Table 6. Comparison of Activity Released from Various Types of Fuels
for Same Temperature Structure
Diffusion of    Total
Fuel                                  Diffusion of  Recoil Activity  Activity
Type        Method of Fabrication     Activity from  from Graphite  Released
UO2  (curies)   (curies)     (curies)
A    Impregnation                     1.2 x 106    97.6 x 106   98.8 x 106
B    HGCR-1 matrix and U02 and graphite  3.45 x 106  6.1 x 106  9.55 x 106
(200-p U02)
C    U02 clad with graphite          3.45 x 106        '0       3.45 x 106
surface temperature the relative release of activity from fuel B or C would depend upon
the particle size of fuel B and upon the dimensions of the U02 in fuel C. An increase
in the particle size from 200 to 2000   would essentially result in activity releases
similar to those of fuel C without imposing the penalty of the increased temperature
difference associated with fuel C.
DISPOSITION OF ACTIVITY IN PRIMARY SYSTEM
In addition to decay, the other possible processes for removing nuclides from the
circulating gas are (1) deposition on primary surfaces, (2) leakage from the primary
system, and (3) removal to a bypass purification system. There is insufficient experi-
mental information available to predict deposition rates for removing the fission products.
Since the leakage from the system must be minimized, this process for removal will not
be significant. Most of the nuclides listed in Table 5 have sufficiently short half lives
for a purification rate of the order of >>1% bypass to give significant reductions in
activity during operation (see Fig. C.5, App. C). Since the problems and cost associated
with handling such a large volume of gas would be excessive, the reduction of impor-
tant gamma-emitting activity during operation by this process is impractical. The
operation of the purification system would be important for reducing the activity of the
long-lived nuclides. Although this would not significantly decrease the gross activity
during operation, it might bring about large decreases in after-shutdown activity.
The purification system proposed for the HGCR-1 is similar to the system described
for the GCR-2 (see Chap. 5). With a flow rate of 1% per hour, the reduction in the
operating activity in the system would be only about 15% (see Fig. C.5, App. C), if the
effect of deposition were neglected. However, with this flow rate and with the long-
lived nuclides neglected, the La14 activity would be reduced by a factor of 5, and the
Ba137 and Y90 activities would be essentially eliminated. The reduction or elimination
of these long-lived activities would, of course, reduce the dose rate after shutdown
quite significantly and would simplify the problems associated with maintenance.

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