The NCG’s strategy required a detailed assessment of the potential damage to the containment, fuel and nuclear shutdown/hold down components of the two nuclear reactor systems.
The determinant of a safe and complete shutdown was, primarily, the ability of each reactor’s fuel and shut down systems to function adequately during or immediately following the high levels of impulse loading from the torpedo explosions. Each reactor is supported in a resilient cradle providing shock absorption to cater for an impulse well in excess of 50g, defined by military operations of the boat. To determine the actual loading in the immediate aftermath of the explosions, forensic examination was undertaken on two of the casualties recovered from the stern section, these being identified as the reactor control room operators. These casualties had sustained skeletal damage indicative of body shock loading of just over 50g. This provided a degree of reassurance that the reactor resilient mounts, being below or about the design limit, had not been damaged and that the reactors could have closed down automatically as intended after power supplies had been lost.
Other factors relating to the condition of the reactor systems included:
i) The shock level (~50g) would have also temporarily disoriented the reactor control personnel who would not have been expected to recover for several minutes by which time most, if not all, of the power required for operator intervention would have been lost;
ii) the shock would have caused opening of electrical circuit breakers leading to loss of power, e.g. to the main coolant pumps;
iii) automatic reactor shutdown sequences would have been initiated probably by the above power loss — this sequence causes the insertion under gravity, with spring and pneumatic assistance, of two diverse means of neutron absorption which then lock into place, and the initiation of decay heat removal which makes use of the large thermal capacity of the water in the shield tank — this sequence cannot easily be interrupted by the operator and it was unlikely that its essentially passive role was impeded by the shock loading;
iv) the reactor shutdown and decay heat removal equipment had a design basis for an inclination of the submarine in excess of 45° for forced cooling during pump spin down, and thereafter a lesser angle of inclination for heat dissipation by natural circulation — the depth of water (108m) in conjunction with the submarine’s length (155m) precluded a larger inclination — in view of the perceived accident sequence it is probable that the maximum trim angle was much less than 45º, although this may have been exceeded for a few seconds or more when the hull responded to the expansion of the explosive gas products; and
v) the design provides four barriers to the escape of fission products (or particulates) from the nuclear fuel. These comprise the fuel assembly cladding, the reactor primary circuits, and the reactor shield boundaries.
To confirm the state of the reactor the RF deployed gamma spectroscopy in the range 4 to 8 MeV (characteristic of reactor operation) in the lower regions outside the pressure hull and the thermal gradient in the flood hull space was profiled to detect any thermal input. Negative results suggested that:
i) the reactors remained shutdown;
ii) there was effectively no contamination (e.g. fuel particulate) in the shield tank, suggesting that the reactor primary circuit containment is complete;
iii) there is no contamination between the shield tank and the pressure hull, suggesting that the shield tank containment is complete; and
iv) the lack of any thermal gradient indicated that no significant heat was being generated in either of the reactor compartments.
On this basis, the NCG’s criterion that at least two of the reactor containments be in place was satisfied.