Level 3 PSA - EDF Hinkley Point

HPC-NNBOSL-U0-000-RES-000028 Version 1.0
Level 3 PSA
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The overall annual risk arising from LOSAs represents an additional 5 % of the SDO-4
BSO.
5.6.5 Worker Cohorts – Risk Summary
According to the current assumptions the most at risk individual worker on site is found
to be the generic worker, with an annual risk that is 36 % of the target of 1 x 10 -6 y -1 set
by SDO-4. The Fuel Building worker cohort is assessed as having a risk which is close
to the generic worker, at around 33 % of the SDO-4 BSO. The MCR worker cohort risk is
only 1/3 of the generic worker cohort risk when including LOSAs.
5.6.6 Effect of Having Two EPR Units on Site
It is possible to make a preliminary estimate of the effect on the overall risk to a generic
worker of having two EPR units on a single site. This effect can be taken into account
simplistically by making the following assumptions, and then re-calculating the overall
risk for the generic worker cohort (In this discussion, the EPR unit at which the generic
worker is employed is referred to as EPR Unit 1 and the adjacent EPR unit on the site is
referred to as EPR Unit 2):
A worker who is employed at EPR Unit 1 will be at risk from the local (in-building)
consequences of accidents occurring at EPR unit 1 (i.e. the local consequences of
Level 1 PSA, PCC and expert review accidents). The risk arising from LOSAs will
also be unchanged, as in all cases the in-building consequences of accidents and
performance of LOSAs occurring at EPR Unit 2 will have no effect on workers at
EPR Unit 1 [AS-014] and that the occupancy factors used for the calculation shall not
be impacted by a second unit on site [AS-009].
In the case of the on-site consequences of the Level 1 PSA accidents a worker at
EPR Unit 1 would also be affected by an external (on-site) release from EPR Unit 2,
so the contribution from this source of risk is effectively doubled.
In the case of the PCC and EXRV accidents, a worker at EPR Unit 1 could be
affected by the external release (i.e. on-site consequence) of accidents at EPR Unit
2, but only when the worker is not in a specific building. Therefore, this contribution is
effectively doubled [AS-015].
In the case of the severe (core damage) accidents assessed in the Level 2 PSA, a
worker at EPR Unit 1 is assumed to be equally affected by accidents occurring at
EPR Unit 2 due to the severity of these accidents and the magnitude of the release,
so the risk contribution from these accidents is effectively doubled [AS-015].A
summary of the risk contributions is shown in Table 7 below.
These results indicate that in terms of risk to an individual worker, the effect of having
two EPR units on the same site is minimal and would lead to a relatively small increase
in risk of the order of 13%. This is due to the fact that the risk to workers is dominated by
the risk arising from being exposed to the radiological release from accidents in the
building where the worker is present. The only significant effect of having two EPR units
on site in terms of worker risk is the effective doubling of the contribution from severe
core damage accidents.
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